Processing of Plastic
Plastics are polymers obtained from petroleum by-products such as ethane and phenol. These raw materials are generally referred to as resins or monomers.
When a great number of monomers are combined through chemical reactions, polymers are formed. This polymer is referred to as plastics.
The process of combining monomers is known as polymerization. The major component of plastic is resin. Resin is a material that softens, flows, and can be moulded.
The two types of plastics are thermoplastics and thermosets.
Thermosets are plastics which on heating, melt into a soft form which can not be remoulded.
Thermoplastics are plastics which when heated, melt into a soft form that can be remoulded.
There are many ways of making plastics from molten materials. The choice of method depends on the plastic product to be made and its use.
Some methods of production of plastic include:
- Injection molding
- Compression molding
- Extrusion molding
A popular method of making thermoplastics is injection moulding.
1. The polymer resin is poured by the machine operator into the hopper. The polymer resin is usually in the form of pellets or powder (The polymer is already coloured and other times there is a colour concentrate added into the feed hopper)
2. The hopper feeds the resin into the extruder.
3. The resins are heated in the heating chamber between the hopper and the mould.
4. The right amount of the molten materials are then forced by the plunger into the mould, by the screw.
5. The plastic article is formed in the mould.
6. The article stays in the mould until it is completely cool; then it is removed.
This method produces thermoplastics.
Plastic products such as valves, rulers, set-squares, and nylon hair combs, are made by the injection moulding method.
Compression molding is a forming process in which a plastic material is placed directly into a heated metal mold and then is softened by the heat and therefore forced to conform to the shape of the mold, as the mold closes.
Extrusion is a plastic manufacturing process that is used to make hoses, pipes, drinking straws, and rods. The process usually begins with a thermoplastic in the form of pellets or granules. The granules melt into a liquid which is forced through a die forming a long ‘tube-like’ shape. The shape of the die is determined by the shape of the tube. The extrusion is then cooled and forms a solid shape.
Examples of plastics produced by extrusion are rods, pipes, etc.
Calendering is a finishing process used on cloth and fabrics. It is used to smooth, coat, or thin a material. The fabric is passed between rollers, at high temperatures and pressures.
Examples of plastics produced by extrusion are hoses and plastic pipes.