• 2023-06-29

Classification of Mold Forming

(1) Injection molding

Plastic is first added to the heating cylinder of the injection machine, which is heated and melted. Under the push of the injection machine screw or plunger, 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 a cycle consisting of injection, pressure holding (cooling), and plastic demolding processes, thus injection molding has the characteristic of periodicity. Thermoplastic injection molding has a short molding cycle, high production efficiency, and low wear of the mold caused by molten materials. It can form complex shapes, clear surface patterns and markings, and high dimensional accuracy plastic parts in large quantities; However, for plastic parts with significant changes in wall thickness, it is difficult to avoid forming defects. Anisotropy of plastic parts is also one of the quality issues, and all possible measures should be taken to minimize it.

(2) Compression molding

Commonly known as compression molding, it is one of the earliest methods for forming plastic parts. Compression molding is the process of directly adding plastic to an open mold cavity with a certain temperature, then closing the mold, and under the action of heat and pressure, the plastic melts into a flowing state. Due to physical and chemical processes, plastic hardens into plastic parts with a certain shape and size that remain unchanged at room temperature. Compression molding is mainly used to form thermosetting plastics, such as phenolic molding powder, urea formaldehyde and melamine formaldehyde molding powder, glass fiber reinforced phenolic plastic, epoxy resin, DAP resin, silicone resin, polyimide, etc. It can also form and process unsaturated polyester (DMC), sheet molding compound (SMC), prefabricated monolithic molding compound (BMC), etc. In general, compression molds are often divided into three types based on the matching structure of the upper and lower molds of the compression film: overflow type, non overflow type, and semi overflow type.

(3) Extrusion moulding

It is a molding method that allows plastic in a viscous flow state to pass through a die with a specific cross-sectional shape under high temperature and certain pressure, and then shape it into a continuous profile with the desired cross-sectional shape at a lower temperature. The production process of Extrusion moulding is the preparation of molding materials, extrusion molding, cooling and shaping, traction and cutting, and post-treatment (quenching and tempering or heat treatment) of extruded products. In the process of Extrusion moulding, pay attention to adjusting the temperature, screw revolution, traction speed and other process parameters of each heating section of the extruder barrel and the die of the head, 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 the molten material is low, the extruded material has a smooth surface and uniform cross-sectional shape; However, when the extrusion rate of the molten material reaches a certain limit, the surface of the extruded material will become rough, lose its luster, and exhibit phenomena such as shark skin, orange peel patterns, and shape distortion. As the extrusion rate further increases, the surface of the extruded material undergoes distortion, even branching and breaking into melt fragments or cylinders. Therefore, the control of extrusion rate is crucial.

(4) Injection molding

Also known as casting and pressing forming. It is the process of adding plastic raw materials to the preheated feeding chamber, and then placing pressure columns into the feeding chamber to lock the mold. By applying pressure to the plastic through the pressure columns, the plastic melts into a flowing state at high temperature and pressure, and gradually solidifies into the mold cavity through the pouring system. This forming method, also known as transfer molding. Pressure injection molding is suitable for plastics that are lower than solid plastics and can be compressed into shape in principle. It can also be formed by pressure injection molding. But it is required that the formed material has good fluidity in the molten state when it is below the curing temperature, and has a large curing rate when it is above the curing temperature.

(5) Hollow forming

It is a processing method of fixing a tubular or sheet-like billet made by extrusion or injection, which is still in a plasticized state, into the molding mold, immediately introducing compressed air to force the billet to expand and adhere to the wall of the mold cavity. After cooling and shaping, it is demolded to obtain the required hollow product. The plastics suitable for hollow forming include high-pressure polyethylene, low-pressure polyethylene, hard polyvinyl chloride, soft polyvinyl chloride, polystyrene, polypropylene, polycarbonate, etc. According to the different methods of blank forming, hollow forming is mainly divided into two types: extrusion blow molding hollow forming and injection blow molding hollow forming. The advantage of extrusion blow molding hollow molding is that the structure of the extruder and extrusion blow molding mold is simple, but the disadvantage is that the wall thickness of the blank is inconsistent, which can easily cause uneven wall thickness of plastic products. The advantage of injection blow molding hollow molding is that the wall thickness of the blank is uniform and there are no flash edges. Due to the bottom surface of the injection blank, the bottom of the hollow product will not produce any seams, which is not only beautiful but also has high strength. The disadvantage is that the molding equipment and molds used are expensive, so this molding method is mostly used for large-scale production of small hollow products, and is not widely used as the extrusion blow molding hollow molding method.

(6) Die casting mold

Die casting molds, also known as transfer molding molds. Add plastic raw materials to the preheating charging chamber, and then apply pressure to the pressure column. The plastic melts at high temperature and pressure, and enters the mold cavity through the pouring system of the mold, gradually hardening and forming. This forming method is called die casting molding, and the mold used is called die casting molding mold. This type of mold is often used for the molding of thermosetting plastics.

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