Cementing

Each material requires its own type of cement. Cast phenolic resins are best cemented with a special preparation, supplied by the manufacturers, which consists of partially cured syrupy phenolic resin. A small quantity of this is mixed with two or three drops of muriatic acid, and used immediately, setting under clamps. This makes a chemical combination that is as strong as the material itself. The only other method of cementing is to use one of the all-purpose household cements, applied to roughened surfaces. This does not always provide a permanent bond, however.

The best adhesive for acetate is the film cement used for splicing moving-picture film. This comes in a convenient bottle with an applicator, takes hold immediately and disappears completely, leaving a physical union of the two pieces by means of its solvent action. Photographic film usually has a cellulose acetate baset and film is, in fact, often used as the raw material for articles which can be made of thin sheets. The gelatin-and-silver emulsion may be removed from films in order to obtain the clear plastic base by soaking them in warm water to which an alkali such as sodium carbonate, lye, ammonia or clorox has been added, and scrubbing them. Other common substances which will fasten acetate in the absence of film cement are acetone or glacial acetic acid. These solvents evaporate quickly, especially from thin sections, and the cemented article may be handled within a few minutes.

Various cements, of both the solvent and glueing types, are supplied by the manufacturers of acrylic resins, and these are especially recommended, but quite successful results can be obtained by the use of plain glacial acetic acid, a preparation used in photography and available at photographic stores. This is a solvent which evaporates completely in a few minutes to a couple of hours, depending on the amount used and the thickness of the material. Paint both pieces to be joined with the acid, allow it to soak in a minute or two, then clamp the two pieces together, being sure to squeeze out all air bubbles. If the surfaces are irregular and do not mate perfectly, the two parts should be soaked in the solvent until they have absorbed enough to soften the material so that when clamped both surfaces will flow into each other. In such cases, the joint must se( several hours before it can be handled. Very little pressure is required when clamping.

Cementing is frequently used to build up larger sections of material from small or thin pieces. For instance, if a 3/4 inch round rod is desired and the only material available is a inch sheet, three strips inch wide are cut and cemented together to form the rod, which is then filed or turned to a round. Likewise, a string of washers may be cut out of thin material stacked and cemented together to form a bigger piece of any shape, which can then be worked. Cigarette holders are often made from tooth-brush handles, combs and other odd bits of acetate by cutting out a large number of small squares, drilling a hole through each one, stringing them on a heavy wire, soaking them in acetone and then clamping the string of them together on the wire until they set. When the wire is removed, this gives a rough tube with a hole through the middle, which is then filed round and polished. Alternating colors of washers may be used. Such holders will stand the heat as long as the live coal does not come into contact with the material.

Turning

Any of these plastics may be turned on a wood or metal-working lathe, using a brass-turning technique, that is. a scraping rather than a cutting action; otherwise the too! will dig right into the material.

Woodworking chisels must be used as scrapers. The tool is held horizontal or with the handle a little above horizontal to get the proper action. When the material comes off in fine ribbons, rather than as a powder, the angle is correct. With metal-working tool-bits, there should be no back-rake or even a little negative rake, and the tool should be at or below center. Speeds and feeds are the same as for soft metals.

Polishing

Polishing plastics can be very easy or very difficult, depending on the care that is taken with each succeeding step in the polishing routine. The roughly-worked portions must first be filed to remove every tool mark. Then the file marks must be removed with a medium-fine sandpaper and the sandpaper marks taken off with the finest sandpaper obtainable (about 7-0). These marks are in turn removed with buffing-compound, a grease stick containing fine abrasive. The last step is to polish the article with a wax-base polishing compound.

Experience alone will show when any of these steps may be omitted, but in 9 cases out of 10, time will be saved by going through each step carefully in order to insure success. If a power buffing-wheel is available, some of the preliminary steps may be omitted on the softer acrylic or acetate resins, particularly if the deeper scratches have been previously removed. The last step can likewise be omitted if a very high polish is not needed.

For hand polishing, rub the article with a rag, a piece of felt or brussels carpet lightly coated with the buffing compound and then with the polishing compound. Different rags must be used for each operation. Regular brass-polishing preparations or special compounds for plastics sold by the supply houses may be used. Even household polishes will work fairly well. The grease sticks are best, however, as the liquid preparations may contain ingredients with a solvent action on the surface. Deep scratches can be filled in somewhat by daubing them with a solution of acrylic resin or acetate chips dissolved in acetic acid, then sanding and polishing.

For power polishing, soft cotton buffing wheels should be used, with two or three outer rows of the stitching removed. A 6- or 8-inch wheel on a 1750 rpm, 1/4 or 1/2 hp motor provides about the proper speed, and care must be used to keep the object moving in order not to "burn" or soften the material by the heat generated. For best work, three wheels should be used, one for the buffing compound, one for the polishing compound and one clean wheel for the final polishing.

Tinting And Lettering

No solvent will attack cast phenolic resins, so there is no way of dyeing or lettering them except possibly by the use of colored lacquers, which will adhere only to roughened surfaces. Both the acetates and acrylics, however, can be tinted or lettered with inks or dyes containing solvents such as acetone or acetic acid. The dye only penetrates slightly below the surface, so that interesting effects can be obtained by cutting designs through the dye layer into the transparent material, or by masking the design with tape or grease-crayon and dyeing various areas in different colors. The dye should be filtered before use to avoid black spots, and the solution should consist of 80 percent acetone and 40 percent water, plus the dye. Household Duco lacquer may also be used on any of these materials.

Cleaning

Use only soap and water for cleaning plastics as many prepared cleansers contain solvents or abrasives which will attack the surface.