IMM 8/97

How do you differentiate between splay due to wet material, excessive heat, regrind pellet size, or contamination? Can parts with splay (except due to contamination) be reground?

Wet material will cause splay at the end of the part. Excessive heat will cause splay at the gate. This is even more evident when using regrind. Regrind-if not reprocessed or compounded-will melt differently, causing heat splay. Regrind in powder form will cause even more splay because it typically will absorb moisture, which is why the material should be dried, if possible. Parts with splay can be reground.
-M. A. Billings, LarSan Chemical, Akron, OH, (800) 837-5510.

Wet material will often produce parts with silver streaking-fine silver-colored lines not confined to the gate area. The use of a moisture analyzer on the material is suggested. Sometimes an air shot at the press will yield a material that is obviously too wet to mold. Excessive heat usually produces discoloration, easy to see in tan or cream-colored materials as a color degradation towards deeper tan. (It's hard to see in black parts.) The use of a melt pyrometer is suggested.

Regrind pellet size, if too large, will show up as a line, usually half moon in shape, near the gate. It is basically unmelted material. Contamination usually results in delamination of material in the part, near the gate, or sometimes at a weld line in the part. Some impact at the gate will frequently reveal this delamination.

It is not recommended to reuse material that has been molded wet or at excessively high temperatures because the polymer has been degraded.
-W.E.Foster, Tessy Plastics Corp., Elbridge, NY, (315-689-3924.

Here's copy from our molding manual concerning splay:

First, there are three types of splay: temperature or volatile splay, jet effect or jetting, and moisture splay. Splay can be caused by problems in the machine. (Note that the same things that cause discoloration can cause splay.) For instance, jet effect splay is caused by obstruction in the nozzle, sprue bushing, gate, and runner. Splay can also be caused by an overly fast injection speed; too much decompression; plastic flakes or debris from undercuts that fall in the flow stream; or hang up spots in the screw, check valve, end cap, or nozzle (material becomes stagnant and degrades, causing discoloration and / or splay); and in-house contaminated material or bad materials from the supplier.

In addition, high temps in the nozzle, barrel, sprue bushing, probes / hot runner, or mold can cause heat splay. It can also occur if the mold is too cold or if there is too much decompression.

Here are a few ways to fix splay. Increase the cold slugs, runner, and gate; polish the runner, gate, sprue; increase the vents; change the gate location or style / type; and change the part design that affects or restricts flow.

Note that parts with splay on some materials can be ground and the material can be used again at 20 percent blend, if the part specs allow it. If the material is the type where hydrolysis occurs, such as polycarbonate, the only reason to grind them is so the parts do not make up so much room in the landfill; the material is useless.
-Alex Mora, Formula Plastics, Ontario, CA, 909-947-4734.