Q. Write short note on following :

            (i) Compression moulding of plastics.

            (ii) Injection moulding of plastics.

                                   OR

       Explain the compression moulding and transfer moulding process for plastics.

                                   OR

       Write down-in detail about the plastic processing methods.

                                   OR

           Explain the following process :

(a)  Injection moulding

(b) Calendaring

Ans. Plastic processing methods : The following methods are commonly employed for processing plastic into various usable articles :

        (1) Compression moulding           (2) Transfer moulding

        (3) Injection moulding                   (4) Extrusion

        (5) Slush moulding                       (6) Calendaring

        (1) Compression moulding : Most of the thermosetting plastics and (few thermoplastic can be moulded through the process. The process consists of placing a correct amount of plastic compound in a heated mould. A punch, called force, compresses the compound from top into the required shape and density. The mould is kept closed for sufficient time to allow the chemical change polymerization) to complete, so that the product is sufficiently hardened. Although loose compound can be used, but for faster production a previously shaped cold-compressed tablet, called perform is used. An example of compression moulding is shown in fig.

        (2) Transfer moulding : It is also known as extrusion moulding or gate moulding. It is actually a modified form of compression moulding. In this, the heat and pressure are applied to the compound separately outside the mould and when the latter becomes fluid it is transferred to the mould under pressure, through a sprue and gate, where it cures finally. This mould is consider, but the operation is easier and enables trouble free production of intricate parts with this sections as the mould is not directly subjected to the compression force. 

        (3) Injection moulding : It is very commonly used for thermoplastic plastics. The process illustrated in figure given.

        The powdered plastic compound is first heated to drive off moisture and then fed into the hopper. When the ram is drawn back, some of the powder drops down into chamber. After closing the mould the ram is moved forward, applying pressure behind the power.

        The compresses the material and forces it forward through the thin space left around the heated torpedo. The material, being in contact with the heated surfaces of the torpedo and the chamber and of the same being under compression, softens and is forced into the cold mould through the nozzle. During heating in the chamber the temperature of the material rises to between 177ºC and 274ºC. After the plastic has been cooled and sufficiently hardened in the mould, the mould is opened and the produced part knocked out. It is a faster process and suits best for large-quantity production.

         (4) Extrusion : it is also known as extrusion moulding. Practically al thermoplastic materials can be extruded into various shapes like tubes, rods, sheets, films, pipes, ropes and other profiles. Thermosetting plastic are, generally, not suitable for extrusion. A sectional view showing the extrusion process is shown in figure. The process consists of feeding the powdered plastic from the hopper into the heated chamber. A rotating crew carries this material forward and forces it out through the heated orifice of the die. The orifice carries the shape corresponding is suitably cooled by water or air blast and carried away by a running belt.

          (5) Slush moulding : It resembles casting in that no pressure is applied. The process consists of preparing a slurry of thermoplastic resin and then pouring the same into a preheated mould. On account of the heat of the mould a uniform layer of resin sets all along the surface of the mould cavity. Excess slurry, if any, is then poured out. Additional heat is provided for curing the resin set in the mould, followed by hardening of the product by chilling and removing the same from the mould. Flexible toys and artificial flowers, etc., are produced by this process.

         (6) Calendaring : This process is vastly used for making plastic films and sheets. In this process a heated doughtily paste of plastic compound is passed through a serious of hot rollers, where it is squeezed into the form of thin sheet of uniform thickness, as in figure. The last roll is water cooled and is called the chilling roll.