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PROCESSING OF RUBBER AND PLASTICS

A) Plastic

Plastics are manufactured through the combination of several different compounds obtained chiefly from cellulose, coal and petroleum. There are numerous techniques or methods for producing plastic objects. Some of these are Blow moulding, Injection moulding, Rotational moulding, Thermoset moulding, Thermoforming, Tube, Film and Sheet extrusion, Casting, Laminating, Calendering, Foamed plastics production.

Generally, moulding methods involve forcing heated and molten plastic material into a mould, which has been made into the shape of the desired object. A cooling process is used while the plastic material is still in the mould to ensure that the desired shape is retained.

Plastic pipes, hoses and tubes, because of their desired flexibility are usually made of thermoplastic material and are produced using the technique called extrusion. Extrusion involves feeding plastic granules into a heating chamber where it is turned into liquid. This liquid is then passed into the extrusion mould where forced cooling causes it to solidify.

Thermoplastic sheets are usually made using the method called calendering. In this case,plastic material is heated until it becomes a paste. This paste is then passed through a series of rollers, which flatten the paste into large sheets of required thickness. In the vast majority of plastic production techniques, air or water is used for the cooling process.

 

B) Rubber

Natural rubber and synthetic rubber are the two classes of rubber currently available. Natural rubber is obtained from latex. Latex is a milky fluid, which is found under the bark of many tropical trees. However, the tree almost exclusively used for commercial production of latex is the Para rubber tree (Hevea brasiliensis).

The technique of processing natural rubber begins by tapping a rubber tree for its latex. Tapping is done by cutting at several points along the erect stem, narrow channels running for about half the circumference of a tree trunk. These channels are made in a spiralling manner. Containers into which latex runs are positioned at the outlets of the channels.

After collection, the latex is sieved to remove solid impurities and then mixed with water. Next, the rubber is made to coagulate (thicken) out of the latex by the addition of dilute acetic or formic acid, and they are deposited on aluminium partitions in horizontal tanks. These partitions allow the rubber to be formed as slabs rather than in a single lump. This makes handling easier. The rubber may be further treated into any grade desired. Some grades of natural rubber are Pale creep, Smoked sheet and Skim rubber.

Synthetic rubber is processed from organic compounds often derived from petroleum. Petroleum is mined from the ground either on land or underneath the sea. There are several synthetic rubbers and they are usually preferred to natural rubbers for certain specialized purposes. For example some are more resistant to oil (e.g. acrylonitrile rubber), much less permeable to air (e.g. butyl rubber) and withstand higher temperatures (e.g. silicone rubber) than natural rubber. Some other synthetic rubbers are butadiene, polyisoprene, polysulfide, ethlyene-propylene, and redox.

 

Choice of Material

There are several kinds of materials available for use in technology. However, a material needs to be carefully selected for a particular purpose in order to reap the maximum benefits and avoid waste. This selection or choice is made after a number of factors have been considered. Principal among these factors are the following:

  1. The purpose which the object to be manufactured is to serve. The relevant questions to which adequate answers should be found in this area include these: Is it a house hold utensil? Will the object come into contact with corrosive fluids? Is it needed for electricity or heat conduction? For how long will the object be required to serve the intended purpose? All these pertinent questions as to the intended purpose of the object should be asked and adequately answered.
  2. Characteristics of different available materials. All useable materials should be considered. Their properties should be fully taken into account. Only those materials whose characteristics fit into the desired usage of the object should be chosen. Compromise should never be allowed in the choice of material because doing that will result in the production of inferior objects. Furthermore, there may be grave consequences by way of failure.
  3. Cost of manufacturing and maintenance. One of the key aims in manufacturing is to produce goods of the highest quality with least overall costs. Two or three different materials which suit the intended purposes often come with widely varying costs. Obviously, in such an instance, the cheapest should be preferred.
  4. Other important considerations. Some of these are aesthetic quality (beauty, attractiveness) fire resistance, speed of construction or production and others.

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