In the injection moulding products injection moulding products have air bubbles is a lot of people in the production and processing of the problem, in general, there are many reasons for the emergence of such a problem, but if you can't deal with it in a timely manner, not only will make the plastic moulding is bad, but also may produce more defects, so when there is an injection moulding products have air bubbles, it must be resolved in a timely manner. The appearance of air bubbles will make the product filling, uneven surface. Vacuum bubbles, this bubble is mainly due to the plastic products shrinkage caused by the wall thickness of the plastic products in the cooling, due to the surface of the cooling rate than the middle of the cooling rate is faster, which will lead to the centre of the plastic products at the lack of glue, the formation of vacuum-like. The solution to this type of vacuum bubbles can be improved by reducing the injection speed, increasing the injection pressure, increasing the replenishment, and using materials with high melt viscosity grade.

When the bubble defect occurs frequently, the bubble can be divided into two kinds of bubbles and vacuum bubbles. Generally speaking, bubbles occurring on transparent products can be directly observed, while bubbles occurring on opaque products sometimes cannot be seen from the outside, and can only be possibly found by cutting them open or using other means. Air bubbles are generally generated due to the injection speed is too fast, the plastic flow can not force all the air in the mould cavity from the exhaust slot, air mixed into the plastic to form bubbles. Vacuum bubble is due to the wall thickness of the product at the centre due to slow cooling, surface cooling rapidly and contraction tends to pull the material over, the uneven volume contraction caused by the thickness of the part of the molding contraction has produced a cavity; raw materials have cold to hot when the moisture and air contained in the speed of becoming a bubble.

1. Improper control of moulding conditions  

Many process parameters have a direct impact on the generation of bubbles and vacuum bubbles. Setting the injection pressure is too low, the injection speed is too fast, the injection time and cycle is too short, the amount of material is too much or too little, insufficient pressure, uneven cooling or insufficient cooling, as well as the material temperature and mould temperature control is not appropriate, will cause bubbles in the plastic parts. Especially when high-speed injection, the gas inside the mould can not be discharged in time, resulting in too much residual gas in the melt, in this regard, the injection speed should be reduced appropriately. If the speed is reduced too much, the injection pressure is small, it is difficult to exhaust the gas within the melt, it is easy to produce bubbles and depression and under-injection, therefore, the adjustment of the injection speed and pressure should be particularly careful. In addition, air bubbles and vacuum bubbles can be avoided by adjusting the injection and holding pressure time, improving the cooling conditions, and controlling the amount of dosing. If the cooling conditions of the moulded parts are poor, the moulded parts can be put into hot water immediately after demoulding to make the cooling rate inside and outside the mould consistent. When controlling the mould 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 contraction, forming bubbles or shrinkage holes; if the temperature is too low, it will result in insufficient compaction of the material, and the plastic parts are easy to produce voids inside, forming bubbles. In general, the melt temperature should be controlled slightly lower, mould temperature control slightly higher. Under such process conditions, it is not easy to produce a large amount of gas, but also not easy to produce shrinkage holes. When controlling the barrel temperature, the temperature of the supply section should not be too high, otherwise it will produce reflux return material caused by gas bubbles.

2. Mould defects  

If the gate position of the mould is incorrect or the gate cross-section is too small, the main flow channel and manifold is long and narrow, there is a dead space for gas storage in the flow channel or the mould is poorly ventilated, it will cause bubbles or vacuum. Therefore, it should first determine whether the mould defects produce air bubbles and the main cause of vacuum bubbles. Then, adjust the structural parameters of the mould for the specific situation, especially the location of the gate should be set at the thick wall of the moulded part. When choosing the form of gate, the phenomenon of vacuum hole generated by direct gate is more prominent and should be avoided as much as possible. This is due to the fact that after the end of pressure holding, the pressure in the cavity is higher than that in front of the gate, and if the molten material at the direct gate is not yet frozen at this time, the phenomenon of backflow of the molten material will occur, which will cause the formation of a hole inside the moulded part. In the case that the form of the gate cannot be changed, it can be adjusted by prolonging the holding pressure time, increasing the amount of material supply and reducing the taper of the gate.   The cross-section of the gate should not be too small, especially when moulding several pieces 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 moulded piece, otherwise, the larger pieces are prone to produce air bubbles.   In addition, it should shorten and widen the slender and narrow runner, eliminate the dead space of gas storage in the runner, and eliminate the fault of poor exhaust of the mould. When designing the mould, should try to avoid the shape of the plastic part of the body has a special thick part or too large a difference in thickness.