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What should I do if there are bubbles in the injection molding product?

by:Vowin Rapid Prototyping     2023-02-14
Causes and Countermeasures of Bubble Defects
In injection molding products, air bubbles are a problem for many people in production and processing. Generally speaking, there are many reasons for these problems, but if they cannot be dealt with in time, it will not only cause poor plastic molding, but also produce more defects. Therefore, when bubbles appear in injection molded products, they must be resolved in time. The appearance of air bubbles will make the product full of dissatisfaction and uneven surface. Vacuum bubbles, which are mainly caused by the shrinkage of plastic products. When a plastic product with a thick wall is cooled, because the cooling rate on the surface is faster than that in the middle, it will cause the center of the plastic product to lack glue and form a vacuum. This vacuum bubble can be improved by reducing the injection speed, increasing the injection pressure, adding extenders, and using materials with high melt viscosity grades.

Bubble defects When there are bubble defects, the bubbles can be divided into air bubbles and vacuum bubbles. Generally speaking, the air bubbles on transparent products can be directly observed, while the air bubbles on opaque products sometimes cannot be seen from the outside, and can only be found by cutting or other means. The generation of air bubbles is usually due to the injection speed being too fast, the plastic flow cannot force all the air in the mold cavity out of the vent tank, and the air mixes into the plastic to form air bubbles. Vacuum bubbles are caused by the slow cooling of the center of the product wall thickness, and the rapid contraction of the surface cooling often pulls the material, resulting in uneven volume shrinkage, resulting in cavities in the thickness part; the raw materials include hot and cold water and air, which quickly become bubbles.
1. Improper control of molding conditions

Many process parameters directly affect the generation of bubbles and vacuum bubbles. The set injection pressure is too low, the injection speed is too fast, the injection time and cycle are too short, the amount of feed is too much or too little, the pressure is not enough, the cooling is uneven or insufficient, and the temperature of the material and the mold are not properly controlled. Causes air bubbles in the part. Especially during high-speed injection, the gas in the mold has no time to escape, resulting in too much residual gas in the melt. Therefore, the injection speed should be appropriately reduced. If the speed drops too much and the injection pressure is small, it is difficult to discharge the gas in the melt, and it is easy to produce bubbles, depressions and insufficient injection. Therefore, special care should be taken when adjusting injection speed and pressure. In addition, air bubbles and vacuum bubbles can be avoided by adjusting the injection and holding time, improving cooling conditions, and controlling the amount of feed. If the cooling condition of the plastic part is poor, the plastic part can be demolded immediately and put into hot water to cool slowly, so that the internal and external cooling speed tends 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 cavities; if the temperature is too low, it will lead to insufficient charge compaction, Plastic parts are prone to gaps, forming air pockets. 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 not easy to produce a large amount of gas, nor is it easy to produce shrinkage cavities. When controlling the temperature of the barrel, the temperature of the feed section should not be too high, otherwise it will cause reflux and cause air bubbles.

2. Mold defects

If the gate position of the mold is incorrect or the cross section of the gate is too small, the sprue and runner are long and narrow, there is a dead corner of gas storage in the runner or the mold is not exhausted well, it will cause air bubbles or vacuum. Therefore, it is first necessary to determine whether mold defects are the main cause of air bubbles and vacuum bubbles. Then, according to the specific situation, adjust the structural parameters of the mold, especially the position of the gate should be set on the thick wall of the plastic part. When selecting the gate form, due to the prominent vacuum hole phenomenon caused by the direct gate, it should be avoided as much as possible. This is because after pressure protection, the pressure in the cavity is higher than the pressure before the gate. If the melt at the direct gate is not frozen, the melt will flow back and holes will form inside the plastic part. If the gate form cannot be changed, it can be adjusted by prolonging the holding time, increasing the feeding amount, and reducing the taper of the gate. The cross-section of the gate should not be too small, especially when forming several plastic parts of different shapes at the same time, it must be noted that the size of each gate is proportional to the weight of the plastic part, otherwise larger plastic parts are prone to air bubbles. In addition, the slender and narrow runners should be shortened and widened to eliminate the dead angle of gas storage in the runners and eliminate the failure of poor mold exhaust. When designing molds, try to avoid extra thick parts on the shape of plastic parts or excessive thickness differences.

3. The raw materials do not meet the requirements for use
If the content of moisture or volatile substances in the molding raw materials exceeds the standard, the particles are too small or the size is uneven, resulting in too much air mixing during the feeding process, the raw material shrinkage is too large, the melt index is too large or too small, and the recycled material content is too much , will affect plastic parts to produce air bubbles and vacuum bubbles. In this regard, methods such as pre-drying raw materials, screening out fine materials, replacing resins, and reducing the consumption of recycled materials should be used.

Bubbles usually occur in the last joint or thicker part of the plastic product. The product in the product is surface cooled first, when the outer surface cools, the central part is still in a hot state. The central part cools and shrinks, and the plastic surface cools down, so bubbles can only be formed by the inner shrinkage. There are many thick parts without air bubbles in the mold, but after tens of seconds, air bubbles appear in the center part.

There are many reasons why injection molded products have air bubbles. When this happens, we should exclude them one by one. Besides, we should find ways to solve them. When the injection molding product has air bubbles, we cannot be careless. If we don't know, we can also ask professional masters to operate.
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