Defect type

Defect appearance

Typical cause

Sink marks

Depressions on the part surface

Uneven shrinkage

Burn marks

Brownish discoloration of the part surface

Excessive melt temperature or too long residence time

Air streaks (air marks, air traps)

Dull/brownish/silvery/white streaks in wall thickness transitions and gates

Air in cavity is overrun and trapped by the melt

Moisture streaks

Large dull, streaky areas

Moisture in the cavity or in the granulate

Color streaks

Uneven coloration of the part

Insufficient mixing, incompatible masterbatch

Glass fiber streaks

Dull, rough part surface

Flow-induced orientation, too rapid cooling/shrinkage

Air inclusions (bubbles)

Voids on or in the molded part, near-surface bulging

Air not vented / injected, too low holding pressure, too high/fast decompression

Voids / vacuum cavities

Airless cavities in the molded part

Insufficient shrinkage compensation

Gas inclusions

Bubbles on/in the part surface

Material decomposition, excessive residence time or shear

Weld lines

Optical/mechanical weak spot (notch or color change visible)

Flow fronts meet with insufficient pressure/temperature

Gloss differences

Excessive, insufficient, or uneven gloss on the part surface

Different cooling or shrinkage conditions

Jetting

Snakelike, rough/dull line on surface – from the gate

No fountain flow, too high pressure, sudden cross-section enlargement, vertical mold

Records effect

Finely ribbed surface – concentric, parallel to flow path end

Melt temperature, injection speed, or mold temperature too low

Dull gate area

Dull halo (concentric rings) around gate

Gates too small, injection speed too high

Incomplete filling

Outer contour not completely formed (usually far from gate)

Premature freezing, too low injection pressure/speed, poor venting

Diesel effect / burn spots

Burn marks and unfilled areas

Insufficient venting

Flash

Film-like edge beyond intended contour

Inaccurate cavity, low viscosity melt, low clamping force, high cavity pressure

Stress whitening / cracks

Whitish discoloration, oriented cracks

Exceeding deformation/stress limit

Ejector marks

Bulging, gloss differences, markings

Processing, geometry, thermal or mechanical causes

Demolding deformation

Cracks, breaks, overstretching (typical for undercut parts)

Part cannot withstand demolding forces

Drag marks

Damaged (often textured) surface

Low mold rigidity, excessive shrinkage, insufficient draft angle

Surface flaking

Material layers peel off

Additive incompatibility, contamination, moisture, inhomogeneity, high injection speed

Cold slug

Comet-shaped mark near gate

Cooled melt from nozzle or hot runner enters cavity

Stringing

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Insufficient cooling in gate area

Black specks / dark spots

Dark spots on surface

Mold wear, thermal degradation, contamination

Deposit formation

Gloss and surface topography defects

Low-molecular deposits on mold surface

Plating defects

Blisters, pimples, poor adhesion

Often caused by molding process/handling

Defective film hinge

Stress whitening, torn or cracked hinge

Material overstressed during demolding, frozen-in mass

Warpage

Deviation from intended shape

Shrinkage differences or demolding deformation