Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)

    Blister
 Blistering Raised or layered zone on surface of the part, the causes are that Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater

  Burn marks
 Air Burn/ Gas Burn, Black or brown burnt areas on the part located at furthest points from gate,the causes are that Tool lacks venting, injection speed is too high
 Color streaks (US), Colour streaks (UK) Localized change of color/colour,the causes are that Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only. Previous colored materal "dragging" in nozzle or check valve.

   Delamination
 Thin mica like layers formed in part wall,the causes are that Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond

   Flash
 Burrs Excess material in thin layer exceeding normal part geometry,the causes are that Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.

   Embedded contaminates
 Embedded particulates Foreign particle (burnt material or other) embedded in the part,the causes are that Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection     Flow marks
 Flow lines Directionally "off tone" wavy lines or patterns,the causes are that Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times)

  Jetting
 Deformed part by turbulent flow of material,the causes are that Poor tool design, gate position or runner. Injection speed set too high.

  Polymer degradation
 polymer breakdown from hydrolysis, oxidation etc,the causes are that Excess water in the granules, excessive temperatures in barrel

  Sink marks
 sinks Localized depression (In thicker zones),the causes are that Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high

  Short shot
Non-fill / Short mold,Partial part,the causes are that Lack of material, injection speed or pressure too low

  Splay marks
Splash mark / Silver streaks,Circular pattern around gate caused by hot gas,the causes are that Moisture in the material, usually when hygroscopic resins are dried improperly. Trapping of gas in "rib" areas due to excessive injection velocity in these areas.
Stringiness
Stringing,String like remain from previous shot transfer in new shot,the causes are that Nozzle temperature too high. Gate hasn't frozen off

  Voids
 Empty space within part (Air pocket), the causes are that Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness).

   Weld line
  Knit line / Meld line / Transfer line Discolored line where two flow fronts meet,the causes are that Mold/material temperatures set too low (the material is cold when they meet, so they don't bond). Point between injection and transfer (to packing and holding) too early.

  Warping
Twisting,Distorted part,the causes are that Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)  

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