Why Do Burn Marks or Scorching Appear on Injection Molded Products?

During the injection molding process, the appearance of black or yellow scorch marks on a product’s surface is a common defect. This issue leads to scrapped parts and reduced production efficiency. It also serves as a clear signal of underlying problems with the equipment or process parameters. Understanding the root cause of scorching is the first step to completely resolving it. Therefore, this article will systematically analyze the four fundamental causes of this defect.

1: Machinery and Equipment Issues

• The Core Problem: The plastic material decomposes inside the barrel. This happens because of abnormally high temperatures or from stagnating for too long.
• The Deeper Reason:
  1. Temperature Control Failure: A malfunctioning heating coil or thermocouple can cause a localized area of the barrel to overheat. This temperature spike far exceeds the set value.
  2. Screw and Barrel Wear: Wear and tear on the screw, check ring, or barrel wall can lead to melt reflow. It can also create dead spots. As a result, some plastic remains in the high-temperature zone for too long and burns.
  3. Nozzle Blockage: A nozzle orifice that is too small or blocked increases injection resistance. This generates excessive shear heat and causes the plastic to scorch.
• The Solution: You should regularly calibrate the temperature control system. Additionally, inspect and promptly replace worn screw components and keep the nozzle clean.

2: Mold Conditions

• The Core Problem: The machine cannot successfully vent the air trapped inside the mold cavity.
• The Deeper Reason (The Diesel Effect): This is the most common cause. During high-speed, high-pressure injection, the air inside the cavity gets rapidly compressed if the mold’s vents are blocked or poorly designed. This adiabatic compression causes the air temperature to spike instantly. The principle is the same as fuel ignition in a diesel engine. Consequently, this process burns the plastic at the leading edge of the melt flow. You will typically find this defect in the last area of the part to be filled.
• The Solution: Inspect, clean, or deepen the venting slots to ensure proper air discharge. You can also consider adding inserts or ejector pins at the burn locations to assist with venting.

3: Plastic Raw Material Issues

• The Core Problem: The raw material itself is contaminated or impure.
• The Deeper Reason: Dust, impurities, or other types of plastics can mix with the raw material. These contaminants may have a much lower decomposition temperature. Therefore, they burn at normal processing temperatures.
• The Solution: Ensure cleanliness during material storage and transportation. When adding material, you can install a magnetic frame at the hopper inlet to adsorb any potential metal fragments.

4: Molding Process Condition Issues

• The Core Problem: Improper process parameters are generating excessive shear heat.
• The Deeper Reason: Even if the equipment and mold are in perfect condition, incorrect process parameters can cause scorching.
  1. Excessive Back Pressure: If you set the back pressure above 300MPa, the screw will encounter significant resistance. This generates excessive friction heat (shear heat) and causes the plastic to partially decompose.
  2. Excessive Screw Speed: When the screw speed exceeds 90 rpm, it can also generate a large amount of friction heat. This happens due to the high shear rate and leads to scorching.
• The Solution: Reduce the back pressure and screw speed as much as possible. However, you must do this without compromising product quality.

How Does MINHUI Prevent Scorching at the Source?

We understand the importance of stable production. That’s why MINHUI’s vertical injection molding machines help customers prevent scorching issues from the very beginning:

• Precise PID Temperature Control System: This system ensures that the actual barrel temperature is highly consistent with the set value, eliminating abnormal overheating.
• High-Quality Screw and Barrel Components: We use wear-resistant, precision-machined components. This helps to reduce dead spots and abnormal shearing.
• Stable and Reliable Control System: Our system allows operators to make precise, repeatable settings for critical parameters like back pressure and screw speed.

Scorching on injection molded products is a complex issue. It is caused by multiple factors. However, through systematic troubleshooting—from the equipment, mold, and raw materials to the process itself—it is entirely preventable and solvable.

If you are currently struggling with scorching issues in your production, or if you are looking to improve production stability by upgrading your equipment, contact the expert team at MINHHUI today. We are ready to provide you with a professional solution.