8 Common Defects in Injection Molding & Analysis_Technology

8 Common Defects in Injection Molding & Analysis

Author:SWITEK

Release Date:2026.08.25

8 Common Defects in Injection Molding & Analysis Deeply understand defect causes for precise troubleshooting

1 Sink Marks (Depression)

Localized depressions or voids forming on the surface of plastic parts. Commonly found in thicker material areas such as ribs and screw bosses.

Characteristic: Caused by uneven wall thickness and insufficient holding pressure

2 Flash / Burrs (Flash)

Excess overflow material produced when plastic melt enters the mold parting line or fit clearances during molding, usually located at part edges or joints.

Characteristic: Insufficient clamping force or excessive fit clearance

3 Warpage Deformation

Anisotropic internal stress generated inside the plastic during injection, causing the part to bend, twist, or exhibit flatness out of tolerance after demolding and cooling.

Characteristic: Caused by uneven cooling and residual stress

4 Short Shot (Underfill)

The melt solidifies before the cavity is completely filled, resulting in incomplete filling at the flow end or thin-wall areas. Multi-cavity molds are prone to uneven filling across cavities.

Characteristic: Poor flowability or insufficient pressure/speed

5 Ejector Marks / White Ejector Marks

Whitening, protrusions, pits, or marks appearing at positions corresponding to ejector pins. Mainly caused by excessive ejection force, uneven ejection force distribution, or insufficient cooling.

Characteristic: High demolding resistance or concentrated ejection force

6 Burn Marks (Burning / Gas Trapping)

Air inside the mold cannot be vented in time and is compressed by high pressure and high speed, causing adiabatic temperature rise (up to 3000°C), which burns the plastic at high temperature and forms dark black spots.

Characteristic: Poor venting and adiabatic compression heating

7 Weld Lines (Knit Lines)

Physical line marks left at the confluence of multiple plastic streams due to excessively low temperature or entrapped air not fully fused, occurring with multiple gates or split flow around holes.

Characteristic: Poor fusion of multiple flow fronts, affecting strength

8 Flow Marks (Wavy Patterns)

Unstable melt flow velocity in the cavity, with the surface layer cooling first, forming concentric circular, spiral, or wavy patterns centered on the gate on the product surface.

Characteristic: Melt front cooling too fast or uneven injection speed

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