7 Common Injection Molding Defects: Causes and Prevention

Injection molding produces billions of parts every year. Most of the time it works great — until it doesn't. Defects can appear suddenly due to process drift, gradually due to tool wear, or persistently due to design oversights. For sourcing managers and engineers working with overseas manufacturers, understanding these defects means faster root-cause conversations and fewer rejected shipments.

Here are the seven defects that account for the vast majority of injection molding quality issues, along with what causes them and how to prevent them.

1. Sink Marks

Small depressions on the part surface, usually opposite thick features like ribs or bosses. The outer skin solidifies first, then the shrinking core pulls it inward. They're cosmetic on non-visible parts but unacceptable on Class A surfaces.

Fix it: Keep rib thickness at 50–60% of the adjoining wall. Increase pack pressure and hold time. Move the gate closer to thick sections. For unavoidably thick areas, consider gas-assist molding.

2. Warping

Part distortion after ejection — flat surfaces bow, edges twist, dimensions go out of spec. Root cause is always non-uniform shrinkage, whether from inconsistent cooling, varying wall thickness, or asymmetric flow patterns.

Fix it: Uniform wall thickness is the single biggest factor. Balance cooling channels. Use symmetrical gating. Allow adequate cooling time. If warping is critical, consider lower-shrinkage resins (ABS vs. PP).

3. Flash

Thin excess plastic that squeezes out along the parting line, around ejector pins, or at insert boundaries. Causes: insufficient clamp force, worn parting surfaces, or too-high injection pressure.

Fix it: Verify clamp tonnage for projected area. Inspect and maintain parting line surfaces. Reduce injection pressure if flash persists. Check mold alignment.

4. Jetting

Snake-like, wavy flow lines on the part surface near the gate. Instead of forming a smooth melt front, the plastic shoots into the cavity as a narrow stream that cools on contact with the mold wall and doesn't blend with subsequent flow.

Fix it: Redesign the gate so the melt immediately contacts an opposing wall (fan gate, tab gate). Reduce initial injection speed. Increase melt temperature. Use a larger gate cross-section.

5. Short Shots

Incomplete parts with missing sections. The mold didn't fill. Causes range from insufficient shot volume and inadequate pressure to blocked flow paths and trapped air.

Fix it: Check shot size and cushion. Increase injection pressure and speed. Raise temperatures. Add vents at end-of-fill locations. Redesign thin flow restrictions.

6. Weld Lines

Visible lines where two melt fronts meet and solidify. Structurally weak — tensile strength at a weld line can drop 10–25%. Common in parts with holes, multiple gates, or complex geometry that splits flow.

Fix it: Higher melt temperature so fronts are hotter at convergence. Faster injection speed. Relocate gates to push weld lines to non-critical areas. Use overflow wells. Run mold flow simulation during design.

7. Burn Marks

Dark brown or black marks at the end of fill or in trapped-air zones. Compressed air heats to the point of charring the plastic (diesel effect). Often found at the last areas to fill, ribs, and deep bosses.

Fix it: Add or deepen vents at last-to-fill locations. Reduce injection speed. Redesign geometry to eliminate air traps. Keep existing vents clean — residue buildup blocks airflow.

What This Means for Sourcing

When you're sourcing injection-molded parts overseas, defect prevention starts with supplier selection. A capable molder runs mold flow analysis before cutting steel, monitors process parameters in real time with SPC, and has a structured corrective action process for when issues arise.

At Dewin, every injection molding project includes pre-production mold flow simulation, first-article dimensional inspection, and ongoing SPC monitoring. Our defect rates run below 0.5% on high-volume programs because we catch issues in simulation and process setup, not on the inspection table.