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@Farjand • 06 Mar, 2011
Friends can you imagine life without plastics? Well possibly no. Humans have invented plastics recently may be about 1800 A.D. Well so let us discuss the process of plastic molding. Plastic molding in simple words is a process of molding plastics in to various forms. The principle of plastic molding is to place molten polymer in a mold and compressing it and later to cool it. There are some common processes to achieve that. The most common is Injection molding.

[​IMG]

CONSTRUCTION
In Plastic injection molding, the apparatus consists of:

1. hopper, 2. barrel, 3. reciprocating screw, 4. nozzle, 5. mold platen, 6.stationary platen, 7. nozzle and 8. sprue.

The system uses a gun barrel system so that when plastic pellets(Polymer resin) are fed through hopper.The system is often driven with a hydraulic motor externally to rotate the screw. The screw is so constructed that it has got helical grooves on its surfaces, so that whenever it is rotated it will give a forward motion to the plastic pallets.

MATERIALS USED FOR THE PROCESS:
Plastic injection molding uses thermoplastic materials (plastics that soften and fluidize when heated.) The plastics used in the injection molding process are liquefied at temperatures between 220° C and 270° C.

PROCESS
The process starts with clamping of the arrangement.The whole assembly viz. the mold and barrel is set in a proper position.The pellets are fed through hopper and are pushed by the screw forward in the barrel. The barrel is normally heated by electric coils in such a manner that the temperature come closer to the melting temperature of plastic as we approach towards the nozzle.

The reciprocating screw is thus divided into three zones basically
1. The feeding zone
2. The transition zone
3. The metering zone
The outside diameter of the screw remains constant throughout these zones; but the depth of the screws' flights decreases from the feeding zone to the metering zone. The nozzle acts as a sealing between the mold and sprue bushings. The mold’s sprue bushing has a concave radius. The nozzle fits into this radius with the aid of a locating ring. When the plastic is then collected in the die the pressure thus applied in the process is in the range of 10000 to 30000 psi due to the screw. It is then allowed to cool and become solid.

Advantages:
The advantages of process outnumber its disadvantages.
1. The process is quite rugged and is used to produce 30% of plastic products in the industry.
2. The process is a high efficiency process with practically negligible amount of loss.
3. The scrap is practically nil as the products so manufactured meet high tolerances.
4. High productivity as the process can be made fully automatic.

Disadvantages:
1. The disadvantages lie mainly for small manufacturers as the equipments needed are quite expensive.
2. The cooling process consumes about 85% of total process time. However this cannot be avoided to achieve quality.

Injection molding products:
The products can consists from jars, sockets,cans etc. Literally speaking any product whose mold can be made available can be easily made by the process.
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