Except for occasional temporary fastenings bearing no load, the only cement that can be used on cast resins is that furnished by the plastics companies, and which consists practically of the raw unvulcanized resin itself. Glue, amberoid, duco and various types of cements recommended for glass can be used for temporary joints, and after their purpose has been served, can easily be cleaned off completely.
The raw resin, a thick, clear syrupy substance, (which practically never dries in case it is spilled on anything) has gone through the same processes as the finished material except the vulcanization, (or more properly, "polymerization"), the principle purpose of which is to drive off the water which is chemically contained in the syrup. The cement is only partially dehydrated, leaving a portion of the water still in it, and this is driven off at the time of using the cement by some artificial means of supplying heat to the cemented joint. The most common method of doing this is to add a few drops of muriatic (hy- drochloric) acid to the cement. The effect of this is to cause extreme heat in contact with the very small amount of water present, the net result being that the cement is "cooked" into a solid hard resin in a matter of a few moments.
The amount of acid used depends on the job in hand, as the time of setting varies with the amount of acid used. For ordinary purposes, to make a strong joint that will be rock-hard within an hour or so, one drop of acid to a puddle of cement about the size of a nickel is recommended. More acid will cause a quicker set, and too much acid might result in a brittle or crumbly joint—too little acid gives a joint that never sets really hard. A little experimentation will quickly demonstrate the proper amount to use.
The acid serves a physical purpose—not a chemical one— its sole purpose being to create a localized and easily - controlled source of heat, this being the normal reaction when acid is added to water. Any source of heat could be used, or the entire object could be placed in a temperature of 200 degrees and the cement would harden in about 12 hours. If the cement fails to set, it is usually a sign that it is stale and has lost its water, becoming extremely stiff and finally hardening, in which case it is useless and might as well be thrown away. Cement should be stored in a cool place, well covered, and not kept over 3 months.
The effect of the sudden application of heat by means of the acid is to turn the cement white, and in the course of time, the chemicals remaining in the joint, composed of artifically-cooked cement, acting on the phenol which is one of the constitutents of the cement will cause it to turn slightly pink in the air. For this reason it is advisable to keep the joints as thin as possible or to keep them in covered places, if you are working on dark materials where a whitish or pinkish streak in the wrong place would be objectionable. One method of overcoming this is to mix a little powdered cast resin with the cement before adding the acid, although this does not always have the expected effect after setting, as the powder from any color of the material is always whiter than the solid mass, and the whiteness of the cement added to this results in a very pale coloring effect. Occasionally, aniline dyes can be added to the cement satisfactorily, or lamp-black if the joint is between black material, but as a rule the favored proceedure is to arrange the design so that the joint is not visible.
The proper method of mixing the cement is to pour a little of it on a glass plate, and mix the acid with it with a glass stirring rod, which is used to apply it. Another method often used is to dip the glass mixing-rod into the cement, and then dip the rod, cement and all, into the acid, which picks up just about the proper proportion of acid, then mix it on the plate. Joints should be as thin as possible, as is the case with any cement. In the main however, it will be found that the cement is as hard and as strong as the material itself. It can be sanded, sawed, filed, buffed and polished in the same way as the material, and except for its color cannot be told from the balance of the project. It is frequently used to fill in chipped places and cracks, where the color is unimportant or where a proper match can be obtained.
The cement will hold on wood and some other materials, and occasionally on metal which is clean and slightly roughened, but the plastics-to-metal joint is not always dependable. In such cases, other means of fastening the metal are usually used, such as screws, etc., or the cement is flowed around the metal or through a hole in it, serving as an anchor. This method is illustrated in Figure 16 showing a method of anchoring a glass gem in a ring, and also in Figure 17, showing a plastic ornament cemented to a metal sheet. In the latter case, two small holes are drilled thru the metal base on which the plastic ornament will sit, and the cement is flowed thru these holes and spread out a little on the under side, forming a physical anchor which will hold satisfactorily. If this projection on the bottom is objectionable, the holes can be countersunk from the bottom and the cement cleaned off flush with the surface of the bottom side of the sheet.
Fig. 16. Glass ornaments may be cemented to plastics by undercutting a depression, and Ailing this space with cement, which acts as a wedge.
Fig. 17. Plastics may be cemented to metal by flowing the cement thru countersunk holes in the metal, the cement thus operating as a mechanical, rather than as an adhesive fastening.
All cementing should be done before the final polishing, if possible, as it must be done on a rough sanded surface, and the excess trimmed and sanded off. It can be wiped off with a wet rag before it hardens, or filed or sanded off after it hardens.
Cement can be thinned, to make it easier to handle, by the addition of up to 10% of grain alcohol (#5 ethyl), but this alcohol must eventually be gotten rid of by evaporation before the joint really sets.