An injection molding machine is a machine for converting the plastic material into a liquid state, injecting into the cavity of the mold, opening the mold, and taking out the solidified molded article. A large number of manufacturers in the major industrial countries of the world today are manufacturing and selling various types of injection molding machines, all of which are not being manufactured to any standardized specifications, but different types are being manufactured according to the purpose of use, size of the machine, accuracy, etc. The common types of injection molding machines that are in widespread use in the market have the following structures.

1. Material used: Thermoplastic resin
2. Plasticizing - injection structure: Screw in-line type
3. Power source: Hydraulic type, electric motor driven type, hybrid type (hydraulic and electric motor drives are used together)

In general, an injection molding machine is constituted from the following functional units

1. Plasticizing Unit

The raw materials of the common thermoplastic resin are sold in the market in the form of pellets, and to carry out injection molding, it is necessary to heat and melt the pellets and to convert them into a fluid state. The unit used for this purpose is the plasticizing unit. Plasticizing is melting the material so that it is soft and can be deformed easily in the plastic state. The following structures are used most commonly for a plasticizing unit.

1. Plunger type
2. Inline screw type
3. Pre-plunger type
4. Screw pre-plunger type

In all methods, pellets are loaded into a cylinder and plasticized by heating using an electric heater surrounding the cylinder.

The method of supplying the pellets to the heated part can be of the plunger type or of the screw type. The plunger type has a structure in which a plunger moves forward and supplies the pellets to the heated part. In contrast with this, the screw type has a structure in which a screw with a helical groove is rotated thereby making the pellets move forward through the helical groove and are supplied to the heated part. These two methods have their own advantages and disadvantages, and the selection is made based on factors such as the material, molded article, cost of investment, etc.

The parameter that needs to be studied in a plasticizing unit is the plasticizing capacity. The plasticizing capacity is the volume of injectable molten plastic resin that can be prepared in unit time, and this affects the volume of the molded article that can be prepared. Therefore, in the case of a molded item that requires a large volume of plastic, it is necessary to prepare a plasticizing unit having a large plasticizing capacity.