Engineering Chemistry S Datta

POLYMERS 447
Polymer solution
Wind-up
roll
Control
valve
Polymer solution
spreads as a film
Solvent evaporates
Continuous
polymer film
Endless metallic belt
Fig. 20.3 A schematic representation of the film casting machine.
(iv) Compression moulding. This method is applicable for both thermosetting and thermoplastic
materials. The mould consists of two halves. The compounding material in
definite proportion fills the gap between the two pieces of mould, which are moved
relative to each other. Heat and pressure applied. The cavities get filled with fluid
plastic, two halves are closed slowly. Finally curing is done by heating (for thermosetting)
and cooling (for thermoplastic). Furniture handles, telephone parts etc., are
made by this process.
Pressure
Guide pins
Top moulding part
(male die)
Bottom moulding part
(female die)
Moulding plastic
ingredients in cavity
Extraction
pin
Fig. 20.4 Compressing moulding of plastics.
(v) Injection moulding. This process is applicable for thermoplastics only. Plastic powder
is fed into a hot cylinder and then injected by a piston plunger into a tightlylocked
mould with the help of a screw arrangement. The mould is cooled for curing
and finally the half of the mould is opened for the ejection of the article. The whole
process is automatic. Buckets, bowls, furniture parts are made by this process.