surfaces of forms

SURFACES OF FORMS

The surfaces of forms should be free of waves and other variations in contour which might cause optical distortions in the finished part. Surfaces of forms are usually covered with soft cotton flannel cloth, flannelette, velvet, or billiard felt. The nap of these cloths helps prevent markoff which might otherwise result from small dirt particles or irregularities in the surface of the mold. For covering threedimension forms use suede rubber or flocked rubber sheeting which can be stretched to the contour of the form.

While the fabricator is generally interested in minimizing markoff, very attractive designs can be intentionally embossed in the plastic surface during forming of decorative parts. Coarsely woven cloths, wire meshes, patterned metals, and other materials, applied to the surface of the form can impart pleasing texture to the Chemcast® GP and enhance the appearance of a formed part.

Forms used for translucent Chemcast® GP should contact the inside surface only since markoff does not show on the outside. Where female portions of molds are required to obtain the desired shape, they should be relieved so only the perimeter contacts the hot Chemcast® GP. An important exception is translucent Chemcast® GP lighting fixture pans and shields. The exterior surface of the pans can be made nonspecular by forming against a sandblasted aluminum mold or similar form. (See Surface Embossing.) Prepainted Chemcast® GP should be formed so that the nonpainted surface contacts the mold whenever possible.

GREASE COVERED FORMS

Felt mold covers saturated with grease can be used to minimize markoff and eliminate objectionable optical distortion. To prepare a grease form, cover the mold with a felt blanket, heat the covered mold and work grease into the felt until it is thoroughly saturated.

In use, the mold must be kept hot almost to the point where the grease runs. The temperature of the Chemcast® GP sheet should be higher than when using uncovered forms. The best parts are produced when the form is heated to 170°F. for Chemcast® GP II, G and 55; and 130°F. for Chemcast® GP lA, and the surface of the grease is heated to approximately the sheet temperature. It is usually necessary to apply a fresh layer of grease after forming each part because much of it is removed by the Chemcast® GP. The form should be mounted below the Chemcast® GP to avoid dripping of grease on the Chemcast® GP while the sheet is being clamped in Place.

After forming, wash the grease from the formed parts with kerosene, hexane or aliphatic naphtha. Do not use chlorinated or aromatic hydrocarbons, lacquer thinners or other solvents which are harmful to Chemcast® GP.

Acrylic Forming Table of Contents