Defect guide
The 21 most common injection molding defects: symptoms, likely causes and the order of solutions. Start with the no-cost settings first.
Short shot (incomplete fill)
The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.
Flash
Plastic escapes through the parting line or around the ejector pins.
Sink mark
A dent on the surface opposite a thick section; the surface is drawn inwards.
Void (internal vacuum void)
Internal cavity in a thick section of the part; the outer surface may look perfect.
Weld line (knit line)
A line and a weak zone where two melt fronts meet.
Splay (silver streaks)
Fine silvery streaks on the part surface in the flow direction.
Burn mark (dieseling)
Usually a brown-black burn at the last point to fill; trapped air heats up and burns the plastic.
Jetting (snake marks)
The melt shoots into the cavity like a jet and fills it like a wavy thread without spreading against the wall.
Warpage
The part distorts or bends after it leaves the mold.
Flow marks
Gloss differences in the form of waves or rings on the surface.
Delamination (peeling)
The surface layer peels off the part and separates in flakes.
Black specks (contamination)
Black or dark-colored specks and streaks in the part.
Brittleness / low impact strength
The part breaks more easily than expected; sometimes the surface looks fine.
Ejector marks / whitening
Marks, whitening or punch-through where the ejector pins are located.
Sticking / drag marks
The part is hard to eject; scuffs or scratches form on the surface during ejection.
Color variation and color streaks
The color is uneven from part to part or within a part.
Dimensional variation and shrinkage differences
Part dimensions fall outside the tolerance or vary from cycle to cycle.
Dull, hazy surface (blush, haze)
The expected gloss is missing; the surface becomes dull or hazy.
Cold slug mark
Melt that has cooled in the nozzle enters the cavity and creates a ring or a dull zone around the gate.
Nozzle drool and stringing
Drooling and strings form at the nozzle or at the hot runner gate.
Stress cracking
The part cracks during assembly, on contact with oil/solvent or over time; the crack usually starts at stressed areas.
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