IMM 10/00

Does anyone have suggestions on how to reduce or eliminate flaking on subgated acetal parts? The flakes appear to be small pieces of resin (about 7 mm, or .028 inch) left in the tool after ejection, which are then smashed in subsequent cycles.

It is not clear if the subgates were installed with a tapered cutter or by EDM. In either case, there could be a fine feather edge protruding from the subgate lip where it enters the mold cavity, causing small pieces of material to shear or flake as the parts are ejected. Remove any feather edge by gently polishing the subgate internally and inside the cavity wall with a small set of ruby stones. While the mold is down, also check your runner system for sharp edges, especially at the junction from one runner to another, and break and blend all sharp corners that would cause material to flake upon ejection.
-S. Hatfield, Accent Plastics Inc., Corona, CA, (909) 273-7777.

We had a similar problem while molding acetal mascara rods. You need to make the angle of the subgate steeper and probably increase its diameter as well. Leave the gate orifice the same size.
-S. Freer, Risdon-AMS (USA) Inc., Middletown, NY, (845) 341-5513.

The flakes are caused by the gate flexing as it is pulled out by the runner system. Providing a relief in the gate area should help to reduce this problem.
-D. Prague, Risdon-AMS (USA) Inc., Danbury, CT, (203) 743-8741, dprague@crowncork.com