Feed System Design OBJECTIVES IN FEED SYSTEM DESIGN DPT 321 INJECTION MOLD DESIGN COURSE CONTENT & COURSE OUTCOMES. Introduction of Feed System

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1 COURSE CONTENT & COURSE OUTCOMES School of Manufacturing Engineering COURSE CONTENT Chapter 5: Gate, Runner and Venting Design DESCRIBE and EXPLAIN the type of gates, and gate selection. DESCRIBE and EXPLAIN the types of runner, runner layout, and ANALYZE the runner balancing. DESCRIBE and EXPLAIN the venting design, and venting selection. Feed System Design COURSE OUTCOMES CO3: Able to design gate, runner, air vent and cooling system in plastic injection mould. Introduction of Feed System The feed system (cold runner twoplate mold) comprises four main components: 1. The sprue 2. Cold slug well 3. The runners 4. The gates Polymer Entrance Point Gate Sprue Primary Runner Part Cavity OBJECTIVES IN FEED SYSTEM DESIGN Cold Slug Well Secondary Runner Conveying the Polymer Melt from Machine to Cavities The primary function of the feed system is to convey the polymer melt from the nozzle of the molding machine to the mold cavities. In most molding applications, the polymer melt must traverse portions both the mold height (down) and mold width (across). Impose Minimal Pressure Drop The feed system must be designed so that there is sufficient melt pressure to drive the polymer melt throughout the mold cavities. A feed system with large flow resistance will incur a substantial pressure drop during the molding process. The flow rate of the polymer melt will begin to decay when the molding machine reaches the maximum allowable pressure. If the flow rate decreases substantially before the end of the mold filling process, then a short shot or other defects are likely to occur. Two plate mold Three plate mold PPD 1

2 Consume Minimal material Control Flow Rates To achieve the best feed system design, the mold designer should specify the diameter of the feed system to jointly minimize the pressure drop and the feed system volume. From graphs: As the diameter of the various segments of feed system increase, the pressure drop decrease below the specified maximum. However, increasing the diameter also results an increasing the volume of the feed system. Application: In Multi-cavity (family) mold, the molding application may require different pressure drops in each leg of feed system to cause the different mold cavities to fill at same time. In multi-gated mold, a common objective in the feed system design is to control the polymer melt flowing through the feed system to alter the melt front advancement in a multi-gated mold. Introduction A sprue is a portion of the hot or cold runner system. The sprue is the entry point for the polymer into the mold. For cold runner systems, the sprue is tapered, with the smallest end the entry point. Sprue Design Determining Sprue Dimensions Sprue bushing are typically standard off the shelf items. Typically, the requirement dimension are: The orifice diameter, O Determined by the injection molding machine s nozzle orifice diameter. The sprue orifice diameter must be slightly larger than the nozzle s diameter (0.5mm larger than nozzle s diameter) Length, L Measured from the bottom of the spherical radius to the bottom of sprue. Included angle The included angle ranges from Cold Slug Wells PPD 2

3 Cold Slug well Design A cold slug well is an extension of a sprue and runner system past the last branch. The purpose of a slug well is to capture the cold slug of polymer that may form in the nozzle between shots. Most of the time, if the slug exists is trapped at the bottom of the sprue. The amount they should extend past the branch is 1.0 to 1.5 diameters. Runner Design Runner Types There are two types of runner; Cold Runner A cold runner is a system of polymer flow channels in an injection mold that are ejected with each cycle of the mold. The mold temperature for cold runners is about the same as the part. Suitable for two-plate and three-plate molds. Hot Runner A hot runner, sometime called a runnerless system, is a system of the polymer flow channels that are not ejected with each cycle. The hot runners maintain the polymer at a melt temperature. Suitable for hot runner mold. Common Runner Layout (Cold Runner) Standard Runner Design The layout goes several names, including nongeometrically balance, herringbone, fish bone, ladder, three or artificially balanced. To be artificially balanced, a runner balance analysis must be done to change the size of the secondary runner, so all cavities fill at the same time. Common Runner Layout (Cold Runner) Geometrically Balanced Runner System This layout is also called natural balanced or H pattern. Common Runner Layout (Cold Runner) Radial Runner System This layout is also called a star layout. PPD 3

4 Example of Runner Layout Example of Runner Layout Cross-Sectional shape (Cold Runner) Runner shape (Machining process) The most efficient runner section is the full round and this should be used wherever possible. For three-plate mold, the trapezoidal runner has to be used. Good substitute is trapezoidal or modified trapezoidal Depth should be equal to round and sides tangent to circle The included angle should be The semi circular or half round runner severely restricts flow and should be avoided although it is frequently seen in production mold. Square and rectangular section runner should never be used. Runner Diameter The diameter of runner depends on: Viscosity of the material melt Volumetric flow rate of the material Flow length of runner Typically a wide range is given Material Diameter Diameter Material mm inch mm inch ABS, SAN /16-3/8 PET /8-5/16 Acetal /8-3/8 Polyethylene /16-3/8 Acrylic /16-3/8 Polypropylene /16-3/8 Nylon /4-3/8 Polystyrene /8-3/8 Polycarbonate /16-3/8 PVC /4-5/8 Runner Diameter (Calculation) Diameter of runner (D) D M 4 L 3.7 Which is: D = diameter of runner (mm) M = mass of the molding (g) L = length of runner (mm) Diameter of branched runners (d) On geometrically balanced runner Keeps pressure gradient about equal D feed = d branch x N 1/3 D feed = Runner closer to sprue d branch = Runner farther from sprue N = No of branches PPD 4

5 Runner Diameter (Calculation) Runner Length (Calculation) Example: Given diameter of primary runner is 6.350mm, calculate the diameter of secondary runner. D primary = d secondary x N 1/ = d secondary x 2 1/3 There are three equations that may be used to compare the pressure, shear flow rate and length. γ = 4Q πr 3 τ = ηγ Where : 1 γ shear rate ( s ) 3 QQ flow rate ( m / s) r gate radius d secondary = 5.04mm Note: In a geometrically balance runner system, the number of branches will always be two. 2L P r shear stress ( MPa) material vis cosity at melt P pressure drop ( MPa) L runner length ( m) temp.( Pa. s) Example: Example: Molding are to be produced from polycarbonate using a melt temperature of C and a flow rate through the runner of 2.85cm 3 /s. The runner length is 120mm and the diameter is 4mm. The viscosity is 1000Pa.s. Calculate; (i) Shear rate and (ii) shear stress. Is the runner length satisfactory or not? The shear stress and shear rate from calculation shows that the value are not exceed the maximum limit from Table below. So, the runner length is satisfactory. Shear rate, 4Q 3 r 4x2.85x10 3 (2x10 ) 6 3 Shear stress, 1000x454Pa 0.454Mpa 454sec 1 Design Target DESIGN PRINCIPAL Unidirectional and controlled flow pattern Flow balancing Constant pressure gradient Maximum shear stress Uniform cooling Positioning weld and meld lines Avoid hesitation effects Avoid underflow Balancing with flow leaders and flow deflectors Controlled frictional heat Thermal shut off for runners Acceptable runner/cavity ratio PPD 5

6 Unidirectional and Controlled Flow Pattern Plastic should flow in one direction with a straight flow front throughout filling Produces a uni-directional orientation pattern Orientation is different directions, flow marks, high stresses, & warping. Orientation in one direction, Uniform shrinkage, & stresses. Flow Balancing (For runner system) All flow paths within a mold should be balanced, Equal fill time and pressure Naturally balanced runner system Also called geometrically balanced Same distance and conditions between the nozzle and all cavities All cavities filling at the same time pressure and temperature Flow Balancing (For runner system) Artificially balanced runner system Flow length is different between sprue and the parts Sizes of the runners are different All cavities at the same pressure & time Flow Balancing (For runner system) Artificially balanced runners Limitations: Very small parts Pressure to fill runners is higher than parts Parts with very thin sections Parts where sink marks are important Smaller molding window than naturally balanced system The higher ratio of runner lengths More difficult to balance Weld and Meld Lines Hesitation Effects Eliminate if possible Position in the least sensitive areas, Weld Lines Formed when two flow fronts meet head on Meld Lines Formed when two flow fronts meet and flow in the same direction Slowing down of the flow front Limiting hesitation Make wall thickness uniform Position gates far from thin features Fill faster Gate far from rib Hesitation in rib Gate near rib PPD 6

7 Avoid Underflow Controlled Frictional Heat Weld Line moves inside frozen layer 48% filled 70% filled 87% filled Flow front Arrows show direction plastic moving at the instant of fill Runners should be sized so there is shear heat in the runner Reduces part Fill pressure Shear stress Reduces melt temperature at machine nozzle Optimize temperature at part Reduce temperature at sprue so temp at part correct Thermal Shutoff of Runners Runners should freeze relative to the part freeze No less than 80% - To prevent packing problems No more than 200% - To prevent controlling the cycle time Acceptable Runner/Cavity Ratio Design runner systems for high pressure drops Minimizes material in the runner Lower ratio runner to cavity volume Smallest runner is OK Largest runner and sprue may possibly control the cycle time The volume of the runners should be 20% or less of the part volume Volume of parts: 5.4 cc Volume of feed system: 4.6 cc Feed system: 85% of part volume PPD 7