Packing at a gate can be difficult when the process conditions are not optimized for the plastic being molded. This set of concentric rings around the gate at the bottom center of the part, is evidence of cold melt and/or cold mold restricting flow into the injection cavity, resulting in under packed plastic and an underperforming molded part.
When molds start to wear, or injection pressures are extreme, flash can occur. In this part the flash seen at bosses and ejector pins can be seen. In this case some flash was near an arc source and started a fire.
Under packing a plastic during molding produces a less dense and weaker part. This surface evidence shows shrinkage across the entire surface, indicating that the polymer structure below the surface is not well consolidated and prone to failure at lower than expected stresses. Such parts may also contain internal voids or porosity.
Here is an example of an acetal nut which failed in service. The rough surface is strong evidence of processing problems such as slow injection speed, cold melt, and cold mold temperatures used during production.
