This is also called as moulding of plastics into articles. To give shapes to plastics, several methods of fabrication are used. They are 1. Compression moulding 2. Injection moulding 3. Transfer moulding 4. Extrusion moulding

1. FABRICATION OF PLASTICS
BY
CH. SAI SRI RAMA CHANDRA MURTHY

2. Fabrication or Moulding of plastics
This is also called as moulding of plastics into articles. To give
shapes to plastics, several methods of fabrication are used. They are
1. Compression moulding
2. Injection moulding
3. Transfer moulding
4. Extrusion moulding

3. Compression moulding:
This method is applied to both thermoplastic and thermosetting
resins. A desired quantity of compounded plastics resin is filled in the
cavity present in the bottom mould, the top mould and bott0m mould are
capable of being moved relative to each other. When heat and pressure are
applied according to specifications, the cavities get filled with fluidized
plastic. The two moulds are closed tightly and curing (the time given to
the polymer for setting in the mould) is done either by heating in case of
thermoplastic resins or by cooling in case of thermoplastic resins.

4. After curing the moulded article is taken out by opening the mould.
This method is commonly used for manufacture of cylinders, gears,
handles, etc.
Compression moulding

5. Injection moulding:
Injection moulding is generally carried out for the fabrication of
thermoplastic resins. The compounded plastic material is added from a
hopper from where they move through a tube having proper dimensions for
accurate quantities to be charged to the lower cylinder.
The hopper moves to and fro in the cylinder to send the plastic
material into the spreader in a heating chamber where the materials are
converted to viscous liquids pushed through the nozzle (injected) into the
cold mould maintained at room temperature.

6. Injection moulding

7. The molten thermoplastic material sets is the mould which is sometimes
water cooled and proper pressure is maintained. The finished article is
taken out by opening the mould.
Advantages of injection mould:
1. This method is characterised by high speed production at low mould
cost.
2. Loss of materials will be le0ss.
The disadvantages of this is a large number of cavities cannot be
filled simultaneously, so there is a limitation to the designed articles.

8. Transfer moulding:
Transfer moulding is a method which uses the principle of injection
moulding for thermosetting plastics. The powdered compounded plastic
materials is in a chamber applied with minimum temperature and high
pressure till it begins to become soft and semisolid. Then it is injected into
a mould by a plunger working at high pressure, due to high friction
developed at the nozzle, the soft plastics materials is ejected from the
orifice into the mould which is heated upto the curing temperature
required for settings the fabricated article is then ejected out mechanically.

9. Transfer moulding

10. Advantages:
1. Since the plasticized mix flows very slowly into the mould, very
delicate articles with intricate shapes can be produced.
2. The articles are free from flow marks and thick pieces of the article can
be cured very slowly, completely and uniformly.
3. Blistering is almost eliminated.

11. Extrusion moulding:
Extrusion moulding is used for moulding of the thermoplastic
materials into articles of uniform cross section like tubes, rods, wires,
cables etc. The thermoplastic ingredients are heated to plastic state
(semisolid state) and then pushed by means of a screw conveyer into a die,
having the required outer shape of the article to be fabricated. The
extruded article gets cooled due to atmospheric exposure or artificially by
air jets, or by water sprayer in a long conveyer which away the cooled
product.

12. Extrusion moulding

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