Injection Molding Process Of Plastic Making

Injection molding is the most widely used method of producing parts of thermoplastic and more recently, thermosetting resins as well. The process resembles the hot chamber die casting of metals : the die, split to allow removal of the solidified product, is kept shut with an appropriate press force and ejectors are provided for removing the molded component. The difference between metals and plastics lies in the supply of the polymer, which is usually fed in a solid form, pellets or powder, through a hopper to a injector screw, a die-end of which is surrounded with heaters that gradually brings the polymer to the required temperature. There the material is softened.

The process starts with feeding plastic pellets into the hopper above the heating cylinder of the machine. The resin falls into and is pushed along the heated tube by reciprocating screw (feeder) until a sufficient volume of melted plastic is available at the injection nozzle end. This may take from 10 sec to 5 or 6 min per shot. The entire screw is then plunged forward to force the plastic into the mold. each shot may produce one or several parts, depending on the die used. The ram is held under the pressure for a few seconds so that molded part can solidify (cool). It then retracts slightly and the mold opens. Knock-out pins eject the molded piece. The sprue and runners are trimmed off , usually in a separate trimming press.

The addition to reciprocating screw machine, there are also conventional single -stage plunger type, and two stage plunger or screw plasticizer type machines .

Injection molding provides the highest production rate of producing plastic parts at a low cost.

The time required per shot will vary with the material and size of the mold, but 300 to 400 per shots hour in a fully automatic equipment are not un common.

Jet Injection Molding :

A modified version of the injection molding is also used and the process is also known as jet molding. By this process the plastic is preheated to about 93*C in the cylinder surrounding to nozzle. It is then further heated as the plunger forces the resin through the nozzle. After the mold has been filled, the nozzle is cooled by running water to prevent polymerization of the remaining material.

Reaction Injection Molding :

It is a newer development of Injection molding, In this process two resins (monomers) are injected together in the mold. Just before they enter in the mold , A chemical reaction takes place between the resins at low heat and a polymer is created at that moment. This process does not require heat before molding.

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