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September 22, 2026

Troubleshooting Short Shots & Burn Marks in Injection Molding

Injection molding engineering thermoplastics requires precision. Minor deviations in pressure, venting, or material rheology can lead to costly scrap. Following our previous guide on sinks, warpage, and jetting, this article focuses on diagnosing and resolving two of the most frustrating molding defects: Short Shots and Burn Marks (Dieseling).

At Jay Plast, we help molders optimize their processes for materials like PC, ABS, Nylon, and POM. Here’s how to eliminate these common defects.

1. Short Shots (Incomplete Fill)

Visual Symptom: The molded part is incomplete because the molten plastic solidified before entirely filling the mold cavity. This typically occurs at the furthest flow distances, thin-walled sections, or thin ribs.

Root Causes

  • Material Viscosity: The plastic is too stiff to flow through the cavity before freezing.
  • Inadequate Pressure/Speed: The injection pressure or injection velocity is too low.
  • Undersized Gates/Runners: Constrictions in the feed system prevent sufficient material flow.
  • Trapped Air: Lack of venting causes air pressure to build up at the end of the fill, preventing the plastic from advancing.

Proven Solutions

  1. Increase Injection Pressure & Speed: Start by increasing injection pressure. If the machine is pressure-limited, increase the injection speed to capitalize on the material's shear-thinning behavior (especially effective for ABS and Nylon).
  2. Elevate Melt and Mold Temperatures: Increase barrel temperatures by 5°C–10°C to reduce melt viscosity. Concurrently, increase mold temperature to prevent premature skin freeze-off.
  3. Verify Shot Size and Cushion: Ensure the machine is actually delivering enough plastic. Check that the melt cushion (usually 3–6 mm) remains at the end of the injection stroke.
  4. Improve Venting: If the short shot always happens at a blind pocket or the end of the flow path, check the mold vents for blockages (like burned gas deposits) and clean or enlarge them.

2. Burn Marks (Dieseling)

Visual Symptom: Black or dark brown charred spots, often accompanied by a distinct smell of degraded plastic, appearing on the edges of the part or at the end of a flow path.

Root Causes

  • Gas Trapping (Dieseling): As molten plastic races into the mold, it compresses the air ahead of it. If the air cannot escape through vents, the rapid compression superheats the air (often > 500°C), igniting and charring the surrounding plastic.
  • Material Degradation: The plastic was overheated in the barrel or subjected to excessive shear stress through a tiny gate.

Proven Solutions

  1. Clean and Expand Vents: Dieseling is almost always a venting issue. Clean the mold vents and ensure they are deep enough (typically 0.02mm for Nylon, 0.04mm for PC and ABS) without causing flash.
  2. Reduce Injection Speed: Slowing down the injection speed at the very end of the fill phase (using a multi-stage injection profile) gives trapped air more time to escape.
  3. Check Barrel Temperatures: Ensure the material is not degrading in the barrel. Verify that actual melt temperatures match the machine setpoints and ensure residence time isn't too long.
  4. Avoid Excessive Screw RPM: High screw rotational speeds create intense shear heat, degrading shear-sensitive polymers like POM or PC before they even enter the mold.

Need High-Quality, Consistent Granules?

Inconsistent melt flow from low-quality regrind is a major cause of short shots. At Jay Plast, we supply prime virgin and tightly controlled reprocessed granules (PC, ABS, Nylon) with consistent Melt Flow Index (MFI) to ensure stable molding cycles.

Contact us today to upgrade your raw material supply and reduce your scrap rates!