Why Do Plastic Parts Swell or Form Blisters After Demolding?

Some injection molded plastic parts may swell, blister, or bulge shortly after demolding, especially on the back side of metal inserts or in areas with excessive wall thickness. This type of defect is usually related to insufficient cooling and incomplete solidification inside the part. When the molded part is released from the mold, internal gas or thermal stress may continue to act on the still-soft material, causing localized expansion or blister-like deformation.

Although this problem may first appear as a simple surface defect, it is often closely related to cooling efficiency, wall thickness design, molding temperature, filling condition, and packing control. In actual production, these factors should be evaluated together in order to find a stable solution.

Main Causes of Swelling or Blistering After Demolding

When a plastic part is not fully cooled and hardened before ejection, the internal material may still contain heat and trapped gas pressure. After the mold opens, this internal pressure may be released gradually, which can cause swelling, bubbling, or bulging in localized areas.

This defect is more likely to occur in parts with metal inserts, thick sections, or large wall thickness variation. In these areas, cooling is usually slower and the internal heat is more difficult to remove. As a result, the outer surface may appear stable at first while the inside is still not fully solidified.

Common Methods to Solve the Problem

1. Improve Cooling Efficiency

   Effective cooling is one of the most important solutions. The mold temperature can be reduced appropriately, the mold opening time can be extended, and the material drying temperature or processing temperature can also be lowered where necessary. Better cooling helps the molded part solidify more completely before demolding.

2. Reduce Filling Speed and Flow Resistance

   Reducing the filling speed may help lower internal stress and reduce the chance of gas-related defects. At the same time, improving flow conditions and reducing unnecessary flow resistance can help create a more stable filling process. In some cases, the molding cycle should also be reviewed and adjusted according to the actual product structure.

3. Increase Holding Pressure and Holding Time

   Appropriate packing pressure and holding time help improve the density and stability of the molded part. If the internal structure is not packed properly, localized voids or unstable areas may remain inside the product, increasing the risk of swelling or blistering after demolding.

4. Improve Excessive Wall Thickness or Sudden Thickness Change

   Part design should be reviewed carefully if the wall is too thick or if there are large wall thickness changes between different areas. Uneven thickness often causes non-uniform cooling and makes internal defects more likely. A more balanced wall thickness design can significantly reduce this risk.

Why Thick Areas and Metal Inserts Increase the Risk

Blistering and swelling often appear near metal inserts or thick wall sections because these areas create more difficult cooling conditions. Metal inserts change the local heat transfer behavior, and thick sections retain heat longer than surrounding areas. If the part is ejected too early, internal pressure may still remain in the material and later appear as swelling or bulging on the surface.

For this reason, both product design and mold cooling layout should be reviewed carefully when insert molding or thick-wall molding is involved.

How to Prevent This Defect in Production

To prevent swelling or blister defects after demolding, manufacturers should control mold temperature, material temperature, injection speed, holding pressure, holding time, and cooling time in a coordinated way. If process adjustment alone is not enough, the part design should also be reviewed to reduce excessive wall thickness or major thickness transitions.

In many cases, this type of defect cannot be solved by a single parameter change. A stable solution usually requires combined optimization of part design, mold cooling, and molding conditions.

Injection Mold and Production Support from FITMOLD

FITMOLD supports custom injection mold development and plastic part production for projects that require better manufacturability, practical cooling design, and stable molding quality. For products with blistering, swelling, or post-demolding deformation risk, we help evaluate mold structure, wall thickness design, and process settings in order to improve production stability and reduce defect risk.

If you are looking for a manufacturing partner for custom injection molds and plastic part production, FITMOLD can support your project from design review to tooling and production.

Contact us: sales@fitmold.com