MATERIALS FOR FORMS

Since most drape forms for Vical acrylic 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 Vical acrylic. 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 Vical acrylic 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 Vical acrylic must be made when building the original. If the final form will require finishing to obtain the smooth surface necessary for forming Vical acrylic, 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 Vical acrylic 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.

Forming Acrylic Table of Contents