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When small bubbles appear inside plastic parts, many teams first try to increase pressure, dry the resin longer, or adjust the machine. Sometimes that works. But in many injection molding projects, the real problem is not a simple setting issue. The defect may be a true gas bubble, a shrinkage void, a sink mark, or a blister.
Among common plastic molding defects, injection molding bubbles are tricky because they may look similar to other injection moulding faults. A clear part may show the problem immediately. An opaque part may only reveal it after cutting, assembly, or testing. Before changing process settings, the first step is to identify the defect type and location.
What Are Bubbles in Injection Molding?
Bubbles in injection molding are gas pockets or hollow areas inside or near the surface of molded plastic parts. They may come from trapped air, moisture vapor, resin degradation, or poor venting. In some cases, what looks like a bubble is actually a vacuum void caused by shrinkage during cooling.
This distinction matters. A true bubble is usually linked to gas, air, moisture, or venting. A shrinkage void is usually linked to thick sections, poor packing, early gate freeze, or wall design. Treating both problems the same way often leads to the wrong correction.
Bubbles vs Voids vs Sink Marks vs Blisters
Bubbles, voids, sinks, and blisters are often grouped together as plastic molding defects, but they are not the same. They may look similar, especially on thick or transparent plastic injection molding products, but the correction path is different.
| Defect | What It Looks Like | Common Cause | First Check |
| Bubble | Small gas pocket inside or near the surface | Trapped air, moisture, resin gas | Drying, venting, melt condition |
| Vacuum void | Hollow area inside a thick section | Shrinkage and poor packing | Wall thickness, hold pressure, gate freeze |
| Sink mark | Surface depression | Thick wall or local shrinkage | Part design and packing |
| Blister | Raised surface bubble or layered swelling | Gas, overheating, surface separation | Material, temperature, cooling |
If the defect is in a thick area and appears after cooling, check shrinkage and packing first. If it appears randomly in clear parts or comes with silver streaks, check drying, material storage, and contamination. If it is always near the end of fill or a weld line, check trapped air and venting.
How to Identify the Type of Bubble Before Changing Settings
Before changing injection pressure or melt temperature, look at where the defect appears. A bubble near the gate may suggest shear heat, gate restriction, or resin degradation. A bubble at the end of fill may point to trapped air. A hollow area inside a thick boss is often a shrinkage void rather than a gas bubble.
Cutting the part is sometimes necessary, especially for opaque plastic injection molding products. I have seen parts that looked clean on the outside but had voids inside thick ribs or bosses. In a functional part, this can reduce strength and cause cracks during screw assembly or drop testing.
| Bubble Location | Likely Direction to Check |
| End of fill | Venting, injection speed, trapped air |
| Near weld line | Flow-front meeting, venting, gate layout |
| Near gate | Shear, gate size, melt degradation |
| Inside thick wall | Shrinkage void, packing, cooling |
| Random in clear part | Moisture, contamination, drying record |
| Around rib or boss | Wall thickness, packing, local shrinkage |
Some molding teams use a controlled heating check to help distinguish gas bubbles from voids. If the area swells when heated, trapped gas may be present. If the area collapses or becomes a sink, it may be closer to a vacuum void. This method is only a supporting check, not the only basis for judgment.
Main Causes of Bubbles in Plastic Parts
Injection molding bubbles may come from material, process settings, mold design, or part design. In real production, more than one factor is often involved. The best correction starts from the most likely source, not from the easiest machine setting to adjust.
Moisture in the Material
Moisture is one of the most common causes of bubbles, splay, and silver streaks. Hygroscopic materials can absorb moisture from the air. During the plastic molding process, moisture may turn into vapor in the barrel and form gas pockets in the melt.
Materials such as nylon, PC, PBT, PET, TPU, PMMA, and other engineering plastics should be dried according to the resin supplier’s recommendation. Drying time, temperature, dryer condition, open bag time, and hopper exposure should be checked before changing the mold or machine settings.
Trapped Air During Filling
Trapped air happens when air in the mold cavity cannot escape before the melt seals it inside the part. It often appears at the end of fill, around deep ribs, in corners, near inserts, or where two flow fronts meet.
For this type of issue, simply increasing hold pressure is not always useful. The air still needs a way out. Better venting, a gate location change, a flow path change, or staged injection speed may be more effective.
Material Degradation and Gas Formation
Material degradation can produce gas inside the melt. This may happen when melt temperature is too high, residence time is too long, screw speed is excessive, or shear is too high. A small gate or restricted runner can also create extra shear heat near the gate.
Degradation-related bubbles may appear with discoloration, black specks, odor, burn marks, silver streaks, or weak surfaces. In this case, actual melt temperature, screw recovery, back pressure, residence time, and hot runner temperature should be reviewed.
Poor Packing and Shrinkage Voids
Some “bubbles” are not gas bubbles. They are shrinkage voids. These are common in thick sections, thick bosses, rib bases, and areas far from the gate. The outside of the part freezes first, while the center continues to cool and shrink.
For shrinkage voids, drying the material longer will not solve the root cause. The team should check holding pressure, holding time, cushion stability, gate size, gate location, mold temperature, and wall thickness. If the section is too thick, a design change may be needed.
Mold Venting and Gate Design Problems
Mold venting is critical for reducing trapped-air bubbles. Vents need to be located where air is pushed during filling, such as last-fill zones, weld line areas, deep ribs, parting lines, ejector areas, and around inserts.
Gate design also affects bubbles. Gate size affects shear and packing. Gate location affects flow path, weld line position, and pressure transfer. A plastic mold manufacturer should review gate and venting together, not as separate details.
Part Design Problems
Part design can create bubble and void risk before the mold is built. Thick walls, sudden wall transitions, oversized bosses, deep ribs, closed corners, and long flow paths can trap air or make packing difficult.
For production plastic injection molding products, small internal defects may become strength or assembly problems later. If the part has thick sections that cannot be avoided, gate location, venting, packing path, and cooling should be planned early.
How to Get Rid of Bubbles in Injection Molding
The right fix depends on the defect type. A moisture bubble, trapped air bubble, shrinkage void, and blister do not need the same correction.
| Root Cause | Practical Fix |
| Moisture in resin | Dry material according to supplier recommendation |
| Random bubbles in clear parts | Check drying record, storage, contamination |
| Trapped air at flow end | Improve venting, adjust injection speed, review flow path |
| Bubbles near gate | Check gate size, shear, melt temperature |
| Shrinkage void in thick wall | Improve packing, check gate freeze, reduce thick section |
| Poor venting | Clean, add, or relocate vents |
| Material degradation | Reduce excessive heat, shear, or residence time |
| Low back pressure | Increase carefully to improve melt consistency |
| High back pressure | Reduce if shear heat or degradation is suspected |
The wrong fix can create another defect. Increasing injection pressure may reduce a void, but it can also increase flash, residual stress, or part sticking. Lowering melt temperature may reduce degradation, but it may also weaken weld lines or cause short shots.
Troubleshooting Workflow for Injection Molding Bubbles
A practical troubleshooting workflow should start with the part, not the machine screen. First confirm the defect type, then check the material, location, process, mold, and part design.
Step 1: Confirm Whether It Is a Bubble or Void
Inspect the part location and cut the part open if needed. Check whether the defect is a gas pocket, shrinkage void, sink mark, or blister.
If the team skips this step, process changes may go in the wrong direction. A void in a thick boss and a moisture bubble in a clear part need different fixes.
Step 2: Check Material Drying and Storage
Review resin grade, drying time, drying temperature, dryer condition, open bag time, regrind ratio, colorant, and contamination risk. For transparent and hygroscopic materials, drying records are more reliable than operator memory.
If the issue is random and appears with silver streaks or surface marks, material moisture or contamination should be checked early.
Step 3: Check Bubble Location
If the defect is near the gate, check shear and gate size. If it is at the end of fill, check venting and flow path. If it is inside a thick wall, check shrinkage and packing.
If bubbles appear randomly across the part, check moisture, contamination, and melt quality. Location often saves more time than blind parameter adjustment.
Step 4: Review Process Settings
Check melt temperature, injection speed, holding pressure, holding time, back pressure, screw speed, cushion, and cooling time. Actual melt temperature is more useful than only reading barrel settings.
Avoid changing several settings at once. One controlled change is easier to judge than five random changes.
Step 5: Inspect Mold and Part Design
Review venting slots, gate size, gate location, runner balance, hot runner temperature, wall thickness, rib base, boss design, and flow-end areas.
If the design creates a local thick section or trapped air pocket, machine settings may only hide the defect for a short time.
FAQs About Bubbles in Injection Molding
How do you get rid of bubbles in injection molding?
Start by checking the defect type and location. Then review drying, material storage, venting, gate design, melt temperature, injection speed, holding pressure, and wall thickness. Different plastic molding defects need different corrections.
Are bubbles and voids the same defect?
No. A bubble is usually related to gas, air, or moisture. A void is often related to shrinkage in thick sections and poor packing. They may look similar after cutting the part, but the fixes are different.
Can moisture cause bubbles in plastic parts?
Yes. Moisture can become vapor during molding and form bubbles, splay, or silver streaks. This is especially important for hygroscopic materials. Drying conditions should follow the material supplier’s recommendation.
Why do bubbles appear in transparent plastic parts?
Transparent parts make internal defects easier to see. Moisture, contamination, material degradation, poor venting, gate shear, thick sections, and internal stress can all show up as visible bubbles or marks in clear parts.
Conclusion
Bubbles are common plastic molding defects, but not every internal hollow mark is a true bubble. Some defects come from trapped air. Some come from moisture. Others are shrinkage voids, sink marks, or blisters. Before changing process settings, the team should first identify the defect type and location.
For stable plastic injection molding products, the better approach is to check material drying, melt condition, venting, gate design, packing, wall thickness, and mold structure together. If you have injection molding needs or want to review bubbles, voids, or other injection moulding faults before production, you can contact HingTung for project discussion.