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Following are some of the common defects in Injection Molding. The solutions mentioned are possible solutions and must be tried out with safety in mind. Please always follow the material manufacturer and machine manufacturer's guidelines for safety.




Possible Reason

Possible Solution


  Short Shot

Molten Plastic is not reaching the mold cavity section

  • Increase melt temperature
  • Increase mold temperature
  • Increase injection speed
  • Increase injection pressure if process is pressure limited
  • Check if the mold is vented in the area of short shots
  • Increase gate and runner sizes



Molten Plastic is flowing into unwanted sections of the mold cavity

  • Check for Mold shut-off and mold damage
  • Decrease melt temperature
  • Decrease mold temperature
  • Decrease injection speed



Plastic is shrinking as it cools but additional plastic cannot be for further compensation of the shrinkage

  • Increase pack and hold pressures
  • Increase pack and hold times
  • Decrease mold temperatures
  • Decrease melt temperatures





A layer/ streak of a unwanted gaseous byproduct from the melt or moisture in the material comes in between the melt flow and the cavity walls preventing the texture from being picked up and in addition eventually leaving a residue

  • Dry plastic to suggested moisture levels
  • Decrease injection speeds
  • Decrease melt temperature
  • Decrease screw rotation speeds
  • Decrease back pressure
  • Increase mold temperatures
  • Increase venting
  • Increase gate sizes



A differential cooling rate of the melt in two sections of the molded product

  • Decrease melt temperature
  • Experiment with differential mold temperatures on the fixed and movable sides
  • Increase pack and hold pressures
  • Increase pack and hold times
  • Increase cooling time


  Burn Marks

When air and gasses get trapped inside the mold cavity during plastic injection, the high pressure results in the dieseling of the plastic resulting in the burning of the plastic.

  • Increase venting in the mold
  • Decrease injection speed
  • Decrease melt temperature
  • Decrease Screw Rotation Speeds



  Black Specs


This can be degraded plastic and/or foreign material that can get mixed with the plastic that needs to be molded.

  • Reduce melt temps
  • Reduce injection speeds
  • Reduce screw speeds
  • Find source of foreign material



Usually occur with the parts are thick. The walls solidify and the plastic melt shrinks towards the wall and therefore sucks a vacuum void on the inside of the part

  • Reduce melt temperature
  • Reduce mold temperature
  • Reduce injection speed
  • Increase pack and hold pressures
  • Increase pack and hold times



When moisture and/or a gaseous byproduct gets mixed with the melt and is injected in the mold cavity this moisture or gas if embedded inside the melt can show up as bubbles

  • Dry material to suggested moisture levels
  • Increase back pressure
  • Reduce melt temperature


  Gate Blush

Shows up at the gate when the material is sheared differently compared to the rest of the part

  • Slow down the injection speeds in the gate area
  • Profile the injection speeds if necessary
  • Experiment with increasing and decreasing melt temperatures or hot tip temperatures in case of hot runner molds



Usually seen with the part is thick causing the injected plastic to ‘snake fall’ on to the cold mold surface and start freezing immediately. When the fresh plastic come in it does not blend well with the first material causing the jetting marks


  • Reduce injection speed
  • Increase melt temperature
  • Change gate location


  Weld Lines

The melt flow front is usually cold due to the exposure to the cold cavity. When two flow fronts meet as in a flow around a mold pin they do not fuse uniformly causing a weld line. Sometimes air can also get trapped in to form the defect

  • Increase melt temperature
  • Increase mold temperatures
  • Increase injection speeds
  • Increase venting


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