Datasheet

Thursday, 17 July 2003 MiniProject: Design Aspects Colin K McCord
EEE512J2 – Electronic Product Design Page 32 Chapter 6: Physical Design
Compression moulding uses heat and pressure to shape plastics. The process is commonly used to mould
thermosetting plastics.
Injection moulding shoots molten plastic material under pressure into a mould. It is the most widely used
method of moulding thermoplastics.
Blow moulding produces hollow objects. It uses air or steam to expand a tube of molten resin, forcing the
material against a mould’s walls.
Casting does not depend on any external pressure to shape the plastics. In the casting process, melted
resin is simply poured into a mould. Manufacturers use casting to produce thick, solid objects.
Extrusion is used to produce such continuous forms as pipe, rods, fibbers, wire coatings and supermarket
plastic bags. Rotating screw force the plastics through a heated barrel, in which they melt, then force them
out through a specially shaped die.
Calendering produces plastic sheets by pressing molten plastic material between two rollers. Manufacturers
also feed fabric, paper, or other materials through the rollers to produce such items as tablecloths and
playing cards.
Laminating involves coating sheets of such materials as wood, paper, or metal foil with plastics. The sheets
are then stacked and pressed together to make such products as plywood, electronic circuit boards, and
tabletops.
Foaming is any of several methods that produce solid plastics filled with air spaces. To make Styrofoam, for
example, manufacturers use beads of thermoplastic resin containing a chemical that forms a gas when
heated during moulding.
Thermoforming is used to mould items from sheets of plastics. A sheet is clamped over a mould and heated
until it becomes soft. A vacuum pump sucks air out through tiny holes in the mould, drawing the sheet into
the mould.
6.4. Health and Environmental Hazards of Plastics
Because plastics are relatively inert, they do not normally present health hazards to the maker or user.
However, some monomers used in the manufacture of plastics have been shown to cause cancer. Similarly,
benzene, which is an important raw material for the synthesis of nylon, is a carcinogen. The problems
involved in the manufacture of plastics parallel those of the chemical industry in general.
Most synthetic plastics are not environmentally degradable; unlike wood, paper, natural fibres, or even metal
and glass, they do not rot or otherwise break down over time. (Some degradable plastics have been
developed, but none has proven compatible with the conditions required for most sanitary landfills.) Thus,
there is an environmental problem associated with the disposal of plastics.