What Causes Porosity in Injection Molded Products?




If you've ever seen tiny bubbles, voids, or spongy areas inside a plastic part, you've witnessed porosity. In injection molding, porosity is a common defect that can compromise a part's strength, appearance, and overall integrity. Understanding what causes these internal voids is the first step to preventing them.

The Main Culprits Behind Porosity

Porosity is primarily caused by inconsistent cooling and material shrinkage, which prevent the plastic from filling the mold evenly and solidifying properly. The key factors are related to the material, the mold design, and the process itself.

1. Material Issues

  • Excessive Moisture: Many plastic resins are hygroscopic, meaning they absorb moisture from the air. If the material isn't dried properly before molding, the trapped water can vaporize under the high heat of the barrel, creating gas bubbles that become voids in the final part. This is a prevalent cause, especially with materials like Nylon (PA) and Polycarbonate (PC).
  • Degraded Material: Using recycled or improperly stored material can lead to thermal degradation. When plastic breaks down, it can release gases that get trapped within the part, causing porosity.

2. Mold Design

  • ProblemsInadequate Gate Size: The gate is the small opening where molten plastic enters the mold cavity. If the gate is too small, it can freeze off (solidify) prematurely before enough plastic has been packed into the cavity to compensate for shrinkage. This lack of "packing pressure" leaves a void in the center of the part.
  • Poor Venting: During the injection process, air is trapped inside the mold cavity. If there aren't enough vents or they are improperly placed, this air has nowhere to go and gets compressed. As the plastic cools, the compressed air expands, forming bubbles or voids, often near the end of the part's fill path.

3. Processing and Machine Settings

  • Insufficient Holding Pressure: Holding pressure is applied after the mold is filled to "pack" the material and push more plastic into the cavity as it shrinks. If this pressure is too low or the holding time is too short, the material won't be fully compressed, leading to a void.
  • Melt Temperature Too High: While you might think higher temperatures would help fill the mold, an excessively high melt temperature can cause the plastic to degrade, releasing gas. It can also cause the material to shrink more drastically as it cools, making it harder to compensate with holding pressure.
  • Fast Injection Speed: Injecting the plastic too quickly can cause the material to "race" through the mold, trapping air and leading to an uneven flow front. This can result in voids, especially in thick sections.

How to Fix and Prevent Porosity?

Preventing porosity requires a holistic approach, addressing all three areas.
  1. Optimize Material Handling: Ensure all hygroscopic materials are properly dried in a hot air or desiccant dryer before processing. Follow the material supplier's recommendations for drying temperature and time.
  2. Review Mold Design: Work with your toolmaker to ensure proper gate sizing and placement, especially on thick parts. Critically review the venting strategy to ensure all trapped air can escape.
  3. Adjust Process Parameters: Begin by adjusting the holding pressure and time. Gradually increase both until the voids disappear. If the problem persists, try reducing the melt temperature or injection speed.

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