7. Runner & Gate Bong-Kee Lee Chonnam National University Delivery System Delivery System (Feed System) sprue (for a cold runner mold) cold slug well (for a cold runner mold) runner gate basic feed system
Delivery System Sprue should be tapered 3~5 inclusive angle being pulled out of the mold more easily should also be highly polished in the line of draw to assist withdrawal to induce more efficient flow diameter at the narrow end should be larger than the machine cylinder nozzle opening Delivery System Cold Slug Well to trap the cooler advancing front of the melt, thus permitting hotter melt to reach the cavities and gates snatch or pull pin or sucker pin underneath it to positively pull the sprue out of the sprue bush diameter design based on the diameter of the sprue where it meets the runner to avoid any obstruction to the melt flow
Delivery System Cold Slug Well for ejection pin systems with a hardened bush to minimize wear the least desirable one due to large mass and long cooling time capture the colder advancing front well normally preferred one sometimes tends to hang up, but preferred for brittle materials Delivery System Cold Slug Well for stripper plate ejection
purpose of runner to transport the melt from the sprue to the gates basic parameters for runner geometry cross-sectional shape diameter cavity layout cross-sectional shape full round cross-section: the most efficient design trapezoidal one: for three-plate molds semi-circular or half round one: severely restricts flow although frequently used square and rectangular ones: should never be used formation of dead zones in the runner design rheologically inefficient and wasteful on material and energy
cross-sectional shape diameter (size of runners) based on the thickness of the molding wall section should be large enough to provide adequate pressure to all the cavities no packing pressure shortfall and adequate control over the molding conditions alternative method: based on an appropriate pressure drop along the length of the runner
cavity layout beneath runner intersections cold slug well for improving melt flow: length of cold slug well ~ runner diameter ejection pin variation of runner diameters cavity layout incorrect design correct design incorrect design correct design
design rules runners must be designed to fill the cavity rapidly runner design must provide for easy ejection and easy removal (de-gating) from the molded part for a multi-cavity system, balanced runner layout is preferred for the best uniformity and part quality runner balancing may be achieved by changing the runner size and length changing the gate dimensions may seem to give a reasonably balanced fill but this will affect the gate freeze-off time smaller runner sizes are preferred to larger ones to minimize scrap volume and generate viscous heating high barrel temperature may cause material degradation design rules (continued) the cross-sectional area of the runner should not be smaller than that of the sprue, to permit rapid, unaltered flow to the gates the diameter of the branch runner should be smaller than that of the main runner as more economical way D dn 1/3 :(main runner diameter) (branch runner diameter)(number of branch runners) the depth of a trapezoidal runner is approximately equal to its width with a 5~15 taper or draft angle on each sidewall the minimum recommended runner diameter for most materials is 1.5mm the runner surface and sprue should be polished 1/ 3
design rules (continued) it is desirable to have multiple sprue pullers and ejection locations in extended runner systems the selection of a cold runner diameter should be based on standard machine tool cutter sizes for hot runner systems, it is advisable to consult the suppliers for availability and recommendations for the correct manifold and drops calculation of runner length (assumption) maximum pressure drop along the length of the runner ~ 70MPa (50MPa for filled material in most cases) safe working figure with which to calculate runner lengths 4Q 3 r -1 :shear rate[s ] 3 Q :flow rate[m /s] r : runner radius[m] : viscosity at melt temperature[pa s] P : pressure drop [MPa] :shear stress [MPa] L : runner length [m] 2 L P r
calculation of runner length (example) polycarbonate using a melt temperature of 310 C (viscosity of 1000Pa s) and a flow rate through the runner of 2.85cm 3 /s runner length of 120mm and diameter of 4mm 4Q 2 L P 3 r r 6 4Q 4 2.85 10-1 454[s ] 3 3 3 r 2 10 1000 454 0.454[MPa] 2 L 2 0.454 120 10 P 3 r 2 10 3 54.48[MPa] Runnerless Molding runnerless molding usually includes sprueless molding basic antechamber designs: melt flows through an insulated cold slug well heated hot sprue bushes or nozzles: internal or external heating insulated runner systems insulated semi-insulated hot runner systems
Runnerless Molding typical antechamber design semi-insulated runner design Hot s advantages over cold runner molds no runner system to be removed from the mold shorter cycle time with no cold runner to be cooled reduced mold opening stroke reduced (or eliminated) cost for storing and regrinding runners lowered risk of material contamination gates may be balanced more easily lowered injection pressure using the larger runner diameters ~ greater number of impressions, utilization of smaller machines smaller shot weight ~ reduced metering times and injection times
Hot s disadvantages significantly more expensive 24-hour operation is required for maximum economic production heat-sensitive materials may be difficult to process gate blockages can be time-consuming and expensive to remedy Hot s hot runner manifold separated unit carrying the runner and nozzle gating systems insulated from the main body of the mold
Hot s hot runner manifold nozzles and gate bushes pin and edge gating, valve gating, thermal sealing heating with band, coil, cartridge, tubular heaters temperature sensing and control thermal expansion and efficiency issues Hot s insulation water cooling in the core support and guide of stripper plate ejection
Hot s hybrid hot runner/cold runner system flash gate ~ uniform melt flow efficient air venting through a parting line (cf. multi-point gating ~ complex flow) evenly spread force due to the molded part off-centered hot runner manifold large number of cooling channels mold design for a suitcase half Gate small opening (or orifice) through which the polymer melt enters the cavity gate design: gate type, dimensions, and locations part geometry (wall thickness, etc.) part specifications (appearance, tolerances, etc.) material used fillers used cycle time de-gating requirements
Gate number of gate for the cavity single gate multiple gates ~ if the length of melt flow exceeds practical limits weld and meld lines cross-section of the gate typically smaller than that of the runner and the part related to the de-gating (separation from the molded part), the material freezing off (during the post-filling stage), and the viscous heating Basic Gate Terminology
Gate Location should be selected in such a way that rapid and uniform mold filling is ensured and weld/meld lines and air vents are positioned properly should be positioned away from load-bearing areas because the high melt pressure and high velocity of flowing material causes the area near the gate to be highly stressed Common Types of Gates direct or sprue gate tab gate edge or side gate overlap gate direct feed of material into the cavity rapidly and with minimum pressure drop the gate has to be trimmed off and a large gate witness is left on the part typically used for flat, thin parts to reduce the shear stress in the cavity high shear stress is confined to the tab which is trimmed off after molding located at the parting line of the mold and fills the cavity from the side, top or bottom of the part similar to the edge gate except the overlaps the wall or surfaces
Common Types of Gates fan gate disc or diaphragm gate ring gate spoke or spider gate similar to a wide edge gate with a variable thickness permits rapid filling of large parts or fragile mold sections through the large entry area creates a uniform flow front frequently used for gating cylindrical or round parts that have an open inside diameter for concentricity and unacceptable weld line for gating round or cylindrical parts material flows freely around the core before it flows down as a uniform front to fill the cavity a four point or cross gate for tubular parts and offers much easier degating than the ring gate possibility of weld lines and out of roundness Common Types of Gates film or flash gate hot probe gate pin gate submarine or tunnel gate similar to a ring gate but is used for flat, straight parts consists of a straight runner and a gate land across either the entire length or width of the cavity or a portion of it normally used to deliver material directly into the cavity through heated runners resulting in runnerless moldings generally used in three plate and hot runner molds to permit rapid gate freeze off and easy de-gating mainly used in two plate molds enables automatic degating of the part from the runner during the ejection stage
Common Types of Gates sprue gate tab gate fan gate flash gate Common Types of Gates ring gate disc gate pin gate submarine gate
De-gating manually trimmed gates automatically trimmed gates for two-plate molds submarine gates De-gating hook gates gating onto the top surface or side of a component is not acceptable difficult and expensive to make material must be flexible C: cavity insert B: separate additional insert cross-section of the gate must decrease toward the parts A: extended runner to enable the full length of the gate to be withdrawn from the mold
Design Rules gate design should deliver a rapid, uniform and preferably unidirectional mold filling pattern gate location should allow the air present in the cavity to escape during the filling stage if the gate location is likely to cause weld or meld lines, it should be positioned so that these will occur at appropriate positions to preserve part quality and appearance gate location and size should avoid the possibility of jetting Design Rules freeze-off time at the gate should ensure maximum cavity packing time and also prevent back flow gate location should be at the thickest area of the part gate length should be as short as possible to avoid an excessive pressure drop across the gate gate thickness is normally 50~80% of the gated wall section thickness fiber-filled materials require larger gates to minimize breakage of the fibers
Flow Analysis direct sprue gate (center-gated design) ~ part warpage double edge gate ~ weld line at the center of the part single edge gate ~ unidirectional filling pattern with uniform molecular and fiber orientation Gate Sizing length : should be as short as possible diameter rough approximation 4Q r 3 r flow analysis using computer software empirical approximation d NC 4 A 3 4Q A : total surface area of the product [mm wall thickness [mm] 0.75 1.00 1.25 1.50 1.75 2.00 C 0.178 0.206 0.230 0.242 0.272 0.294 N: 0.6 (PE, PS); 0.7 (PC, PP, acetal); 0.8 (Nylon); 0.9 (PVC) 2 ]