China Electronic Mould & Plastic Enclosure with 25 year experience

Forming classification of mold

(1) Injection molding

Firstly, the plastic is added into the heating cylinder 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, small wear of molten material on the mold, and can form large quantities of plastic parts with complex shapes, clear surface patterns and marks, and high dimensional accuracy; However, for plastic parts with large wall thickness changes, 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 earliest methods to form 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 plastic, epoxy resin, DAP resin, silicone resin, polyimide, etc. it can also be used for molding unsaturated polyester pellets (DMC), sheet molding compounds (SMC), prefabricated monolithic molding compounds (BMC), etc. Generally, according to the matching structure of the upper and lower mold of the compression film, the compression mold is divided into three types: overflow type, non overflow type and semi overflow type.

(3) Extrusion molding

It is a forming 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 shape into a continuous profile with the required section shape at a lower temperature. The production process of extrusion molding is the preparation of molding materials, extrusion molding, cooling and finalization, 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, the number of screw revolutions, and the traction speed, so as to obtain qualified extruded 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 extruded material 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, lose luster, and appear shark skin, orange skin texture, shape distortion and other phenomena. When the extrusion rate increases further, the surface of the extrudate will be distorted, even split and broken into melt fragments or cylinders. Therefore, the control of extrusion rate is very important.