Engineering Chemistry S Datta

448 ENGINEERING CHEMISTRY
Plastic feed
Top moulding
part (cold)
Hopper
Nozzle
Hinge
Moulding
powder
Archimedian
screw
Heater
Fluidized
plastic
Bottom
moulding
part (cold)
Extraction pin
Fig. 20.5 Injection moulding of plastics.
(vi) Blow moulding. A hot softened tube of thermoplastic is placed between the two
halves of mould and compressed air is blown whereby the thermoplastic takes the
shape of the mould to form hollow articles like bottles, jerrycans etc. The mould is
cooled and the article taken out.
(vii) Extrusion moulding. This process is used for continuous moulding of thermoplastics.
The powder compounded material is introduced and electrically heated. In plastic
condition, it is pushed by screw conveyer into a die whose shape is acquired by it and
finally cooled.
Plastic feed
Plastic
Metallic
wire
Hopper
Heater
Screw
conveyer
Plastic coated
cable
Fig. 20.6 Moulding of insulated electric cable by vertical extrusion moulding.
(viii) Thermoforming. This process is used to obtain three-dimensional articles from
two-dimensional sheets. The thermoplastic sheet is heated and softened and placed
between two halves of a metal die. The sheet is compressed between the die to the
desired shape. Sometimes, vacuum is also applied so that the sheet sticks to the
shape of the mould.