Many older electric molding machines (and a few new ones) have a programming glitch which causes the screw to retract if the packing or back pressure is set too low.
Question: When trying to make a short shot, I set my packing pressure to 0 and the machine pulls the screw back during packing. How can I make an injection-only short shot?
Answer: Turn packing time to 0 (or the minimum allowed time) and use additional cooling time (if necessary) to maintain the same overall cycle time.
The Cause: The screw is retracting due to a programming error on the machine. The machine senses that the pressure in front of the screw at the end of injection is higher than the set packing pressure (0) and it begins to retract the screw. The same thing often occurs during screw recovery. This misbehavior also happens on some machines when the mechanical resistance to movement is higher than the pressure setpoint. If the machine is not well lubricated and maintained, the minimum packing pressure and back pressure necessary to prevent screw retraction can often be very high. In such cases you should consider having a service representative review the condition of the machine and the software to determine if something can be done to rectify this situation.