Moulding of Plastics and Power Metallurgy

 PLASTIC:  Introduction:

Plastic or Polymers are classified into two categories:

1. Thermoplastics

2.  Thermosets

Thermoplastics:

Thermoplastic polymers soften when heated and can be reshaped, the new shape being retained on cooling. The process can be repeated many times by alternate heating and cooling with minimal degradation of the polymer structure.

Thermosets:

Thermosetting polymers (or thermosets) cannot be softened and reshaped by heating. They are plastic and moldable at some state of processing, but finally set to a rigid solid and cannot be resoftened. Thermosets are generally stronger and stiffer than thermoplastic.

Plastic  Processing:

Though there are a wide variety of plastic product manufacturing processes in use, the discussion of all of them is beyond the scope of this book. We shall describe three common methods. These are

(i) Injection moulding, 

(ii) Extrusion, and

(iii) Blow moulding.

 Injection  moulding:

This is the method used for large scale production of thermoplastics components. The plastic-powder is filled into a hopper connected to a cylinder-piston mechanism. As the piston withdraws, some plastic powder is inducted into the cylinder and the piston then moves it forward by exerting pressure on it. The cylinder is heated so that the plastic powder gets heated to a temperature between 175–275°C. Under the action of heat and pressure, the plastic softens and is forced through a nozzle into a water cooled die.

After the plastic part has cooled and solidified, it is ejected out of the die and the cycle starts all over again.

Extrusion:

This process is also called extrusion moulding. This method is also suitable for thermoplastics, thermosetting plastics are generally not suitable for extrusion. By extrusion, solid rods, pipes, tubing and different sections can be made. A hopper feeds polymer material into a chamber, which is kept heated. A screw rotates in the centre of this chamber feeding the polymer material forward. Under the action of heat and pressure, it starts flowing. In the front portion of the chamber, a (heated) die is fitted, which provides the only exit for the material. As more and more material is screw fed, a continuous stream of material is squeezed out from the die, its cross-section acquiring the shape of the die. The material coming out of the die is cooled and carried off by a suitable belt conveyer. The scheme of extrusion process is shown in Fig.4.2

Blow  moulding:

This process has been used for making plastic bottles, toys, hollow dolls and similar other items. The blow moulding begins with a heated tubular piece of plastic, which is called PARISON. The parison is positioned in between a two piece mould as shown in Fig. 4.3.

The bottom opening of the parison gets pinch closed and sealed as the two piece mould closes. Air is then injected into the parison under pressure (0.7–10 kg/cm2) thereby forcing the plastic to acquire the shape of the mould. The mould is opened and the part formed inside the mould is removed after it has cooled sufficiently. The above process is similar to making articles of glass by blowing air into a mass of molten glass.