Parts made in a plastic injection molding process can have their own unique set of possible defects. That is why all of our employees are trained to spot these problems. The following is a listing of the most common defects associated with the plastic injection molding and transfer/compression molding processes:

Sink Marks: Sinking is caused by the outer skin of plastic solidifying while the material inside is still molten and viscous. As it cools and solidifies, the material compacts . The best way to avoid these dimples is to design the part with a consistent wall thickness. However, many times the effect of the sink mark can be minimized by adjusting molding parameters such as injection pressure and time.

Gas marks (burning): When molten plastic is injected into a mold cavity, the air that is in that cavity needs a place to escape. If the hot compressed air cannot escape, it may leave burn mark on the part. The best way to avoid gas marks is to strategically locate air vents of the proper depth within the mold. Adjustments to process parameters such as injection speed and screw speed can be altered to reduce or eliminate burning.

Flash: When two halves of a mold come together, the sides of the cavities must press together tight enough that no molten plastic leaks out of the cavity, resulting in a thin layer of plastic protruding from the part called flash. The plastic injection molding press must have enough tonnage, and the mold must be properly built and maintained to avoid this problem. However, some materials flash more easily than others. In fact, most thermoset materials will flash regardless of the press and mold. These parts need to be “deflashed” as a secondary operation.

Short shots: If too little plastic injection molding material flows into the mold, the cavity will not fill properly, which we refer to as a short shot. This can be corrected by changing the plastic injection molding parameters.

Knit Lines (or weld lines): If a part has a hole or insert, the molten plastic will flow around the opening and meet on the other side. Where the two sides meet, the part will be weaker, and a knit line may be visible. Often this effect can be minimized by adjusting processing parameters such as injection speed and mold temperature.

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Plastic Injection Molding

Plastic Injection Molding