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MATERIALS FOR FORMS Since most drape forms for Chemcast® GP sheets are not
subjected to great pressure, they may be constructed of wood,
plywood, sheet metal or sheet laminates bent over wooden or metal
formers. Except for long production runs, forms need not be made
of metal or materials generally used for sheet metal forming.
Either convex or concave forms can be used but usually the convex
or male shape is preferred. Forms for compound threedimensional
or deepdrawn shapes are more difficult to make, but are made
the same way as patterns for castings. Wood Forms The best woods for forms are kilndried hardwoods such as
birch or cherry. Welldried soft woods such as white pine,
poplar or Philippine mahogany may also be used if sound and free
of knots. All woods must be sealed to prevent changes in shape
and dimensions with atmospheric changes. Synthetic resins, high
temperature varnish or casein should be used since shellac or
regular varnish will soften at forming temperatures. Wooden forms
should be made so that side grain of the wood is on the surface
throughout since end grain may produce distortions in the
finished pieces. Resin Impregnate Forms Masonite die stock, a compressed lignin bonded material, cloth
or paper base phenolics and Impreg or Compreg, special
resinimpregnated woods, can also be used for forms for
Chemcast® GP. These materials can be built up, laminated, and
finished to produce excellent molds with polished, "rainless
surfaces which will resist wear and will not be affected by
normal moisture changes. Metal Forms Forms can be made of cast metal or fabricated from plate and
bar stock. Some lowmelting alloys can be cast to good finish
for small forms. These alloys are especially convenient for model
work. Aluminum, bronze, brass, and highmelting alloy molds can
be cast by regular foundry techniques. They should be careful!,
finished because pits, blowholes, and other surface defects
will be reproduced in the formed Chemcast® GP parts. Provision
must be made to control the temperature of metal forms. Cast Forms Casts of various materials can be made from the original
wooden form. Allowances for the shrinkage of the casting as well
as for the contraction of the Chemcast® GP must be made when
building the original. If the final form will require finishing
to obtain the smooth surface necessary for forming Chemcast® GP,
allowance must also be made for the material to be removed during
finishing. Gypsum Forms Satisfactory forms can be cast from high strength gypsum
products. Specific instructions for their use are supplied by the
manufacturers and should be followed exactly. No finishing is needed if a good smooth surface is applied to
the original pattern and the casts are properly made. Gypsum
forms should be made with a hollow shell from 2u to 4"
thick, not solid. They should be reinforced with steel rods and
wire mesh welded into a shape to take any tension or bending
stresses. Use steel strips around the edges and at clamping
points to strengthen and stiffen the form and prolong its life.
Do not use large masses of wood, burlap, sisal, or other moisture
absorbing materials as fillers since they may shrink or swell and
crack the form. Fiberglas or shredded asbestos are suitable
fillers Resin Forms Very good forms can be cast from phenolics and other
thermosetting resins. The dimensions of phenolic molds should be
checked from time to time. The form should be cast from a carefully finished female mold,
according to directions supplied by the manufacturer. Polyester
and epoxy glass reinforced lay-ups can be used for either
male or female molds. A smooth gel coat on the surface against
which the Chemcast® GP is to be formed will help reduce
markoff. Resin forms may be more expensive than gypsum forms,
but provide a smoother, tougher surface when properly made from
the correct materials. |
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Copyright © Plastiglas de Mexico 1999
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