Injection Moulding process for shaping plastic components

Injection Moulding steps and important parts of a mould

An injection mould  is a machine that converts molten plastic to desired shapes of high accuracy which are difficult to form by conventional machining operations. This process is used to produce almost every plastic component used in our daily lives. Some small metal components are also manufactured with this process if high level of accuracy is required for mass production. The surface finish of the components made by injection moulding is also very good and products are almost ready to use i.e no further polishing or machining is required, though tumbling is done to remove the marks of ejector pins and runner gates and lines left by joined parts. 

Components produced by injection moulding posses high mechanical strength due to better molecular interlocking in the material of the component. When the molten material is injected in the cavity with high pressure then molecules of the component being form, come closer to each other and interlock together.

Since the material is injected with high pressure, therefore there is uniform pressure in all parts of the component, making it homogeneous.

Though initial cost of a mould is very high but moulding process results to be very economical because of huge output from a single mould.

This process is widely used in all plastic industries as internal complicated shapes can't be formed so economically and easily with any other operation.

Components of Injection mold

Any mould can't be imagined without core and cavity. These form the inner and outer contour of the component and designing of a mould starts with designing of core and cavity.

Core is also called the male part of the mould and cavity is the female one.

Core always forms the inner contour of the component. In case of very complicated shapes or holes, we use core inserts or side cores. Cores are generally on the movable platen of the mould in order to ease the ejection of the component. Some shrinkage allowance is always subtracted from the actual size of the desired component to get the size of the core, so that component comes to the desired shape. A core may be easily machined by Computerized Numeric Controlled/NC milling or lathe operation.

Cavity is the female part of the mould and it makes the outer shape of the desired component. A cavity is made by CNC/NC milling or lathe or Electric Discharge Machine. EDM is a non-conventional operation. It forms the desired shape of the cavity by sinking the discharged electrode in the cavity plate. This is also called spark erosion. For this a graphite or copper or zinc pattern is made by panto-graph or CNC and is used as electrode. It is of the same shape and size as that of the cavity.

There is always some shrinkage allowance added to the size of the desired component to get the size of cavity. In case of highly complicated shapes, Die sets or multiple dies or split dies are used. In die set system all the cavity parts are assembled in the die plate to form a single cavity of the desired shape. Cavity generally lies on the fixed platen of the mould and material is injected from this side.
Other important parts are ejection pins, runner, gate, hot air vent, guide pillars, guide bushes, cooling channel, movable platen and fixed platen.

Process

The core is always mounted on the movable platen and cavity is housed on the fixed platen. First the mould is brought to closed state and molten material is injected with pressure through the nozzle, which flows through the runner system and enters the cavity through the gate which is a small opening between the runner and cavity. The air trapped inside the cavity leaves through the air vent allowing molten material to fill the space. After the cavity is filled, the fluid in the cooling channel cools the mold, solidifying the molten material. After this, core is withdrawn. Since the material shrinks due to drop in temperature, it sticks with the core and needs to be ejected properly, so ejector pins are fixed on movable platen which doesn't move until the core is moved to a certain distance and component is successfully ejected from it. After this movable platen also moves back allowing the component to fall in the bucket or on the conveyor belt, for further operations.