Injection molding defects guide — Causes and solutions
An injection molding defect is an imperfection on a molded part caused by incorrect machine parameter settings (temperature, pressure, speed), a material issue (moisture, degradation), or a mold problem (venting, parting line). This guide covers the 30 most common injection molding defects with their causes and solutions, validated by plastics engineers with 20+ years of hands-on experience.
Each defect is linked to the material properties in use. Browse our 600+ polymer technical datasheets or the technical terms glossary (70+ terms) to go further.
The 15 most frequent defects
- Flash — Material that escapes through the parting line. Causes: insufficient clamping force, mold wear, injection pressure too high
- Sink Marks — Surface depressions at thick wall sections. Causes: insufficient holding pressure, cooling time too short
- Silver Streaks — Silvery streaks caused by moisture or trapped gases. Causes: insufficient material drying, melt temperature too high
- Short Shot — Mold cavity not completely filled. Causes: insufficient shot size, injection pressure too low, melt temperature too low
- Burn Marks (Diesel Effect) — Black marks at the end of fill. Causes: trapped air compression, blocked vents, injection speed too high
- Weld Lines — Weak junction where two melt fronts meet. Causes: melt temperature or mold temperature too low, injection speed insufficient
- Warpage — Distortion after cooling. Causes: asymmetric cooling, internal stresses, differential shrinkage
- Jetting — Serpentine appearance on the part surface. Causes: gate too small, injection speed too high at the start of fill
- Flow Marks — Visible waves near the injection gate. Causes: mold temperature too low, irregular injection speed
- Delamination — Surface peeling in layers. Causes: material contamination, incompatible regrind, melt temperature too low
- Voids / Bubbles — Internal cavities in thick sections. Causes: insufficient holding pressure, mold temperature too high
- Color Streaks — Non-uniform color distribution. Causes: poor masterbatch mixing, uneven temperature, worn screw
- Ejector Marks — Marks at ejector pin locations. Causes: premature ejection, mold temperature too high, poorly positioned ejectors
- Brittleness — Part breaks easily. Causes: material degradation, moisture, weld lines in a critical stress area
- Orange Peel — Wavy, rough surface texture. Causes: mold temperature too low, injection speed too slow
15 additional defects covered
Black specks, blisters, brown streaks, cold slug, gloss halo, plate-out, record grooves, shark skin, spider webbing, sprue sticking, mold sticking, stress cracking, unbalanced flow, dimensional instability, and more.
For each defect: likely causes ranked by probability, machine parameters to check, and step-by-step recommended solutions.
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