Injection Molding Defects Glossary

November 20, 2015

Molding Defects Alternative Name Description Causes
Blister Blistering Raised or layered area on surface of the piece Tool is not properly cooled and/or has a faulty heater, leaving the material hotter than it should be.
Burn Marks Air burn/gas burn/dieseling Black or brown marks from where the air is trapped Tool is not properly vented, or injection speed is too fast.
Color Streaks Localized color change The plastic additive Masterbatch does not properly mix or the material runs out and is pushed through naturally; material from a previous run is dragging in the nozzle of valve.
Delamination Thin mica like layers form in the wall of the piece Caused by repeated cyclic stresses and impact. The material becomes contaminated and should not be used for a safety-critical application because it is unable to bond to other materials.
Flash Burrs Excess material in thin layer exceeding the thickness of the rest of the piece Overloading of the mold or damaged parting line on the tool; injection speed too fast; clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.
Embedded Contaminants Embedded particulates Foreign particle embedded in the piece Particles on the surface of the tool, material contamination or foreign debris in the barrel, or material having been burnt from overheating prior to injection.
Flow Marks Flow Lines Directional wavy lines or patterns Slow Injection Speeds
Jetting Deformed piece from flow of material Poor design in tool, gate position, or runner, causing jetting from too little of die swell. Injection speed is set too high.
Knit Lines Weld Lines Small lines on the rear end of core pins or windows in pieces that resemble lines Caused by the melt-front flowing around an erect object in a plastic piece as well as at the end of fill where the melt-front joins back together.
Polymer Degredation Polymer breakdown Excess water in the granules, excessive temperatures in barrel, excessive screw speeds causing high shear heat, material being allowed to sit in the barrel for too long, too much regrind being used.
Sink Marks Localized depression of thicker zones Minimal holding time; pressure too low; not enough cooling time; gate temperature set too high; excessive material or walls too thick.
Short Shot Non-Fill/ Short Mold Partial piece produced Short on material, injection speed or pressure too low, mold not warm enough, short on air vents
Splay Marks Splash mark / Silver streaks Circular pattern around gate caused by hot gas Hygroscopic resins are dried improperly, leaving moisture; gas being trapped in ribbed areas due to excessive injection velocity in these areas; material overheating.
Stringiness Stringing String like remnant from previous shot transfer in new shot Overheating of nozzle; gates and sprues remain attached; screw does not decompress; poor placement of the heater bands inside the tool.
Voids Air pocket Not enough holding pressure to form out the piece. Edges of the piece unable to set because of mold filling too quickly; mold halves not properly aligned.
Weld Lines Knit line / Meld line / Transfer line Discolored line where two flow fronts meet Mold/material temperatures set too low, making the material too cold to form a bond; not enough time for transition between injection and transfer.
Warping Twisting Distortion of piece Material is not given enough time to cool, bowing the piece inward toward the heated side of the tool.

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