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What if there are voids and bubbles in plastic injection molded parts?


The gap inside the plastic product is in the form of round (bubble, vacuum bubble) or elongated bubble. When injection molding products, bubbles and voids appear, and the voids inside can be seen from the outside of generally transparent products; This is because after the molten material enters the injection molding machine mold, the product will form a hard cooling layer and develop inward or fast or slow according to the cooling degree of the mold. In the thick wall area, the central part still maintains high temperature and workability and continues to shrink.

How to solve the internal gap of injection molded parts?
1、 Injection molding process
Many process parameters have a direct impact on the generation of bubbles and vacuum bubbles. Too low injection pressure, too fast injection speed, too short injection time and cycle, too much or too little feeding, insufficient pressure holding, uneven or insufficient cooling, and improper control of material temperature and mold temperature will cause bubbles in plastic parts. Especially during high-speed injection, the gas in the mold is too late to be discharged, 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 small, it is difficult to exhaust the gas in the molten material, and it is easy to produce bubbles, depressions and underinjection. Therefore, special care should be taken when adjusting the injection speed and pressure.
In addition, bubbles and vacuum bubbles can be avoided by adjusting the injection and pressure holding time, improving the cooling conditions and controlling the feeding amount. If the cooling conditions of plastic parts are poor, the plastic parts can be put into hot water for slow cooling immediately after demoulding, so that the internal and external cooling speed tends to be the same.

2、 Injection mold
If the gate position of the mold is incorrect or the gate section is too small, the main flow channel and shunt channel are long and narrow, there is a dead corner of gas storage in the flow channel or poor exhaust of the mold, bubbles or vacuum will be caused. Therefore, we should first determine whether the mold defects produce bubbles and the main causes 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 selecting the gate form, the phenomenon of vacuum hole in the direct gate is prominent, so it should be avoided as far as possible. This is because the pressure in the cavity is higher than that in front of the gate after the pressure maintaining. If the melt at the direct gate is not frozen at this time, the melt backflow phenomenon will occur, resulting in the formation of holes in the plastic part. When the gate form cannot be changed, it can be adjusted by extending the pressure holding time, increasing the feeding amount and reducing the gate taper.
3、 Injection molding raw materials
If the content of moisture or volatile matter in the molding raw materials exceeds the standard, the material particles are too small or uneven, resulting in too much air mixed in the feeding process, too large shrinkage of raw materials, too large or too small melt index of molten materials, and too much recycled material content, it will affect the generation of bubbles and vacuum bubbles in plastic parts. Therefore, the problems should be solved by pre drying raw materials, screening fine materials, replacing resin and reducing the amount of recycled materials.

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