How to Get Rid of Bubbles in Injection Molding: Causes, Fixes, and Prevention

Injection molding bubbles may come from moisture, trapped air, material degradation, or shrinkage inside a thick section. Because these problems can look similar, immediately changing pressure, temperature, or drying conditions can send troubleshooting in the wrong direction.

At HingTung, we normally start with the defect itself: where it appears, whether it repeats in the same location, and whether anything changed before it appeared. This helps narrow the problem to the material, molding process, tooling, machine, or part design.

What Are Bubbles in Injection Molding?

Bubbles in injection molding are hollow pockets inside or near the surface of a molded plastic part. True bubbles may contain trapped cavity air, moisture vapor, or gases generated as the resin is processed. However, some apparent bubbles are actually vacuum voids caused by shrinkage during cooling.

A controlled heating check can provide another clue. Trapped gas tends to expand when heated, while a vacuum void may shrink or collapse. Location also matters: a hollow area inside a thick boss points more toward shrinkage, while a defect where two flow fronts meet suggests trapped air.

What Causes Bubbles in Injection Molding?

When investigating what causes bubbles in injection molding, first separate gas-related defects from shrinkage-related voids. Most problems can then be traced to material condition, trapped cavity air, plasticizing conditions, or insufficient shrinkage compensation.

Moisture or Gas From the Material

Hygroscopic plastics such as PA, PC, PBT, PET, TPU, and PMMA can absorb moisture that turns into vapor inside the barrel. Excessive melt temperature, residence time, screw speed, or shear can also degrade resin and generate gas.

Completing the specified drying cycle does not prove that the resin is sufficiently dry. When moisture is suspected, verify the actual moisture content where appropriate. Resin can also pick up moisture again during conveying or while sitting in the press hopper.

Trapped Air During Filling

The cavity contains air before every shot. When trapped air in injection molding cannot escape ahead of the advancing melt, it can contribute to bubbles, burn marks, short shots, or weak areas.

Part geometry can make this happen even when vents already exist. Melt may race around ribs or lower-resistance paths before separate flow fronts reconnect and enclose air. In this situation, injection mold venting design, gate position, flow path, and local geometry should be reviewed together.

Air Introduced During Plasticizing

Air can also enter or remain in the melt during screw recovery. Back pressure is not a one-direction adjustment: too little may contribute to an inconsistent melt, while excessive back pressure can increase shear heating and material degradation.

Other conditions worth checking include:

• Screw speed and decompression
• Nozzle and nozzle-to-sprue sealing
• Hot-runner conditions
• Actual melt temperature rather than barrel setpoints

Shrinkage Voids Mistaken for Bubbles

Some bubble defects in injection molding are actually shrinkage voids. They commonly form in thick walls, boss bases, and heavy rib intersections when the outer material freezes while the hotter center continues to shrink.

For these defects, check holding pressure and time, gate freeze, cooling, gate design, and injection molding wall thickness. Additional resin drying will not correct a geometry-driven void.

What Does the Bubble Location Tell You?

Before changing machine settings, check whether the defect follows a repeatable pattern. Bubble location and cavity distribution often narrow the investigation faster than changing several process parameters.

One affected cavity usually points toward a local tooling condition. If several established molds or presses begin developing bubbles around the same time, look for something they share, such as material handling, drying, cooling, or another facility-level change.

How to Get Rid of Bubbles in Injection Molding

There is no universal machine setting for how to get rid of bubbles in injection molding. The correction should follow the root cause identified from the defect pattern, material condition, and molding process.

Avoid changing several parameters at once. Increasing pressure may reduce a shrinkage void but create flash or residual stress. Slowing injection may help one air trap while creating filling problems somewhere else.

Injection Molding Bubble Troubleshooting Workflow

For injection molding bubble troubleshooting, work backward from the molded part instead of starting with random machine adjustments. The objective is to identify which part of the process changed or is allowing the defect to form.

1. Identify the defect. Confirm whether it is a gas bubble, vacuum void, sink mark, or blister.
2. Check the pattern. Record the location, frequency, affected cavities, and whether the bubble moves when process conditions change.
3. Ask what changed. For a previously stable process, review resin lots, maintenance, vents, dryers, conveying, cooling, and hot-runner conditions.
4. Verify the material. Check actual moisture where appropriate instead of relying only on drying time and temperature.
5. Run a short-shot study. Observe how the melt fronts approach the affected area. This can reveal race tracking, converging flow fronts, unexpected last-fill zones, or trapped-air pockets.
6. Verify actual outputs. Check melt and mold temperature, fill time, transfer pressure, cushion, recovery time, and other relevant process data.
7. Inspect the mold and design. If process changes cannot provide a stable solution, an injection molding DFM review can help identify whether gate location, venting, cooling, wall thickness, or geometry is making the defect difficult to control.

If bubbles appear near the end of fill together with burning or incomplete filling, poor mold venting should move higher on the troubleshooting list. For an established process that previously ran well, finding what changed is usually more useful than immediately rebuilding the process window.

How to Avoid Air Bubbles in Injection Molding

The most reliable way to avoid air bubbles in injection molding is to control the material, mold, and process before the defect becomes a recurring production problem.

• Verify resin drying and handling, especially for hygroscopic materials.
• Keep mold vents clean and include them in preventive maintenance.
• Maintain a validated process window and monitor actual process outputs.
• Review last-fill areas, flow-front convergence, gates, and local wall thickness during tooling development.
• Record material and process changes so new defects can be traced more quickly.

For new OEM parts, HingTung reviews these relationships during DFM and mold trials. Addressing filling, venting, packing, and geometry together is usually more reliable than trying to compensate for a tooling or design problem later with machine settings.

FAQs About Injection Molding Bubbles

Can injection molding bubbles reduce part strength?

Yes. Internal bubbles and voids reduce the effective material section and can create local stress concentrations. Their impact depends on size, location, material, and load. A defect inside a loaded boss or fastening feature is generally more significant than the same defect in a non-critical cosmetic area.

Can bubbles be acceptable in an injection molded part?

Possibly. Acceptance should be based on drawing requirements, cosmetic standards, functional loads, and defect location. An internal defect in a structural, sealing, or assembly area requires more attention than a small defect in a non-critical region.

Why are bubbles sometimes visible only after molding?

Some internal defects are hidden in opaque parts and only become visible after cutting, machining, assembly, or failure testing. Others become more noticeable as the part cools and shrinkage develops. This is why appearance at mold opening alone may not be enough for defect evaluation.

When does a bubble problem require a mold or part design change?

A design or tooling review becomes more important when acceptable parts can only be produced within an unusually narrow process window or when bubbles repeatedly return after small material or environmental changes. Gate location, venting, cooling, or local geometry may be making the process inherently unstable.

Conclusion

Injection molding bubbles are easier to solve when the defect is diagnosed before process settings are changed. Bubble location, material condition, flow behavior, cavity pattern, and changes to a previously stable process can help distinguish trapped gas from moisture, degradation, and shrinkage voids.

HingTung’s plastic injection molding services cover DFM, mold development, trial molding, process optimization, and repeat production. For recurring bubbles or voids, the goal is not simply to hide the defect, but to establish tooling and process conditions that remain stable in production.