PP with low shrinkage

[Monomer]

I know PP tend to have a large variance in the shrinkage from one grade to the next/One manufacturer to the next. Does anyone have any recommendations for PP with low shrinkage rates?

[Suhas]

The only thing you can do is to add a filler in there to stabilize your shrink. Not sure if there is a low shrink PP.
On the other hand why are you changing manufacturers so often?
Suhas

[brentb]

Also molding conditions affect greatly PP shrinkage, and if the parameters are not stable and consistent, shrinkage will not be either!

KOM

brent

[JayDub]

Shrink rate depends largely on degree of crystallinity, and for PP degree of crystallinity also defines component physical properties. So maybe you could look at the problem from the opposite direction. Pick a grade of PP, then run your cycle as long as is economically feasible to maximize crystallinity/shrink. Better part to part consistency, better physicals (strength, toughness, etc.) and better post-mold dimensional stability.

[Monomer]

We have placed a foaming agent in there to help pack the part out greater (do not know how this will effect the shrinkage). We also got this as a transfer part from another molder an we have no idea what grade of PP was being used. Would something like a nucleating agent help with the shrinkage to help crystalize the PP first?

[rickbatey]

So what exactly is the problem? Too tight of a dimensional range causing too many rejects and parts out of tolerance? The CFA will help with packing out sinks and deflection or warpage but you must process it like the handout states or you'll not have a consistent part of process. Do you have a shut off nozzle or press software to maintain screw position after plasticizing? Without them your process will be be out of control.
A PP with talc should save you money and help with shrinkage but if processed poorly you can have talc showing on the part surface. You can then raise mold temps and at times barrel as well to bury the talc. Glass bubbles for filler can help control shrinkage without a shut off nozzle. I wonder if your issues are from the press or from a process that isn't fully optimized?
Rick.

[Monomer]

Two problems. One is warping and the other is dimensions being too small.

[rickbatey]

What is the melt and mold temps? Machine settings? Valve gates or open drops? Multiple gates? Is it a feature near end of fill too small? Not a hole too small is it? Warpage and too small of dimensions should be fixed with CFA. How are you molding the CFA and who is the supplier? I think a nucleating agent will only make the issues worse. Please answer these questions plus give exact data as to how much deflection and how much the dimensions are off. That will help point out the root cause!
Rick.

[Monomer]

The part is a family mold with two separate parts. One of the parts a lid was experiencing warping while the bottom cover (the other part) was experiencing the dimensional shrinkage (around a larger core) roughly a 1 mm of excess shrink. The melt temperature was set at 400 front, 390 middle and 380 rear. Mold temp was set at 60 degrees. In terms of the foaming agent we are currently using 1% foaming agent fed through a dye feeder at 1%. We have tried using larger foaming percentages up to 2% put that leads to excess buldging on the outside of the parts.

[rickbatey]

So now what are the machine settings? I ask because the melt should be ok but might be on the cold side depending on pack/hold pressure and time. With the CFA you should run as low as possible hold time and psi to allow the foam to expand. Also screw recovery should be max or near it back psi with as slow a recovery time as you can get and finish at cooling time; little to no decompression on screw. Have you ran a gate freeze study? Shear rate study? I do think the mold temp is too low but how many circuits, bubblers or straight through water circuits? Does the lid have ribs only on one side? Is the core in the other part large enough before shrinkage to meet the part requirements? I've seen cores too small to start with and no processing techniques will create an acceptable part.
Rick.

[rickbatey]

These parts must assemble to one another, correct? Do you think they ran them one at a time to get good parts? I have seen it done in the past.
Rick.

[Monomer]

I don't believe there is a shut off in the mold to run one part at a time individually.