Autodesk Moldflow Communicator Modeling

Transcription

1 Autodesk Moldflow Communicator 2012 Modeling

2 Revision 1, 23 March 2011.

3 Contents Chapter 1 Modeling Recommended modeling details How plastic fills a mold Cycle time How thickness affects flow Tapering walls Mold Inserts Mold types Injection locations Design rules of polymer injection locations Number of polymer injection locations Selecting polymer injection locations Injection location temperature during packing and cooling Injection locations on 3D mesh Injection time Feed system Selecting the number of cavities iii

4 Sprue Gates Runners Cooling system Cooling considerations Cooling circuits Cooling components Coolant iv

5 Modeling 1 The flow simulation will only be as good as the model you create. This section describes the tools that are available to assist in modeling, provides detailed instructions on how to perform various procedures, and highlights pitfalls to avoid in order to ensure that your model can be filled. Recommended modeling details Injection molded parts often have small complex features so the construction of models is usually the most time-consuming aspect of model analysis. It is not always necessary to create a complete model. Simplifications that save modeling and analysis time can often be made. The fundamental principle is that the model must be equivalent to the flow in the real part. Flow is dictated by the following elements, listed according to their significance. Part thickness Flow length Volume The aim should therefore be to create a model with the same thickness, flow length and volume as the real part. When good practices are observed, a simple model will generally provide similar results to a more complex model of the same part. How plastic fills a mold There are three distinct processing phases in injection molding. The three phases in the molding process are as follows: Filling phase Packing phase Cooling phase 1. Filling phase During the filling phase, plastic is pushed into the cavity until the cavity is just filled. As plastic flows into the cavity, the plastic in contact with the mold wall quickly freezes. This creates a frozen layer of plastic between the mold and the molten plastic. At the interface between the static frozen layer and the flowing melt, the polymer molecules are stretched out in the direction of flow. This alignment and stretching is called orientation. 1

6 The following diagram shows how the flow front expands as material from behind is pushed forward. This outward flow is called fountain flow. The edges of the flowing layer freeze as they come into contact with the mold wall in a near-perpendicular direction. The molecules in the initial frozen layer are therefore not highly orientated, and when they are frozen, the orientation will not change. The red arrows in the diagram show the flow direction of the molten plastic. The dark blue layers show the layers of frozen plastic against the mold walls. The green arrows indicate the direction of heat flow from the polymer melt into the mold walls. The frozen layer gains heat as more molten plastic flows through the cavity, and loses heat to the mold. When the frozen layer reaches a certain thickness, equilibrium is reached. This usually occurs early in the injection molding process, after a few tenths of a second. 2. Packing phase The packing phase begins after the cavity has just been filled. During this phase, further pressure is applied to the material in an attempt to pack more material into the cavity. This is intended to produce a reduced and more uniform shrinkage with reduced component warpage. When the material has filled the mold cavity and the packing phase has begun, material flow is driven by the variation of density across the part. If one region of a part is less densely packed than an adjacent region, polymer will flow into the less dense region until equilibrium is reached. This flow will be affected by the compressibility and thermal expansion of the melt in a similar way to which the flow is affected by these factors in the filling phase. The pvt (pressure, volume, temperature) characteristics of the material provide the necessary information to calculate parameters such as density variations with pressure and temperature, compressibility, and thermal expansion data. When combined with the material viscosity data, an accurate simulation of the material flow during the packing phase is possible. The following diagram shows the difference between the end of the filling phase (left) and the end of the packing phase (right). 2 Modeling

7 In practice, due to the limitations of pressure and available unfrozen flow channel, it is not possible to pack enough material into the mold to fully compensate for shrinkage. The uncompensated shrinkage must be allowed for by making the cavity bigger than the desired part size. 3. Cooling phase Although the cooling of the plastic occurs from the commencement of the filling phase, the cooling phase is the time from the end of packing to the opening of the mold clamps. This phase is the extra time that is required to cool the part sufficiently for ejection. This does not mean that all sections of the part or runner system have to be completely frozen. The material at the center of the part reaches its transition temperature and becomes solid during cooling time. The rate and uniformity at which the part is cooled affects the finished molding quality and production costs. Mold cooling accounts for more than two-thirds of the total cycle time in the production of injection molded thermoplastic parts. Cycle time Cycle time is the total time required to complete all the stages of the injection molding cycle. The cycle time is made up of the following stages: Fill time. Packing time The time required to fill the mold with polymer. The injection molding machine controls the velocity (flow rate) of the molten polymer entering the mold during this stage of the cycle. The stage of the injection molding cycle when pressure is applied to the polymer melt to compress the polymer and to force more material into the mold. This compensates for the shrinkage that occurs as the polymer cools from the melt temperature to ambient (room) temperature. From 5 to 25 percent more material can be added to the mold during the packing stage. The gate should freeze during the packing time to prevent material from exiting the mold. The Packing time is also known as the Holding time. Modeling 3

8 Cooling time. Mold open time. The cooling time is the stage of the injection molding cycle when there is no more pressure being applied to the polymer. The mold is held shut and the polymer continues to cool until the part can be ejected. The cooling stage is normally the longest part of the molding cycle and can account for up to 80 percent of the total cycle time. The time for which the mold is open before the next molding cycle begins. This time includes the following: Opening the mold Ejecting the part Preparing for the next cycle, such as loading inserts (not always part of the cycle) Closing the mold How thickness affects flow Molten plastic will preferentially flow through thicker sections of a mold, but subtle changes to the part geometry can help to balance the flow. Flow leaders and deflectors A flow leader is an increase in thickness along a flow path to increase the rate of flow along that path. A flow deflector is a decrease in thickness along a flow path to decrease the rate of flow along that path. Flow leaders and deflectors can be used to ensure all flow paths within the cavity fill at the same time to achieve balanced flow paths. Often the most suitable polymer injection location will not define equal flow paths, and the use of multiple polymer injection locations creates extra unwanted weld lines. Altering thicknesses within the design specifications is then the most appropriate way to balance the flow paths. The following diagram of a square plate of uniform thickness with a polymer injection location in the center will demonstrate flow. The part on the left shows a radial flow pattern with unbalanced filling causing areas that fill early to be overpacked, which results in distortion problems. In the part on the right, the thickness of the plate has been increased from the center to the corners of the part, decreasing the resistance to flow in these directions The following animation shows how changing the thicknesses in some sections of a part can create a more balanced flow pattern. The flow leaders 4 Modeling

9 and deflectors will create a fill pattern that is closer to a balanced flow, and additional refinements can be made to balance the flow further. NOTE: Where possible, use flow deflectors instead of flow leaders to minimize the weight of the part. Shrinkage is a function of thickness so always consider the effects that changes in thickness might have on warpage. Changes such as those above can only be made using a licensed Autodesk Moldflow Adviser or Autodesk Moldflow Insight product. Tapering walls Some mold surfaces must be tapered so that the plastic part can be ejected from the mold when it has cooled sufficiently. Untapered mold walls The walls of the mold in the following diagram, which are marked with red, are not tapered. When the ejector pins push the finished part out of the mold, the force applied must overcome the friction between the mold wall and the plastic part. As shown in the following animation, when the mold walls are not tapered, the frictional resistance continues throughout the ejection process. Plastic parts that do not have tapered walls can be impossible to eject from the mold. Even if the part is ejected, the surface can be scuffed in the process, making the part visually unacceptable. Modeling 5

10 Tapered mold walls The walls of the mold in the following diagram, which are marked in red, are tapered. As shown in the following animation, when the ejector pins push the finished part out of the mold, there is an initial resistance due to friction which then is reduced to zero when the part is moving. Depending on the surface finish of the part, a draft angle of 1.5º for highly polished surfaces up to 6º-8º for a leather like surface will enable the part to be easily ejected from the mold. Mold A mold is a series of machined steel plates containing a cavity or cavities into which molten plastic is injected at high pressure. When the plastic cools, it solidifies into the shape of the cavity defined by the steel plates. 6 Modeling

11 Inserts There are two types of inserts, mold inserts and part inserts. Mold inserts Mold inserts are made of metal and are attached to the mold. They are used to create a cavity in the part. Mold inserts can be created with a Midplane, Dual Domain or 3D mesh. Modeling the mold insert as a volume improves the accuracy of the heat transfer calculations. When you assign a mold material to elements instead of modeling the insert, a default small thickness is used for the insert material, which may be less accurate. This approach may be used when investigating mold design, but for more precise results you should model the insert as a volume. Consideration of inserts in Midplane, Dual Domain, and 3D analyses The following table list the types of inserts that are considered by Midplane, Dual Domain, and 3D analyses where applicable. Inserts considered in Midplane analyses Part insert Fill+Pack Cool BEM Warp Mold insert Not applicable Not applicable In-Mold label Not available Not applicable List item. on page 7 Core Not applicable List item. on Not applicable page 7 NOTE: 1 In-Mold labels do not have a significant effect on warpage. 2 In Midplane analyses, a core is considered as a mold element and included in the mold cool analysis. Modeling 7

12 Inserts considered in Dual Domain analyses Part insert Fill+Pack Cool BEM Not considered Warp Mold insert Not applicable Not applicable In-Mold label Not available Not applicable List item. on page 8 Core Not applicable List item. on page 8 Not applicable NOTE: 1 In-Mold labels do not have a significant effect on warpage. 2 In Dual Domain analyses, a core is considered as a mold element and included in the mold cool analysis. Inserts considered in 3D analyses Part insert Fill+Pack Cool BEM Cool FEM Warp Mold insert Not applicable In-Mold label Not applicable List item. on page 8 Not applicable List item. on page 8 Core List item. on page 8 Not applicable NOTE: 1 Although In-Mold labels are ignored in a Cool (FEM) analysis, it is possible to use the HTC conditions to simulate the label's insulating effect. 2 In-Mold labels do not have a significant effect on warpage. 3 The core temperature is calculated when the Calculate internal mold temperature option is enabled in the 8 Modeling

13 Cool 3D solver parameters. This calculation uses default mold steel material properties unless another material was defined for the core. Mold inserts Inserts are parts of the mold that are created separately from the mold cavity block. Inserts are inserted in the block to achieve a desired cavity shape or cooling effect. The effect of inserts on heat transfer An insert will only assist heat transfer if a cooling channel is located in or near the insert. Inserts can be used to modify the rate of cooling in specific areas of the mold. A common example is a part with ribs that are thinner than the main surface. There is a natural tendency for the part to deflect away from the thinner ribs as the main surface has higher area shrinkage. By running the rib area hotter (conductivity insert), the part can be deflected back to the required shape. In some cases, an insert of lower conductivity or containing a separate cooling circuit, can be used to form the ribs. This gives better control of the rib temperature. Modeling inserts Inserts are modeled with regions representing each of the faces of the insert. In its simplest form, the complete insert is effectively a closed volume defined by the six surfaces of a cube. Inserts can have complex cross-sections to match features on the cavity model, in which case they consist of more than the basic six surfaces. Inserts can be added to inserts to create more complex shapes. When small inserts abut onto larger inserts, the surface of the larger insert must have an internal boundary for the smaller insert to connect with. When the edge of a plastic surface runs across an insert surface, the insert surface should have an internal boundary added to ensure that the mesh on the plastic surface and the mesh on the insert surface are compatible. Inserts must not contact the mold outer surface. NOTE: Each insert must be meshed and correctly oriented. Modeling 9

14 Modeling mold inserts Inserts are modeled with flat shell surfaces representing each of the faces on the insert. The complete insert is effectively a closed volume defined by the top bottom and side surfaces. Inserts can have complex cross sections to match features on the cavity model. Inserts can also be joined to one another to create more complex shapes. Modeling requirements Do not apply a thickness values to the insert surface(s). Inserts must not contact the mold outer surfaces. When a small insert butts onto a larger insert, the surface of the larger insert must have an internal boundary where the smaller insert contacts it. When the edge of a plastic surface runs across an insert surface, the insert surface should have an internal boundary added to ensure that the mesh on the plastic surface and the mesh on the insert surface are compatible. Where an insert projects from a feature such as a boss on the plastic part, the insert surfaces should either have internal boundaries coincident with the open edge of the boss or the insert should be modeled as two inserts stacked end to end, with the junction occurring at the height of the end of the boss. This will ensure that the mesh on the insert surface is compatible with that on the coincident plastic surface. This modeling requirement is illustrated in the following figure. Modeling mold inserts examples This topic provides examples of an insert that is in poor contact with the surrounding mold, an insert that is in good contact with the mold, and a stepped insert. 10 Modeling

15 Inserts in poor contact with the mold This modeling assumes there is poor contact between the insert and the rest of mold material due to clearance for ease of mold assembly. This picture illustrates how inserts in poor contact with the mold material are modeled. The surface in contact with the air gap is assigned a very low interface conductance value. The insert is made from three surfaces, each with eight sides, creating three independent closed volumes. Inserts in good contact with the mold This modeling assumes there is a good contact between insert and rest of the mold material. NOTE: To satisfy the requirement for a discrete boundary on the insert surfaces, where they are crossed by the edge of the surfaces forming the plastic boss, you can create one insert which is the length of the boss and then another, as shown in the image above: from the end of the plastic boss into the mold to represents the total insert depth. These inserts can be most easily created in Autodesk Moldflow Insight. Alternatively, create a single insert of the required length with internal points or boundaries. Stepped inserts Model stepped inserts by creating one insert to represent the small end and a second to represent the larger end. The end face of the larger insert must have an internal boundary the same size as the smaller insert to ensure connectivity between the inserts. Modeling 11

16 NOTE: Meshing must be completed before analysis commences, and so cannot be changed from inside Autodesk Moldflow Communicator. Mold types You can design different mold types, depending upon whether you want to produce a single part or multiple parts from a single mold. Single-cavity molds Two-plate mold A single-cavity mold has only one cavity, so it can only produce one finished part per injection cycle. A single cavity mold is suitable for producing large parts, or for short part runs. The most commonly used mold type is the two-plate mold, which is cheaper and simpler to make, and has a lower cycle time than the three-plate mold. Many single-cavity molds use a two-plate design. If ther part can be made with a single injection location and no runners are required, the polymer can be injected directly into the cavity via the sprue. Multi-cavity mold Two-plate molds A multi-cavity mold has two or more cavities. The cavities may be identical, so that they produce multiple copies of a part in a single injection cycle, or they may be different, so that they produce the related parts of a family mold in a single injection cycle. Multi-cavity molds are suitable for producing parts in high volume, or for producing small parts. The two-plate mold is the most commonly used mold type. Compared to the three-plate mold, the two-plate mold is cheaper and simpler to make and has a lower cycle time. Before you design a multi-cavity mold, you should analyze the single part to determine the gate location. In a two-plate mold, the runners and gates must be in the parting plane so they can be ejected when the mold opens. It is possible to use a two-plate mold for multi-cavity or family molds, but 12 Modeling

17 only if the parting plane can be aligned with the gate, as illustrated in the figure below. When designing a multi-cavity mold, it is important that you design the runners so that the flow of polymer is balanced. With some multi-cavity molds it might not be possible to balance the flow of material using a conventional two-plate mold, so you might have to use a three-plate mold, or a two-plate mold with hot runners, instead. Two-plate mold with hot runners In a two-plate mold with hot runners, the material remains in a molten state through the sprue, runners, and gate and only starts to freeze when it enters the cavity. Hot runners are ideally suited to multi-cavity molds where the molded parts are small. If there are many small parts, a conventional runner system can waste a lot of material, particularly if no regrind of materials is possible. In a two-plate mold with hot runners, once the material in the cavities has frozen and the mold has been opened, only the molded parts and any cold runners are ejected. When the mold is closed again, the molten polymer that is still in the runners fills the cavity again. Runners in a mold can be a combination of both hot and cold runners. Two-plate molds with hot runners can be used as an alternative to a three-plate mold because the runners don't need to be ejected. In the following diagram, the injection location for this mold has been placed at the center of the cavity, to avoid marking the part on a visual side; therefore, the runners cannot be positioned on the parting plane. Modeling 13

18 The advantages of hot runner systems tend to outweigh the disadvantages when producing large quantities of high-quality parts. Sometimes the best results can be achieved by using a combination of hot and cold runners. Advantages of hot runners Less material waste and no regrinding Less obvious gate marks Degating may not be required Shorter cycle time Greater control over the filling of the mold and flow of the polymer Disadvantages of hot runners Higher initial setup costs More difficult to change material color More prone to breakdown, especially the heating control system Not practical for thermally sensitive materials Three-plate mold A three-plate mold is used when part of the runner system is on a different plane to the injection location. The runner system for a three-plate mold sits on a second parting plane parallel to the main parting plane. This second parting plane enables the runners and sprue to be ejected when the mold is opened. In the following diagram of a three-plate mold, the runners will be ejected separately to the cavities. The runner system can be placed on a different plane from the injection location in the following situations: The mold contains multiple or family cavities A complex single cavity mold requires more than one injection location The injection location is in an awkward position Achieving a balanced flow requires the runner to be outside the parting plane 14 Modeling

19 You can use a two-plate mold with hot runners to overcome these problems, but three-plate molds have some advantages over hot runner molds. Advantages of three-plate molds Cheaper to build than a hot-runner mold Less likely to break down than a hot-runner mold Thermally sensitive materials are less likely to degrade Disadvantages of three-plate molds Higher cycle time due to ejection of the runner system More material is wasted Greater injection pressure is required to fill the mold Injection locations The injection location represents the position where polymer is injected, enabling the software to simulate the flow pattern inside the mold cavity. This topic describes the considerations affecting the selection of injection locations when running a Fill+Pack analysis. To mold the best part possible, you must identify the optimum injection location. This will create a balanced filling pattern by allowing the extremities of the mold to fill at the same time and under the same pressure. You must also select the number of injection locations required and their positions so that the volumetric shrinkage at the end of the flow is close to the design value. NOTE: There is no gate size associated with the injection cone; it simply represents the mathematical starting point of an analysis. A gate should be modeled to ensure accurate results. If the initial analysis indicates that the fill pattern is unbalanced, you should alter the injection location, or add another one, to solve the problem. Modeling 15

20 Figure 1: Unbalanced flow (above) Balanced flow (below) It may be necessary to use several injection locations, or to even change the type of gate being used, for example, to an edge gate, to produce more uniform orientation effects in the product. Gate Location Analysis The Gate Location analysis is used to identify the most suitable injection location to create a balanced filling pattern. Using the Advanced Gate Locator algorithm, the Gate Location analysis determines the best locations for the specified number of gates. Prohibited gate location regions are excluded from the analysis. Using the Gate Region Locator algorithm, if there are no injection locations specified, the Gate Location analysis will determine the best place for a single gate, given the selected material. If one or more injection locations already exist, the result suggests the best place for the next gate location given the selected material. Gas-assisted Fill+Pack analysis To use the Gas-assisted Fill+Pack analysis, you must identify the node at the gas injection location. Gas, like the molten polymer, always flows toward the area of lowest pressure. Therefore, select the gas injection location to ensure that the gas stays in the gas channel, and that the area of lowest pressure is near the end of the gas channel. Design rules of polymer injection locations The positioning of injection locations greatly affects material orientation and part warpage. In some cases, changing the gate position is the only way of controlling the orientation effects and producing a satisfactory design. Each polymer injection location injects the material under the same pressure. Unless an end-of-fill spike occurs, the pressure increase during the injection period is generally linear. The aim of proper gating is to avoid problems associated with overpacking, such as variation in shrinkage and product sticking in the cavity. 16 Modeling

21 Design Rules Gate centrally to provide equal flow length Gate symmetrically to avoid warpage Gate into thicker sections for better filling and packing Centralized gates provide equal flow lengths to all extremities of the part. This results in more even packing in all directions and a lower shrinkage difference, which leads to a higher quality part and lower rejection rate. Symmetrical parts should be gated symmetrically to achieve balanced flow and avoid differential shrinkage and subsequent warpage of the part. Place polymer injection locations in thicker regions of the part, preferably at a spot where the function and appearance of the part are not impaired. This causes the material to flow from the thickest areas to the thinnest areas, and helps maintain the flow and packing paths. Gating into thinner sections can result in hesitation or sink marks and voids. The following animation shows how restrict material flow through a thin section can lead to material flow problems. Gate long, narrow parts from an end for uniform flow When a long narrow part is centrally gated, packing near the gate and variable molecular or fiber orientation throughout the part will cause differential shrinkage, which will warp the part. Gating a long part at one end will provide a uniform molecular and fiber orientation in the length direction. Although the end with the gate will be packed more than the opposite end, the resulting difference in shrinkage will not cause warpage. The following figure shows the prefered injection location. Modeling 17

22 Position the gate away from load-bearing areas Hide the gate scar The high melt pressure and high velocity of flowing material at a gate causes that area to be highly stressed. For this reason, you should locate the gate away from load-bearing areas. The removal of the gate will leave a mark on the part, which could be visually unacceptable. Place the gate so that the gate scar can be hidden or easily disguised. Vent properly to prevent air traps Gate for proper weld-line location and strong weld lines The gate location should prevent air traps by enabling the air in the cavity to escape during injection. Failure to vent the air will result in a short shot, a burn mark on the molding, or high filling and packing pressure near the gates. The gate location should cause weld and meld lines, if any, to form at appropriate positions that are not objectionable to the function, external load, or appearance of the part. Place the gate so that weld lines are formed early in the filling phase or at a high pressure area to ensure proper melding of the weld lines. Multiple gates shorten flow lengths Add gates so that flow paths are within the flow limits for the thickness, distance, and process conditions. Each gate should have equal flow rates and volumes. NOTE: Changes such as those above can only be made using a licensed Autodesk Moldflow Adviser or Autodesk Moldflow Insight product. 18 Modeling

23 Number of polymer injection locations Because each part is different, there are no specific rules for determining the number of polymer injection locations; however, there are some general factors that must be considered. Flow length Flow length refers to how far the polymer must flow from the polymer injection location. Generally, parts with thicker walls can have longer flow paths than thin-walled parts because the material will flow more easily in the thicker regions, as shown in the following diagram. The flow characteristics of the plastic material affect how far a material will flow for a given thickness. The shorter the flow length, the more gates required to fill the part. Each material has its own flow length. The materials datasheets from material suppliers contain information about flow lengths that can be achieved for each specific material at a range of thicknesses. Very large parts, thinner walled parts and higher viscosity materials will typically require more gates to fill a part. Part volume Generally, parts with larger volumes require more polymer injection locations to fill properly. Selecting the correct number of injection locations First, try a single gate in the flow centroid of the mold, and check that all flow paths fill at nearly the same instant in time. If this criteria cannot be met with a single gate, then try using multiple gates. Mentally, divide the part mold into sub-moldings, with a gate positioned at the flow centroid of each sub-molding or in the middle of one side. The runner system dimensions should be set up so that each sub-molding is filled at nearly the same instant in time, as shown in the following diagram. Modeling 19

24 The positions of the gates should achieve both uniform and acceptable values of shrinkage across the part. Where thick and thin sections are present, it is good practice to position the gate nearer to thicker sections. This avoids insufficient packing, which is caused by premature freezing of the material between the gate and the thicker regions. NOTE: Changes such as those above can only be made using a licensed Autodesk Moldflow Adviser or Autodesk Moldflow Insight product. Selecting polymer injection locations Because injection locations greatly influence the way in which the plastic flows into the mold cavity, their postioning directly affects part quality. One of the goals when selecting polymer injection locations is to ensure that all flow paths in the cavity fill at the same time. This prevents overpacking along the flow paths which might otherwise fill first. The three possible polymer injection locations in the following diagram of a model show how the polymer injection location can be used to help balance flow The polymer injection location can also be used to change the position of weld lines and air traps, and reduce hesitation and other molding problems. The above animation shows how polymer injection locations 1 and 2 cause a weld line to form on the right of the model, and polymer injection location 3 causes a weld line to form on the bottom right of the model. In 20 Modeling

25 some cases it is necessary to have more than one polymer injection location. Other methods can be used to help balance flow paths, such as including flow leaders or flow deflectors. NOTE: Changes such as those above can only be made using a licensed Autodesk Moldflow Adviser or Autodesk Moldflow Insight product. Injection location temperature during packing and cooling The injection location temperature moves toward the mold temperature during the packing and cooling phases because there is negligible flow from the barrel during these phases. The injection location temperature affects the flow of the melt into the part during packing, usually by decreasing the flow. To achieve realistic warpage values, it is important to take the injection location temperature into account during the packing and cooling phases. NOTE: The injection location temperature cools when using thermoplastic materials and heats when using thermoset materials. Two different calculations are used depending on the type of model that you are analyzing. Midplane and Dual Domain Dual Domain 3D The initial melt temperature constraint at the injection location is ignored when filling is complete. The injection location temperature change is calculated by using the mold temperature as well as convection and shear heating information. The initial melt temperature constraint at the injection location is ignored when filling is complete. The injection location temperature change is calculated using the mold temperature as well as convection and shear heating information. The mold temperature and the cylindrical diameter of the injection location are used to calculate the injection location temperature change during packing and cooling. The injection temperature at the end of fill is used for the initial temperature in the calculation, and only heat conduction is considered. When injecting into a beam element, the calculation uses the diameter of the beam element. When injecting directly into the cavity, the contact diameter of the gate is used. Smaller injection location or beam diameters result in faster temperature changes. Modeling 21

26 Injection locations on 3D mesh The injection location represents the position where polymer is injected, allowing the software to simulate the flow pattern inside the mold cavity. This help topic describes some injection location considerations when running a Fill+Pack analysis on a 3D model in Autodesk Moldflow Insight. 3D Fill+Pack analysis When you select an injection location on a tetrahedral mesh, the location is associated with a node on the mesh. When a Fill+Pack analysis is performed with no gate modeled, the gate size is automatically assigned based on the part geometry, or in the case of small parts the average facet size of the tetrahedral elements on the surface around the injection location. The actual area used for material injection is defined by the number of nodes which fit inside the virtual gate diameter. If a fine mesh is used, the actual injection area will be closer to the virtual gate size than for a coarse mesh. For example, in the first image below the fine mesh size fits well into the virtual gate diameter, but the second image shows that a coarse mesh could produce a smaller than expected gate size. 22 Modeling

27 Injection time The injection time is the time it takes for the mold to fill completely. When you set up a Fill+Pack analysis sequence, the software can be set to either calculate a machine injection time automatically, or on the basis of a user-specified value. By default, the injection time is calculated automatically. Automatic injection time If you set the injection time to Automatic, the analysis finds the injection time which gives the lowest injection pressure. The following graph shows the results from nine analyses on the same part. The blue points represent the analyses where the injection time was set to a particular value. The red point represents the analysis where the Automatic injection time check box was selected, which shows the lowest possible injection pressure for the part. Modeling 23

28 MPa Figure 2: Injection pressure as a function of time The variation of injection pressure against injection time has two influences. Firstly, as the injection time increases from zero, the pressure required to force the molten plastic through the part decreases. Secondly, as the injection time increases, the polymer temperature decreases due to heat transfer to the mold, which causes the viscosity and frozen layer thickness to increases, which in turn increases the injection pressure. Specified injection time If a specified injection time is entered, the Summary lists an actual injection time that is slightly higher than the value entered. The extra time is to account for material compressibility. Feed system There are a number of factors to consider when designing the feed system, including the gate locations, the number of cavities, the shape of the runner system components, and flow balance. The first step in designing the feed system is to determine the gate locations for each part in the mold. The rest of the components will fit into place depending upon each gate location. The objective when designing the feed system is to design it with balanced flow so that each part in the mold fills at the same rate. The creation of a well-balanced feed system requires careful consideration the following elements: Single-cavity, multi-cavity, or family mold Cavity layout Location of the sprue Runner system layout Shape of the sprue, runners, and gates In general, make runners as short as possible, with the lowest possible shot weight. In the following diagram, the flow length for every part is the same. This is a naturally balanced runner system. 24 Modeling

29 Selecting the number of cavities A number of factors should be considered when deciding how many cavities can be filled from a runner system. The number of cavities which can be filled from any given runner system depends on the following: Machine size (available clamp force) Available shot volume Available production time Required product quantity Shape and size of the moldings Mold costs The number of cavities filled from a particular runner system can affect the ease with which the program arrives at a runner balancing solution. The possible combinations of pressure drops in the runner system itself and in filling the cavity volumes can rapidly increase the complexity of the balancing problem. Following are simple formulas for determining the number of cavities. Use the minimum value derived from these formulae. Production schedule If the dimensional tolerance of the part is not very critical and a large number of moldings are required, multi-cavity molds are preferred. Number of cavities = LKtctm where: L = number of parts in the lot K = the reject factor, expressed as K = 1/(1-reject rate) tc = cycle time to produce a single set of parts tm = available time to supply a lot of parts Shot capacity Number of cavities = SW where, Modeling 25

30 S = 80% of machine capacity W = part weight Plasticizing capacity Number of cavities = PXW where, P = machine plasticizing capacity X = estimated number of shots per minute W = part weight Sprue The sprue is the extension of the injection nozzle into the mold. In a single cavity mold with a single injection location, the sprue can meet at the cavity wall. The sprue is usually connected to a runner system as shown in the following diagram. The angle of the taper on a sprue should be large enough for it to be easily ejected, but not too large because the cooling time and the required amount of material increases along with the size of the sprue diameter. Gates Gates connect the runner system to the cavity and are the orifices through which the melt enters the mold. When you design gates, you should consider the following: The final appearance of the molded part Removal of the gate Complexity of the cavity The material used The volume of the material injected into the mold Before designing the runner system, you should run a Gate Location analysis for each cavity to find out the best gate locations. For parts where appearance is important, the gates should be narrow to prevent large 26 Modeling

31 blemishes on the surface of the part. A smaller opening will also make gate removal easier. Make gates short, to prevent large pressure drops and avoid sharp angles between gates and runners, which could contribute to a pressure drop in the system. Make corners rounded, so that the melt flow is not inhibited. The cross-sectional shape you choose for the gate depends on the shape of the runners. The gates are highlighted in the following diagram. Manually trimmed gates Gates can have many different configurations but they are broadly classified based on the method of gate removal into manually trimmed and automatically trimmed gates. Manually trimmed gates require an operator to separate the parts from runners during a secondary operation. Manually trimmed gates are used for the following reasons: The gate is too large to be sheared from the part as the tool is opened. Some shear-sensitive materials, such as PVC, should not be exposed to the high shear rates inherent to the design of automatically trimmed gates. Simultaneous flow distribution across a wide front to achieve specific orientation of fibers or molecules often precludes automatic gate trimming. Manually trimmed gate types include: Direct or sprue gate Disc or diaphragm gate Edge or standard gate Fan gate Film or flash gate Overlap gate Ring gate Spoke or spider gate Tab gate Modeling 27

32 Direct or sprue gate A direct gate is commonly used for single-cavity molds, where the sprue feeds mater directly and rapidly into the cavity with minimum pressure drop, as shown in the following diagram. Disc or diaphragm gate The disadvantage of using this type of gate is the gate mark left on the part surface after the sprue is trimmed off. Freeze-off is controlled by the part thickness rather th determined by the gate thickness. Typically, the part shrinkage near the sprue gate w be low; shrinkage in the sprue gate will be high. This results in high tensile stresses near the gate. Dimensions: The starting sprue diameter is controlled by the machine nozzle. The sprue orifice diameter here must be about 1mm larger than the nozzle exit diameter. Standard spru can have tapers from 0.5 degrees to 1.5 degrees (1.0 degrees to 3 degrees included ang with a common size of about 1.2 degrees taper angle (1/2 inch per foot included angl Therefore, the sprue's orifice diameter and length will control the diameter of the spr where it meets the part. Typically, the sprue diameter will be well over double the w thickness of the part, controlling the molding cycle time. A smaller taper angle (a minimum of one degree) risks not releasing the sprue from t sprue bushing on ejection. A large taper wastes material and extends the cooling tim Non-standard sprue tapers will be more expensive to machine, with little gain. A disc gate is often used for gating cylindrical or round parts that have an open insi diameter. This gate is useful when concentricity is an important dimensional requirement, and the presence of a weld line is objectionable. These gates are typica difficult and expensive to trim from the part. As shown in the following diagram, the disc gate has a thin land around the inside edge of the part, which facilitates the removal of the gate. Since the disc is fed from concentric sprue or hot drop, uniform flow to all parts of the gate is easy to mainta 28 Modeling

33 Dimensions: The typical gate thickness (H) is 0.2 to 1.3 mm. Edge or standard gate An edge gate is located on the parting line of the mold, as shown in the following diagram. The gate cross section is rectangular and can be tapered in width and/or thickness between the part and runner. Modeling 29

34 Dimensions: The typical gate thickness (H) is 25 to 75 percent of the part thickness, and the wid is typically two to ten times the thickness. The gate land should be short, typically 0 to 1.0 mm in length. Larger parts can have longer land lengths. Fan gate A fan gate is a wide edge gate with variable thickness, which permits rapid filling o large parts or fragile mold sections through a large entry area. Fan gates are used to create a uniform flow front into wide parts where warpage and dimensional stabilit are main concerns. As shown in the following diagrams, the fan gate tapers in both width and thickne to ensure the following: The flow front velocity will be constant across the entire width 30 Modeling

35 The entire width is being used for the flow The pressure is the same across the entire width Film or flash gate Dimensions: Well-designed fan gates have a narrow land, typically 2.0 mm or less. This land will very thin, typically under 1 mm. The gate width is typically 25 mm to as wide as th part. The main body of the gate will be thin in the center and thick on the edges to promo flow to the outer edges. A film gate consists of a straight runner and a gate land across the entire width of t cavity or a portion of the cavity, as shown in the following diagrams. Modeling 31

36 It has the same objectives as a fan gate but it is more difficult to achieve. The thin la of the gate has areas that cause hesitation, and it is very sensitive to the thickness o the land, diameter of the runner and flow rate. Dimensions: The typical gate size is small, approximately 0.2 to 1.0 mm thick (H). The land area (gate length (L)) must also be kept small, typically under 1 mm. Overlap gate An overlap gate is similar to an edge gate but a portion of the gate overlaps the part shown in the following diagrams. 32 Modeling

37 Dimensions: The typical gate thickness(h) is 25 to 75 percent of the part thickness, and the wid is typically 2 to 10 times the thickness. The gate land should be short, typically mm in length. Larger parts can have longer land lengths. Ring gate With a ring gate, the material flows freely around the core before it moves down as uniform tube-like extrusion to fill the mold, as shown in the following diagrams. Modeling 33

38 NOTE: In practice this is difficult to achieve because the gate relies on hesitation in t thin gate land to achieve a balanced fill. Uniform fill is sensitive to the gate land, runn around the part and the injection time. Dimensions: The typical gate thickness (H) is 0.2 to 1.5 mm. Spoke or spider gate The spoke gate, which is also called a four-point gate or cross gate, is shown in the following diagrams. This gate is used for tube-shaped parts and offers easy gate remo and material savings. Disadvantages are the possibility of weld lines and the fact th perfect roundness is unlikely. 34 Modeling

39 Dimensions The gate cross section can be rectangular like an edge gate and will have similar nomin dimensions, or it can have a circular cross section and be configured like a circular tapered gate. Tab gate A tab gate is typically employed for parts that require low shear stresses, lsuch as opti parts. The high shear stress generated around the gate is confined to the auxiliary t which is trimmed off after molding. A tab gate, which is shown in the following diagra is used extensively for molding PC, acrylic, SAN, and ABS material types. Modeling 35

40 Dimensions: The typical minimum tab width (W) is 5 mm. The typical minimum tab thickness ( is 75 percent of the depth of the cavity. NOTE: Ranges of typical dimensions are given for different gate types. Actual gate dimensions will vary depending on the selected material, part geometry, and number of gates. Automatically trimmed gates Gates can have many different configurations but they are broadly classified according to their method of degating into manually trimmed and automatically trimmed gates. Special features are incorporated into automatically trimmed gates so that the gates are trimmed or sheared when the mold opens and the parts are ejected. Automatically trimmed gates are used to avoid gate removal as a secondary operation, and to minimize gate scars Automatically trimmed gate types include: Hot-runner or hot-probe gate 36 Modeling

41 Pin gate Submarine, tunnel, or chisel gate Valve gate Hot-runner or hot-probe gate A hot-runner gate, which is shown in the following diagram, is generally used to deliver hot material through heated runners directly into the cavity to produce runnerless moldings. The gate or gate tip can have many different configurations from full round to annular. The geometry and size of the gate tip will determine how the gate freezes and the gate scar formed. Pin gate The pin gate is used in a three-plate mold design, where the runner system is on a secondary mold parting line and the part cavity is in the primary parting line. Reverse taper runners drop through the middle plate, parallel to the direction of the mold opening as shown in the following diagram. As the mold cavity parting line is opened, the small-diameter pin gate is torn from the part. A secondary opening of the runner parting line ejects the runners. Alternatively, the runner parting line opens first. An auxiliary, top-half ejector system extracts the runners from the reverse taper drops, tearing the runners from the parts. Dimensions: Typical gate sizes are 0.2 to 1.5 mm in diameter. The design is particularly useful when multiple gates per part are needed Modeling 37

42 to assure symmetric filling, or where long flow paths must be reduced to assure packing to all areas of the part. Submarine, A submarine gate is used in two-plate mold construction. tunnel, or An angled, tapered tunnel is machined from the end of the chisel gate runner to the cavity, just below the parting line, as shown in the following diagram. As the parts and runners are ejected, the gate is sheared at the part. If a large diameter pin is added to a non-functional area of the part, the submarine gate can be built into the pin, avoiding the need of a vertical surface for the gate. If the pin is on a surface that is hidden, it does not have to be removed. Multiple submarine gates into the interior walls of cylindrical parts can replace a diaphragm gate and allow automatic degating. The out-of-round characteristics are not as good as those from a diaphragm gate, but are often acceptable. Dimensions: The typical orifice diameter of the gate is 30 to 75 percent of the part wall thickness. The gate is tapered at a minimum of 10 degrees per side to ensure proper ejection. It is common to have the gate taper to the diameter of the runner. Valve gate The valve gate adds a valve pin to the hot runner gate. Valve gates have a larger gate diameter and they can be opened and closed as needed as shown in the following diagram. This smooths over the gate scar. 38 Modeling

43 Since the packing cycle is controlled by the valve pin, better control of the packing cycle is maintained with more consistent quality. NOTE: Ranges of typical dimensions are given for different gate types. Actual gate dimensions will vary depending on the selected material, part geometry, and number of gates. Runners The runners are the feed channels that connect the sprue to the gates. The design of the runners is important to ensure even filling of the cavities. The design of the runners is dependent on whether the mold is single cavity, multi-cavity, or part only. This topic explains how to lay out the runners and avoid uneven filling, hesitation, and overpacking. Runner balancing During a runner balance calculation, runner dimensions are altered within, or as close as possible without affecting the balanced fill, any constraints set, to ensure that cavities fill at the same time and that the volume of the runner system is minimized. Considerable savings in material usage can be made by creating a well balanced runner system. You can only balance runner systems for single-gated parts. When you create the original runner system, make the runner sizes too big. This will help the runner balance calculation arrive at a sensible solution. It is important to ensure that you are satisfied with each of the following aspects from the Fill analysis before you run a runner balance: Molding conditions. Gate locations. Runner layout. Modeling 39

44 The aim is to achieve runner dimensions which: Have the same pressure drop in all flow paths, so that all cavities fill at the same moment. Minimize the volume of runner material, relative to cavity volume, by achieving the highest possible pressure drops in the runner system. Generate controlled shear heating, to minimize stress levels without using a high melt temperature. Runner properties The shape and diameter of the runners are important factors in successful mold design. The shape of the runner affects the volume of material that remains molten, and the diameter affects the temperature of the melt in the runners and, thereby, the quality of the product and material waste. Effects of shape The cross-sectional shape of the runners affects the flow of the polymer through the runner system. When the hot melt hits the cold metal of the runner a layer freezes and forms a skin on the surface of the runner. The center of the runner remains molten while the polymer is being injected into the mold. The following diagram shows the molten core for different shaped runners. A circular cross section provides the greatest proportion of polymer in a molten state. Runners with a curved or angular cross section require less force to remove from the mold than rectangular or square runners. Although circular runners are the best choice for material flow and ejection, they are also the most expensive. This is partly because the runner needs to be cut into both plates and it is difficult to cut both halves of the runner so that they meet exactly. A trapezoidal cross section can be used as a compromise. Trapezoidal runners often provide acceptable flow and ejection characteristics, and are cheaper to produce than round runners. If you do decide to use a circular runner, extra care is needed to align the two halves of the circular runner to avoid an increase in injection pressure due to the reduced effective flow cross-section. In the following diagram, the runner on the left is correctly aligned, but the runner on the right will have a smaller molten center that will restrict flow. 40 Modeling

45 Effects of diameter A small runner diameter causes shear heating in the runners so the plastic temperature is higher in the runners than in the barrel. Higher melt temperatures reduce residual stress levels and the tendency of parts to warp, but high barrel temperatures can cause degradation of the material. To minimize material waste and decrease the barrel temperature required, design the runners with a small cross-sectional area. NOTE: Changes made to the diameters of runners should be gradual. Avoid creating a large difference in size between the runner diameter and the gate diameter, or the gate diameter and the part surface thickness. Sizeable changes in thickness at these intersections may cause the following molding problems: Rapid changes in flow resistance Flow instabilities Increased injection pressure Designing the runner layout The combination of sprue, runners, and gates is used to transport the melt from the injection nozzle to the injection location for each part. The design of the runner layout affects the amount of material used and the quality of the parts produced. If the flow within each cavity is unbalanced, overpacking and hesitation can lead to poor part quality. Long or poorly designed runners can cause large pressure drops and require a larger injection pressure to fill the part. The following diagram shows a typical runner system for a multi-cavity two-plate mold. When designing the layout of the runner system, you must: Determine the number of cavities required Determine the material type Modeling 41

46 Determine the processing conditions Adjust any flow leaders or deflectors as necessary Flow balance the cavities Runner aspect ratios The runner balance calculation is set to maintain the same geometry aspect ratio of runner dimensions when balancing runners. For example, if the ratio of end diameters for a tapered runner is 2:1, the program will try to maintain this ratio for the balanced runner, as shown in the following diagram. Similarly, the program will try to maintain the ratio of the sides for rectangular runners, as shown in the following diagram. Exceptions to this rule occur when you have specified standard runner sizes. Runner sizes Runner sections Generally, the minimum dimension of a runner cross-section should be 1.5mm greater than the thickness of the part in millimeters. This enables the cavity to pack evenly and produce an even volumetric shrinkage. The selection of runner dimensions may be limited by the type of material and the design of the part. For example, a 1mm diameter runner of styrene may snap when the part is being ejected from the mold, but a 1mm diameter runner of nylon may flex. The results also depend on the type of ejection system that is used. Before modeling runner surfaces, it is important to understand how runner sections are used by the runner balance calculation. Runner balancing ensures that all the cavities finish filling simultaneously. The aim of a runner balance analysis is to achieve the balanced filling of all cavities in the mold by using different runner diameters. 42 Modeling

47 In the following diagram, (A) shows a runner system where the runner on the left has a single runner section, while the runner on the right has two sections. Diagram (B) shows how the Runner Balance tool might balance the system by assigning a single size to the left runner section, and assigning different sizes to the two sections in the right runner. This will result in a balanced runner system that has varying thicknesses in the secondary runners which may or may not be desirable depending on the mold requirements. You can control the size increase or decrease of the runner dimensions by carefully considering the size of the runner surfaces. Runner cross-sections - Shape The shape of the runner affects the volume of material that remains molten. Effects of shape The cross-sectional shape of the runners affects the flow of the polymer through the runner system. When the hot melt hits the cold metal of the runner a layer freezes and forms a skin on the surface of the runner. The center of the runner remains molten while the polymer is being injected into the mold. The following diagram shows the molten center of different runner shapes. A circular cross section provides the greatest proportion of polymer in a molten state. Runners with a curved or angular cross section require less force to remove from the mold than rectangular or square runners. Although circular runners are the best choice for material flow and ejection, they are also the most expensive. This is partly because the runner needs Modeling 43

48 to be cut into both plates and it is difficult to cut both halves of the runner so that they meet exactly. A trapezoidal cross section can be used as a compromise. Trapezoidal runners often provide acceptable flow and ejection characteristics, and are cheaper to produce than round runners. If you do use a circular runner, extra care is needed to align the two halves of the circular runner to avoid an increase in injection pressure due to the reduced effective flow cross-section. In the following diagram, the runner on the left is correctly aligned, but the runner on the right will have a smaller molten center that will restrict flow. Runner cross-sections - Diameter The diameter of the runners influences the temperature of the melt in the runners and thereby the quality of the product and the amount of material waste. Effects of diameter A small runner diameter causes shear heating in the runners so the plastic temperature is higher in the runners than in the barrel. Higher melt temperatures reduce residual stress levels and the tendency of parts to warp, but high barrel temperatures can cause degradation of the material. To minimize material waste and decrease the barrel temperature required, design the runners with a small cross-sectional area. NOTE: Change made in the diameters of runners should be gradual. Avoid creating a large difference in size between the runner diameter and the gate diameter, or the gate diameter and the part surface thickness. Sizeable changes in thickness at these intersections can cause the following molding problems. Rapid changes in flow resistance Flow instabilities Increased injection pressure Hot runners Hot runners have heated runners. The material in the runners remains in the molten state and are not ejected with the molded part. Hot runner 44 Modeling

49 systems, which are also referred to as hot-manifold systems or runnerless molding, minimize flash and gate stubs and reduce material waste. Hot runners - heated runners Heated runners are a type of hot runner in which heat is supplied to maintain the material in a molten state. Hot runner systems can be internally heated or externally heated. Internally heated runners Internally heated runner systems have annular flow passages, with the heat being furnished by a probe and a torpedo located in the passages, as shown in the following diagram. This system takes advantage of the insulating effect of the plastic melt to reduce heat transfer loss to the rest of the mold. Externally heated runners The advantages of an internally heated runner system are improved distribution of heat and better temperature control. The disadvantages of this type of heated runner system are higher costs and a more complicated design. Externally heated runner systems consist of a cartridge-heated manifold with interior flow passages, as shown in the following diagram. The manifold is designed with various insulating features to separate it from the rest of the mold, thus reducing heat loss. The advantages of an externally heated runner system is its sophisticated heat control. The disadvantages of this type of heated runner system are higher costs, a more complicated design, and the thermal expansion of various mold components that has to be taken into account. Modeling 45

50 Hot runners - insulated runners Insulated runners are oversized passages formed in the mold plate. As shown in the following diagram, the oversized passages allow an open, molten flow path to be maintained because of the insulating effect of the plastic that freezes on the runner wall, combined with the heat applied to the system with each shot. The advantages of an insulated hot runner system include simple design and low mold cost. The disadvantages of this type of hot runner system include the following: Unwanted freeze at the gate Fast cycle time needed to maintain melt state Long start-up periods needed to stabilize melt temperature Problems in uniform mold filling Runner systems for single cavity molds When the design of the runner system and the position and number of injection locations is optimal, an evenly filled part will be produced. When the number and positioning of injection locations is not ideal, or the runner system is unbalanced, problems like overpacking and hesitation will occur. Unbalanced flow in single cavity molds When a part has complex geometry, it may require more than one injection location. You must ensure that the flow of the material is balanced to avoid molding problems. There are two stages in balancing flow. The first stage is determining how many injection locations are needed and where they should be. The second stage is designing the runner system so that the part fills evenly. Injection locations It is helpful when you are balancing flow paths to split the part into imaginary sections that will fill simultaneously. The following diagram shows a part that requires three injection locations to fill effectively. The three sections separated by the red lines show the sections will all fill at the same time. The pale blue arrows in the diagram indicate the flow paths of material, and the yellow cones indicate the injection locations. 46 Modeling

51 NOTE: When you design the sections in a mold, avoid putting section boundaries where weld marks would be undesirable. Runner system The next stage is to design the runner system so that each section fills simultaneously. For a part with two equal sections, the runners should be the same diameter, length and distance from the sprue. For parts with unequal sections, the pressure in each runner and section should be the same and all the flow fronts should meet at the same time. The following diagram shows a runner system designed to finish filling all three sections at the end of fill. Overpacking in a single cavity mold Overpacking occurs in complex parts that have more than one injection location when flow fronts meet before the part has filled. In the following animation, the material from the central gate merges with the flow from the other two gates before the part has filled, causing underflow and overpacking. To overcome this type of overpacking, divide the part into imaginary sections in which the flow fronts meet at the end of fill. Design the runners to ensure that the pressure in each section is equal. In the following animation, the gates have been moved so that each section of the mold fills simultaneously. Modeling 47

52 Runners can be designed to achieve balanced flow and prevent underflow by equalizing the pressure where the flow fronts meet. You can use the pressure results to observe the pressure in the cavity. Avoiding hesitation Hesitation occurs in parts of various thicknesses when the melt moves preferentially into thicker areas and melt in the adjacent thin area lies stagnant. The stagnant melt looses heat while the thicker area continues to fill. Hesitation can usually be avoided by using multiple injection locations with a balalnced runner system. The following diagram shows a part that requires multi-gating because of the two thin ribs in the design. If the gate was located as shown, hesitation would occur in the thin rib near the gate. The plastic in the rib would freeze of while the thick area is being filled. The hesitation, which is indicated by the red arrow, is caused by restricted flow. The gating shown in the following diagram would only be marginally better because the polymer still flows more readily in the thicker section than the thinner section, causing hesitation in the thin section indicated by the red arrow. The solution for this problem is using two gates with an artificially balanced runner system, as shown in the following diagram. The gates are positioned so that the thin ribs are at the end of flow paths, which prevents hesitation. 48 Modeling

53 Runner systems for multi-cavity and family molds The runner system should be designed so that all the parts finish filling simultaneously. Unbalanced runners can result in hesitation, underflow, or overpacking. You might have to artificially balance the runners, or rearrange the cavities to create a naturally balanced system. Unbalanced flow in family molds The runner system should be designed so that all of the parts finish filling at the same time. Each part should be analyzed before you design the runners for a family mold. When you have confirmed that each cavity will fill, you can design the runner system to create balanced fill paths in each cavity. Unbalanced runners can result in molding problems such as hesitation, underflow and overpacking. The following diagram shows an unbalanced family mold. The runners are all the same length ad diameter, but because the cavities are of different sizes, the flow will be unbalanced. The smaller part (bottom left) will fill first, resulting in overpacking, and the largest part will fill last. The following diagram shows a balanced family mold. The smallest cavity has the thinnest runner, restricting plastic flow into it. This means that the four cavities will all fill at the same time, reducing the possibility of molding problems. You can use a Runner Balance Analysis to balance the runners. Modeling 49

54 Unbalanced flow in multi-cavity molds It is important for the feed system in multi-cavity molds to be balanced so that the plastic melt fills each seperate mold cavity simultaneously. Non-uniform fills can result in some cavities producing good parts and other cavities producing inferior parts due to short shots, overpacking or flash. Before designing a multi-cavity mold, you should analyze each cavity without the runner system. When you know how each cavity will fill, you can design the runner system to create balanced fill paths. The following diagram shows a conventional way of filling a multi-cavity part, which may cause molding problems because the flow path for the outer parts is much longer than the flow path for the inner parts. In this example, the runners must be balanced. In the following diagram, the runners in this naturally balanced runner system all have the same flow length which means that the polymer will reach the gate of each part simultaneously. 50 Modeling

55 Another method of balancing flow paths is to use artificial runner balancing, where the runners have different diameters to promote flow to the more distant cavities. If you have a conventional runner layout, you can artificially balance the runners using this method. Overpacking in multi-cavity and family molds Overpacking in multi-cavity or family molds occurs when one part fills before another. Overpacking might occur because: One part is significantly larger than the other The runners are unbalanced, so the flow lengths vary in parts with a similar size. The pressures at the injection location of each part vary. The runner system is poorly designed. Overpacking can be avoided in multi-cavity or family molds by ensure that each part fills correctly as a single part. Once you have found the best injection location for each individual cavity, design the runner system so that all parts finish filling simultaneously. The Fill Time result will help you do this. Modeling 51

56 In the following diagram, the smaller part will fill first and will overpack because pressure must be maintained to fill the larger part. Hesitation at the gates of multi-cavity and family molds When designing a mold for a multi-cavity part, ensure the flow of the material is balanced. With unbalanced runners, hesitation can occur in the gates of the parts closest to the sprue. Hesitation causes underpacking and short shots. In the following animation, the material reaches the first gate before the runner system has filled. The higher pressure at the gate will cause the polymer to flow more readily through the runners. Polymer flow will slow at the first gate and then freezes, as indicated by the red arrow. To avoid hesitation at the gates, the runners should be balanced so that the flow reaches the gates simultaneously. You can use the Fill time result to ensure that the flow paths are balanced. Hesitation at the gates might lead to underpacking or a short shot. Cooling system The cooling circuit is comprised of the cooling channels, various components added to the cooling channels to facilitate cooling, and the coolant which flows through the system. The overall requirements for cooling the part will always be a compromise between uniform cooling to assure part quality, and fast cooling to minimize 52 Modeling

57 production costs. The degree to which consideration outweighs the other is dependant on the functional requirements of the part. To channel coolant into high heat-load areas, it may be necessary to design intricate cooling systems. Cooling system components The following diagram of a cooling system illustrates typical cooling elements. A = Collection manifold B = Mold C = Supply manifold D = Pump E = Cooling Channels F = Hoses G = Baffles H = Temperature controller Cooling considerations Cooling time It is important that when designing plastic parts you consider the various aspects of cooling, and how they affect the finished product. Cooling time is the time after the end of packing until ejection. Usually, the material at the center section of a part wall reaches its freeze temperature (Vicat softening point) and becomes solid during cooling time. Cooling time usually represents 80 percent of the total cycle time. Modeling 53

58 Two major factors that affect the cooling time are melt temperature and mold temperature. Both may need to be optimized to obtain a high quality part. Increasing either the melt or mold temperature increases the cooling time because it takes longer for the frozen layer to reach the required thickness. Lower mold temperatures = shorter cycle time Lower mold temperatures result in shorter cycle times, which leads to higher productivity. The following diagram illustrates how increasing the mold temperature increases the cycle time. Part thickness Cooling time increases rapidly with wall thickness, so avoid thick part walls to maintain an economically acceptable cooling time. Part thickness should be as uniform as possible. In the following diagram, the part on the left has a thick wall section. This part will take longer to cool than the part on the right Ejection temperature The ejection temperature is the temperature below which the plastic is solid. When the whole of the part has reached the ejection temperature, the part can be ejected from the mold with no adverse effect on its quality. 54 Modeling

59 Heat transfer Since the cooling time frequently comprises some 80 percent of the cycle time, any reduction in cooling time will significantly reduce the cycle time. Frequently, the cycle time is determined by the time taken to reduce the part temperature to a level at which it can be safely ejected without compromising the quality of the part. The temperature at which a part can be ejected from the mold is affected by a number of factors. Successful ejection requires the part to be stiff enough to resist any tendency to warp caused by shrinkage variations and residual stresses, and stiff enough to resist the local forces on the part from the ejection system. The geometry of the part, the mold finish, and the degree to which the cavity has been packed during the filling and packing phases can all affect the cycle time. The overall requirements for cooling the part will always be a compromise between uniform cooling to assure part quality, and fast cooling to minimize production costs. The mold can be thought of as a heat exchanger because heat passes into the mold and transfers out of it. The mold's primary energy input is hot plastic injected into the cavity. Hot runners can also be a an energy source. During mold filling, heat is lost by the following three mechanisms: Coolant flow in the cooling circuits Conduction to the injection molding machine mold Convection and radiation to the atmosphere About percent of the heat introduced into the mold by the plastic melt is transferred by conduction through the metal to the surface of the cooling channels and dissipated into the heat transfer fluid. Most of the heat will be extracted by coolant flow. When running the mold hot, the heat lost through the mold and to the atmosphere may exceed heat input from the plastic melt. Convection on the mold surfaces and conduction into the molding machine are of only minor importance, accounting for typically 5-15 percent of the total heat flow. The radiative heat transfer should be considered only when the temperature of the mold is high, that is, greater than 85 C, because the radiative heat flow is typically less than 5 percent of the total amount. In addition to the heat input from the plastic melt, the hot runners and manifolds can also contribute heat to the mold. Cases where the coolant is at a temperature well above the ambient temperature also contribute heat. Modeling 55

60 Air gaps Uniform cooling A layer of air can impair the effective transfer of heat. Therefore, take steps to eliminate any air gaps between the mold insert and molding plates, and any air pockets in the cooling channels. Temperature difference The temperature difference on opposite sides of the part should be kept to a minimum and should not exceed 10 C for parts that require a tight tolerance. Uniform cooling helps you achieve effective heat transer. Effective heat transfer is achieved when the following conditions are met: The coolant flow is turbulent. The area of the cooling surface is great enough. The temperature difference across the coolant/metal interface is 2 5 C. The inlet/outlet temperature of the coolant varies between 2 3 C. When these conditions apply, the temperatures on the mold surface are determined by heat transfer through the metal to the coolant/metal interface. This is a function of the conductivity of the metal and the geometry of the mold. Surface temperatures are controlled by the position of the cooling circuits relative to the surface of the mold. In the following diagram, the surface temperature is highest at the internal corners and lowest on the thin top bar. To achieve a uniform temperature, the cooling channels must be placed as near as possible to hot spots and away from surfaces that have a tendency to run cold. where: a - Isotherms b - Hot spot 56 Modeling

61 c - Cooling circuit In the following diagram, an extra cooling channel has been placed as close as possible to the surface of mold. The hot spot in zone 1 has been eliminated. The adjustment of cooling channels around zone 2 accounts for the heat load which comes from the thick leg section of the part. where: a - Isotherms b - Cooling circuit z1, z2 and z3 - Zone 1, zone 2, and zone 3 Effective heat extraction Effective heat extraction can reduce the cycle time. Effective heat extraction requires the consideration of the following variables. Inlet/outlet temperature The coolant flow rate should be high enough to ensure that the temperature at the outlet is within 2-3 C of the inlet temperature. Turbulence Effective heat extraction requires the fllow of coolant flow to be turbulent. Turbulence is indicated by the Reynolds number, which is calculated from the diameter of the cooling channels, the flow rate, and viscosity of the coolant. A Reynolds number of 10,000 is recommended. Surface area The surface area of the cooling channel must be great enough to ensure that the temperature rise across the channel is 2-5 C. Increasing the length or number of the channels improves the area available for heat transfer. This results in a higher pressure drop in the channel. If Modeling 57

62 the diameter of the cooling channel is increased, a higher flow rate is required to achieve turbulence. A balance has to be struck between the diameter and length of the cooling channels, and the pressure and volume characteristics of the cooling pump. When these conditions have been optimized, the temperature rise across the metal is controlled by the placement of cooling circuits. Although, ideally, this should be no more than 5 C, it is more realistic to expect a temperature rise across the metal of C. Heat transfer due to coolant flow The effect of heat transfer increases as the flow of coolant changes from laminar flow to turbulent flow. For laminar flow, heat can be transferred only by means of heat conduction from layer to layer. However, in turbulent flow, the mass transfer in the radial direction enables the heat to be transferred by both conduction and convection. As a result, the efficiency increases dramatically. Figure 3: Left: Laminar flow, Right: Turbulent flow Since the increase of heat transfer will diminish as the coolant flow becomes turbulent, there is no need to increase the coolant flow rate when the Coolant flow on page 72 exceeds 10,000. Otherwise, the small, marginal improvement in heat transfer will be offset by the higher pressure drop across the cooling channels, along with more pumping expense. Once the flow becomes turbulent, a higher coolant flow rate brings diminishing returns in improving the heat flow rate or cooling time, while the pressure drop and pumping expenses are drastically increasing. This concept is shown below. Max heat flow rate Cooling time Heat flow rate Pressure drop Figure 4: Flow rate NOTE: It is important to make sure that the coolant reaches turbulent flow everywhere in the cooling system. Autodesk Moldflow analyses can help you identify and correct problems such as stagnated cooling channels, 58 Modeling

63 by-passed cooling channels, and high pressure drops in some cooling circuits. Restrictive flow plugs Coolant will take the path of least resistance to flow. Use a restrictive flow plug in certain cooling channels to redirect the flow of coolant to other cooling channels that have a high heat load. NOTE: Changes to process settings can be made only with a licensed Autodesk Moldflow Adviser or Autodesk Moldflow Insight product. Cooling system equations Theoretically, cooling time is proportional to the square of the heaviest part wall thickness and the largest runner diameter raised to the power of 1.6, and inversely proportional to the thermal diffusivity of the polymer melt. Cooling time The thermal diffusivity of polymer melt is defined as: R 2 p c v where: R2 is the thermal conductivity p is the density cv is the specific heat constant volume In other words, doubling the wall thickness quadruples the cooling time. Reynolds number and coolant flow The type of coolant flow can be determined by the Reynolds number (Re), as listed in the following table. Reynolds number (Re) 10,000 < Re 2,300 < Re < 10, < Re < 2,300 Re < 100 Type of flow Turbulent Transition Laminar Stagnated The Reynolds number (Re) is defined as: Re=pUd where: p is the density of the coolant U is the averaged velocity of the coolant d is the diameter of the cooling channel is the dynamic viscosity of the coolant Modeling 59

64 Cooling circuits Cooling circuits are used in a Cool analysis to deliver coolant to areas of the mold that would not otherwise cool effectively. A cooling circuit consists of a set of connected two-point beam surfaces which make up the cooling system. The placement of cooling channels is restricted by mechanical constraints such as ejector pins and metal inserts. Information from Cool analyses can be used to evaluate each design. When you are designing a cooling system, consider the coolant inlet, the circuit type, and the circuit location. Coolant inlet Prior to running a Cool analysis, you need to identify and set all cooling circuit inlets. Circuit type A series circuit that can achieve even coolant flow rate and heat transfer is usually preferable to a parallel circuit. If it is necessary to use a parallel circuit, each branch should be balanced for local heat load. Poorly designed parallel circuits can have branches where there is little or no flow. The flow in each branch should be controlled so that all of the cooling circuit coolant flowing through them, and that the flow is turbulent for maximum cooling efficency. The following diagram illustrates a series cooling circuit on the left and a parallel circuit on the right. Circuit location In general, locate cooling circuits at a distance of about 2.5 times their diameter from the plastic. This will give fairly uniform cooling over the part. In some cases, however, it may be necessary to locate the line closer or further from the part, depending on how much heat is to be removed. Cooling circuits should be close in areas that concentrate heat, such as 60 Modeling

65 internal corners and ribs. Cooling circuits can be positioned further away in areas that have less heat content, such as thin sections. Cooling efficiency Often molds contain ribs and cores that are very difficult to cool. Placing bubblers, baffles, or metals of high conductivity in these areas greatly improves conduction through the core to the cooling channel. Do not use small inlet channels to feed larger channels. Remember that the only channels in which turbulent flow is required are the circuits that are actually cooling the part. If a small inlet line feeds a large cooling circuit, achieving turbulence in the large circuit must be accompanied by a large pressure drop over the smaller line. This is a waste of pumping power. Cooling elements must be assigned a value of heat transfer effectiveness to represent their ability to accept heat from the mold. For most situations a default value is applicable. Cooling circuit design Mold cooling accounts for more than two-thirds of the total cycle time in the production of injection molded thermoplastic parts. An efficient cooling circuit design reduces the cooling time, which in turn increases overall productivity. A well designed circuit achieves uniform cooling, improving part quality by reducing residual stresses and maintaining dimensional accuracy and stability. The primary factor governing production costs is cycle time, and the cycle time is governed by the material's ejection temperature. To ensure part quality, consider the following factors: Surface finish Residual stresses Crystallinity Thermal bending The following diagram shows how an effectively cooled part (left) leads to a correctly molded part in a shorter period of time (right). The following diagram shows how a poorly cooled part (left) leads to a low quality part in a longer period of time (right). Modeling 61

66 Cooling system components A cooling system typically consists of the following items: A - Collection manifold B - Mold C - Supply manifold D - Pump E - Cooling channels F - Hoses G - Baffles H - Temperature controller Cooling planes A cooling plane needs to be created before you can model any part of a cooling circuit. Cooling planes are always created parallel to the X-Y plane. Since a mold must contain both a cavity and core, there may be a small gap between them. While this gap is too small for material to flow into, it still acts as a partial barrier to heat transfer. To simulate this effect, you need to model the surface between the cavity and core as a parting plane. 62 Modeling

67 A parting plane represents the resistance of heat transfer across a surface inside a mold, usually where the two halves of the mold meet. By default, the solver assumes perfect conductivity between mating surfaces. Thermal interface conductance is the only attribute that can be specified as a parting plane property. If a parting plane has low contact pressure or a small gap, and it is located between the part and the cooling channels, it should be modeled. TIP: Click Bottom View to display the model in the best orientation for working with cooling planes. Parallel and series cooling circuits Cooling circuits are generally classified as series or parallel circuits. In both types of circuits, the final temperature rise of the coolant is determined entirely by the energy input from the plastic and the volumetric flow rate of the coolant. The most important factors in maintaining effective heat transfer are, therefore, flow rate and circuit design. The following diagram shows a series circuit on the left and a parallel circuit on the right. It is easier to control coolant velocity in a series circuit because the flow rate in each section is the same. It is therefore easier to maintain flow rate conditions that provide effective heat transfer. Modeling 63

68 Parallel circuits Parallel cooling channels are drilled straight through from a supply manifold to a collection manifold. Due to the flow characteristics of the parallel design, the flow rate along each of the cooling channels may be different, depending on the flow resistance of each individual cooling channel. These varying flow rates in turn result in different heat transfer efficiency for each cooling channel. As a result, cooling may not be uniform with a parallel circuit. Typically, the cavity and core sides of the mold each have their own parallel circuit. The number of cooling channels per circuit varies with the size and complexity of the mold. Only use a parallel circuit if your model has one or more of the following circumstances: The pressure drop over a series circuit is too high to be realistic. An area of the mold cannot be effectively cooled with a series circuit. You are simulating the manifold carrying coolant to the mold. When a parallel circuit is used, each branch must be capable of extracting the heat load from the surrounding area. Coolant flow must be regulated by specifying the diameter and length of each branch within the circuit. Each branch should have turbulent flow to give an effective heat transfer coefficient. The surface area of the branch is determined by balancing its length and diameter against the localized heat load. A balanced parallel circuit provides uniform heat extraction; however, parallel circuits also have the following disadvantages: The flow rate in each branch is reduced when extra branches are incorporated. This reduces cooling efficiency unless the total flow rate is increased accordingly. Each cooling channel may have a different flow rate, causing non-uniform cooling. This disadvantage can be minimized by adjusting the diameter of the branches to balance the coolant flow. If one branch is partially blocked by debris, the flow rate in that branch may be dramatically reduced while the flow rate may slightly increase in other branches. This will cause non-uniform cooling. Series circuits Cooling channels connected in a single loop from the coolant inlet to the outlet are called serial circuits. This is the most common type of cooling channel. If the cooling channels are uniform in size, the coolant can maintain its turbulent flow rate through the entire circuit. The coolant will continue to collect heat along the cooling circuit so you should ensure that the temperature rise of the coolant from inlet to outletis minimized. The temperature difference of the coolant at the inlet and the outlet should be within 5 C for general purpose molds, and within 3 C for 64 Modeling

69 precision molds. More than one serial circuit may be required for large molds to ensure uniform coolant temperature and cooling. Due to the problems experienced with parallel circuits, series circuits are generally preferred, but these cannot always be used. Series circuits should not be used in the following situations: The length of the series circuit results in a pressure drop that is too high for the available pump capacity. Physical constraints in the mold design mean that the mold cannot be effectively cooled with a series circuit. Cooling circuit design references The following references provide additional information to help you design efficient cooling circuits. Gastrow, Hans Injection Molds: 108 Proven Designs Edited by K. Stoeckhert. Hanser Publishers, Munich Vienna New York Menges, G. and Mohren, P. How To Make Injection Molds, 2nd Edition Hanser Publishers, Munich Vienna New York Douglas M. Bryce. Plastic Injection Molding: Mold Design and Construction Fundamentals Society of Mechanical Engineers Cooling components In addition to the cooling channels, there are several other components that make up a cooling circuit, such as hoses, coolant inlets bubblers and baffles. Some are required while others are optional, depending upon the complexity of the model. Cooling channels Complex geometry in plastic molds can create areas that are difficult to cool. For example, parts of the mold that project into the cavity, such as bosses and ribs, carry high heat loads because they are surrounded by plastic. They also restrict the area of metal through which heat can escape. To channel coolant into high heat-load areas, it may be necessary to design intricate cooling systems. This means linking cooling circuits together to form a network of cooling channels. Networking requires the inclusion of bends and devices such as baffles and bubblers. It is important to consider how these devices affect the operation of the cooling system. Cooling system design considerations The aim of the mold designer is to design a cooling system with the following characteristic: Uniformly cool the part Modeling 65

70 Achieve the desired target mold temperature for the start of the next cycle Minimize cycle time The mold designer must also consider the following factors, which affect the performance of the cooling system: The physical layout of channels and the mold material into which they are cut The coolant parameters such as coolant type, temperature, flow rate and pressure drop The best location for cooling channels is in the blocks that contain the mold cavity and core. Placing the cooling channels outside the cavity or core block may not provide adequate cooling. The physical design of the cooling system is normally restricted by the mold geometry, positioning of split lines, moving cores, and ejector pins. Bends in cooling circuits The inclusion of a bend in a cooling channel increases turbulence, which results in a large pressure drop and an increase in heat transferability through the bend. In a Cool analysis, bends are handled rather like an extra section of cooling channel that has unique resistance and heat transfer characteristics. These sections are assigned apparent lengths for resistance and heat transfer that are much greater than the actual flow length through the bend. For example, a bend can have a resistance equivalent to a flow length 50 times the diameter of the cooling channel. Heat transfer capacity equates to a channel 10 times the diameter. The apparent lengths are used to calculate the pressure drop and heat transfer capability. These characteristics are then applied to a single point in the flow channel that represents the bend. Turbulence also occurs when there is a change in the diameter of the cooling channel. Position of cooling channel inlet/outlets The inlets and outlets should ideally be positioned on the bottom of the mold. This eliminates the risk of the coolant dripping onto the mold. Cooling channel efficiency Cooling channel efficiency is a measure of how effectively a particular channel section extracts heat from the polymer during the entire cycle. This measurement has no units and is calculated relative to the average performance of the whole cooling system: if there is no variation in the efficiency throughout the system, the efficiency should be equal to one throughout. The cooling channel efficiency of any channel is defined as: = t q n d H c / L t where 66 Modeling

71 t is the cycle time q is the heat flux of each channel d is the channel diameter Lt is the total length of the cooling system Hc is the total heat removed by the cooling system during the entire cycle time From the above definition, it follows that if all channels have the same diameter and heat flux, then the cooling channel efficiency is equal to one (1) for all channel sections. If there is any variation in the amount of heat extracted by the sections, per unit length, then for that section, the cooling channel efficiency will be greater than or less than one. Cooling channel efficiency data helps identify which channels are extracting more heat than others. Channels with a cooling efficiency that is close to zero are not participating in cooling. If these channels are located in a region where there is no heat load, they can be discarded. If they are located in a region where there is a significant heat load but they have a cooling channel efficiency close to zero, the performance of these channels should be improved by changing the cooling system design, or by changing the coolant process conditions. Cooling channels with negative values of efficiency are, on cycle average, releasing heat instead of extracting heat. This can happen when the coolant temperature is higher than the ambient temperature and the cooling time is relatively long. Length of cooling channels Increasing the length of a cooling channel increases the surface area available for heat transfer. In the following diagrams, the blue line represents the cooling channel in the mold. The cooling line in shown in diagram b will be more effective than those shown in diagram a. However, long channels can cause the following problems: Increased pressure drop Excessive temperature rise Difference between inlet and outlet temperature greater than 3 C To avoid these problems, very long circuits should be broken into two or more shorter circuits as shown in diagram c. Modeling 67

72 Distance between adjacent cooling channels The distance between the cooling channels can be decreased to make the mold temperature more uniform. If, however, the overall mold design only permits larger spacing between the channels, the distance from the cavity to the cooling channels should also increase. The optimum spacing depends on the diameter of the cooling channel and the thickness of the part. Preferred cooling channel spacing is shown in the following diagram where the diameter of D is 10-14mm, and the spacing, a, is 3D to 5D. Distance between cooling channel and mold cavity The distance between the cooling channel and the mold cavity can be increased to make the temperature at the cavity surface more uniform. This also increases the temperature rise at the surface of the cavity during injection. Which has positive consequences for the distortion, the mechanical properties of the parts and, to a limited degree, mold filling. The smaller the spacing, the more rapidly heat is removed and the shorter the cycle, but the resulting cavity surface temperature variations can lead 68 Modeling

73 to part quality problems. As a general design guideline, cooling channels should be placed about times the channel diameter from the cavity. The following diagrams show the more even mold surface temperature obtained with correctly positioned cooling lines. Pressure drops in cooling channels At the design stage, you need to know the pressure of coolant available at the plant. If the available pressure is less than what the cooling channels will require once the mold is in place, the cooling of the part could be ineffective due to possible non-turbulent flow of coolant. The pressure drop is directly related to the length of the cooling chanel, cooling channel diameter, and the flow velocity. Pressure requirements will increase with any of the following: An increase in cooling channel length An increase in flow velocity A decrease in cooling channel diameter Hoses Hoses connect the cooling channels and enable the flow of coolant through the cooling circuit. Hoses contribute only to the pressure drop of the coolant flow, and not the heat transfer calculation. Two cooling channels of different diameters can be connected by hoses. The following diagram shows two cooling channels connected by a hose. Modeling 69

74 Bubblers Areas of the mold which cannot be cooled effectively by normal cooling channels may require the use of bubblers. Bubblers divert the coolant flow into areas that would normally lack cooling. Bubblers are created by fitting a tube in the center of a drilled hole, forming an annular channel. The coolant flows into the bottom of the tube and bubbles out of the top, like a fountain. The coolant continues down around the outside of the tube and continues through the cooling channels. The following diagram illustrates the coolant flow through a bubbler. Bubblers provide the most effective cooling of parts with slender cores. The diameter of both sections of the bubbler must ensure that the flow resistance in both sections is equal. The condition for this is inner diameter / outer diameter = Modeling

75 Bubblers are commercially available and are usually screwed into the core. The tubing should be beveled at the end to enlarge the cross section of the outlet for diameters up to 4mm. Bubblers can be used for core cooling and for cooling flat mold sections where drilled or milled channels can not be used. NOTE: Because bubblers have narrowed flow areas, the flow resistance increases. Therefore, care should be taken in designing the size of these devices. The flow and heat transfer behavior can be readily modeled and analyzed by Autodesk Moldflow. Baffles Areas of the mold which cannot be cooled effectively by normal cooling channels may require the use of baffles. Baffles divert the coolant flow into areas that would normally lack cooling. A baffle is a cooling system component which is constructed by inserting a metal plate in the cooling channels. The plate forces the coolant to flow up one side of the baffle and down the other. The baffle interrupts the flow in the cooling channels creating turbulence around bends which improves the heat transfer capability of the coolant. Baffles are modeled by creating two circular beam elements with a gap at the top for the coolant to flow around. The first circular element represents the flow up one side of the baffle and the second element is for the flow down the other side. Yellow is the default color assigned to a baffle. In the following diagram, A indicates the baffle cross-section and B indicates the mold model representation. The baffle is made up of two circular beam elements with HTE (Heat Transfer Effectiveness)=0.5. NOTE: Because baffles have narrowed flow areas, the flow resistance increases. Therefore, care should be taken in designing the size of these devices. The flow and heat transfer behavior can be readily modeled and analyzed by Autodesk Moldflow. Modeling 71

76 Flow control valves Molds often have many individual cooling circuits which could require individual controllers to obtain optimum heat transfer. Since one mold temperature control unit per mold is common practice, it is not possible to control the individual circuits by using variable inlet temperatures. With flow control valves, each cooling circuit can have a different flow rate. This enables you to optimize the heat extracted from different parts of the mold. Coolant Coolant flow A coolant is a fluid which flows through the cooling channels to regulate the temperature of the molten plastic in the mold. An ideal coolant has high thermal capacity, low viscosity, is low-cost, and is chemically inert, neither causing nor promoting corrosion of the cooling system. Coolant flow behaviour affects heat transfer between the mold and coolant. Heat transfer effectiveness is increased when the coolant flow is turbulent and not laminar. Turbulent coolant flow has a better temperature gradient than laminar flow. The temperature gradient from the cavity to the cooling channel has two components: The temperature gradient across the metal, which depends on the conductivity of the metal The temperature gradient across the coolant and metal interface, which depends on coolant flow The heat flow path from plastic to cooling channel The speed of coolant flow can affect the heat transfer. At very low speeds the flow is laminar; the heat has to be conducted through various layers of coolant to the center of the channel. Since coolant is a poor conductor of heat, heat transfer is very inefficient. This is shown in the following graph a below where there is a relatively large temperature difference between the coolant metal interface and the center of the channel. As coolant flow increases, heat transfer increases at a marginal rate until the coolant flow becomes turbulent. There is now a component of coolant velocity perpendicular to the channel which causes a dramatic improvement in heat transfer. The greater heat transfer shown in following graph b results in a lower temperature at the cavity wall for coolants with a turbulent flow. In these graphs, the mold elements are represented by the following: a - Coolant b - Water/metal interface 72 Modeling

77 c - Cavity wall d - Plastic part Laminar and turbulent flow The relationship between heat transfer and coolant flow can be expressed as a power factor. Coolant flow is either laminar or turbulent, or in transition between laminar and turbulent. For laminar flow, heat transfer increases proportional to the cube root of the flow rate. This means that doubling the coolant flow increases the heat transfer by about 24 percent. For fully turbulent flow, the heat transfer is proportional to the square of the cube root of the flow rate. Therefore, in the turbulent zone, doubling the coolant flow increases heat transfer by 59 percent. The power required to pump coolant around the system is proportional to the flow rate cubed. This means that doubling the coolant flow will require eight times the pumping power. When turbulent flow has been fully developed, greater increases in flow rate will increase the molds capacity to extract heat. However, the amount of heat that can be extracted is limited by the amount of heat passing into the mold, and heat extraction may not be improved beyond this limitation. The most effective condition for heat transfer is to ensure that coolant flow is turbulent, and the capacity to extract heat does not greatly exceed the amount of heat available for extraction. The following graph shows heat extraction from the mold as a function of the coolant flow rate. In region A heat transfer is by conduction, in region C heat transfer is by convection, while B is a region transition. Heat Extraction Capacity Actual Heat Extraction A B C Turbulent Flow Modeling 73