INJECTION MOULDING PROCESS

Injection molding is a cyclic process of forming plastic into a desired shape by forcing the material under pressure into a cavity. The shaping is achieved by cooling (thermoplastics) or by a chemical reaction (thermosets). It is one of the most common and versatile operations for mass production of complex plastics parts with excellent dimensional tolerance. It requires minimal or no finishing or assembly operations. In addition to thermoplastics and thermosets, the process is being extended to such materials as fibers, ceramics, and powdered metals, with polymers as binders.

Injection moulding of thermoplastics was patented by John and Isaiah Hyatt in 1872 to mold camphor-plasticized cellulose nitrate (celluloid). The first multicavity mold was introduced by John Hyatt in 1878. Modem technology began to develop in the late 1930s and was accelerated by the demands of World War II. A similar surge in the technology of materials and equipment took place in the late sixties and early seventies.

The injection molding process is used to turn plastic stock into finished products. The process involves many steps:

1. Feeding raw material.

2. Plasticize the raw material.

3. Fill the mould.

4. Pack the mould.

5. Hold pressure.

6. Cooling of mould.

7. Opening of mould and Part ejection.

The main factors in the injection moulding are the temperature and pressure history during the process, the orientation of flowing material and the shrinkage of the material. This means that the structure and the properties of injection moulded parts are inhomogeneous and the products have always internal stresses.

Feeding raw material:

This is a primary step of injection moulding process. The moulding machine operator feeds the resin in the form of pellets into the hopper of injection moulding machine either with hands (distributed feeding) or through under pressure pipes (centralized feeding) . Drying and mixing of raw material can be done at the same time. where they fall into an augur-type screw channel, which feeds the pellets forward inside the heated barrel.

Plasticize the raw material:

The resin entered in the barrel through feed throat is plasticized by screw extruder and accumulating it in the forward section of the barrel. The heater bands surrounded on barrel maintains the melt temperature of the resin. This plasticized resin is then injected into the mould after mould closing.

Fill the mould:

After the mould closes, the entire screw moves forward (usually driven by a hydraulic mechanism at the drive end of the machine) and pushes the molten resin out through the end of the barrel and inject melt into the cooled mould. The air inside the mould will be pushed out through small vents at the furthest extremities of the melt flow path. This phase takes time from tenth of seconds to few seconds depending on the plastics grade, wall thickness and the shape of the part. This phase fills about 95% of the mould cavity.

Pack the mould:

After filling the mould with resin reduce the pressure to the pack value and maintain it for a specified time to assure the mould is full. Normally the screw will stay in the forward position until the resin begins to harden in the mould. This creates the pressure in the mould, thus ensuring that the mould fills completely.

Hold pressure:

The screw is held in the forward position for a set period of time, usually with a molten 'cushion' of thermoplastic material in front of the screw tip such that a 'holding' pressure may be maintained on the solidifying material within the mould, thus allowing compensating material to enter the mould as the moulded part solidifies and shrinks. Reduce and maintain the pressure at the hold value while the plastic cools. This holding pressure is only effective as long as the gate(s) remain open.

Cooling:

The cooling process starts immediately upon the injection of the molten polymer, but the cooling time is referred to as the time from the solidification of the gate, when the holding step is finished, to the ejection of the part, when it has reached a temperature low enough to withstand the forces during ejection.

Critical dimensions, surface finish, cycle time, etc., are all affected by mold cooling. Hence mold cooling is a decisive factor if a product will be manufactured with good quality and at a competitive cost.

Opening of mould and Part ejection:

When the cooling phase is complete the mould is opened and the moulding is ejected. This is usually carried out with ejector pins in the tool, which are coupled via an ejector plate to a hydraulic actuator, or by an air operated ejector valve on the face of the mould tool. Ejection is the removal of the cooled molded part from the mold cavity and from any cores or inserts. Once the moulding is clear from the mould tool, the complete moulding cycle can be repeated.