Check Ring Drift vs. Repeatability

In a recent e-mail, I received this question from one of our customers…
Jim M.
Regarding cushion, regardless of dynamic check ring tests (e.g. get 1.3%) if the machine will not hold a cushion, should the culprit be repaired?
For example, the gate freeze test shows 7 seconds & the hold time is set to 8s.   When I transfer over to pack stage, the screw drifts about .020″ during that time.  Is that an OK scenario, or is zero drift the answer?  If I put the pack time at 20s, the part still looks good, but screw keeps slowly moving & doesn’t stop.
My Response
Every check ring drifts. Tests such as the Dynamic Check Ring Repeatability Test test consistency during fill. A 1.3% variation implies that your check ring is wearing very evenly and your process should remain consistent. If the amount of drift makes you uncomfortable, it may be time to inspect and/or replace. But, a long as the drift is consistent, you should be able to maintain a robust process.
Additional Thoughts
There are a variety of check ring designs available to the molding industry… and all of them leak. A properly functioning check ring will leak consistently. If you are running a material with a very low viscosity, you may want to ensure the check ring you are using is appropriate to the applicaton and properly sized for your barrel.
  1. MJ left a comment on 2009/09/30 at 11:28 am

    Hello Jim.
    I’m doing let me say “static” check ring test. I put plate in front of nozzle, move nozzle to touch the plate. Then I adjust 600 bar of holding pressure, and if screw will not move more than 10mm in ten second, check ring is good. Check ring drifts allways, but important is how far.

  2. Andy Routsis left a comment on 2009/09/30 at 11:42 am

    I agree MJ,
    It all depends on the application. A large pipe fitting molded out of PVC will have a different need than a micro-molded medical device.

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