Foam Agents to prevent dimples.

[andrewlamberson]

Today at work I was proofing a new brochure that had a section on Foaming Agents for Engineered Thermoplastics and one of the statements read:

"Foaming agents can also be used to eliminate sink marks, shallow depressions on a part surface created by excessive shrinkage in thick areas during cooling. Using a small amount of foaming agent creates enough internal pressure within the part to offset the forces of shrinkage that cause sink marks to form."

Has anyone experimented with using a small amount of foaming agent to eliminate the ongoing problem on differential shrinking in thick bodied baits ...that cause the "dimple" on the skin of the bait?

I also wonder if by using a foaming agent, you could produce a bait that floats better? Endothermic foaming agents release  carbon dioxide allowing you to reduce the density of the polymer. Since PVC has a density less than water,  baits normally float without a hook. But I'm wondering if you could decrease the density of the bait enough to offset the weight of the hook?

If a foaming agent would work...that would open up a bunch of new bait designs that are limited by thick body sections causing skin dimples.

[412BaitCo]

Hmmmmm. This is very interesting Andy! Do you have the ability to give it a try on something known to dimple easliy?

[ipt]

seems pretty genius to me. although it would seem easier to just make air vents rather than applying foam every time.

[firetiger]

Can the foam be placed in a mold and formed into the shape of a fish head.  Then place the foam fish head in a mold and cover with plastic?  How much does it cost?

[andrewlamberson]

Vents remove the air that was in the mold ...that need to go somewhere when you displace the air with the plastic (I don't think  there is any significant chemical reaction- out gassing with soft PVC)

Foaming agents actually add a gas to the polymer (plastic) ...displacing heavier plastic...with the lighter gas....and create a pressure from the part outward to (hopefully) prevent dimples (not sure what it would do in a small bait mold of PVC...thus the question).

As you may have read in my earlier posts...shooting the bait at a lower temperature will often fix the denting problem...but maybe not... with a bait that has a significant heat reserve in the dense thick part of the bait. As this hot core cools, it shrinks and  sucks in the weakest material ...which is often the skin of the part (bait). This causes warping in solid plastic parts and  "sink marks".

[andrewlamberson]

The foaming agent is actually a chemical/material compound, not a physical piece of plastic or foam. When heated it generates the gas. Kind of like adding Alka Seltzer !


I'm sure there would be some timing issues!!

[pjmcla]

Are these not typically mixed and shot under continuous pressure?   Would the foaming occur while the plastic was in the pyrex and be fully expanded before injection?  I like the idea.  Finding the right agent for our application may be a bear.

[Denny Welch]

This whole thread is better left to you smart guys.  I'm outta here.