Q.3 Explain important plastic processing methods.
Ans. Following are various plastic processing methods :
(a) Compression Moulding: -
This moulding process is used for thermosetting plastics. A predetermined quantity of plastic raw material is forced into a heated metallic mold and pressure is applied. As a result of it, plastic becomes soft and fills the mould cavity to give the final shape of the product.
(b) Injection Moulding: -
Plastic raw material in the form of plastic granules is fed through feed hopper. Required pressure is applied by means of plunger which injects the raw material through a nozzle into the mould cavity as shown in the figure. When the plunger moves in backward direction, required quantity of plastic granules drop down from the feed hopper. The plastic gets softened by heating, so that it may flow through the nozzle. The die is water cooled to accelerate the process of solidification. After a sufficient time of hardening, the plunger is made to come back and mould is opened and the manufactured part is ejected out.
(c) Transfer Moulding: -
It is also called gate or extrusion moulding. It is the process of forming articles in a closed mould, where the fluid plastic material is transported to the mould cavity under pressure from the outside of the mould. The preheated material is placed in a heated transfer pot. When the material is softened enough, the plunger forces the almost fluid plastic through an orifice (sprue) into the closed mould where the final cure takes place.
(d) Blow Moulding: -
It is a process of placing a softened thermoplastic close end tube and applying air pressure to inflate the tube. In this process, the close-end tube is continuously extruded, properly cut and sealed at one end, gripped in a two-piece mould and then air in measured quantity is injected to force the plastic against the wall of the mould which is water cooled. After allowed for sufficient cooling, the mould is opened and the product is ejected out of the mould.
Applications: -
Products manufactured by moulding include bottles, automobile heater tubes, floats, etc.
