About Injection molding machine

 Injection molding is plastics equivalent of metal die casting. This is the mos twidely method of mass producing to close tolerance three dimensional article over a wide range of sizes and variety of shapes (which can be quite complicated), Using most of the thermoplastic material which are available as granules and powders.

Injection moulding process includes plastics material, mold and injection molding machine. In addition to this the man behind the operation is very important, as he should have good understanding of this subject.

In this process the plastic granules are converted into melt in the plasticizing unit of machine and then this melt is injected into a clamped mold by the machine. The mold consists of two halves namely core half and cavity half. The space between core and cavity of the mold is filled with hot plastic melt.The melt than takes the shape provided by the mold on freezing the melt. The freezing of plastic melt in mold is due to the cold water circulation in the core and cavity of the mold. After freezing an ejection mechanism separates the article from the mold.

The plastic melt also experience different temperatures and pressure during the entire process. There for we must understand and plastics melt behaviour when it undergoes shearing and the condition of melt at different temperature and pressure.

Machine operation sequence 

The cavity is packed as the screw continuously moves forward.

The mould closes and the screw begins moving forward for injection.

The cavity fills as the reciprocating screw moves forward,as a plunger.

The mould opens for part ejection.

The mould closes and the next cycle begins.

Injaection molding Process

Softening and melting of plastics granules by feeding through a hopper into the barrel for which screw design is responsible for consistent quality of melt production. The oil pressure behind the clamp ram moves the moving platen closing the mould. The clamp force should be 20% more than they cavity pressure to prevent opening of the mould during injection. After proper clamping the melt is injection into the closed mold. The melt must flow in balanced manner and fill mold uniformly to avoid molded in stresses which cause failure of parts. After injection the cooling time allows the melt to freeze and retain the shape of the space between the core and cavity. Turbulent water flow through cooling channel is desire for better hear exchange. Ejection of solidified part after the mold is opened sufficiently.Various ejection systems are used for different article.

Advantage: -

 Molding are produced in finished state.

 Parts can be produced at high production rate.

 Functional features such as metal inserts, threads, holes, and undercut can be molded in.

 Minimum scrap can result as runner and gates reground and reused.

 Close dimensional tolerance can be maintained.

 Molding are produced in the finished state.

 Parts can be produced at high production rate.

 Large volume production is possible.

 Relativity low labor cast per unit is obtainable.

 Good decoration is possible.

 Minimum scrap losses.

 Can process a wide range of material.

 For many shapes this process is the most economical way to fabricate.

Disadvantage: -

 The molding process has several disadvantages to other molding processes.

 The following is a short list of some of the disadvantage to injection molding.

 This is not a preferred method of manufacturing for short production run.

 This is mainly due to the cost of tooling and the cost of operation.

 However B.J. molding can handle smaller runs than most shops.

 Design and development of parts that will work well with injection molding often takes a very long time.

 Many parts are just not suited to the process.

 High initial equipment investment.

 Part must be designed for effective molding.

 Accurate cost prediction for molding job is difficult.