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Injection Molding Problems: How to Solve the Most Common Injection Molding Problems

Posted by Evan Freemon on May 15, 2017 | Updated on January 21, 2019

Injection molding is the most versatile way to produce parts and products. It’s fast, works with a variety of plastics and can result in a prototype or final product that is both durable and highly detailed. But the process is complex and riddled with challenges. Luckily, many of them are easy to resolve. Our job at iCONN Systems is to help you succeed, so we’ve created a cheat sheet with the most common injection molding problems you might encounter and how to fix them.

Most Common Injection Molding Problems

  • Flow Lines
  • Sink Marks
  • Vacuum Voids
  • Surface Delamination
  • Weld Lines
  • Short Shots
  • Warping
  • Burn Marks
  • Jetting
  • Flash

Flow lines

Definition: Off-tone streaks, patterns or lines that appear on parts.

Causes: The issue might be varying flow rates as the molten plastic moves through the mold or changes in the thickness of the mold’s walls. It also may be that your injection speed is too low, which will cause the molten plastic to solidify at different rates.

Fixes: Increase the injection speeds and pressure to ensure the cavities are filled evenly. You can also increase the temperature of the molten plastic or the mold to prevent the plastic from cooling too quickly. Round any areas of your mold where the wall thickness varies to create a uniform flow-path for the molten plastic.

Sink Marks

Definition: Small depressions located in thicker areas of the injection molding after there is shrinkage in the finished product.

Causes: The cooling time or mechanism is incorrect, and the plastic isn’t cooling down enough while inside the mold. There also may be an issue with the pressure inside the cavity, or with too much heat at the gate.

Fixes: Decrease the mold temperature, increase the holding pressure and time, and allow the plastic to cool and cure longer inside the mold.

Vacuum Voids

Definition: Air pockets trapped inside of or close to the surface of an injection molded part.

Causes: Vacuum voids may be the result of insufficient holding pressure that causes the molten plastic to condense inside the mold. The mold halves also may not be aligning properly. You also could check for uneven curing.

Fixes: Position the gate near the thickest part of the molding. Use a less-viscous plastic to avoid trapping gas (air can escape more easily with thinner consistencies). You also can increase the holding pressure and time, and check to make sure your mold parts seamlessly align.

Surface Delamination

Definition: Thin layers of “material” that appear on a part surface and can be peeled off.

Causes: Contamination. Likely, a foreign material got into the molten plastic. Overuse of release agents also can cause this problem.

Fixes: Pre-dry plastic before molding. Increase the mold temperature and smooth out any sharp corners or turns in the mold to facilitate even melt flow. If you adjust the ejection mechanism in the mold design, you also can reduce or eliminate your need for mold release agents.

Weld Lines

Definition: A seam that appears where two areas of molten plastics meet.

Causes: Poor bondage between two or more flow fonts.

Fixes: Try raising the temperature of the mold or the molten plastic. You can also increase the injection speed, switch to a less viscous plastic or adjust the flow pattern to a single-source flow.

Short Shots

Definition: When the molten plastic does not fully fill the mold cavity (or cavities).

Causes: The calibration or plasticizing capacity might be incorrect, which will result in not enough molten plastic to fill the cavities. The plastic also may be too viscous, which will cause it to solidify before all cavities have been occupied. The wrong degassing or gas techniques also may be to blame, as trapped air can prevent molten plastic from inhabiting space.

Fixes: Work with a less viscous plastic and increase the mold or melt temperature to enhance flowability. Adjust your process to account for any gas getting trapped inside the mold and use proper ventilation.


Definition: When the finished part is unintentionally twisted, uneven or bent in areas.

Causes: The most likely cause is non-uniform cooling; for example, varying cooling rates inside the mold. When plastics cool at different rates, some areas get stressed, which results in warping.

Fixes: Make sure your cooling process isn’t being rushed (both in time and speed). Uniform wall thickness within the mold also will help ensure the molten plastics flow evenly and in a single direction. You also may want to reconsider the type of plastic you’re using. Semi-crystalline materials are prone to warping.

Burn Marks

Definition: Rusty discolorations that appear on the surface of a part.

Causes: If your prototypes have burn marks, it may be excessive heating or injection speeds that are too fast. It also may be trapped air overheating and creating etches on the surface of molded parts.

Fixes: Lower your injection speeds and improve degassing quality. You also can try reducing mold and melt temperatures.


Definition: When molten plastic fails to adhere to the surface of the mold and instead creates wavy folds on the surface of the part.

Causes: It’s likely the problem is a combination of things that happen all at once: An inadequate injection speed, a melt temperature that is too low and a molten plastic with a viscosity that is too high. When the plastic touches the mold’s walls, it cools quickly and the viscosity increases. As new molten plastic pushes past cooling plastic, it creates scrape marks.

Fixes: Increase your mold and melt temperatures and the size of the gate to slow down the injection speed. You also need to make sure the right amount of contact is being made between the molten plastic and the mold.


Definition: When some of the molten plastic escapes from the mold cavity through the parting lines, ejector pins, etc.

Causes: Check your mold clamps; they may not be tight enough. And make sure your molds haven’t expired. The injection pressure also may be too great, forcing molten plastic out of the mold.

Fixes: Increase your clamp pressure and confirm that the mold doesn’t require cleaning and maintenance. While you’re at it, assess your injection speed, injection pressure and mold temperature.

In most cases, the issue you’re having is as simple as adjusting the injection speed or pressure or considering the type of plastic and melt temperature you’re using. It’s also important to use high-quality molds. For more information about the many variables involved in injection molding, check out our free whitepaper.

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