Following are the processes involved in the fabrication of articles of plastics:
Each of the above process will now be briefly described.
(1) Blowing: This method of lubrication of articles of plastics is more or less the same as that one employed in the glass industry. A lump of plastic material is taken and by blowing, it is converted into hollow plastic articles such as jars, bottles, toys, etc.
(2) Calendaring: In this process, the plastic material is allowed to pass between the cylindrical rollers. The process is used to prepare plain flat sheets of plastics.
The process consists of closely placed four revolving cylinders. The first three cylinders are heated and the last one is kept cold. The plastic material passes between first three cylinders and it is converted into thin sheets. It is cooled while passing through the surface of cold cylinder. If cloth is to be provided with plastic coating, the cloth is inserted alongwith plastic material between second and third heated rollers. The roller may be provided with artistic designs which will appear on the finished product.
(3) Casting: This process is similar in principle to that of metal casting. The resin is heated and when it is in plastic form, it is poured into the mould. The curing of articles is then done either with or without the application of heat. During curing, the low pressure may be applied, if necessary.
This process is used to prepare plastics of beautiful colours and it is most suitable for cellulose plastics. The optical properties of transparent plastics are much better, if they are cast. Apart from moulding the useful products, the casting is also widely used for potting and encapsulation, particularly in the electrical industry.
(4) Laminating: In this process, the thermo-setting reinsure just applied on sheets of paper, asbestos, cloth, wood, glass, fibre, etc., and they are subjected to heavy pressure by allowing them to pass through rollers to form plastic laminates. The thickness of sheet varies fromo.12mmto 15mm. They possess excellent mechanical and electrical Due to the pleasing finish surface, they are used for ornamental and decorative purposes.
(5) Moulding: This is the most commonly adopted process for the fabrication of plastic articles. The general process consists in placing the raw materials in a mould and then heating it. The moulding can be done by various methods such as compression moulding, extrusion moulding, injection moulding, jet moulding and transfer moulding. The choice of moulding method will depend on the article to be prepared.
These methods are briefly described as follows:
(i) Compression moulding: In this method, the moulds to receive the plastic material are prepared. The moulds are usually heated and then the plastic material is placed in the moulds. The moulds are closed and they are heated to a temperature of 100 C to 200Â°C under a pressure of 10 to 50 N/mm2. The plastic material gets the shape of moulds on account of heat and pressure. In case of thermo-plastic, the moulds are cooled before the articles are taken out. Thus the moulds are to be heated and cooled alternatively in the preparation of thermo-plastic articles. Thus, for production of thermo-plastics, this method. In case of thermo-setting plastics, it is not necessary to cool the moulds as articles of such plastics get the shape due to chemical action.
(ii) Extrusion moulding: In this method, the resin powder is fed through hopper at the inlet end of the revolving screw. At the outlet end, the material is heated and it is extruded or forced through a nozzle. The plastic material as it comes out from nozzle is received in moulds and it is cooled with air jets or water bath. The method of extrusion moulding is adopted for thermo-plastic resins to form continuous lengths of narrow ribbons, sheets, pips, rod, etc. The method is also sufficiently versatile to be adopted for the production of single items such as bottles.
One of the recent developments in extrusion moulding is the coextrusion moulding in which different materials or various combinations of the same material are extruded simultaneously to produce a laminar composite. It is used for structural reasons to strengthen an inexpensive but work plastic with a in coating of stiff but expensive plastic. It can also be adopted to develop decorative effect by coextruding plastics of different colours. The principle of extrusion moulding is simple and hence it forms a large percentage of the plastic processed throughout the world.
(iii) Injection moulding: This is comparatively a modern method of moulding. The plastic material is loaded, heated and then injected into the mould. It is then allowed to cool before being taken out from the mould. Resin powder is allowed to fall through a hopper and it is then pushed by a piston into a hot cylinder. The plastic material is melted and it is then forced to fall in the cool mould under a pressure of about Cylinder-160 N/mm2 through nozzle. The article gets the shape of mould and becomes solid.
The process of injection moulding is very much suitable for thermo-plastic resins. The complete process is automatic and the articles can be prepared within 10 seconds to one minute. It is thus adopted to prepare the plastic articles of small size on a large scale.
The injection moulding is thus a cyclic process and a high dimensional accuracy can be achieved in most materials although account must be taken of factors such as moisture absorption, post-moulding shrinkage, etc.
The injection moulding machines are usually rated according to their shot size and clamping force. The quality of mould to be used also plays an important role for the success of the moulding operation.
(iv) Jet moulding: In this method, the plastic material is moderately heated. It is then allowed to pass through nozzle, which is preheated to a high temperature. This method of moulding may be adopted for thermo- plastics as well as for thermo-setting materials.
(v) Transfer moulding: When the process of injection moulding is applied to the thermo-setting resins, it is known as the transfer moulding. In this process, the moulds are also heated before the plastic material is injected through the nozzle. Thus the thermo-setting resins are heated in this process in tow chambers, namely, cylinder and mould. The pressure on mould is maintained till the chemical action to prepare the plastic article is completed. The plastic materials are removed from the moulds either mechanically or manually.
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