Installation Guide

21
Solvent Cements
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 Cements
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
cement. If parts fit well it will run under only
1
⁄
16 inch and give a
neat appearance. This should be for indoor use only.
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
solvent cement, limit the soak time to 1
1
⁄
2 minutes to obtain the
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.