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.
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If you have problem with variance of cushion or with pressure at tranfer, you should try to do decompressions analysis. Maybe distance for check ring seating is small (I mean decompression). It helped me with Celanex 2002.