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August 23, 2026

Master Guide to Troubleshooting Common Injection Molding Defects in Thermoplastics (Sinks, Warpage & Jetting)

Injection molding high-performance engineering plastics like Polycarbonate (PC), ABS, Nylon (PA6/PA66), and Delrin (POM) requires precise control over pressure, velocity, mold temperatures, and material preparation. Even slight deviations in processing conditions or resin moisture can lead to costly scrap rates and defective parts.

At Jay Plast, we work closely with injection molders, toolmakers, and plant supervisors. In this technical troubleshooting guide, we detail the root causes and proven solutions for the 5 most common molding defects encountered when processing engineering thermoplastic granules.

1. Sink Marks & Internal Voids

Visual Symptom: Shallow depressions or dimples on the outer surface of thick wall sections, bosses, or rib intersections. Internal voids are hollow pockets inside the core of thick sections.

Primary Root Causes:

  • Inadequate packing pressure or insufficient hold time before gate freeze-off.
  • Outer skin of the plastic cools and solidifies while the molten core continues to shrink away from the cavity wall.
  • Rib-to-wall ratio exceeds recommended guidelines (rib thickness should not exceed 50%–60% of the adjacent nominal wall thickness).

Proven Solutions:

  1. Increase Hold Pressure & Time: Elevate packing pressure in 5% increments and extend hold time until gate freezes.
  2. Lower Melt Temperature: Reduce barrel temperatures slightly so the core cools faster.
  3. Increase Cushion: Ensure a minimum melt cushion of 3 mm to 6 mm remains in front of the screw to transmit packing pressure into the mold.
  4. Enlarge Gate & Runner Size: Relocate or enlarge gates to prevent premature gate freeze.

2. Part Warpage & Distortion

Visual Symptom: Molded component twists, bends, or bows after ejecting from the mold and cooling.

Primary Root Causes:

  • Non-uniform cooling rates between core and cavity sides of the tool.
  • Differential shrinkage caused by polymer fiber orientation (especially in glass-filled resins like PA6 GF30).
  • High residual internal stress resulting from excessive injection speed or cold mold temperatures.

Proven Solutions:

  1. Balance Mold Temperatures: Ensure the differential temperature between core and cavity half is no greater than 5°C–10°C.
  2. Optimize Cooling Time: Allow sufficient in-mold cooling time so the part reaches structural rigidity prior to ejection.
  3. Switch to Low-Shrinkage Alloys: If warpage persists with neat resins, switch to an engineered alloy such as PC+ABS or mineral-filled grades from Jay Plast.

3. Jetting & Snake-Skin Surface Marks

Visual Symptom: A squiggly, snake-like ribbon of solidified resin embedded on the part surface near the gate area.

Primary Root Causes:

  • Molten plastic shoots through a narrow gate into a wide cavity without impinging upon a mold wall, cooling prematurely before filling the rest of the cavity.
  • Excessive injection speed through a small gate.

Proven Solutions:

  1. Use Slow-Fast Injection Profiling: Start injection at a slow speed until the melt passes through the gate and contacts the cavity wall, then accelerate to high speed.
  2. Modify Gate Design: Re-engineer gate placement (such as using a submarine gate, fan gate, or overlap gate) so the melt stream immediately impinges upon a tool wall or core pin.

4. Moisture Splay & Silver Streaks

Visual Symptom: Silver splash marks, streaks, or micro-bubbles radiating outward from the gate across the part surface.

Primary Root Causes:

  • Processing hygroscopic granules (PC, Nylon, PC+ABS, PBT) that were inadequately dried prior to molding. Water turns to steam in the heating barrel, forming gaseous streaks.
  • Degradation of degraded resin caused by extreme melt temperatures or long barrel residence times.

Proven Solutions:

  1. Dehumidifying Desiccant Drying: Always dry PC at 120°C for 4 hrs and Nylon at 90°C for 5 hrs. Do NOT rely on standard hot air hoppers in humid environments.
  2. Verify Moisture Levels: Measure moisture content using a moisture analyzer; target <0.02% for PC and <0.1% for Nylon.
  3. Inspect Hoppers for Condensation: Ensure dry granules are loaded directly into covered hoppers to prevent ambient re-absorption.

5. Weld Lines & Knit Lines

Visual Symptom: A visible line or notch where two flow fronts meet after splitting around a core pin, hole, or rib.

Primary Root Causes:

  • Flow fronts cool excessively before merging, resulting in poor molecular entanglement and mechanical weakness along the seam line.

Proven Solutions:

  1. Increase Melt & Mold Temperature: Elevate mold temperature by 10°C–20°C to keep flow fronts hot and fluid.
  2. Increase Injection Velocity: Faster filling prevents flow fronts from cooling prior to convergence.
  3. Vent the Weld Area: Add deep air vents at the exact point where flow fronts meet to prevent trapped air pockets from cooling the melt edge.

Technical Support & Material Quality from Jay Plast

Defects often stem not just from machine settings, but from inconsistent raw material MFI (Melt Flow Index) or contaminated recycled granules. At Jay Plast, all our virgin and reprocessed engineering plastic granules undergo stringent quality control checks to guarantee consistent viscosity, thermal stability, and clean processing.

Need help selecting the right melt flow grade or eliminating molding defects? Contact Jay Plast's technical team today!