Injection Molding Defects Library
Engineering reference for 13 common injection molding defects — root causes, corrective actions, process parameters and material guidance.
13
Defect Types
225+
Documented Causes
208+
Corrective Actions
5M1P
Diagnostic Method
Flash / Burr
Flash is excess material that escapes the mold cavity through gaps at parting lines, vents, or slides.…
Warpage / Distortion
Warpage is dimensional distortion caused by uneven shrinkage from differential cooling or residual stress…
Sink Marks / Depressions
Sink marks occur when surface solidifies before the core, which then shrinks inward.…
Short Shot / Incomplete Fill
Short shot is failure to completely fill the mold cavity during injection.…
Burn Marks / Dieseling
Two types: Diesel Effect trapped air compresses and self-ignites at 300-400 deg C. Gate shear material de…
Weld Lines / Knit Lines
Weld lines form where two advancing melt fronts meet. If the fronts are too cool or have trapped air the …
Silver Streaks / Splay
Silver streaks are caused by steam moisture air or degraded material stretched along the flow path.…
Cracks / Brittleness
Cracks occur from excessive internal stress improper drying wrong material or ESC from chemical contact.…
Dimensional Variation
Dimensional instability comes from inconsistent shrinkage process instability or tool wear. Plastics use …
Voids / Bubbles / Porosity
Two types surface bubbles from moisture vs internal vacuum voids same mechanism as sink marks core shrink…
Nozzle Stringing / Drooling
Stringing occurs when residual melt in the nozzle tip is too fluid and elongates as the mold opens.…
Ejector Marks / Push Marks
Ejector marks occur when the part is too hot or soft during ejection causing pins to sink in or create lo…
Long Cycle Time
Cycle time is the total time for one complete injection molding cycle. Extended cycle time reduces produc…