which will loosen the adhesive. Sheets so treated
slight yellowish tint when viewed from the edge;
should be washed immediately with clear water.
MIL-P-8257, a thermosetting polyester plastic, has a
bluish or blue-green tint; and MIL-P-5425, a
heat-resistant acrylic, is practically clear. In addition,
stretched acrylic sheets and fabricated assemblies are
CAUTION
permanently marked to ensure positive identification.
Aliphatic naphtha is highly volatile and
Plastic enclosures on aircraft may be distinguished
flammable. You should exercise extreme care when
from plate glass enclosures by tapping lightly with a
using this solvent.
blunt instrument. Plastic will resound with a dull thud
or soft sound, whereas plate glass will resound with a
metallic sound or ring.
Do not use gasoline, alcohol, kerosene, xylene,
ketones, lacquer thinners, aromatic hydrocarbons,
Storage and Handling
ethers, glass cleaning compounds, or other unapproved
solvents on transparent acrylic plastics to remove
Transparent plastic sheets are available in a number
masking paper or other foreign material, as these will
of thicknesses and sizes that can be cut and formed to
soften and/or craze the plastic surface.
required sizes and shapes. These plastics will soften
NOTE: Just as woods split and metals crack in
and/or deform when heated sufficiently; therefore,
areas of high, localized stress, plastic materials
storage areas having high temperatures must be
develop, under similar conditions, small surfaces
avoided. Plastic sheets should be kept away from
fissures called "crazing." These tiny cracks are
heating coils, radiators, hot water, and steam lines.
approximately perpendicular to the surface, very
Storage should be in a cool, dry location away from
narrow in width, and usually not over 0.01 inch in
solvent fumes, such as may exist near paint spray and
depth. These tiny fissures are not only an optical defect,
paint storage areas. Paper masked transparent plastic
but also a mechanical defect, inasmuch as there is a
sheets should be kept indoors as direct rays of the sun
separation or parting of the material. Once a part has
will accelerate deterioration of the masking paper
been crazed, neither the optical nor mechanical defect
adhesive, causing it to cling to the plastic so that
can be removed permanently; therefore, prevention of
removal is difficult.
crazing is a necessity.
Plastic sheets should be stored, with the masking
paper in place, in bins that are tilted at approximately
When it is necessary to remove masking paper
10 degrees from the vertical to prevent buckling. If it is
from the plastic sheet during fabrication, the surface
necessary to store sheets horizontally, you should take
should be remasked as soon as possible. Either replace
care to avoid chips and dirt getting between the sheets.
the original paper on relatively flat parts or apply a
Stacks should not be over 18 inches high, and small
protective coating on curved parts.
sheets should be stacked on the larger ones to avoid
unsupported overhead. Storage of transparent plastic
Machining
sheets presents no special fire hazard, as they are slow
burning.
Machining qualities of transparent plastics are
Masking paper should be left on the plastic sheet as
similar to those of brass or soft copper. Cutting edges of
long as possible. You should take care to avoid
tools should have no rack and perform a scraping
scratches and gouges, which may be caused by sliding
action, rather than a cutting action. Tools and work
sheets against one another or across rough or dirty
should be held firmly to prevent vibration and
tables.
chattering. The feed of the work should be constant: if
Formed sections should be stored so that they are
the work stops, it will be burned. The use of large
amply supported and there is no tendency for them to
amounts of water-based coolant will also prevent
lose their shape. Vertical nesting should be avoided.
burning.
Protect formed parts from temperatures higher than
120F. Protection from scratches may be provided by
REINFORCED PLASTICS
applying a protective coating of masking paper or other
Glass fiber reinforced plastic and honeycomb are
approved materials.
used in the construction of radomes, wing tips,
If masking paper adhesive deteriorates through
stabilizer tips, antenna covers, fairings, access covers,
long or improper storage, making removal of paper
etc. It has excellent dielectric characteristics, making it
difficult, moisten the paper with aliphatic naphtha,
2-41
