Static Check Ring Test

[Rojan]

Hi,

Long story short, if I want to check if my check ring is leaking (or barrel is the cause but that's not the point) I turn off HR (for example) and then at different screw positions I increase the holding pressure incrementally. At some point the screw starts moving forward but slowly. My question is how much leaking is acceptable. I think the borderline is blurry and it is process dependent. I would like to make a standard in my company so that no matter who does this test everybody should have the same judgement regarding this test.

What are your experiences?

Thanks,
Robert

[chrisprocess]

Interesting, could you explain that method a little more in detail? I'm not as familiar with it and trying to understand. Maybe just terminology, but what is HR?

The check ring study I'm familiar with is without any pack and hold (fill only parts) and is measured by shot weight.
Rule of thumb for us is no more than 3% difference by shot weight

[Rojan]

Chris,

I think you mean the Dynamic Check Ring Repeatability Test which checks the repeatability and not tightness of the check ring.
I would like to know how you evaluate if the check ring leaks.
By HR I meant any hot runner system which I turn off before I do this test. The reason for that is not to inject into the mold. When the HR is cold I inject at different pressures but the screw stops and does not move. After I set the holding pressure higher the screw will start moving because there is a leak. The question is what pressure you use to test this. In my opinion this pressure should be a bit higher than the one you hit in the process. But I want this test to be kind of universal and not process-specific.


Robert

[chrisprocess]

Oh I see, so you're injecting into a cold runner system or cold sprue and watching the screw to creep forward as you apply various injection pressure. The concept is if the screw creeps forward, plastic is leaking behind the check ring, yes?

Plastic is compressible so I'd suspect the higher pressures will cause the screw to move forward, respectively, but should stop at a certain point.
When you set the holding pressure higher, does the screw continuously keep moving forward over time or does it stop at a certain point?

Maybe not the best person to answer this but the hold pressure should not exceed the nozzle contact force, but you should (in theory) be able to go maximum hold pressure and have the screw stop at a certain point.
Though I wouldn't ever do this especially into a sealed hot-runner mold.

Don't really know a Rule of Thumb for this but if I encountered something like this on the floor, I'd just run the process in automatic and do something like 25% higher of pack time/pack pressure and weigh parts for consistency. This would indicate a worst-case scenario for static check ring repeatability. Other than that I don't really have a good answer for you sorry!

[Suhas]

Hi Rojan,

The procedure you are following is correct.
The answer to your question 'What is acceptable leakage' depends on what you are molding. If you are molding forks and spoons a little leakage does not matter. However, if you are molding a large quantity of critical components then the little may matter.

Hope this helps.
Regards,
Suhas