The difference between die extrusion and injection moulding

Extrusion

It is a moulding method to make plastics in viscous flow state through high-temperature and certain pressure, through a die with a specific cross-sectional shape, and then at a lower temperature to form a continuous profile of the desired cross-sectional shape. The production process of extrusion moulding is to prepare moulding materials, extrusion moulding, cooling and shaping, traction and cutting, post-processing of extruded products (tempering or heat treatment). During the extrusion moulding process, pay attention to adjusting the temperature, screw rotation speed, traction speed and other process parameters of the heating section of the extruder barrel and the die of the die in order to obtain qualified extrusion profiles. Special attention should be paid to adjust the rate of polymer melt extrusion from the die of the die. Because when the extrusion rate of the molten material is low, the extrudate has a smooth surface and a uniform cross-sectional shape; but when the extrusion rate of the molten material reaches a certain limit, the surface of the extrudate will become rough and lose its luster , Shark skin, orange peel pattern, shape distortion and other phenomena appear. When the extrusion rate is further increased, the surface of the extrudate is distorted, even splitting and breaking into melt fragments or cylinders. Therefore, control of the extrusion rate is crucial.

Injection moulding

Also known as casting. The plastic raw material is added to the preheated charging chamber, and then the pressing column is placed in the charging chamber to lock the mould, and the pressure is applied to the plastic through the pressing column. Gradually solidified into plastic parts. This moulding method is also called transfer moulding. Pressure injection moulding is applicable to plastics that are lower than solid plastics, and in principle can be compression moulded, and can also be formed by pressure injection moulding. But the moulding material is required to have a good flow in the molten state when it is lower than the curing temperature, and a higher curing rate when it is higher than the curing temperature.

Hollow moulding

It is to fix the trend of tubular or shaped blanks made by extrusion or injection that are still in a plasticized state in the forming mould, and immediately pass compressed air to force the blanks to expand and stick to the mould cavity wall surface to be cooled After shaping, the mould is demoulded to obtain a processing method of the hollow product required. Plastics suitable for hollow moulding are high-pressure polyethylene, low-pressure polyethylene, hard polyvinyl chloride, soft polyvinyl chloride, polystyrene, polypropylene, polycarbonate, etc. According to different parison moulding methods, hollow moulding is mainly divided into extrusion blow moulding hollow moulding and injection blow moulding hollow moulding. The advantage of extrusion blow moulding hollow moulding is that the structure of the extruder and the extrusion blow mould is simple, but the disadvantage is that the wall thickness of the parison is inconsistent, which is likely to cause uneven wall thickness of the plastic products. The advantage of injection blow moulding hollow moulding is that the wall thickness of the parison is uniform and there is no flash. Because the injection parison has a bottom surface, the bottom of the hollow product will not be stitched and seamed, which is not only beautiful but also high in degree. The disadvantage is that the moulding equipment and moulds used are expensive, so this moulding method is mostly used for the mass production of small hollow products, and there is no extensive blow moulding method for extrusion blow moulding.

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