Various methods to manufacture the plastic components

MANUFACTURING OF PLASTIC COMPONENTS

In this article students may able to understand about the various manufacturing methods of plastic components

TABLE OF CONTENTS

    1. Single screw extruders

    2. Twin screw extruders

    3. Gas injection moulding process

    4. Injection moulding process

    5. Autoclave process

    6. Structural foam injection moulding process

There are various manufacturing methods are available to manufacture the plastic components. Here these methods are explained with a neat sketch in exam point of view.

1. SINGLE SCREW EXTRUDERS

• Extrusions are produced by forcing a material through the opening of a die to produce the required shape.
• In this, the powdered raw material, usually thermoplastic is fed into a hopper and carried along by a screw conveyor through the heating chamber. The raw material becomes viscous fluid in the heating chamber.
• Then, the fluid material is forced through the heated die.
• The material leaving the die rests on a moving conveyor and is cooled by water, air spray to obtain the shape of the die opening.
• Heavy plastics materials are cut to required shape and the flexible plastics are coiled.

Advantages of single screw extruders

• Less cost
• High production rate

Disadvantages of single screw extruders

• Long uniform sections are only can be formed
• Suitable for mass production only
• Less dimensional accuracy

Applications of single screw extruders

• This process is suitable for making the parts like tubes, rods, sheets, films, pipes, ropes and profiles.

2. TWIN SCREW EXTRUDERS 

• The method of processing of plastics using twin screw extruders is similar as that of single screw extruders. 
• But they have two screws rotating in a heated barrel. 
• The output of a twin screw extruder can be typically three times that of a single screw extruder of same diameter and speed.

The following screw configurations are available.

• Intermeshing counter-rotating
• Intermeshing co rotating
• Non - Intermeshing counter-rotating
• Non- Intermeshing co rotating
• In a counter rotating twin screw extruder, the material is effectively squeezed between counter rotating rolls.
• In a co rotating system, the material is transferred from one screw on the other in a figure of eight pattern. 

Advantages of twin screw extruders

• Output rate is high.
• Mixing efficiency is more.
• High heat is generated for melting the plastic material fastly.

3.GAS INJECTION MOULDING PROCESS

• It is a modern technology, uses inert gas to act as the core in an injection moulded plastic product. 
• Due to this, greater properties like stiffness, reduced stresses and distortion may have achieved.
• First the molten polymer is flowed into mould cavity through feed system. 
• Just before full flow of molten plastic is complete, special nozzle is injecting the gas into mould. 
• By controlling the amount of gas injected into the hollow core, the pressure on the cooling polymer is controlled and maintained until the moulding is packed, 
• The final stage is the withdrawal of gas nozzle.
• It allows the gas geld in the hollow core vent. After this, the mould is opened.

Advantages of gas injection moulding process

• Lower cycle time.
• No surface imperfections
• Clamping forces are much lower

4. INJECTION MOULDING PROCESS

• This process is similar to the hot chamber casting of metals. The following diagram shows a screw injection moulding process.
• In this, the plastic material is fed through a hopper in a measured amount, to the heating zone chamber where the raw material becomes fluid.
• The screw till a suffiecnt volume of melted plastic is available at the injection nozzle end.’
• The screw is fully plunged forward to force the plastic into the mould.’for every plunging action, one or several parts are produced depending on the die used.
• The ram is held under pressure for a certain period till the parts can solidify.
• Then the mould is opened and the knockout pins eject the moulded piece.

Advantages of injection moulding process

• High production rate.
• Low cost.

5. AUTOCLAVE PROCESS

• Autocalve process offers one of the highest manufacturing standard for composites.
• Fabrics and fibers are pre-impregnated by the prepreg manufacturer, under heat and pressure or with solvent, with a pre catalyst is largely latent at ambient temperatures giving the materials several weeks, or sometimes months.
• The prolonged storage life of the material are stored frozen.
• The resin is usually a near-solid at ambient temperature, and so the prepregs have a light stickly feel to them. Such as that of adhesive tape.
• Unidirectional materials take fibre direct from a creel and are held together by the resin alone.
• The prepregs are laid up by hand or machine onto a mould surface, vaccum bagged.
• And then heated to typically 120-1800C.
• This allows the resin to initially reflow and eventually to cure.
•  The autoclave provides additional pressure for the moulding which can apply upto 5 atmospheres to the laminate.

Advantages of autoclave process

• Widely used.
• Low tooling cost
• Simple principle.

Disadvantages of autoclave process

• Resin need to be low in viscosity.
• It requires skilled labour.

6. STRUCTURAL FOAM INJECTION MOULDING PROCESS

• These plastics has cellular core with a high density. 
• Foam effect is achieved by the dispersion of inert gas throughout molten resin directly 

• Before moulding introduction of the gas is usually carried out either by pre blending the resin with a chemical blowing agent which releases gas when heated or by direct injection of the gas. 
• Usually nitrogen is used for this purpose. W
• Then the compressed gas or resin mixture is rapidly injected into the mould cavity. 
• The gas expands explosively and forces the material into all parts of the mould.
• Foamed plastics may be produced with good properties by using screw injection moulding machines. 
• Specialized foam injection moulding machines are available to produce parts with 50kg.