Injection Moulding

Injection moulding: uses material in granule form which is cheapest form of raw material, Here the mould cost is very high, Injection moulding is suitable for large production, Injection moulding operation takes more time, In Injection Moulding decoration is not possible before processing, Holes and undercuts be produced by injection moulding, Injection moulding produces not as large part as thermoforming, Injection moulded in parts don’t require any final trimming operation, Pressure requirement is very high, Scrap production is very low, Higher original machine cost.

Very widely used. High automation of manufacturing is standard practice. Thermoplastic or thermoset is heated to plasticate in cylinder at controlled temperature, then forced under pressure through a nozzle into sprue, runners, gates, and cavities of mould. The resin undergoes solidification rapidly. The mould is opened, and the part ejected, Injection Moulding is growing in the making of glass-reinforced parts. High production runs, low labor costs, high reproducibility of complex details, and excellent surface finish are the merits. High initial tool and die costs; not economically practical for small runs.

This technique is a major processing tech. for converting thermoplastics and now for thermosetting materials of powder regrinds form into a part of designed shape and configuration. In this process, batch process is used and the m/c operates in cycles. It is a process in which granules of plastic material i.e. (all thermoplastic and some thermosets) are heated in the barrel to its processing temp. Then it is forced into the mould through nozzle for desired shape by the application of heat and pressure. Then after cooling the product is ejected from the mould. 

The following variables are controlled during the process:

1. Plastic material feeded from hopper 

2. Pressure applied to the plunger

3. Temperature of barrel

4. Temperature of nozzle

5. Temperature of Mould

6. Cooling time

7. Mould Clamping force. 

Types of injection moulding:

1. Conventional plunger type injection moulding: In this type of moulding plastic granules are poured into hopper of m/c and feed into the heating cylinder or barrel. Barrel is heated by band heaters. The granule material now comes into molten state which is then pushed by plunger. The torpedo present in the middle of the barrel heats the granules from out side. By injection of plunger the melt is injected out through the nozzle and enter into the mould. Then after certain cooling time, the component is ejected out. 

2. Piston type pre-plasticizing moulding: In this type of moulding two units are used. First units contain shot weight of material. The unit is known as pre-plasticizing unit and second unit is known as shooting chamber. In this moulding, a piston A moves forward and pushes the material in front of piston B. Injection piston moves little back-ward until it touch a switch which stops piston A for moving further in forward direction. And next process is same as conventional type moulding. 

3. Screw type pre-plasticizing injection moulding: In this type of moulding there are two chambers. In upper chamber screw rotates and also moves forward due to which sharing is done and plasticize material is pushed into shooting chamber or injection chamber. In injection chamber piston pushes the material which then comes out from nozzle and enter into mould. So after giving cooling time mould is opened and product is taken our from it. 

4. Reciprocating screw type injection moulding: In this type of moulding a reciprocating screw is used for plasticizing the plastic granules. Rotation of the screw picks up material from the hopper; it will pass over the full flight of the screw. As the screw rotates in forward direction, material will naturally moves in forward direction. Then molten material is transferred into mould via locating ring. Then after cooling, mould is opened and component is ejected out. Now a days a newer development of injection moulding is used named 

5. Gas assisted injection moulding: This trick of using injection gas is not entirely new. The first mention of the process was recorded 20 years ago, when Ernst Mohrbach registered a patent for the production of heel for ladies shoe using internal gas pressure moulding. The GAIM is more effective and economical for large complex parts.

Automatic injection mould: In an unscrewing type mould both the core and cavities are rotated to automatically unscrew the moulding from the mould. The unscrewing mechanism is fitted behind the moving mould plate in place of the conventional ejector unit which provided required rotational motion. From the impression construction stand point.