The gap joint technique should only be used with colorless Plexiglas® acrylic sheet, or in cases in which joints will be hidden, or when light leaks are not a problem. Inconspicuous joints in colored sheet are difficult, if not impossible, to achieve. Adding acrylic sign paints to Component A can be effective at times, but may adversely affect the joint strength.
Cement forms or dams may be made with masking tape as long as the adhesive surface does not contact the cement. This is most easily done with a strip of tape placed over the masking tape adhesive. The tape must be chosen carefully. The adhesive on ordinary cellophane tape prevents the cure of PS-30 and Weld-On 40 or 42. Before actual fabrication of parts, sample joints should be tried to ensure that the tape system used will not harm the cement. Since it is important for all the cement to remain in the gap, only contact pressure should be used. Types of joints are shown in Figures 9A, 9B, and 9C (pages 24 and 25). Figure 10 combines the joints into a box construction.
Bubbles will tend to float to the top of the cement bead in a gap joint after the cement is poured. These cause no problem if the bead is machined off. A small wire (not copper), or similar object may be used to lift some bubbles out of the joint; however, the cement joint should be disturbed as little as possible.
Since polymerizable cements shrink as the cement hardens, the freshly poured cement bead should be left above the surfaces being cemented to compensate for the shrinkage. If it is necessary for appearances, the bead may be machined off after the cement has set, as shown in Figures 11 (A and B) and 12 (on page 26).
Figures 13 and 14 (page 27) illustrate two cementing techniques that have proven useful with the furnishings industry. The first shows a thick section butt joint using polymerizable cement, with the thick Plexiglas® acrylic sheet preset into a fixture. The latter Figure shows a system for making inside joints that cannot be routed and finished as described previously.
Procedures for Using Polymerizable Cement
To get good performance from polymerizable cement, the following directions must be closely followed:
If at all possible, store Component B, the catalyst, under refrigeration not below 40OF. If this is impossible, be certain that Component B is stored in a dark place and that it is definitely kept out of direct sunlight. If Component B turns yellow or amber, discard it and obtain a fresh lot from the supplier.
When mixing Components A and B, be sure to use clean containers. Make sure that any disposable containers are not lined with a coating that will affect either component. If the container coating is soluble in either of the cement components, it may contaminate the cement and cause improper curing.
Both Component A and Component B should be at room temperature at the time of mixing. It may be convenient to remove a one day supply from refrigeration the day before mixing.
When Components A and B are combined, be certain that the two components are continuously under agitation for two minutes to insure their complete mixing. Let the air bubbles rise to the surface before using the cement.
5. Combine Components A and B in the ratio of 95 parts Component A to 5 parts Component B, either by weight or by volume. Do not use a mixture containing less than 93 parts Component A or more than 7 parts Component B.