One of our more advanced customers brought to us this observation…
Jim M.
It is interesting that this test does not utilize the pack & hold portion of the cycle; just the fill portion. Experience shows, especially with high 2nd stage pressures, that the screw will drift, and some reflection of that would be a good indication of check ring holding. Your thoughts…?
Our Response
Every check ring will leak and drift. What is most important in maintaining a consistent fill is the repeatability of this leak. The Dynamic Check Ring Repeatability Test is intended to measure this variability in this leak during fill. In most cases, if the drift is variable during 2nd stage, it will also be variable during fill. Although each process is different… if the screw does not bottom out, and cushion varies less than 5% of the overall shot size from shot to shot, usually the amount of drift is not a major factor.
Additional Thoughts
It is always great to hear someone asking such highly technical questions because it means they are focusing on the finer points of processing rather than subjects of general knowledge.
-Andy
Great Comments Andy,
In my experience, I have also found that variations in the screw drift during pack coincide with variations in the repeatability of fill.
This test also makes a great reference when issues occur in the future. If you know the repeatbility was 2% in February and is now 6% in May… then you know the ring is either failing or gone.
-Dan
Brilliant question I would say…
Thanks Jim,
You always have such great insight.
-Andy
Hello Everyone.
My experience is, that if you want to do dynamic check ring test exactly, you need to subtract variation of actual transfer point. With my older machine is sometimes up to 0,15ccm what is with material PA66 app.1,95gram. So from shot to shot I can have variance 1,95gram.
Thanks MJ – Such a calculation is a unique way to compensate for an older pressure-controlled machine running a traditional molding process. I would interested to see what type of data it produces.
For the Dynamic Check Ring Repeatability Test to function best… you should transfer via screw position. In such a case, the screw travels the same distance using closed-loop velocity control during first stage injection with second stage pressure turned off. These steps will ensure you are isolating the dynamic portion of the cycle to measure check ring repeatability.
Hello Andy.
Thanks for comment. I used screw position for tranfer, but actual displayed screw position was different from shot to shot 0,15ccm. If I produced small parts, variation was 7% of weight what is too much for me.
MJ
Hello MJ – Thanks for the clarification. If your actual transfer has such a high variability, you have a complication with your machine, albeit the controller, valve, or potentiometer… even on an older machine, your actual transfer position variation should be nominal with respect to the shot size. -Andy
Thanks Andy for your comments. I like your website and webinars also, it helps me very much.
MJ