INJECTION MOULDING

Injection Moulding Process

It is very similar to pressure die casting. In this process plastic material in a very highly softened state, is forced to flow at high pressure through a nozzle(according to the required shape) into the mould cavity.
The plastic slowly solidifies in the die and then ejected by opening the die. After this the shape of the component is almost in its final form and by this process products can be produced in an extremely fast rate. The cycle times may be in the range of 10 to 30 seconds. Unlike molten metal in die casting process, plastics melts have a very high viscosity and have to be injected with a large force into the hollow mould cavity. More melted plastic most be packed onto the mould during solidification because of avoiding shrinkages in the mould. Injection moulding pressures generally range from 70 MPa to 200 MPa.

Injection moulding is the most widely used plastic-processing method all over the world. Also it can be used to produce wide variety of products. Very complex parts can be produced also whose sizes may vary from very small (50g) to very large (25Kg) with excellent control of tolerances.

So most of the polymers may be type of injection moulded, including thermoplastics, fiber reinforced thermoplastics, thermosetting plastics and elastomers. Structural injection moulding process is also possible in which a core and skin may be made of different polymers. So reaction injection moulding and liquid injection moulding, which are different in manner of mixing ingredients, involve the injection of liquid polyurethane systems that polymerize within the mould. Moulds with moving and unscrewing mandrels are not much unusual, and allow the moulding of parts with multiple cavities and internal and external threads. Additional advantage is that metallic inserts such as screws, pins and strips can also be placed in the mould cavity to become an integral part of the injection moulded product.

Injection Moulding Machine

An injection moulding machine produces components by injection moulding process. So most commonly used machines are hydraulically powered in-line screw machines but electric machines are also appearing and will be more dominant in the market in near future.

The main units of a typical injection moulding machine are the clamping unit, the plasticizing unit, and the drive unit; they are shown in Figure given below. The clamping unit holds the mould. It is capable of closing, clamping, and opening the mould. Its main components are the fixed and moving plates like the tie bars and the mechanism for opening, closing and clamping.

The work of the injection unit or plasticizing unit is to melt the plastic and inject it into the mould. Now the drive unit provides power to the plasticizing unit and clamping unit. Injection moulding machines are often classified on the basis of the maximum clamp force that the machine can generate. This is the force that pushes the two mold halves together to avoid opening of the mould due to internal pressure of the plastic melt in the mould. The clamping force of typical injection moulding machines is in the range of 200 to 100,000 kN.

Injection Moulding Defects

• Ring shaped miniature bands appear on the molded product surface around gates or narrow sections.
• Many times streaks of a silvery white color appear in the material flow direction.
• The material splits off and flows in two or more directions. When the divergent flow fronts converge again, hair-like lines appear where they meet. This results in reduced strength, as well as burn-mark and bubble defects.
• The presence of holes inside moulded products.
• The surface of the moulded product collapses somewhat-something that is particularly common in thicker products.
• Ordinarily a moulded product is removed from the mold by having the product stick to the moving side of the mold when the mold opens, after which the product is ejected using ejector pins. Sticking occurs when the part fails to separate from the stationary mold or from the moving mold. In the case of the latter, the problem could result in broken ejector pins.

Advantages of Injection Moulding

• Almost every polymer can be processed or made.
• High production rate.
• Low labour cost.
• Injection moulded parts have an almost limitless amount of flexibility, you just need to be able to design around certain restrictions outlined below.

Disadvantages of Injection Moulding

• Cost of the machine is very high.
• Tools are costlier.
• Designing of mould is bit difficult and time consuming.