Top Causes of Burn Marks on Injected Molded Parts

When it comes to plastic injection molding defects, mold burns are one of the most common.

These plastic injection molding defects appear as discolorations, usually black, brown or rust-colored. They can show up on injected molded part as streaks or small spots. Burn marks on injected molded parts usually occur near the end of the flow path or where air became trapped.

Top Causes of Burn Marks on Injected Molded Parts

Problem Source Solution
Burn marks at end of filling part Plastic Injection Speed is Too High  Pressure is too high & allowing fill rate controls to be ignored. Lower pressure.
Resin is charring. When it enters the melt stream the charred resin becomes molded into the part. Resin Temperature is too High Check temperature controls & heater bands to ensure they are functioning properly and are accurately calibrated. If so, reduce the melt temperature.
 Burn marks near end of fill Improper Venting Slow down injection rate
Resin burn marks destroying mold Trapped Air in Injection Cavities or Pockets Improper mold construction – not enough vents
Black streaks in molded parts Gate Size is too Small Resin melt flow is too low
Black streaks in molded parts Nozzle Size is too Small Open nozzle orifice
Black specs & color striation Screw and Barrel Wear Replace Screw and Barrel

Unfortunately, burn marks are more than just an aesthetic eye sore, they cause degradation within the material and mold properties. This degradation typically results in a compromise of the component’s structural integrity, leading to eventual part failure.

The right kind of molder will be able to catch and fix these plastic injection molding defects before the parts reach you.

Are You Using the Right Kind of Molder?

There are 2 types of plastic injection molding manufacturers: Commodity or Engineering-Grade.

Most plastic injection molding companies specialize in commodity work (about 90%, in fact). Commodity molders typically do not have the technology or adept knowledge to work with engineering-grade resins and/or complex molding projects.

Today, there are only a few companies around the world who specialize in Engineering-Grade molding projects. Between the rarity of this specialty and the global freight distance, it can be a real challenge to find a capable domestic company to handle these projects.

At PMC, we have helped develop and launch more than 10,000 molded plastic products since our founding in 1975. We are a US plastic injection molding company with in-house Master Molders who uniquely specialize in complex molding projects and engineering-grade resins.

If your current molder is struggling with your plastic injection mold design, turn to PMC.

Contact PMC For Your Molding Project Needs