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Common Defects in Injection Molding

Injection molding is a method of mass production manufacturing and is not without its faults. Defects can be minor surface marks or major areas of weakness or failure. They are often the result of errors in the molding process, poor design, material contamination, or a combination of the three.

In this article, we’ll discuss the common defects you may encounter with injection molding, including how to recognize them, what causes them, and how to avoid them. 

Black Specks

If the surface of your part appears to have specks, or flakes, of another material embedded in it, you may have a contamination problem with your material. However, if these specks are black, they may indicate a different problem. Often, black specks in parts can be caused by the molten material carbonizing inside the injection molding machine’s barrel. This occurs when the material is kept inside the barrel too long and degrades, causing it to turn black and clump together. To avoid this problem, increase injection speed so the time the material is kept inside the barrel is minimized. 

Burn Marks

A part may become burned during molding if the material overheats. This defect is distinguishable from other discoloration issues because it will be black or rust-colored. Severe burns, however, may appear as degraded or destroyed material, not just surface discoloration. Overheating may be caused by insufficient venting or injection speed being too high. Adjusting the venting, injection speed, molding temperature, and cycle time may help negate the possibility for burns marks. 


Your part may have uneven coloring or total discoloration if your colorant isn’t properly mixed with your base material or has run low or if your material is otherwise contaminated. Discoloration by contamination is commonly caused by leftover material from previous molding batches residing in the hopper, barrel, or mold. In this case, it is best to thoroughly clean the machine and mold before continuing production.