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•The production of plastics can be roughly divided into four categories:
1. Acquiring the raw material or monomer.
2. Synthesizing the basic polymer.
3. Compounding the polymer into a material that can be used for fabrication.
4. Molding or shaping the plastic into its final form.
Historically, resins derived from vegetable matter were used to produce most
plastics. This included such materials as cellulose (from cotton), furfural (from
oat hulls), oils (from seeds) and various starch derivatives. Today, most
plastics are produced from petrochemicals which are widely available and
tend to be cheaper than other raw materials. However, the global supply of
oil is exhaustible, so researchers are investigating other sources of raw
materials, such as coal gasification.
Synthesis of the Polymer
The first step in plastic manufacturing is polymerization. The two basic
methods by which polymerization can occur are addition and condensation
reactions. These can occur in the gaseous, liquid and occasionally solid phase.
Sometimes the polymer synthesis can take place at the interface of two
immiscible liquids in which the monomers are dissolved.
Chemical additives can be used in the production of plastics to achieve
certain characteristics. These additives include:
° Anti-oxidants to protect the polymer from degradation by ozone or oxygen
° Ultra-violet stabilizers to protect against weathering
° Plasticizers to increase the polymer’s flexibility
° lubricants to reduce friction problems
° pigments to give the plastic colour
° flame retardant
Plastics are often manufactured as composites. This is achieved by adding
reinforcements such as glass or carbon fibers to the plastics, increasing their
strength and stability. Plastic foam is a different type of composite which
combines plastic and gas. An example of this can be seen in styro foam cups
which are made of foamed polystyrene.
Shaping and Finishing
Compression molding is one of the oldest methods used for converting polymers
into useful materials. It uses pressure to force the plastic into a certain shape.
One half of a two-piece mold is filled with plastic and then the two halves of the
mold are brought together and the plastic is melted under high pressure
Methods used for shaping plastics are
Plastics: Any, especially high density polythene; polystyrene and polyvinyl
chloride; all synthetic fibres
Tooling cost: Moderate
Production volume: High but restricted to minimum order lengths
Uses : Anything with a constant cross section: fibres; tubing; pipes; sheets; films;
cable sheathing; profiles e.g. curtain rails or window frames
Tolerances: Standard industry tolerances usually provide adequate precision for
•Angular tolerances will be ± 1 to 2 degrees
•Flatness tolerance across a profile is ± .004 per inch of width
•Extrusions will be straight within 0.0125 inch per foot of length
•The approximate twist tolerance will be .5 degrees per foot
Material is introduced into the injection moulding machine via a Hopper. Theinjection
moulding machine consists of aheated barrel equipped with a reciprocatingscrew (driven by
a hydraulic or electric motor), which feeds the molten polymer into a temperature
controlled split mould via a channel system of gates and runners.
• The screw melts (plasticizes) (plasticizes) the polymer, polymer,and also acts as a ram
during the injection phase. The screw action also provides additional heating by virtue of
the shearing action on the polymer.
• The polymer is injected into a mould tool that defines the shape of the molded part
Plastics: Commonly all thermoplastics
Marks: The plastic enters the mould through what is known as a gate which
leaves a 'sprue' which is then broken off but leaves a slightly rough, often
circular area; there are sometimes also smooth circular marks left by the ejector
pins used to help release the warm moulding from the mould; mould lines are
sometimes also visible.
Tooling cost: High
Production volume: High
Uses : Precision technique capable of complicated shapes: e.g. medical
components; cheap products produced in very large numbers: Lego; plastic
cutlery; machine housings; washing-up bowl
* Typical values shown first (Feasible values in parentheses)