Causes and solutions of air bubbles in PC injection molding

1、 Cause analysis of bubbles (trapped gas):
1.1. Judging by bubble shape, distribution effect and dispersion degree
A. Bubbles are scattered irregularly on the surface of products, mostly caused by water vapor (insufficient drying of materials).
B. Bubble particles are extremely fine and dense, mainly distributed around the product gate, forming light or fan patterns, which are mostly caused by air.
1.2. Observe mold cavity, insert, gate position, etc
If there are too many corners in the mold cavity, the thickness difference is too large, or there are too many inserts, or the gate position is not right, the melt will flow into the mold cavity, and the air in the mold will be agitated to form eddy current, and the air will be formed at a certain position.
1.3 Observe whether it is the gas generated by the decomposition of PC material
Generally, the main reason for producing decomposition gas is that the melt temperature is too high, and other reasons are as follows:
A. The material itself is of poor quality and easy to decompose;
B. PC materials are sensitive to chemicals and easy to decompose;
C. If the barrel is not cleaned, etc.
2、 Solutions to trapped gas:
2.1. The drying of materials should be sufficient (the drying effect should be confirmed, the moisture content must be below 0.03%, and the dehumidifier dryer should be used if necessary).
2.2. Process adjustment:
A. Slowing down the injection speed is conducive to gas discharge;
B. The ejection [1] position cannot be too large. Too much ejection, too much air entering the front of the nozzle;
C. The material temperature is reduced, and the increased gas due to material gasification is improved;
D. The back pressure is increased to increase the material compactness;
E. Increase the holding pressure.
2.3 Mold adjustment:
Generally, strengthen the mold exhaust and optimize the gate position. In addition, the glue point should be large enough, but sometimes the bigger the glue point is, the better. (Excessive glue injection point will reduce the pressure retaining effect, and countercurrent will occur when cooling and shaping.)
3、 Different types of vacuum bubbles and bubble solutions
3.1 Shrinkage vacuum bubble
When the wall thickness of the product is large, the cooling speed of its outer surface is faster than that of the center part. With the cooling, the resin in the center part shrinks and expands to the surface, resulting in insufficient filling in the center part and vacuum bubbles.
Solutions:
1) Determine reasonable gate and gate size according to wall thickness. Generally, the gate height shall be 50%~60% of the product wall thickness.
2) Until the gate is sealed, a certain amount of supplementary injection material is left.
3) The injection time should be slightly longer than the gate sealing time.
4) Reduce the injection speed and increase the injection pressure,
5) Use materials with high melting viscosity grade.
3.2 Inflatable bubbles
Bubbles are mainly caused by the generation of volatile gases.
Solutions:
1) Pre-dry fully.
2) Reduce the resin temperature to avoid decomposition gas.
3.3 Underpressure bubble
It is mainly due to poor material fluidity, resulting in bubbles caused by under-pressure.
Solutions:
It can be solved by increasing the temperature of resin and mold and the injection pressure.
Attachment: Identification method of bubble and vacuum bubble
Bubbles are different from vacuum bubbles (shrinkage pits). Vacuum bubbles are formed because of the uneven shrinkage caused by the uneven wall thickness of the product after the filling and cooling of injection molding. In fact, there is no air inside the product, that is, shrinkage pits. Bubbles usually appear at the intersection of weld lines or at the end of product filling, because there is too much gas in the mold and it cannot be emptied during filling. Especially when the products with large volume form large bubbles, there will be a bang~bang~explosion after the products are broken.