Overpacking of a part

[Suhas]

How do you test if the part is over packed?
Thanks,
Suhas

[rickbatey]

Suhas,
That is normally accomplished with an agent that attacks the resin of the molded part, looking for high stress that the solvent will show in short order. Other wise, I back off on a mold part until I get non-fills, put the numbers where we like to keep them (hold time per gate freeze, hold psi around 60% or so of the peak injection pressure), and weight the parts.
You're not thinking of soft resins are you? That is another animal. I tend to use hold psi in the 30-50% of peak pressure range, but for really short hold times. I've seen Kraiburg and Santoprene parts that had bumps at the injection points. These were from pushing cold resin from the runner into the part, compressing it, and the bumps showed as soon as you opened the mold.
Hope that helps! Rick.

[brentb]

How do you test if the part is over packed?
Thanks,
Suhas



Many ways. If you are basing the the conclusion of an over-pack condition, on cracking, there are catalysts, as Rick says they can be substances like solvents or surfactants. Many times over-packed parts will quickly succumb to the action of the catalyst and stress crack, which could indicate over-packing. Many stress lines in a transparent part viewed with polarized lenses can accomplish a similar thing.
Dimensional and weight data can indicate an overly packed part also.

If you can't eject the part from the mold, I'd strongly suspect over-packing!


Keep on Molding!

brent

[Suhas]

Thanks Rick, Brent,
I have similar thoughts but I was wondering if there is a certain plastic psi number we can put for each material. I think this is possible with some research. Each material has a PVT curve available and may be we can predict from that. Based on this, we will know the ideal pack and hold. - ANTEC paper?
What do you guys think?
Suhas

[rickbatey]

A paper might make for very interesting reading. I suppose that you could do the math based on the theoritical part volume, resin density, run the numbers, and see where you stand. Then perhaps develop a formula for rule of thumb work. Personally I try to get the most consistent part weight that doesn't require us to throw extra dollars into every box of parts we ship!
Rick.

[brentb]

Packing might be part specific?

Keep on Molding
brent

[Suhas]

Yes, both of you are correct.
1. The issue is resin density changes with temperature. So we need to figure that in. I think we can never get Part weight = Part Vol X Solid material Den .... since the addition of material is already done when the material temp is high and so the density is low.
2. Adding to Brent's comment, the packing is also localized, meaning not all the areas are overpacked. Typically the gate areas are; the thinner areas are if the thick area is right next to it (because of packing of the thick area); the thicker areas can be if you are trying to fill a thin area, etc, etc. So yes, it is also part specific.
This would be an interesting study. Flow simulation softwares have come a long way, but would it not be great if they predict these conditions by doing a virtual DOE? (Patent Pending !! - OK with your names also :) )
Suhas