In the world of plastic injection molding, quality, consistency, and precision are key. However, even the most advanced systems can sometimes produce defects — and one of the most common issues manufacturers face is burn marks on plastic parts. As a company with over 70 years of experience delivering custom plastic injection molding services across Canada and North America, we at UGP Plastics understand the importance of identifying and resolving these issues before they compromise your product integrity or production efficiency.
In this article, we explore the common causes of plastic burn, how to avoid these defects, and what manufacturers should know to maintain high-quality molding standards.
What Are Burn Marks in Injection Molding?
Burn marks, also known as gas burns or dieseling, appear as dark discoloration — usually black or brown — on the surface of molded plastic parts. These burn marks on plastic not only affect the appearance but may also weaken the structural integrity of the part.
Common Causes of Burn Marks in Plastic Injection Molding
Understanding the root cause of burn marks plastic injection molding is crucial for effective troubleshooting and prevention. Below are the most common factors that lead to this defect:
- Trapped Air or Gas: One of the primary causes of burn marks is air or gases trapped inside the mold cavity. When the mold fills too rapidly or lacks proper venting, the trapped air is compressed and ignites, causing a plastic burn.
- Insufficient Mold Venting: Without adequate venting, gases can’t escape during the molding cycle. This buildup leads to elevated temperatures and eventually causes scorching on the part surface.
- High Injection Speed or Pressure: If the injection speed or pressure is too high, molten plastic can trap air aggressively, causing heat spikes and burn marks on plastic. Slowing down the process or adjusting injection profiles can help mitigate this.
- Overheating the Resin: Excessively high barrel or nozzle temperatures can degrade the plastic material. This results in burn marks plastic injection molding when the degraded material enters the mold cavity.
- Dead Spots in Flow Path: Improper runner or gate design can create areas where molten plastic stagnates. These dead spots can lead to thermal degradation and eventually produce plastic burn on finished parts.
How to Prevent Burn Marks on Plastic Injection Molded Parts?
To eliminate plastic burn and achieve high-quality parts:
- Optimize mold venting locations and sizes.
- Fine-tune injection speeds and pressures.
- Reduce barrel and nozzle temperatures if degradation is suspected.
- Ensure proper maintenance and cleaning of molds and hot runners.
- Improve part and mold design to avoid trapped air pockets.
Consistent quality begins with a thorough understanding of the process. At UGP Plastics, we combine decades of knowledge with advanced equipment and precision tooling to deliver defect-free plastic parts — every time.
At UGP Plastics, we’ve been perfecting plastic injection molding for over seven decades. From part design to mold flow analysis, we take every step to ensure your components are free from burn marks, warping, or any other molding defects. Our team specializes in tailored molding solutions that meet the most demanding performance and aesthetic standards.
Contact UGP Plastics today for reliable, precision-molded plastic parts — built with quality and backed by expertise.
Frequently Asked Questions (FAQ):
What causes burn marks in injection molding?
Burn marks typically occur due to trapped air or gases in the mold cavity, high injection speeds, insufficient mold venting, or overheated resins.
What causes stress marks in plastic?
Stress marks are usually caused by uneven cooling, excessive packing pressure, or flow hesitation. Material selection and mold design also contribute to stress development.
What are common injection molding defects?
Common defects include burn marks, warping, short shots, sink marks, flash, weld lines, and bubbles. Each defect has unique root causes and corrective actions.
How to remove burn marks in injection molding?
You can eliminate burn marks by improving mold venting, reducing injection speed and temperature, and ensuring the resin is not degrading due to excessive heat.
How to get rid of burn marks on plastic?
If you’re already seeing burn marks on the final part, refinishing may help. However, true resolution lies in fixing the injection molding process parameters to prevent the defect from occurring.
How can mold design affect burn marks?
Poor vent placement, improper gate design, and narrow flow paths can trap air, leading to burn marks. Collaborating with experienced mold designers helps prevent these issues at the source.
