MIL-PRF-5425E
4.5.3 Formability.
4.5.3.1 Sheets 0.500 inch (12.7 mm) and less in thickness. Two test specimens, 14 inches
(35.6 cm) in diameter, shall be formed to determine conformance to 3.4.1. The heating
temperature and pressure to be used shall be in accordance with the manufacturer's instructions.
4.5.3.2 Sheets over 0.500 inch (12.7 mm) in thickness. Two test specimens, measuring
4 by 36 inches (10 by 91 cm), shall be formed to determine conformance to 3.4.2. Heating
methods, temperatures, rate of forming, and procedures shall be in accordance with the
manufacturer's instructions. Forming shall be done over wooden molds covered with a soft
lintless cloth. After the formed specimens have cooled, a section 1.5 inches (3.8 cm) along the
circumference and 4 inches (10 cm) long shall be cut from one of the formed specimens. This
specimen shall be subjected to accelerated weathering (see 4.5.10.1) and then visually examined
for crazing or other optical defects.
4.5.4 Internal strain. Two conditioned 12- by 18-inch (30- by 45-cm) plastic sheets shall be
tested. Each sheet shall be considered to be a 12- by 12-inch (30- by 30-cm) specimen supported
by the remainder of the sheet. Two fine lines shall be scribed at right angles crossing the center of
the 12- by 12-inch area. Finely scribed gauge marks shall then be placed 2 inches (5 cm) from the
edge of the specimen area on each of these lines. The distance between each pair of gauge marks
shall be measured to the nearest 0.010 inch (0.25 mm) and the data recorded. Each sheet shall be
hung by one short edge in a circulating air oven at 320 ±18 °F (160 ±10 °C) for the time indicated
below:
Nominal Sheet Thickness, Inch (mm)
Minimum Heating Time, Minutes
0.250 (6.4) and less
16
0.375 (9.5)
25
0.500 (12.7)
33
0.750 (19.0)
55
1.000 (25.4)
79
After removal from the oven, the sheets shall be cooled to standard conditions (see 4.4.1) while
hanging vertically. The distance between each pair of gauge marks shall be remeasured. The
dimensional change shall be computed as the percent change in distance between the gauge marks
from the first measurement. The average of the four values shall be recorded and shall meet the
requirement in table II.
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