Acceptable Variation in Fill Time…

I corresponded with a frequent blog contributor late last week…

If my fill time varies 0.02 seconds, is that a machine problem? I have a similar machine with 0.00 seconds variation.

note: after a few emails, it was determined…
  • 200 ton hydraulic machine
  • 38% shot size
  • 0.55 second fill time
  • Possibly a pressure limited process
My Response
As a general rule, a 3-4% variation in fill time is not unusual with a hydraulic molding machine.
Although common, one way to improve this variation is to tailor the process to the machine’s capabilities…
1) Make sure the machine has adequate maximum injection pressure. Also read: Providing a Buffer To Accommodate for

2) Make sure the hydraulic valves and transducers are in good functioning order. Also the hydraulic fluid should be sent to a lab every six months to check for contaminates and breakdown. Also, the filters should be checked and replaced routinely.
3) Your machine may have a hard time during velocity transitions, causing a pattern or cycle of overshoot and undershoot. This may also be seen at the beginning and end of fill. In these cases, a stepped velocity profile will adjust the velocity to one which is easier for the machine to maintain.
Additional Thoughts
Although closed loop controls are very helpful, it is often necessary to adjust the process to compensate for a machine’s shortcomings. Keep in mind, once you identify an issue with your molding machine, you should work with the manufacturer to better understand the cause. They might have encountered the same issue with another customer… and might already have a fix which you might not have considered.

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