Extruder is one of the most common processes used to create plastic components for further processing. Extruder machine is also similar to that of injection moulding machine.
The process consists of feeding the powdered plastic from the hopper into the heated chamber. A rotating screw carries the material forward and forces it out through the heated orifice of the die. The required shape is obtained through the die. The mould is suitably cooled by water or air blast and carried away by a running belt. The raw material is in the form of pellets, granules and powder.
Plastic material with color pigments and resin are melted inside a chamber provided with rotating screws. The plastic melt is passed through a shaped orifice called die, and then it is cooled to get the desired shape. Figure shows a simple extrusion process.
The thermosetting plastics are not suitable for extrusion. For the extrusion of plastics, a single screw machine has completely replaced by a ram type machine. The screw has three distinct sections.
- Feed section connects the material from hopper into central region of the barrel.
- Melt section in which plastic causes melting to begin.
- Pumping section in which additional shearing and melting occurs.
In this process, the material is fed from the hopper and the screw is rotated in a barrel. The screw imparts both axial and rotary motions. The restricting effect of the die will build up a pressure until it is in a plastic state and can be extruded.
Types of extrusion die
- Die for solid section
- Die for hollow section
Die for solid section is simple in construction. It is a machined opening through which the molten polymer is extruded whereas the die for hollow sections is a complex one. Die for hollow section is provided with a mandrel inside to produce the desired shape in the inner side after extrusion. Figure shows few examples of dies used in extrusion.
Applications of Extruder
- It is used to make tubes, sheets, films, pipes, ropes and other profiles.
- Complete shapes with constant cross sections can be extruded with relatively inexpensive tooling.