Why do injection molded parts produce bubbles and vacuum bubbles?

September 29, 2019

Why do injection molded parts produce bubbles and vacuum bubbles?
 

1) Improper molding conditions
Many process parameters have a direct impact on the generation of bubbles and vacuum bubbles. If the injection pressure is too low, the injection speed is too fast, the injection time and cycle are too short, the feed amount is too much or too little, the pressure is insufficient, the cooling is not uniform or the cooling is insufficient, and the material temperature and mold temperature are improperly controlled, which will cause the plastic parts. Air bubbles are generated inside.

 

Especially in high-speed injection, the gas in the mold is too late to be discharged, resulting in too much residual gas in the melt. For this, the injection speed should be appropriately reduced.

 

However, if the speed drops too much and the injection pressure is too low, it is difficult to exhaust the gas in the melt, and it is easy to generate bubbles and depressions and underfills. Therefore, the injection speed and pressure should be adjusted with particular caution.
 

In addition, bubbles and vacuum bubbles can be avoided by adjusting the injection and holding time, improving the cooling conditions, and controlling the amount of feed. If the cooling condition of the plastic parts is poor, the plastic parts can be placed in hot water and slowly cooled after demolding, so that the internal and external cooling speeds tend to be uniform.
 

When controlling the mold temperature and the melt temperature, it should be noted that the temperature should not be too high, otherwise it will cause the melt to depolymerize and decompose, generate a large amount of gas or excessive shrinkage, forming bubbles or shrinkage holes; if the temperature is too low, it will cause the filling pressure. Insufficient, the inside of the plastic part is prone to voids and bubbles.
 

Under normal circumstances, 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 generate a large amount of gas, and it is not 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 backflow and return bubbles.
 

2) mold defects
If the gate position of the mold is not correct or the gate section is too small, the main channel and the branch passage are long and narrow, and there is a dead space in the flow passage or a bad exhaust of the mold, which may cause bubbles or vacuum.

 

Therefore, the main reason for whether the mold defects generate bubbles and vacuum bubbles should be determined first. Then, for 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 the gate form is selected, the phenomenon that the vacuum hole is generated by the direct gate is more prominent, and should be avoided as much as possible. This is because the pressure in the cavity is higher than the pressure in front of the gate after the pressure is maintained. When the melt is not frozen, the melt backflow occurs, and holes are formed inside the plastic part. In the case that the gate form cannot be changed, the method can be adjusted by extending the dwell time, increasing the supply amount, and reducing the taper of the gate.
 

The cross section of the gate should not be too small. Especially when molding several plastic parts with different shapes at the same time, it must be noted that the size of each gate should be proportional to the weight of the plastic part. Otherwise, the larger plastic parts are prone to air bubbles.
 

In addition, the elongated and narrow flow path should be shortened and widened, the dead angle of the gas in the flow path should be eliminated, and the malfunction of the mold exhaust failure should be eliminated. When designing the mold, it should be avoided that the plastic part has a thick part or a large thickness.
 

3) Raw materials/pvc compound/granule/pellets do not meet the requirements for use
If the moisture or volatile content in the molding material exceeds the standard, the pvc compound/granule/pellets are too small or uneven in size, resulting in too much air being mixed in the feeding process, the shrinkage of the raw material is too large, and the melt index of the melt is too large or too small. Too much recycled material will affect the generation of bubbles and vacuum bubbles in the plastic parts. In this regard, the pre-dried raw materials should be used separately, the fine materials should be screened out, the resin should be replaced, and the amount of recycled materials should be reduced.

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