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Common Injection Molding Defects (Sink Marks, Bubbles, Warpage) – Root Causes & Practical Solutions

Created on 05.21

Meta Description: Analyze the root causes of the most common injection molding defects (sink marks, bubbles, warpage) and provide practical, actionable solutions to improve part quality and reduce scrap.

Batch defects in injection molding are a nightmare for customers—they lead to high scrap rates, rework costs, delayed deliveries, and inconsistent product quality. The key to solving these defects is identifying their root causes, which often lie in three areas: part design, mold structure, and injection parameters. As an OEM injection molding manufacturer, we’ve resolved thousands of defect issues, and we’re sharing our expertise to help you avoid these costly problems.

3 Most Common Injection Molding Defects: Root Causes & Solutions

1. Sink Marks (Depressions on Part Surface)

Root Causes: Sink marks occur when plastic shrinks unevenly during cooling. Common causes include: uneven wall thickness, insufficient packing pressure, slow cooling, or poor plastic flow.

Practical Solutions: Optimize part design: Ensure uniform wall thickness and add fillets/ribs to distribute shrinkage evenly. Adjust injection parameters: Increase packing pressure (10-20% higher than injection pressure) and extend packing time (5-10 seconds) to compensate for shrinkage. Improve mold cooling: Add cooling channels near thick-walled areas (channel diameter 8-12mm) to speed up uniform cooling.

2. Bubbles (Air Pockets in the Part)

Root Causes: Bubbles are caused by trapped air in the mold cavity or moisture in the plastic material. Common causes include: insufficient venting, wet plastic pellets, or improper injection speed.

Practical Solutions: Dry plastic pellets: Most plastics (e.g., ABS, PA66, PC) absorb moisture—dry them at the recommended temperature (PA66: 100-120℃, PC: 80-100℃, ABS: 80-90℃) for 2-4 hours before molding. Improve mold venting: Add venting slots (0.01-0.02mm thick, 5-8mm wide) at the end of the plastic flow path to allow air to escape. Adjust injection speed: Use a slow initial injection speed (20-30mm/s) to avoid trapping air, then increase speed (50-80mm/s) for full filling.

3. Warpage (Part Deformation After Ejection)

Root Causes: Warpage is caused by uneven internal stress in the part, which occurs due to uneven cooling, improper gate design, or material shrinkage differences.

Practical Solutions:
Optimize mold cooling: Ensure cooling channels are evenly distributed (spacing 20-30mm) to cool the part uniformly.
Adjust gate design: Use a central gate for symmetric parts to ensure uniform plastic flow and stress distribution.
Select the right material: Choose materials with low shrinkage rates (e.g., PP: 1.0-2.0%, POM: 0.8-1.5%) for parts requiring dimensional stability.
Post-molding treatment: Use annealing (heat treatment at 80-100℃ for 1-2 hours) to relieve internal stress for critical parts.

Key Tip: Prevent Defects with In-Process Quality Control

The best way to avoid defects is to implement strict in-process quality control. Our facility checks every 10th part for defects, adjusts injection parameters in real time, and uses SPC (Statistical Process Control) to monitor production stability—ensuring consistent part quality for every batch.