Part thickness change for one run to another

[joeprocess]

Interesting issue. Had a customer reject for parts being too thick. Part is approx. 5" x 8" flat with some openings. 2 cav. Thickness should be 0.130" but rejected lot is 0.010" thicker across the whole part, but no flash. Both cavities. Material Unfilled PC/PBT.

I suspect the part was over-packed, but only after the part skinned over enough to prevent P/L flash. Tried to duplicate the issue and couldn't. Parts either flashed or were to the right size. Both good and bad lots where ran in the same press.

Some here think maybe the wrong material was used, but the parts look very similar and a burn test seems to lead me away from that conclusion. The only material that may have been misused would be a PC/ABS or ABS. Neither of these have a far enough shrink rate difference to account for 0.010" difference on a 0.130" wall.

Thoughts?

[Suhas]

Hi Joe,
You are right about the skin formation. I would check and see if the tonnage is set the same as before. As you get closer to the required tonnage the mold could start to blow open causing an increase in the part thickness.
Let us know what you find out.
Suhas

[rickbatey]

What about 2-3 seconds more pack time? Hotter or colder mold? Could you have gotten a lot of resin with a much lower MFI?
Rick

[joeprocess]

We run this part with gate seal. Do not have data on either of the other two questions. I am thinking of adding pressure transducers to the mold to get better process monitoring.

[moldingppm]

Definitely sounds like a material issue. Especially if the setup was never changed between passed and rejected parts. How many reject parts were there total?

[Sunny]

Well I think you should check whether your machine is developing same clamping for each turn or you can use another machine and compare production.

[joeprocess]

molding PM. setup was not changed, but the bad parts were produced at a different date. The whole lot was rejected.

[Brisli]

You could put an dial test indicator on your moving platen or on the parting line of the mold to see if it is blowing open during injection.