Plastic Injection Molding: Temperature-Induced Defects & Solutions

Top 3 Plastic Injection Molding Defects Caused by Temperature Issues & How to Fix Them

When it comes to plastic injection molding, temperature plays a vital role in part quality. The most common temperature-induced defects are Warpage, Sink Marks and Jetting.

Below, we explain these plastic injection molding temperature defects and how to fix them:

Mold Warpage

Defined: Plastic injection mold warpage is when the intended shape of the molded part is distorted during the cooling process, causing the part to fold, bend, twist or bow.

Cause: Mold warpage is often caused by insufficient mold temperatures. If the mold temperature is too low the molecules will solidify prior to packing and at different rates. This temperature-induced issue causes plastic injection mold warpage.

Solution: To prevent mold warpage, increase the mold temperature based on the resin supplier’s recommendations and then adjust accordingly. Operators should allow 10 cycles for every 10 degree change in order to allow the process to re-stabilize.

*If temperature is not the cause of your mold warpage, you’ll want to look at Solutions for Injection Molding Warpage, which provides other potential issues and how to solve them.

Sink Marks

Defined: In plastic injection molding, sink marks are categorized as depressions (small craters) that can occur in thicker areas of the molded part.

Cause: Sink marks are most often caused by insufficient cooling time. This usually occurs because the thick sections of the injection molded part take longer to cool than the thin areas.

Solution: Aside from reducing the thickness of the thickest wall sections, you may also need to lower mold temperatures, increase holding & prolong holding time in order to allow for sufficient cooling and curing.


Defined: Jetting happens when molten plastic fails to stick to the mold surface. When the liquid molten plastic solidifies it will exhibit the wavy fold characteristics of the jet stream on the surface of the plastic injection molded part.

Cause: In plastic injection molding, jetting is most commonly caused because the melt temperature is too low & the viscosity of the molten plastic resin becomes too high. This leads to an increased resistance in flow through the mold. When the plastic resin is injected into the mold and makes contact with the mold walls, it’s rapidly cooled & viscosity increases. The resin that flows through behind the viscous resin pushes that viscous resin further, which leads to scrape marks on the finished product’s surface.

Solution: Aside from increasing melt and mold temperatures, you can also increase the mold gate size so injection speed slows. You may also need to optimize your plastic injection mold gate design to achieve sufficient contact between the molten plastic resin & the mold.

Why Choose PMC?

Plastic Molded Concepts is a U.S based company that uniquely specializes in molding with engineered resins. We have been thriving in the plastic molding business for 45 years. We have the experience and certified in-house Master Molders, allowing us to take on projects of any complexity.

PMC offers a comprehensive range of plastic molding services including mold design assistance, modeling, prototyping, manufacturing, and testing. Reach out to PMC if you’re experiencing plastic injection molding temperature defects or request a quote today for your next project.

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