Packing out the void strengthens the sprue so that it will release with the part. Overall cycle time does not have to be extended if you decrease the cooling timer by the amount you raised the hold timer. Sticking can also occur with a conventional runner at the junction of the runner and sucker pin. Again, if the sprue sticks in the mold, utilizing the same methodology will help eliminate the problem. Cushion Size Cushion size should be at the absolute minimum to ensure the screw does not hit bottom and the pack and hold pressures are getting into the part.
Larger cushions can add to hold time in the barrel and aggravate degradation.
1 High back pressures can aggravate drooling into the mold and require additional decompression. In general, use very small or no decompression because it tends to pull air back into the nozzle causing splay in the next shot. Very small amounts of decompression can be used to reduce drool if needed. Screws should be chosen to be compatible with the hardness of the barrel material to minimize wear as with any plastic material. These unfilled materials are generally very mild on screw wear.