By Jason Vander Griendt

January 4, 2018 at 6:52 am

Plastic injection molding is the creation of a plastic product through the manufacturing process of injecting plastic into a mold. The heated plastic is injected into a mold and then cooled.

During the process, it’s possible for defects to occur. These defects affect the quality of the product. Not to mention that the backlash from such product defects can cost manufacturers lots of money.

The defects in plastic injection molding are usually a result of problems with the material used, the process of molding, or the tools used in the molding process. Learning about each defect will help you get the best quality product possible with minimal risk of defects.

The most commonly occurring defects are:

  • Burn Marks
  • Voids
  • Delamination
  • Flash
  • Flow lines
  • Short shot
  • Sink marks
  • Warping
  • Burn Marks

Causes: Burn marks are defects caused by the combustion of heated plastic. When there’s improper ventilation or prolonged heating, the plastic is burned. Another cause of burn marks in plastic injection molding is the burning of the resin itself or the burning of trapped air. When the injector speed is too fast, burn marks tend to appear as well.

The burn marks thus lead to discoloration that appears as black, brown or rust colored marks on the plastic.

How to avoid:

  • Improve ventilation.
  • Reduce injection speed and pressure.
  • Ensure that operators run consistent cycles.
  • Voids

These appear when air bubbles are trapped within or close to the surface of a molded product. In plastic injection molding, this particular defect isn’t viewed as detrimental to the overall strength of the product. However, when voids are bigger and occur more, the molded part is weakened.

Causes: When temperatures are so high in plastic injection molding, it causes the melt stream to trap gases and air. As a result, voids occur. Another cause is excessive moisture in the material. As the resin is heated, the moisture is converted to steam. This steam then forms a gas pocket.

How to avoid:

  • Use less viscous plastic.
  • Make sure that the mold parts have proper alignment.
  • Increase holding pressure.
  • Change ratio of wall thickness to the thin section.
  • Delamination

This defect occurs when a separation exists between the layers of the molded plastic. This appears as a coating on the surface of a finished product. These ‘coatings’ can peel off of the underlying material.

Causes: The major cause of delamination in plastic injection molding is contamination. When the molten plastic is contaminated with foreign materials, the material can’t bond properly. Another cause is over-using of mold release agents.

How to avoid:

  • Increase the speed of injection in little increments.
  • Increase the temperature of the mold.
  • Ensure that only compatible materials are mixed.
  • Ensure the plastic is properly pre-dried before molding.
  • Flash

Flash occurs when there’s an excess amount of plastic during the plastic injection molding process, and there’s a seepage. Once this seepage cools it needs to be removed from the finished product.

Causes: Flash is caused when the mold isn’t held in place with adequate force. Thus, the plastic seeps through the mold. It can also be a result of worn-out or aged molds. Another common cause is excessive injection pressure that pushes the molten plastic out.

How to avoid:

  • Increase clamp force.
  • Ensure proper maintenance of the mold.
  • Adopt proper tooling design into repetitive processes.
  • Reduce injection pressure.
  • Flow Lines

These appear as wrinkles in the finished product. Flow lines occur as the result of variations in cooling speed of the material, where if a new layer is added before the previous one has cooled, wrinkles can appear in the finished product.

  • Short Shot

A short shot appears when the heated plastic doesn’t completely fill the mold during plastic injection molding. The major cause is the inadequate quantity of prepared material for mold injection.

Causes: The major cause of short shot is the inadequate quantity of material. Also, using a very viscous material can lead to this problem since the material solidifies before filling the mold. Improper venting can also lead to a short shot.

How to avoid:

  • Increase mold temperature.
  • Ensure adequate degassing and venting techniques.
  • Increase the flow rate.
  • Use a less viscous material with a better flow.
  • Sink Marks

They appear as depressions in the thicker areas of the finished product. Voids can happen when certain sections become solid first as a result of a low injection pressure. This shrinkage leads to marks in the area where the material is sunk into the void.

  • Warping

Warping is evident in parts of plastic components that are meant to lie flat but somehow end up leaving gaps. This is a result of uneven shrinkage in different parts of the product because, for some reason during the cooling process, some components don’t dry evenly.


Now that you know about the defects that can occur in plastic injection molding, it’s a little easier to avoid them occurring. In many cases, it’s actually easy to prevent the issues mentioned above by choosing the right materials and the right techniques.

Companies that produce high-quality plastic products are well aware of the possibility of plastic injection molding defects and they’re always on the lookout for the new ways to avoid them.

If you need help with that, or any other injection molding queries, please give us a call at 1.888.202.2052 or email We’re here to help!

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Further reading: Is 3D printing cheaper than injection molding?

Jason Vander Griendt

Jason Vander Griendt is a Mechanical Engineering Technician with years of experience working at major companies such as SNC Lavalin Inc, Hatch Ltd. Siemens and Gerdau Ameristeel. He is the CEO of JCAD – Inc., a company he started in 2006 after seeing a gap in the market for businesses who could assist clients through the entire product design and manufacturing process.

Jason has been featured in Forbes, has had his businesses analyzed and discussed in multiple start-up books, was a previous winner of the Notable8 Digital Innovator of the year award, and is a regular guest on business panels and podcasts. Email Jason at or follow him on LinkedIn.

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