Reasons and solutions for voids and bubbles in injection molded parts

1、 Injection molding process
Many process parameters have a direct impact on the generation of bubbles and vacuum bubbles. If the injection pressure is set too low, the injection speed is too fast, the injection time and cycle are too short, the amount of material added is too much or too little, the pressure is insufficient, the cooling is uneven or insufficient, and the material temperature and mold temperature are not controlled properly, all of which can cause bubbles to appear in the plastic parts. Especially during high-speed injection, the gas in the mold cannot be discharged in time, resulting in too much residual gas in the molten material. Therefore, the injection speed should be appropriately reduced. If the speed drops too much and the injection pressure is low, it is difficult to exhaust the gas in the molten material, which can easily cause bubbles, dents, and underinjection. Therefore, special caution should be taken when adjusting the injection speed and pressure.
In addition, methods such as adjusting injection and holding time, improving cooling conditions, and controlling the amount of material added can be used to avoid the generation of bubbles and vacuum bubbles. If the cooling conditions of the plastic parts are poor, they can be immediately placed in hot water for slow cooling after demolding, so that the internal and external cooling rates tend to be consistent.
When controlling the mold temperature and melt temperature, it should be noted that the temperature should not be too high, otherwise it will cause the melt to degrade and decompose, produce a large amount of gas or excessive shrinkage, and form bubbles or shrinkage pores; If the temperature is too low, it will cause insufficient compaction of the filling material, which can easily create voids and form bubbles inside the plastic parts. In general, the melt temperature should be controlled slightly lower and the mold temperature should be controlled slightly higher. Under such process conditions, it is neither easy to generate a large amount of gas nor easy to produce shrinkage cavities. When controlling the temperature of the material barrel, the temperature of the feeding section should not be too high, otherwise it will cause backflow and material return, causing bubbles.
2、 Injection mold
If the gate position of the mold is incorrect or the gate section is too small, the main flow and splitter channels are long and narrow, there are dead corners in the flow channels, or the mold exhaust is poor, it can cause bubbles or vacuum. Therefore, it is necessary to first determine whether mold defects generate bubbles and the main cause of vacuum bubbles. Then, according to the specific situation, adjust the structural parameters of the mold, especially the gate position should be set at the thick wall of the plastic part.
When choosing the gate form, due to the prominent phenomenon of vacuum holes generated by the direct gate, it should be avoided as much as possible. This is because after the pressure is maintained, the pressure in the mold cavity is higher than the pressure in front of the gate. If the molten material at the direct gate has not yet frozen, it will cause the molten material to flow back, causing holes to form inside the plastic part. In cases where the form of the gate cannot be changed, adjustments can be made by extending the pressure holding time, increasing the material supply, and reducing the taper of the gate.
The cross-section of the gate should not be too small, especially when forming several plastic parts with different shapes at the same time. It is important to note that the size of each gate is proportional to the weight of the plastic part, otherwise larger plastic parts are prone to bubbles.
Narrow and narrow channels should be shortened and widened to eliminate dead corners in the channels and eliminate the problem of poor mold exhaust. When designing molds, efforts should be made to avoid excessively thick parts or large differences in thickness on the plastic parts.
3、 Injection molding materials
If the moisture or volatile matter content in the forming raw materials exceeds the standard, the particles are too small or uneven in size, resulting in too much air mixed in the feeding process, the shrinkage rate of the raw materials is too large, the melt index of the melt is too large or too small, and the recycled material content is too high, all of which will affect the production of bubbles and vacuum bubbles in the plastic parts. To address this, methods such as pre drying raw materials, screening fine materials, replacing resins, and reducing the amount of recycled materials should be used separately.