pressed into one sheet depends upon the thickness
CHARACTERISTICS
wanted. The end care of the product dictates the
Plastics are formed from organic materials,
choice of paper, canvas, wood veneer, or glass fabric.
generally with some form of carbon as their basic
Paper-based material is thin and quite brittle and it
element. Plastics are synthetic materials, but they are
breaks if it's bent sharply, but canvas-based material
not necessarily inferior to natural material. On the
is difficult to break. As layers are added to paper-
contrary, they have been designed to perform
based material, it gains in strength, but it's never as
particular functions that no natural material can
tough and strong in a laminated part as layers of glass
perform. Plastics come in a variety of colors, shapes,
fabric or canvas.
and forms. Some are as tough, but not as hard, as
Laminated materials are widely used aboard ship.
steel; some are as pliable as rubber; some are more
F o r example, laminated gears are used on
transparent than glass; and some are lighter than
internal-combustion engines, usually as timing or
aluminum.
idler gears; on laundry equipment; and on certain
Plastic materials fall into two major divisions--
pumps. In comparison with metal gears, plastic gears
THERMOSETTINGS and THERMOPLASTICS.
are quieter in operation, pick up less heat when
It's necessary to know which one you are using if you
friction is generated, and wear longer.
are going to perform any kind of shopwork on
Plastics are identified by several commercial
plastics.
designations, by trade names, and by military and
Thermosettings are tough, brittle, and heat
federal specifications. There is such a large number
hardened. When placed in a flame, they will not burn
of types, grades, and classes of plastics within each
readily, if at all. Thermosettings are so hard that they
major group that you cannot rely only on the
resist the penetration of a knife blade; any such
recognition of a trade. Use the appropriate federal
attempt will dull the blade. If the plastic is immersed
supply catalog to cross reference the military
in hot water and allowed to remain, it will neither
(MIL-P-XXXX) or federal (FED-L-P-XXXX)
absorb moisture nor soften.
designations to the correct procuring data for the
Federal Supply System.
Thermoplastics, on the other hand, become soft
and pliable, or even melt when they are exposed to
heat. When cooled, they retain the shape they took
MACHINING OPERATIONS
under the application of heat. Some thermoplastics
will even absorb a small amount of moisture, if they
Since machining operations on plastics call for
are placed in hot water. A knife blade will cut easily
you to cut parts from sheet or rod stock, use various
into thermoplastics.
metal-cutting saws; remove stock from parts by
rotating tools as in a drill press or a milling machine;
When you test a plastic by inserting it into a fire,
cut moving parts with stationary tools, as on a lathe;
be careful, because thermoplastics will burst into
and do finishing operations.
sudden intense flame and give off obnoxious gases. If
you use the fire test, be sure to hold the plastic piece
Sawing
well away from you.
MAJOR GROUPS
You can use several types of saws--bandsaw,
jigsaw, circular saw--to cut blanks from plastic stock.
You don't need to know the exact chemical
Watch the saw speed carefully. Since the plastic will
composition of the many plastics in existence. But, it
not carry away most of the heat generated, there is
will help if you have a general idea of the composition
always danger that the tool will overheat to the point
of the plastics you are most likely to use. Table 3-4
that it will burn the work.
provides information on some groups of plastics that
a machinery repairman may use.
Laminated plastics are made by dipping,
In drilling plastics, back the drill out frequently to
spraying, or brushing flat sheets or continuous rolls of
remove the chips and cool the tool. A liberal
paper, fabric, or wood veneer with resins, and then
application of coolant will help keep the drill cool. To
pressing several layers together to get hard, rigid,
get a smooth, clean hole, use paraffin wax on the drill.
structural material. The number of layers that will be
3-15
