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Injection Molding Defects and How to Prevent Them

Injection Molding

Injection molding is one of the most cost-effective ways to produce parts. High efficiency, rapid production speed, size and shape flexibility, and operation automation are a few advantages of injection molding. But, unfortunately, injection molding defects are common, and depending on the sort of product being manufactured, there is a range of things that could go wrong.

Injection molding flaws come in various shapes and sizes, but some are more prevalent than others. In this article, we will get to know about numerous injection molding part problems and their causes. It also explains the countless options for resolving them.

Flow LinesInjection Molding Defects and How to Prevent Them

On the narrow section of molded pieces, these are wavy-like patterns. Flow lines are usually a somewhat different color than the nearby regions around them. Flow lines could be seen as ring-shaped bands as well, appearing on the surface of your goods. They’re frequently found near the mold’s locations. Flow lines may not affect the quality of your parts, but they may be inappropriate in some consumer products.

Caused due to:

  • Temperatures in the mold and the substance are both extremely low.
  • Variation in the cooling rate of the material as it moves in different directions
  • Slow injection rate and force
  • The gate’ flow channel’s sizes

How to prevent:

  • Injection pressure, speed, and material temperature should all be increased.
  • More space between the mold gates and the mold coolants is required.
  • Increase the diameter of the nozzle.
  • Convert to a hot runner or reduce the main channels.

Weld Lines

Weld lines are imperfections in plastic molded parts that cause molten material to condense on the item’s surface. The molten material separates into two or more sections in the mold at first. Weak material bonding is frequently the cause of this defect. Its weak connection further weakens the part.

Caused due to:

  • Inconsistent wall thickness.
  • The temperature of molten metal is either too low or too high.
  • On fabricated pieces, there are inserts, holes, and multiple-gate characteristics.

How to prevent:

  • Increase the molten materiality.
  • Adjust the gates and add a venting slot if necessary.
  • Reducing the amount of releasing agents is a good idea.
  • Raise the melting point temperature.

Burn Marks

This injection molding flaw shows itself as a black discoloration on the molded part. In addition, the molded plastic portion may be rust-colored. Burn scars do not usually compromise the structural integrity of items. However, when the part is burned to the point of degeneration, it poses a concern.

Caused due to:

  • The time it takes to remove the air from the cavity is too long.
  • The melting temperature is too high.
  • Extreme injection speeds or a high-speed cast screw.
  • The incorrect design of the runner system.

How to prevent:

  • Lower the temperature of melting and molding.
  • Reduce the injection rate.
  • Add exhaust systems to the mix (gates and gas vents).
  • Reduce the number of mold cycles in the system.


Color streaking is another name for this problem. When your molded item arrives with a hue that isn’t isn’t what you expected, it’s discoloration. These discolorations are usually limited to a small area. You might notice some streaks of strange shade on your part from time to time. This flaw does not impact the product, but it does harm its appearance.

Caused due to:

  • In the hopper or nozzle, leftover pellets or residual resins.
  • Molds of various sorts from a prior cycle of manufacture.
  • Inadequate colorant thermal stability.
  • The masterbatch was not properly mixed.

How to prevent:

  • Between production runs, ensure staff clean the hopper, nozzle, and mold thoroughly to remove any leftover pellets or base material.
  • To remove the extra color from the machine, consider using a purging compound.
  • Make sure you or your supplier is using a colorant that can withstand high temperatures.
  • Ensure that the masterbatch is evenly mixed.
  • Ascertain that the colorant has acceptable thermal stability.

Final Thoughts

Injection molding often necessitates a considerable upfront investment in tooling. That is to get your mold design perfect the first time, rather than having to start again after discovering major defects. On the other hand, defects in the molding process or material are usually easier and less expensive to fix. 

Regardless of the reason, defects in molded items can have a significant negative impact on your bottom line. Many of these, however, can be readily avoided by implementing effective design methods. Others necessitate alterations to the materials used and the storage conditions.

Now that you recognize what to look for when it comes to common injection molding defects, you can take steps to avoid them in the future and ensure that your products meet or surpass your customers’ expectations for quality.

Contact Karma Product Development

Technologies keep on improving gradually. We at Karma Product Development provide you the concept of modeling and traditional designs. With us, you will confidently work with the experts that successfully outsource your projects contact us on 8776508890.