Total change in length
Enter the panel length, temperatures, and fastener size.
- Movement at each end–
- Mounting-hole diameter–
- Gap at each end of a frame or channel–
- Channel depth–
Same length, same swing
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Acrylic's coefficient of thermal expansion is about 0.00004 in. per inch per °F (7 × 10⁻⁵ per °C). The change in length is:
change = 0.00004 × length × temperature swing
A 48 in. panel over a 100 °F swing changes by 0.00004 × 48 × 100 = 0.192 in., almost 3/16 in. (4.9 mm). Over the same swing, aluminum (0.0000128) moves about 0.06 in., steel (0.0000065) about 0.03 in., and glass (0.000005) about 0.02 in. That difference is why acrylic screwed tight into an aluminum frame or a wall can bow, pull its screws, or crack at the holes.
Mounting holes
We assume the panel is held near its middle (or spread evenly between fasteners) and installed somewhere in the middle of its temperature range, with each screw centered in its hole. Each end then moves by half the total change, so:
hole diameter = screw diameter + change ÷ 2 + 1/32 in.
The 1/32 in. (0.8 mm) margin covers drilling and layout tolerance. When the movement at each end is more than about 1/8 in., or the oversize would be bigger than the screw itself, use slots running along the length instead, so the panel can slide without the washer losing its grip. Use washers or standoffs that cover the oversized hole, and snug the screws rather than cranking them down; the panel has to be able to slide.
Frames and channels
For a panel captured in a channel or frame, leave a gap at each end of half the total change, and make the channel deep enough that the panel still has its full bite when it shrinks: channel depth = the edge engagement you need + half the change. Don't caulk or glue the panel solid into a rigid frame; use a flexible sealant or gasket that allows movement.
For more, see acrylic thermal expansion, how to mount acrylic to a wall, how to drill acrylic without cracking, and using acrylic outdoors.
Questions about acrylic expansion
How much does acrylic expand?
About 0.00004 in. per inch per °F. A 4 ft panel grows or shrinks about 0.2 in. over a 100 °F swing, roughly eight times as much as glass.
How big should mounting holes in acrylic be?
Add half the expected movement plus about 1/32 in. to the screw diameter. For long outdoor panels, slotted holes work better than large round ones.
Does expansion matter indoors?
Less, but it still counts for big panels near windows, heaters, or lights. A 30 °F swing moves a 48 in. panel about 0.06 in., enough to bow a panel that's screwed tight at both ends.