Inconsistent check ring

[Joel JS]

Hello to all,

(My second attempt at posting.... my apologies if it appears twice.) Was wondering if anyone has encountered this.

I was sampling a mold with different materials and the check ring was doing its job (consistent parts and cushion.) I successfully ran a 20% talc filled PP, then switched to a PP copolymer, no talc with no issues.

I then switched to a HDPE and kept the barrel temps at 440 - 420F. I made a few full parts then kept getting short shots. I adjusted the shot size for the short shots, and a few shots later the mold was flashed. I was using ample suck back (10mm) I lowered the barrel temps 10 F and repeated, and got the same results.

So,.... is the check ring worn, (although it worked fine previously)? If I had dropped the barrel temps more, and increased the material viscosity, would that have cured the problem? Have others seen this happen with different materials?

Thx! Joel

[Suhas]

Joel,
The next time you run the machine (any mold) run about 30 consecutive short shots. Zero pack and hold with about 10 seconds of charge delay time. Weigh all those parts and see how much variation you have withing the 30 samples. It will start giving you some food for thought.
-Suhas

[Joel JS]

Joel,
The next time you run the machine (any mold) run about 30 consecutive short shots. Zero pack and hold with about 10 seconds of charge delay time. Weigh all those parts and see how much variation you have withing the 30 samples. It will start giving you some food for thought.
-Suhas

Suhas,

I am not in that plant very often, so it is difficult for me to perform such, I may be able to ask another tech to do such as it is a good idea. But, in you professional opinion, does this inconsistent check ring lead you to believe that it is worn? (And it did appear fine with a different material.... full parts, no flash, consistent cushion.) Thanks for your reply!
Joel S

[rickbatey]

The talc filler raises the viscosity of the melt so odds are filled resins run ok but unfilled show how inconsistent the NRV function truly is.
Rick.