One of our more advanced customers just asked these questions…
Monitor the shot-to-shot variation (cushion)… if it is not consistent, can I increase the back pressure?
What tolerance should I be able to hold on cushion, e.g. I am hitting 0.20” cushion, what should I be expecting to repeat, i.e. what min & max. on the alarm?
If the cushion is varying greatly from shot to shot, then back pressure may be an issue.
The cushion will vary over time…and many companies will set a variation of approx. +/-5% of the material used to fill the mold.
An example would be this:
Overall shot size = 110mm
Target cushion = 10mm
Material used = 100mm
Upper cushion limit = 15mm
Lower cushion limit = 5mm
The purpose of a cushion is to give your process the breathing room it needs to compensate for material variations so the screw does not bottom out. It’s similar to keeping a safe distance between your car and the car in front of you in traffic to compensate for variations in traffic and one’s response time. This acts as a buffer to prevent a wreck from happening.