IMM 5/97

Does anyone recall the process (and sources) of gasketing mold parting lines to pull a vacuum prior to injection? We are trying to eliminate voids (air entrapment) in thick nylon parts.

All the venting in the world, including evacuation of the mold, will not overcome this problem as the voids in these parts are not trapped air but contain a vacuum created by material shrinkage. Only changing the part design, modifying the gate position, or adding flow leaders will help. This problem is caused when:

• A thick section is filled from a thinner section.

• Insufficient injection hold time is used.

• Gate freezes off too soon.

-R. Evans, R.E. Promotions Services, Ltd., Bridgtown, Cannock, U.K., (011) 44 1543 466 585.

First, vacuum generation-we use an air valve and Venturi generator. When the mold closes (or ejector plate is pulled back, or barrel is forward if using sprue break), the plunger on the valve is depressed and the vacuum is generated. Air is sucked out through a vented ejector pin. My first response to an included void would not normally point to a volatiles problem. Nylons (I assume crystalline types) have high shrinkage. If you can have a gate equal to, or .005 inch/.010 inch larger than, the gate thickness and you can run a hot mold (depending upon the type of nylon used, hot could mean 300F to 400F), you can pack the part on a long cycle and eliminate the voids. Minimizing stock temperature also helps.

Here's a possible solution that would be significantly less expensive than the modifications required to install a vacuum system. This would be to use our tungsten disulfide coating. We know from experience it makes resins flow easier and generally packs out parts with lower pressures and temperatures while filling without voids. We also know it is not a solution for poor mold design, though it could help some. The process is inexpensive and molds can generally be returned the same day they are received.
-D. Brown, Russell Products Co., Dayton, OH, (937) 849-1601.