（27） Bubbles (vacuum bubbles) Injection Molding Process Defect Handling Manual

Bubbles are vacuum bubbles in which the gas is very thin. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. The formation of vacuum bubbles is due to insufficient injection of plastic or low pressure. Under the rapid cooling action of the mold, the fuel at the corner of the mold cavity is pulled, resulting in volume loss.
Bubble refers to a phenomenon of bulging on the surface of a molded product.
Bubbles are prone to occur in the following two situations: when the product surface starts to gradually bulge when taken out of the mold after injection molding, and when the molded product surface bulges due to thermal expansion. In either case, when the surface of the molded product becomes soft due to high temperature, the internal gas will expand due to heat and lift the surface of the molded product, forming bubbles. If a large amount of air is involved during metering, bubbles are easily generated. Specifically, when the screw speed is fast, the back pressure is low, and the amount of molding is large, bubbles are easily generated. In addition, during the cavity filling process, some flow patterns can sometimes be drawn into air, resulting in bubbles. If the bonding between the surface layer and the core layer is weak, or there are small voids or cracks, it is easy to generate bubbles from this point of view. Specifically, in forming thin-walled products, strain remains in the product due to forced filling, or the mixing of cold materials or spray marks. Especially in liquid crystal polymers, it is easy to generate bubbles due to the low interlayer strength (which is an inherent property of resins).
From the perspective of forming conditions, when the injection speed is high, the bubbles tend to deteriorate. In addition, when the gate is too small, bubbles tend to worsen due to the formation of spray lines and the residual strain caused by high shear forces.
The large amount of gas generated in the resin is also prone to bubbles. When the barrel temperature is too high and the residence time is too long, the generated gas will increase, which is also prone to generate bubbles. In addition, insufficient drying and excessive moisture content in the material can also produce bubbles.
Solution:
(1) Increase injection energy: pressure, speed, time, and material volume, and increase back pressure to make mold filling full.
(2) Increase material temperature and smooth flow. Reduce the material temperature to reduce shrinkage, and appropriately increase the mold temperature, especially the local mold temperature at the location where vacuum bubbles are formed.
(3) Set the gate at the thick part of the workpiece to improve the flow conditions of the nozzle, runner, and gate, and reduce pressure consumption.
(4) Improve mold exhaust conditions.
To reduce air entrainment in metering, the following conditions should be changed:
• Reduce screw speed
• Increase back pressure
• Do not set too much molding amount
If air entrainment occurs during cavity filling, it is necessary to adjust the shape, gate position, and injection speed. This should be specifically addressed according to the situation of the formed product. Through short short filling, grasp the flow pattern, and then establish corresponding countermeasures on this basis. Changing the pressure holding has no effect on improving air bubbles. Rather, reducing the shear force during filling so that the material can smoothly fill the mold cavity will be more effective in eliminating air bubbles. Specifically, the following molding conditions can be changed:
• Increase mold temperature
• Slow down the injection speed
• Increase the gate
• Increase thickness (only for parts that are too thin)
• Avoid spray marks