Plastic forming is the process of making plastics of various shapes (powder, granule, solution and dispersions) into automatic moulding machine products or blanks of desired shapes. There are more than thirty methods of forming.

The selection of plastic moulding depends primarily on the type of plastic (thermoplasticity is still thermosetting), the initial shape, and the shape and specifications of the product. Extrusion, injection molding, calendering, blow moulding and thermoforming are the common methods of processing thermoplastics. Molding, transfer moulding and injection moulding are commonly used for processing thermosetting plastics. Plastic forming is the process of making products or blanks of various shapes (powder, granule, solution and dispersion) of plastics into desired shapes. There are more than thirty methods of forming. Laminating, moulding and thermoforming are the processes of plastics forming on a flat surface. The above methods of plastics processing can be used for rubber processing. In addition, there are also liquid monomers or polymers as materials such as casting. Among these methods, extrusion and injection moulding are also the basic moulding methods.

Plastic products are made from the mixture of synthetic resin and various additives by injection, kneading, binding and pouring. Plastic products in the moulding together, but also achieved the ultimate function, so plastic moulding is the key process of production.

Injection Mold

Injection molding, also known as injection molding, is the use of injection machine to quickly inject melted plastic into the mold, and solidify to obtain a variety of plastic products. Almost all thermoplastics (fluoroplastics outside) can use this method, but also can be used for the formation of some thermosetting plastics. Injection forming accounts for about 30% of the production of plastic parts. It has the advantages of forming disordered parts in one time, accurate specifications and yield. However, the cost of equipment and moulds is high, and it is mainly used in the production of large quantities of plastic parts.

There are two types of needle forming machine: plunger type and screw type. The right one is the sketch of screw needle forming. Principle of needle forming: Particulate powder material from hopper into barrel, when plunger is pushed forward, material is pushed into heating zone, then through shunt shuttle, molten plastic is injected into mould cavity through nozzle, and plastic products are obtained after cooling. After the plastic injection parts are removed from the die cavity, proper post-processing is usually needed to eliminate the stress, stability criterion and function of the plastic parts during forming. In addition, there are cutting edges and gate, polishing, surface finishing and so on.

Extrusion molding

Extrusion forming is the process of extruding plasticized plastics into the die successively by means of screw rotating pressure. When passing through the relative shape of the die, the plastic profile corresponding to the shape of the die can be obtained. Extrusion forming accounts for about 30% of plastic products. It is mainly used for various plastic profiles with relatively long cross-sections, such as plastic pipes, plates, rods, sheets, strips, materials and irregular cross-sections. It is characterized by continuous forming, high yield, simple die structure, low cost, compact organization and so on. Except fluoroplastics, almost thermoplastics can be extruded, and some thermosetting plastics can also be extruded.

The right figure is a sketch of screw extrusion. Granular plastics are fed into the screw propulsion chamber from the hopper, then melted in the heating zone by the rotating screw, and compressed. Under the action of the screw force, it is forced to pass through the extrusion die with relative shape to obtain the section with the die.

Binding molding

Restraint forming, also known as compact forming, compression forming, die forming, etc., is the method of taking solid particles or prefabricated sheets into the die, heating and pressing to soften and melt them, and filling the cavity under the action of pressure to obtain plastic parts after solidification. Bond forming is mainly used for thermosetting plastics, such as phenolic, epoxy, silicone, etc. It can also be used for binding thermoplastic PTFE products and polyvinyl chloride (PVC) records. Compared with needle-punching forming, the binding forming equipment and die are simple and can produce large-scale products, but the production cycle is long, the power is low, it is difficult to complete automation, and it is difficult to produce thick-walled products and products with disorderly shape.

The following figure is a sketch of binding forming. Generally, the binding forming process can be divided into feeding, closing, exhaust, solidification and demoulding stages. Plastic parts should be treated after demoulding. The treatment method is the same as that of injection moulding plastic parts.

Blow molding

Blow moulding (attributed to the secondary processing of plastics) is a method of processing hollow plastics billets by means of compressed air to inflate and deform, and to obtain plastic parts after cooling and setting. The main methods are hollow blow moulding and film blow moulding.

The right one is the drawing of extrusion and blow forming for hollow parts. The tube billet with relative temperature is placed in the opposite blow mould. The mould is closed and the compressed air is injected through the blow tube. The billet is blown up and made close to the die wall. The hollow parts are taken out after holding pressure and cooling.

casting molding

The casting process of plastics is similar to that of metals. The active polymer material or monomer material is injected into a specific die to react and solidify under relative conditions, and the processing method of plastic parts consistent with the die cavity is obtained. This forming method has simple equipment, no need or slight pressure, low requirement for die strength and low output cost. It can be applied to thermoplastic and thermosetting plastic parts of various specifications. But the precision of plastic parts is low, the yield is low and the forming cycle is long.

Gas-assisted injection molding

Gas-assisted injection molding (GAIM) is a new method in the field of plastic processing. Gas-assisted forming process is generally acceptable.