Fabrication is giving desired shape to plastic material with the help of mould at desired temp and pressure.

a. INJECTION MOULDING :

This method is applicable mainly to thermoplastic resins. The predetermined quantity of resins powder is fed into a heated cylinder.

From there it is injected into the tightly locked mould at a controlled rate by means of a screw arranged or by a piston plunger.

The mould is kept pressure of 1750kg/cm2 is used for injection. The mould is kept cold to allow the hot plastic to cure and become rigid.

When Material has been allowed curved sufficiently, half of the mould is opened to allow the injection of the finished articles without any deformation. Heating is done by oil or electricity.

ADVANTAGES: It is widely used for moulding of thermoplastics.

This method has high speed production, low moulding cost, very low loss of material and low finishing cost.

DISADVANTAGES : Large number of cavities cannot be filled simultaneously; there is limitation of design of article to be moulded.

b.Compression moulding :

This is most common method for moulding thermosetting materials which can withstand high temp and pressure. This process consists of compressing the molten resinous material into the desired shape by the use of moulds, heat and pressure.

In this predetermined quantity of resin powder is preheated to 1200 C before the cavity of the heated mould is filled with it. After charging the mould, the two parts of mould are successfully brought together under low pressure. It is then compressed by hydraulic pressure of 2000 to 10000psi.

This pressure and heat allow the resin to melt and flow, thereby filling the cavity between the two parts of mould. The material in the mould is kept for the specified time under correct temperature and pressure for proper curve.

The curing is done by either heating or cooling. After curing, the moulded articles are taken out by opening the mould apart.

The electric switch boxes, radios and television cabinets are manufacture by this method.

Advantage of Compression Molding

1. Waste of material in the form of sprue, runners and transfer-culls is avoided, and there is no problem of gate erosion.

2. Internal stress is the molded article is minimized by the shorter and multidirectional flow of the material under pressure in the mold cavity.

3. A maximum number of cavities can be used in a given mold base without regard to demands of spur and runner system.

4. Compression molding is readily adaptable to automatic removal of molded articles.

5. This technique is useful for thin wall parts that must not warp and must retain dimensions.

6. For parts weighing more than 3pounds, compression molding is recommended because transfer or screw injection equipment would be more expensive for larger parts.

7. For high-impact, fluffy materials, compression molding normally is recommended because of the difficulty in feeding the molding compound from a hopper to the press or performer.

In general compression molds usually are less expensive to build than transfer or injection types.

Disadvantages of Compression Molding

1.In the case of very intricately designed articles containing undercuts, side draws, and small holes, the compression method may not be practicable, because of the need for complicated molds and the possibility of distorting or breaking mold pins during the flow of the material under high pressure.

1. For heavy handle, compression molding would be slower than transfer or injection because in transfer the plastic is thoroughly heated and is precompressed almost to its final density prior to entering the mold.

c. Extrusion moulding method

The thermoplastic materials are moulded by this method. They undergo continuous moulding to form articles of uniform cross-section.

These articles include tubes, rodes, strips, insulated electrical cables. In this method, the thermoplastic materials are heated to plastic conditions and then pushed by means of a screw conveyor into a die having the required outer shape of the article to be manufactured.

Here the plastic mass gets cooled due to the atmospheric exposure. A long conveyor carrier away the cooled product continuously.

*d.Transfer moulding method : *

This method uses the principle of injection moulding for thermosetting materials. The moulding powder is placed at a heated chamber.

The chamber is maintained at the minimum temperature at which the moulding powder just begans to become plastic. This plastic material is then injected through an orifice into the mould by a plunger working at a high pressure.

Due to greater friction developed at the orifice, the temperature of material rises, so that the moulding powder becomes almost liquid. Thus, it flows into the mould which is being heated upto the curing temp required for setting.

The mould article is then ejected mechanically.