Installation Guide
31
allowed to cool too much before forming is completed tend to
relax at lower annealing temperatures.
In addition to increased dimensional stability and resistance to
crazing, annealing increases the strength of certain cemented joints.
Annealing also lessens the effect of solvent smears and “runs”
that may result from errors in cementing. Proper annealing will
eliminate any tendency toward immediate crazing or cracking of
parts subjected to brief exposure to solvents, e.g., parts that are
to be painted. If the solvent makes the Plexiglas
®
acrylic sheet
sensitive to crazing, as do some types of paints, the parts should be
annealed after each exposure.
When only surface stresses are present, only the surface needs
to be heated. This will greatly reduce annealing times for thick parts.
Drilled holes may be considered a special case of surface
machining. Parts with through holes must be placed in the oven
so that the air flow is along the hole passage.
Caution: Protective spray masking coatings must be removed
from Plexiglas
®
acrylic sheet parts before they are annealed. If
such coatings are not removed prior to annealing, optical distortions
may appear in the annealed part in areas where the thickness or
surface of the coating is irregular.
Annealing Procedures
Plexiglas
®
acrylic sheet parts to be annealed should be clean
and dry and should be supported so that they are not under stress
while being annealed. This is particularly true when clamps are
used
to hold cemented assemblies together during the annealing
or curing
period. The weight of the clamps or excessive clamping pressure may
set up local stresses, which may warp or even craze the parts
unless proper precautions are taken.
Room should be provided between parts to permit free circulation
of air. This will avoid traps or pockets of dead air where solvent
vapors can settle.
Slow Cooling After Annealing
The rate of cooling must be slower for thick sections than for
thin sections. Table 7 lists suitable cooling rates for various
thicknesses of Plexiglas
®
acrylic sheet.
Annealing cycles can be worked out in many ways. Some fabricators
allow their forming ovens to cool to annealing temperature near
the close of the day’s work, place the fabricated parts in the oven
and hold them at the annealing temperature for the specified
time, then adjust the oven to cool during the night at the specified
cooling rate.
Cooling Times for Plexiglas
®
Acrylic Sheet Parts (All Formulations)
Maximum
Thickness
(Inches)
0.060 to 0.177
0.236 to 0.354
0.472 to 0.708
0.944
1.500 to 1.750
2.0
Cooling Rate
(
°
F/Hr)
140
54
25
18
12
10
221
°
F
(105
°
C)
3
⁄
4
1
3
⁄
4
4
5
1
⁄
2
8
10
Notes: (1) Parts are usually held in the forced circulation air oven and the temperature of the oven dropped at the cooling rate.
(2) As in heating, the air should circulate around each part.
230
°
F
(110
°
C)
3
⁄
4
2
4
1
⁄
2
6
9
11
Time (Hours) to Cool Plexiglas
®
Acrylic Sheet Parts from the Indicated Temperature to 120
°
F
212
°
F
(100
°
C)
1
⁄
2
1
3
⁄
4
3
3
⁄
4
5
8
9
203
°
F
(95
°
C)
1
⁄
2
1
1
⁄
2
3
1
⁄
4
4
1
⁄
2
7
8
TABLE 7