One of the main causes of defects in cosmetic molded parts is air bubbles. This troublesome part defect not only causes appearance problems, but also damages physical properties. Bubbles are a common phenomenon, but they are usually difficult to resolve.

When eliminating air bubbles, many molders mistakenly guess what the air bubbles are, and then immediately start to adjust the process parameters to eliminate the bubbles.

There are only two possibilities:

1. Residual gases, including air, moisture, volatiles of resins or decomposition gases of polymers or additives.

2. Vacuum gap.

It is important to determine what type of bubbles the part has and the root cause. Determining the bubble type will allow you to pinpoint the cause and determine the next course of action to eliminate the problem. How do you test to determine if it is a gas or a vacuum void? Many people claim that you can tell the difference by the shape, position or other characteristics of the bubbles. But you may be easily fooled by this method. You should use a simple test instead, which takes less than 15 minutes, but you still need a little patience.

Test your part by gently heating the part area with one or more bubbles until it softens. Because some operators tend to pick up the nearest torch and aim it at the part.

Instead, use a heat gun or similar tool. Then, when you gently heat the area of the part where the bubbles are, the bubbles should be deformed. If it is a bubble, the gas will heat up and expand, raising the surface, and often eject when the surface of the part becomes soft. If there is no air in the bubble, but a vacuum gap, the bubble will collapse due to atmospheric pressure pushing the soft surface of the part.

The test must meet certain conditions. Ideally, find bubbles with a diameter of at least 3 mm (about 0.125 inches) or larger, and ensure that the life of the part does not exceed 4 hours. The bubbles may start as voids, but over time, air will migrate through the plastic and the voids will become bubbles.

Residual gas

Let’s start the troubleshooting discussion, assuming that your test proves that it is indeed a bubble, that is, the bubble has expanded or even burst. Air bubbles may come from flow front problems, such as convergence fronts, injection or mold/machine problems, such as unvented core rods, poor exhaust (try vacuum exhaust), excessive pressure reduction, or resin degradation due to overheating or prolonged periods of time Residence time. The gas may come from water vapor, resin volatiles or decomposition by-products. When parts are filled or piled up, air trapped in ribs or unvented protrusions outside the nominal wall will be pushed out, leaving traces of air bubbles. In most cases, determining the source of the gas is more important than knowing the composition of the gas.

The first step of the process is to cancel the hold or the second stage by lowering the hold pressure to a very low value and see if the bubbles still exist. If so, at least you don’t have to worry about the parameters involved in the second stage. Assuming you still see air bubbles, the next step is to understand the filling method to determine if air is trapped in the gas when filling the part.

Please do a short research when you close the second stage and complete the 99% volume fraction. That is, the lens size is reduced from full 99% to 5% in increments of 10%. Don’t start from scratch, don’t increase the injection volume, because you may get a different flow method. In addition, this test requires speed control of the first stage of the injection process. If the first stage is pressure-constrained, the consistency required for accurate results may not be obtained.

When and where do the bubbles appear? Check the flow pattern of each part to see if the plastic flow front surrounds itself, or if there is hesitation in the flow front when filling thin sections of the part. Are the bubbles always in the same area? If so, it means there is a fixed position to bubble. Pay attention to whether there is race tracking effect or jetting phenomenon, which may cause air to be trapped in the polymer.

Check for any protrusions on the ribs or nominal wall. If they are short, it means that there is air in the area, and when the ribs are filled, the air will be pushed out to form bubbles. Sometimes, you will actually see traces of bubbles from this projection. Do bubbles appear only after the part is 85% full? If so, it may be a leak. Check the vents.

A very strange source of bubbles is the Venturi effect. There are many possibilities for the Venturi effect to draw air into the melt flow: ribs, ejector pins, poor fit between nozzle tip and gate bushing, nozzle misalignment, and separation plates in the hot runner. These are difficult to detect, but when you exclude other sources, you must check the tool. Apply bluing agent near the droplets of the hot runner and on the mating surface of the plate, taking care not to let any bluing agent enter the flow path. If the blue agent is displayed at startup, it indicates where the problem is. Another common source of bubbles is excessive pressure reduction, especially in hot runner molds.

Another source is screws, more specifically the rear area or feed section. General purpose screws with an L/D of 18:1 or lower may be the culprit. Try to use a lower back zone temperature and/or higher back pressure. Another solution might be to draw a vacuum on the mold before injection.


When the part is inside or outside the mold, the cooling process usually creates voids in the thicker part. In the thicker part of the part, the center cools slowly, and the polymer shrinks more, pulling away from itself to form bubbles. If you heat the mold to a higher temperature, the bubbles disappear, but the result is a sink, which indicates that your bubbles are empty. Voids and depressions are signs of internal stress and are warning signs that the part may not perform as expected.

Insufficient plastic is the main cause of dents or voids, so it is recommended to fill more material into the cavity. Make sure you have a stable cushion and do not bottom the screws so that you can pack the parts correctly. A higher holding pressure or longer holding time may help, but many times the door is frozen before the center of the nominal wall can be fully tightened.

To resolve voids or depressions, try to reduce the filling speed, use gas back pressure, or increase back pressure. Make sure that the runner or gate does not freeze prematurely, and a longer holding time will allow more packaging in the second stage. If the gate freezes too quickly, you may need to open the gate slightly, as small changes in diameter will cause a longer gate sealing time. If possible, try to lower the melt temperature.

Another way to eliminate voids or depressions is to thin the nominal wall. The thickness of plastic parts is not always strong. The thicker nominal wall should be redesigned to make it thinner and have stiffeners to increase strength. This will save plastic and cycle time.

If possible, core the thick part. Change the gate position first to fill the thicker areas in the mold, and more polymer can be introduced into the part before the gate freezes. You can also try to increase the mold temperature and/or eject the part faster, which can prevent voids by collapsing the outer wall during the cooling process, although this may cause sinking.

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