Depending on technique, cast acrylic surfaces to be joined are softened, and in some cases swollen into a cushion, by contact with commercially available solvent cements. Plexiglas® MC acrylic sheet should not be soaked in cement as long as Plexiglas® G acrylic sheet, for it tends to dissolve rather than swell into a cushion.
Since solvents evaporate at different rates, the composition of cements made from a mixture of several solvents should be periodically checked to maintain correct proportions. In many cases, checking the specific gravity with a calibrated hydrometer may be adequate.
The surfaces to be cemented may be left as cast, sanded (either wet or by hand), scraped, or machined according to methods described in Machining Plexiglas® Acrylic Sheet, page 11. The edges to be cemented should not be polished, as this may cause crazing when the cement contacts the edges. The pieces should fit accurately without forcing or flexing. Well-fitted parts produce stronger joints.
If a part is to be immersed in solvent, the softening action of the cement must be confined to the area of the joint by masking the surrounding Plexiglas® acrylic sheet. Tapes, gelatin solutions, or commercially available masking compounds that are impervious to the action of the cement, and do not affect the hardening or color of the cement, may be used to mask the material. The part to be masked is coated with the mixture, so a fairly thick film adheres to it. The coated piece is hung until it cools and dries. When the film is no longer tacky, it is stripped from the areas to be cemented with a razor, taking care to avoid scratching the surface of the plastic. The edge of the part is then soaked and assembled, and the remaining film is stripped from the material. The glue may be reused several times.
The temperature and humidity conditions in the cementing room, and the temperature of the parts to be cemented are important. Unthickened cements should not be used at temperatures below 65°F. Low temperatures reduce the solvent action and increase the soak time, increasing the possibility of crazing. Adding 10 percent diacetone alcohol to the solvent cement will reduce blushing in humid conditions.
Thickened solvent cement is made by adding clean Plexiglas® acrylic sheet chips or Plexiglas® acrylic molding resin to a solvent to produce a syrupy cement that can be applied like glue. Plexiglas® VS-100 acrylic molding resin dissolves more readily than other molding resin grades or cast sheet chips, producing a cement with a higher solids content. The solvent in the thick- ened cement swells both surfaces to be cemented, to permit the formation of the bond. The joint hardens as the solvent evapo- rates or dissipates through the material. The viscous cement acts as a carrier for the solvent, since the solids content is too low to fill any sizeable voids without developing bubbles. The parts should fit as closely as possible to minimize bubbles and maxi- mize joint strength.
Thickened cement should primarily be used as a perimeter c e m e n t . I f p a r t s f i t w e l l i t w i l l r u n u n d e r o n l y 1 ⁄ 1 6 i n c h a n d g i v e neat appearance. This should be for indoor use only. a
In general, thickened solvent cement produces an inferior joint compared to straight solvent cement in the important areas of strength and appearance. Thickened cements should only be used to minimize run-under by capillary action when perimeter-cementing appliques, to increase solvent attack of a solvent-resistant material or to join two parts that cannot be feasibly made to fit closely. If the part to be cemented requires soaking in a tray or on a saturated pad of
s o l v e n t c e m e n t , l i m i t t h e s o a k t i m e t o 1 1 ⁄ 2 m i n u t e s t o o b t a i n t h e
best joint appearance (minimal squeeze-out) and optimum set time.
If areas are cemented, they should be small – large areas show many bubbles when dry. Large areas may require eight hours or more to dry.