PLASTICS MOLDING PROCESSES
There are various methods of producing components from
the plastics materials which are supplied in the granular, powder and other
forms. Various plastics molding processes are:
1. Compression Molding.
2. Transfer Molding
3. Injection Molding.
4. Blow
Molding.
5. Extrusion Molding
6. Calendaring.
7. Thermoforming.
8. Casting
Two major processes from the above are discussed as
under.
Injection die Molding
In this process, thermoplastic materials
soften when heated and re-harden when cooled. No chemical change takes place during heating and cooling. Fig. illustrates the injection molding process. The process involves
granular molding material is loaded into a hopper from where it is metered out
in a heating cylinder by a feeding device. The exact amount of material is delivered
to the cylinder which is required to fill the mold completely. The injection ram pushes
the material into a heating
cylinder and doing
so pushing bushes
a small amount of heated
material out of other end of cylinder through the nozzle and screw bushing and
into cavities of the closed mold. The metal cooled in rigid state in the mold.
Then mold is opened and piece is ejected
out material heating temperature is usually between 180°- 280°C. Mold is cooled in order to cool the
mold articles. Automatic devices are
commercially available to maintain
mold temperature at required level. Injection molding is generally limited to
forming thermoplastic materials, but equipment is available for converting the machines for molding thermosetting plastics and compounds of rubber.
Typical injection molding |
Extrusion Molding
Generally all thermo plastic materials are highly suitable for extrusion
in to various shapes such as rods,
tubes, sheets, film, pipes and ropes. Thermosetting plastic is not suitable for extrusion molding. In this process the
powder polymer or monomer is received through hopper and is fed in to the
heated chamber by a rotating screw
along a cylindrical chamber. The
rotating screw carries the plastic powder forward and forces it through the
heated orifice of the die. As the
thermoplastic powder reaches towards
the die, it gets heated up and melts. It is then forced through the die
opening of desired shape as shown in the sectional view of the extrusion
molding process through Fig. On leaving the product from the die, it is
cooled by water or compressed air and is finally carried by a conveyor or belt.
The process is continuous and involves
low initial cost.
Schematic extrusion molding |
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