Plastic Manufacturing: Extrusion, Injection, and Compression

Extrusion Process

Extrusion is primarily used for thermoplastics. It shares the same basic process as injection molding; however, the difference is that in extrusion, the piece is shaped by an extrusion die rather than a mold. In this process, plastic material—usually in the form of powder or granules—is stored in a hopper and fed into a long heating chamber. The material is moved by the action of a screw, and at the end of the chamber, the molten plastic is forced out under pressure through a preformed extrusion die. The transverse configuration of the die determines the shape of the piece.

As the plastic is extruded through the die, it feeds onto a conveyor belt where it is cooled, usually by fans or immersion in water. This procedure produces items such as:

• Pipes
• Rods
• Sheets
• Films
• Wires

For wire and cable coating, the thermoplastic is wrapped around a continuous length of wire or cable, which passes through the die alongside the plastic. After cooling, the wire is wound onto drums.

Injection Molding

The injection molding process involves injecting a polymer in a molten or rubberized state into a closed mold under pressure and cooling through a small gate. Inside the mold, the material solidifies and begins to crystallize (in the case of crystalline polymers). The final part is obtained by opening the mold cavity and removing the molded piece.

Injection molding is a popular technique for manufacturing a wide variety of parts. It is a more environmentally friendly process compared to paper manufacturing, logging, or chrome plating, as it does not directly pollute the environment, emit gases, or produce significant aqueous waste, and it operates with low noise levels.

Compression Molding

Compression molding is the most widely used method for unitary and small-series production. This procedure is suitable for molding thermosetting resins, obtained as a powder or granules. A mold is prepared in the shape of the piece (usually consisting of a male and female part), heated, and coated with a release agent before the precise amount of resin is deposited inside.

After closing the mold, the resin is distributed using heat and pressure (typically 140°C–170°C and 100 bar or more). The heat and pressure cause the plastic to fill the mold entirely. To harden the resin (polymerize or cure), the mold must be cooled before the piece is removed. The polymerization or curing is a permanent chemical change. To obtain the necessary heat, various procedures are used, such as electrical resistors, infrared, or microwaves, while the applied pressure is obtained through mechanical or hydraulic presses. The time spent applying heat and pressure to the closed mold is a function of the part design and resin composition. This procedure is used to produce simple parts and solids of revolution, such as cups, plates, radio keys, and light piping boxes.