Avoiding sink marks

Sink marks are like invading pests, once you realize you have one, getting rid of it will be difficult.

Sink marks can be defined as a depression in the plastic part; it can even resemble a dimple or a groove. It is caused by excessive localized heat with resultant contraction (sinking) of the resin/composite as the finished part cools. While sink marks can occur during start-up, more problematic is when the defects appears during a run. The causes can run the gamut of part design, mold, machine, and process.

Primarily, work with your mold designer and molder (even better if they are one in the same) as up front as possible. Potential for sink marks comes from concentrated heat. Part features like excessive thickness where mating walls meet and non-uniform and excessively thick wall sections can be recognized and eliminated to prevent sinks from forming.

Even with the part design optimized, the mold design must take into account those areas, i.e. wall joints, boss bases, support ribs, where heat can build in the mold over the course of the run. The mold designer can apply cooling to those areas to draw away heat and not allow it to build as the production run progresses. Also, while many designers like to be steel safe, if the gates(s) and runner are not adequate size, resin flow can be restricted and the part not allowed to adequately fill. The result is a part that is not packed out and the plastic does what it wants, mainly move, and sinks are one byproduct.

Therefore, while processing can contribute, the best prevention for sink marks is pre-planning. Because once you see sinks, you may be doing some significant back tracking to eliminate them.