How to deal with bubble problems in plastic product processing

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Bubbles often occur at the final seams of plastic products or in the thicker parts of the products. Within the mold, the product first cools on its surface; once the outer surface has cooled, the central part remains hot. The central part contracts as it cools, while the surface of the plastic has already cooled; therefore, bubbles can only form as a result of contraction from within. There are many thick parts that have no bubbles when they come out of the mold, but after several dozen seconds, bubbles appear in the central part. There are several types of bubbles, each with different manifestations and causes, as described below: (1) In thicker products, the surface becomes bulged; bubbles can be found at the cut surface. The possible reasons for this are: 1) The injection speed is too fast, allowing air to get trapped and unable to be expelled, thus forming bubbles. 2) Excessive or insufficient feeding amount, as well as too high injection pressure, can also cause this phenomenon; adjusting the feeding amount appropriately can improve it. 3) The cooling time is too short (especially for thick-walled products); a solution is to use cold water and cold molds outside the machine for cooling. 4) There are black lines of decomposition in the product, which are caused by excessive temperature. If the situation is not severe, the temperature can be reduced appropriately; if the entire barrel breaks down, the machine must be disassembled for cleaning. 5) A too small pouring gate in the mold and poor venting can also cause this phenomenon. (2) The solid products have voids and show blackening. When forming solid products, the injection pressure should be low and the injection speed should be slow; otherwise, air is burned due to friction under high pressure and gets trapped within the product, resulting in black voids inside it. Therefore, by appropriately reducing the injection pressure and injection speed, the voiding phenomenon will disappear. It is also possible to check whether the amount of material added is too large; reducing it appropriately can also help to improve this phenomenon. (3) When the thickness of plastic products varies, small bubbles appear in the thicker parts of the product, or large black bubbles are present inside the thickest sections. When molding several products of different shapes at the same time, it is necessary to ensure that the size of each gate is appropriate to the weight of the product; otherwise, thick products are prone to developing bubbles. At the same time, attention must also be paid to venting in the mold. Products that remain on the punch side often make venting more difficult, which can lead to the aforementioned issues. This phenomenon can be improved by modifying the mold or increasing the holding time and cooling time, in order to compensate for insufficient pressure at the cross-section. More importantly, when designing a product, efforts should be made to avoid having overly thick sections in it.