Cause analysis and solution for bubble generation in plastic products

April 3, 2019

 

Cause analysis and solution for bubble generation in plastic products

 

Plastic products will generate bubbles after molding, forming pores on the plastic parts. Bubbles are defects that form a small volume or a series of pores inside the plastic parts; the dark bubbles are vacuum holes generated inside the plastic parts, also called vacuum bubbles.
Causes of defects and countermeasures
The main reason for the generation of bubbles is that the gas enters the melt and is cooled and formed together with the melt;
The main reason for the generation of dark bubbles is that when the plastic parts are cooled and solidified, the cooling speed is different inside and outside the plastic parts, the outer speed is slow, the outer layer is solidified first, and the inner melt is cooled and contracted. At this time, vacuum holes are generated inside the plastic parts. Become a dark bubble. The specific analysis is as follows.
1, mold aspects
(1) If a sprue is used, the pressure in the cavity is higher than the pressure in front of the gate after the plastic pressure is maintained. When the melt at the sprue is not solidified, melt reflow occurs. A dark bubble forms inside the plastic part. In this regard, the use of sprues should be avoided as much as possible. If the mold adopts a sprue design, in order to reduce the generation of dark bubbles, the method of appropriately maintaining the dwell time and increasing the feed amount of the melt can be used for remediation.
(2) In a multi-cavity mold, if several plastic parts with different shapes are formed at the same time, it must be noted that the size of the gate is proportional to the mass of the plastic part. Otherwise, larger plastic parts are prone to bubble defects.
(3) If the exhaust system of the mold is defective or stagnate, the gas is mixed into the melt, resulting in the generation of bubbles. In this regard, the exhaust system of the mold should be carefully checked to clear the passage of the Dussel.
(4) If the flow path is too long and too thin. Or the dead angle of the gas storage in the flow path will cause the mold to be poorly exhausted. In this regard, the narrow flow path should be shortened and widened, and there should be no dead corners in the flow path.
(5) If the gate cross-sectional area is too small, or the position is not correct, it is easy to cause bubbles and dark bubbles. In this regard, it is necessary to increase the cross-sectional area of ​​the gate, especially the gate position should be placed at the thick wall of the plastic part.
2, raw materials
(1) If the moisture content of the raw materials is too high, or the volatile components exceed the standard, they will generate a large amount of gas after being heated, which will add an extra burden to the exhaust system of the mold. If they cannot be driven away, they will be mixed into the melt to form bubbles. . In this regard, these materials should be pre-dried. ,
(2) If the particles of the raw material are too small or the particle size is different, so that too much air is mixed in the feeding process, the chance of gas entering the melt increases, and bubbles are easily generated. In this regard, the raw material granules should be controlled and the fines removed.
(3) If the shrinkage of the resin is too large, it is liable to cause dark bubbles during molding and curing. For this, the resin should be replaced.
(4) If too much recycled material is mixed in the raw material, the volatile gas is increased, the shrinkage rate is also increased, and bubbles and dark bubbles are apt to occur. In this regard, the amount of recycled material should be controlled, and the addition of a maleic anhydride grafting compatibilizer suitable for Nanjing Plastics can improve the strength and toughness of the material.
3, molding operation
(1) If the injection speed is too fast, the injection time is too short, and the gas in the cavity is too late to be discharged, resulting in too much residual gas in the melt, resulting in bubbles. In this regard, the injection speed should be appropriately reduced and the injection time should be increased.
(2) If the melt temperature and the mold are too high, it will cause the melt to degrade, and generate a large amount of gas and excessive shrinkage to form bubbles and dark bubbles. In this regard, the melt and mold temperatures should be controlled.
(3) If the melt temperature and the mold temperature are too low, the filling and compaction will be insufficient, and voids will be formed inside the plastic part to form bubbles. In this regard, the melt temperature and mold temperature should be increased.
(4) If the feeding amount is too much or too little, and the holding time is insufficient, it is easy to cause bubbles in the plastic parts. In this regard, the appropriate amount of feed should be controlled and the dwell time should be extended.
(5) If the cooling is uneven or the cooling is insufficient, it is easy to cause bubbles and dark bubbles in the plastic parts. In this regard, the cooling conditions should be improved.
(6) When controlling the temperature of the barrel, the temperature of the feed section should not be too high. If it is too high, the melt will cause backflow and return bubbles.
5, plastic parts structure
In the design of plastic parts, it is necessary to avoid the parts of the plastic parts that are extremely thick or have a large difference in thickness. Bubbles and dark bubbles are most likely to occur in such places.

 

 

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