Molding classification of molds

(1) Injection molding

Firstly, the plastic is added into the heating barrel of the injection machine. The plastic is heated and melted. Driven by the screw or plunger of the injection machine, it enters the mold cavity through the nozzle and mold pouring system, and hardens and shapes into injection molded products due to physical and chemical effects. Injection molding consists of injection, pressure maintaining (cooling) and plastic part demoulding processes, so injection molding has the characteristics of periodicity. Thermoplastic injection molding has the advantages of short molding cycle, high production efficiency and small wear of molten material on the mold. It can form plastic parts with complex shape, clear surface patterns and marks and high dimensional accuracy in large quantities; However, for plastic parts with large changes in wall thickness, it is difficult to avoid forming defects. Anisotropy of plastic parts is also one of the quality problems. All possible measures should be taken to minimize it.

(2) Compression molding

Commonly known as press molding, it is one of the methods of early molding plastic parts. Compression molding is to directly add the plastic into the open mold cavity with a certain temperature, and then close the mold. Under the action of heat and pressure, the plastic melts into a flowing state. Due to physical and chemical effects, the plastic hardens into a plastic part with a certain shape and size that remains unchanged at room temperature. Compression molding is mainly used for molding thermosetting plastics, such as phenolic molding powder, urea formaldehyde and melamine formaldehyde molding powder, glass fiber reinforced phenolic plastics, epoxy resin, DAP resin, silicone resin, polyimide, etc. it can also be used for molding and processing unsaturated polyester mass (DMC), sheet molding compound (SMC), prefabricated integral molding compound (BMC), etc. Generally, according to the matching structure of the upper and lower die of the compression film, the compression die is divided into three types: overflow type, non overflow type and semi overflow type.

(3) Extrusion molding

It is a molding method to make the plastic in viscous flow state pass through the die with specific section shape under high temperature and certain pressure, and then set it into a continuous profile with the required section shape at a lower temperature. The production process of extrusion molding is to prepare molding materials, extrusion molding, cooling and setting, traction and cutting, and post-treatment (quenching and tempering or heat treatment) of extrusion products. In the process of extrusion molding, pay attention to adjusting the process parameters such as the temperature of each heating section of the extruder barrel and the die of the die, the number of screw revolutions and the traction speed, so as to obtain qualified extrusion profiles. Special attention should be paid to adjusting the extrusion rate of polymer melt from the die. Because when the extrusion rate of molten material is low, the extrudate has smooth surface and uniform section shape; However, when the extrusion rate of molten material reaches a certain limit, the surface of extruded material will become rough and lose luster, and shark skin, orange skin grain, shape distortion and other phenomena will appear. When the extrusion rate is further increased, the surface of the extrudate is distorted, even branched and broken into melt fragments or cylinders. Therefore, the control of extrusion rate is very important.

(4) Injection molding

Also known as cast molding. It is to add plastic raw materials into the preheated charging chamber, then put the pressure column into the charging chamber, lock the mold, apply pressure to the plastic through the pressure column, the plastic melts into a flowing state under high temperature and high pressure, and enters the mold cavity through the pouring system to gradually solidify into plastic parts. This forming method is also called transfer molding. Pressure injection molding is applicable to plastics lower than solid plastics. In principle, plastics that can be compressed can also be molded by pressure injection method. However, when the molding material is lower than the curing temperature, the melting state has good fluidity, and when it is higher than the curing temperature, it has a large curing rate.

(5) Hollow molding

It is a processing method to fix the tubular or sheet blank made by extrusion or injection and still in the plasticized state in the molding mold, immediately inject compressed air to force the blank to expand and stick to the wall of the mold cavity, and demould after cooling and shaping, so as to obtain the required hollow products. The plastics suitable for hollow molding are high-pressure polyethylene, low-pressure polyethylene, rigid polyvinyl chloride, soft polyvinyl chloride, polystyrene, polypropylene, polycarbonate, etc. According to different parison molding methods, hollow molding is mainly divided into extrusion blow molding and injection blow molding. The advantage of extrusion blow molding hollow molding is that the structure of extruder and extrusion blow mold is simple, but the disadvantage is that the wall thickness of parison is inconsistent, which is easy to cause uneven wall thickness of plastic products. The advantage of injection blow molding hollow molding is that the wall thickness of the parison is uniform and there is no flash. Because the injection parison has a bottom surface, there will be no splicing and seam at the bottom of the hollow product, which is not only beautiful but also high strength. The disadvantage is that the forming equipment and molds used are expensive, so this forming method is mostly used in the mass production of small hollow products, and there is no extrusion blow molding in use. The hollow forming method is widely used.

(6) Die casting mould

Die casting mold is also called transfer mold. Add plastic raw materials into the preheated feeding chamber, and then apply pressure to the pressure column. The plastic melts under high temperature and high pressure, enters the cavity through the pouring system of the mold, and gradually hardens and forms. This forming method is called die casting forming, and the mold used is called die casting forming mold. This kind of mold is mostly used for the molding of thermosetting plastics.

In addition, there are foam plastic molding, glass fiber reinforced plastic low pressure molding, and so on.