Common Injection Molding Defects: Root Causes & Engineering Solutions

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Injection molding defects are among the most costly problems in plastic parts manufacturing — causing production downtime, scrap, and customer returns. Understanding the root causes of the most common defects, and knowing which process parameters and mold design factors to adjust, is essential knowledge for both mold designers and production engineers. This guide covers the seven most common injection molding defects with engineering-level root cause analysis and solutions.

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1. Short Shot

A short shot occurs when molten plastic fails to completely fill the mold cavity, leaving a partial or missing section of the part.

Root Cause	Diagnosis	Solution
Insufficient injection pressure/speed	Part always short at the same location	Increase injection velocity in first 70% of fill; check hydraulic pressure limit
Blocked or inadequate venting	Burn mark at short shot location	Clean vents; add vents at last-fill area; reduce injection speed at end of fill
Gate too small	High pressure drop across gate	Increase gate width or depth by 20%; consider additional gate
Melt temperature too low	High viscosity; incomplete fill	Increase barrel temperature in 5°C increments; check material drying
Wall too thin for flow length	L/T ratio > 150	Add flow leaders; increase gate count; consider material with higher MFI

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2. Sink Marks & Voids

Sink marks are depressions on the part surface caused by insufficient material packing. Internal voids occur when the outer skin solidifies before adequate material fills the core.

• Wall thickness variation — Thick sections adjacent to thin sections create differential shrinkage. Redesign ribs to 60% of nominal wall thickness
• Insufficient pack pressure — Increase pack pressure in 10% increments until sink marks disappear or gate freezes
• Pack time too short — Increase pack time until no further improvement in sink depth (gate freeze-off point)
• Gate too small — Small gate freezes before adequate packing. Increase gate size or use hot runner valve gate
• Cooling too fast at thick section — Add or reposition cooling channels to maintain pack pressure path from gate to thick section

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3. Weld Lines

Weld lines form where two flow fronts meet and the melt is too cool to fully re-fuse. They appear as visible lines and represent a structural weakness (typically 10–50% strength reduction at the weld).

• Relocate gate — Moving the gate changes where flow fronts meet. Place weld lines in low-stress, non-cosmetic areas
• Increase mold temperature — Higher mold temperature keeps the melt warmer at the weld, improving molecular re-entanglement. Increase by 10–20°C and re-evaluate
• Increase melt temperature — Lower viscosity at the flow front improves weld quality. Stay within material supplier’s recommended processing range
• Overflow wells — Add a small overflow pocket beyond the weld line location to flush the cold flow front material out of the main cavity
• Ultrasonic welding (post-mold) — For structural weld line concerns, design the part to be two pieces and ultrasonically welded at the weld line location

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4. Flash

Flash is excess plastic that squeezes into the parting line, vents, or ejector pin clearances, creating thin fins on the part that require removal.

Root Cause	Diagnosis	Solution
Insufficient clamp force	Flash appears on parting surface across entire part	Verify clamp tonnage: 0.3–0.5 T/cm² projected area; increase or reduce injection pressure
Parting surface damage	Flash localized to specific area	Inspect and stone damaged area; replace if chipped
Excessive injection pressure	Flash correlates with pressure setting	Reduce peak injection pressure; check fill/pack transfer point
Vent too deep	Flash in vent location	Reduce vent depth to below material flash threshold (typically 0.02mm for PA)

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5. Warpage & Dimensional Instability

Warpage is distortion of the part from its intended flat or curved geometry after ejection. It is caused by residual stress from non-uniform cooling, orientation effects, or differential shrinkage.

• Non-uniform cooling — Temperature differential across the part creates differential shrinkage. Improve cooling circuit to achieve ±3°C surface temperature uniformity
• Fiber orientation (GF materials) — Glass fibers align with flow direction and cause anisotropic shrinkage (different shrinkage parallel vs perpendicular to flow). Reposition gate to change flow pattern
• Mold temperature asymmetry — Different temperatures on cavity and core sides cause the part to bow toward the cooler side. Balance cooling on both sides
• Insufficient cooling time — Ejecting the part before the core temperature drops below heat deflection temperature causes post-ejection warpage. Increase cooling time
• Post-ejection fixturing — For difficult-to-control warpage, eject parts into a cooling fixture that holds geometry while the part fully cools to room temperature

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6. Burn Marks

• Diesel effect — Trapped air compresses adiabatically to temperatures above 300°C, burning the plastic. Solution: add vents at burn mark location; reduce injection speed at end of fill
• Material degradation — Excessive melt temperature or residence time degrades the resin. Reduce barrel temperature; ensure full shot size utilization
• Hot runner overheating — Check hot runner zone temperatures; replace faulty thermocouples; reduce hot runner temperature by 5–10°C

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7. Silver Streaks / Splay

• Moisture — The most common cause. Undried hygroscopic resins (PC, PA, ABS) generate steam that creates silver streaks. Dry material per supplier specification: PC 120°C/4hr, PA 80°C/4hr
• Trapped air at gate — Air entrainment during injection at the gate. Reduce injection speed at fill start (velocity profiling)
• Contamination — Foreign material or incompatible regrind creates streaks. Purge machine thoroughly; eliminate regrind

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Systematic defect troubleshooting starts with one change at a time, documented and measured. At BuildMold, our T1 mold trial reports include defect photographs, root cause analysis, and the specific parameter adjustments made — providing full transparency on the path from first shot to production-ready mold.