Full Product Manual

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.
23
Procedures for Using Polymerizable Cement
To get good performance from polymerizable
cement, the following directions must be
closely followed:
1. If at all possible, store Component B, the catalyst,
under refrigeration not below 40
O
F. 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.
2. 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.
3. 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.
4. 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.
TABLE 4