Cooling Times for Plexiglas® Acrylic Sheet Parts (All Formulations)
Time (Hours) to Cool Plexiglas® Acrylic Sheet Parts from the Indicated Temperature to 120°F
Maximum Thickness (Inches)
0.944 1.500 to 1.750 2.0
Cooling Rate (°F/Hr)
140 54 25 18 12 10
2 41 6 9 11
1 4 5 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.
1 3 5 8 9
1 3 4 7 8
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.
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.
When only surface stresses are present, only the surface needs to be heated. This will greatly reduce annealing times for thick parts.
Slow Cooling After Annealing
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.
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.
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 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.