Injection molding

In injection molding production, yellow-brown or black burn marks appearing locally on products are a common defect that troubles many factories. When customers see burn marks, their first reaction is often "the temperature is too high," so operators lower the barrel temperature again and again—yet the burn marks remain, and short shots or cold slugs appear instead. What is the problem? The issue is that "color burn" in injection molding usually has two completely different root causes: high temperature generated by compressed entrapped air, and thermal degradation of the material itself. The two look similar, but their causes and troubleshooting directions are entirely different. Treating them as the same thing will only make adjustments more chaotic.

I. The Two Major Root Causes of Burn Marks

1. Entrapped-Air Burn (Diesel Effect)

This is the most common cause of burn marks in injection molding. The air in the mold cavity, along with gases released when the plastic is heated, cannot escape during injection and is rapidly compressed by the high-speed incoming melt. In a very short time, these gases are compressed to extremely high pressure, and the temperature surges enough to "scorch" the plastic in contact with them. This phenomenon is also called the "diesel effect" in the industry, because the principle is similar to compression ignition in a diesel engine. Its typical feature is that the burn marks appear in regular locations, usually at weld lines where two melt streams meet, deep-cavity dead corners, the front ends of thin ribs, and the last-filled positions—exactly where air is most likely to accumulate and hardest to exhaust.

2. Material Degradation Burn

This type of burn is not a mold venting problem; the plastic has already "deteriorated" in the barrel. Common triggers include: barrel temperature set too high, melt residence time in the barrel too long (long cycle time, low screw speed, high back pressure), screw speed too fast causing excessive shear heat, or too small a gate and too fast an injection speed, so that the melt generates local high temperature due to severe friction as it passes through the gate and burns right near the gate. Heat-sensitive materials (such as PVC and POM) are especially prone to problems, with a narrow temperature window where degradation and blackening occur with little warning.

II. How to Diagnose: Look at Location, Do an Isolation Test

Step 1: Look at where the burn occurs

Location is the best clue. Burn marks near the gate most likely indicate shear overheating—the gate is too small or the injection speed is too fast, and friction generates heat as the melt passes through, causing degradation. Burn marks at weld lines, deep cavities, and rib ends most likely indicate entrapped air—these positions are where air easily accumulates and cannot escape. If black spots appear randomly across the entire product, suspect dead corners with accumulated material in the barrel or screw, where old material carbonizes and is carried out by new material.

Step 2: Do an isolation test

If the location is inconclusive, a simple test can be done: reduce the injection speed. If the burn marks noticeably decrease or disappear, it is an entrapped-air or shear-heat problem—at lower speed, the air has more time to escape through the vents, and shear heat is correspondingly reduced. If the burn marks persist or even worsen after reducing speed, check the barrel temperature and residence time to confirm whether the material has already degraded in the barrel.

III. Targeted Solutions

If it is entrapped air:

·Cut vent grooves: Add or deepen vent grooves at the burn locations (usually the last-filled positions), and clean them regularly to prevent blockage by oil stains or residual material. Vent groove depth should be reasonably designed according to material flowability—too shallow and the air cannot escape; too deep and flash is likely.

·Adjust the process: Reduce injection speed so that the air has more time to escape through the vents instead of being instantly sealed in the cavity. If necessary, use multi-stage injection and slow down during the sections prone to air entrapment.

·Modify the mold: Add ejector pins or inserts at the air-entrapment locations to use the clearance for venting; or use mold flow analysis to predict air-entrapment locations in advance and design venting properly at the mold-making stage.

If it is degradation:

·Check the temperature range recommended by the material supplier and appropriately lower the barrel temperature and back pressure.

·Shorten the injection cycle, or switch to a smaller barrel to reduce melt residence time.

·Check the gate size and appropriately enlarge the gate if necessary to reduce shear rate.

·If accumulated material in the barrel is suspected, thoroughly clean the screw and barrel with a dedicated purging compound.