The switch-over from ram speed control to packing pressure typically takes place before the cavity is filled. The remainder of filling takes place at the constant pressure achieved at the time of the switch-over from filling to packing/holding, or at the specified packing/holding pressure. In either case, the ram speed typically decreases.
The significance of the switch-over point is best illustrated by considering the consequences of switching too early or too late.
- Switching too late can lead to:
-
- Mold opening and flashing due to build up of excessive cavity pressure towards the end of fill
- Burn marks as the plastic slams into the end walls of the part
- Damage to molding machine and/or mold as ram bottoms out
- Switching too early can lead to:
-
- Short shot due to insufficient ram displacement
- Longer cycle times
Note: If the switch-over is occuring earlier than expected, look for a short shot, or check that material compressibility has been taken into account when setting the velocity/pressure switch-over point.