Bubbles issues are also know as vacuum bubbles, in which gases are very thin. In general, if the bubbles happen at the moment when you open the moulds, the bubble is a matter of gas interference. Vacuum bubbles are formed, because the plastics fillings are insufficient or pressure is low.
Solutions
(1) Increase injection volumes; adjust injection speeds, holding pressure and temperatures of injection materials; increase the moulds’ temperature to improve short shots of the products.
(2) Place the pouring gate at the thick part of the workpiece; improve flow conditions of nozzles, runners and pouring gates; reduce consumption of flow.
(3) Improve exhaust conditions of the moulds.
(4) Avoid great differences of the products’ wall thicknesses.
(5) Improve structures of injection molded products to avoid great differences of the cavities’ parts which can be filled with molten plastics.
(6) Reasonably adjust the molding process to avoid impacts on the plastic moulded products due to gases generated by plastic degradation.
Size changes of plastic products are caused by different shrinkage of the plastics essentially. Any variable operation of material temperatures, molds, pressure and production cycles will lead to size changes of plastic products, especially for products made of materials such as PP, PE, nylon, etc. which have larger crystallinities.
The main reasons are as follows.
1. Machines
(1) Select machines with the large plasticizing capacity if the plasticizing capacity is insufficient.
(2) Check whether voltage fluctuates, components of injection systems wear or hydraulic valves have problems or not, when feeding instability happens.
(3) Check whether motors have problems, screws and charging barrels wear, hydraulic valves are stuck, or voltage is stable or not, when instability of screw speed happens.
(4) Temperature is out of control; proportional valves and general pressure valves are not working properly; back pressure is instable.
2. Moulds
(1) Moulds should have enough strength and rigidity; we should adopt wear resistant materials for the cavities.
(2) When dimensional accuracy is high, we should try not to use a multi-cavity mould.
(3) Ejection systems, pouring systems and cooling systems should be set reasonably so as to ensure stability of production conditions.
3. Plastics
(1) A mixture of the new material and the reworked material should be consistent.
(2) Dry conditions should be consistent; particles should be uniform.
(3) Take into full account the impact of shrinkage on dimensional accuracy.
4. Processing
(1) The higher the temperature is, the smaller the changes of products’ sizes will become.
(2) The mould temperature should be lower when it comes to crystalline plastics.
(3) The moulding cycle should be stable without too much fluctuation.
(4) Shot weight of injection mold products should be stable.