Short answer: Acrylic's coefficient of linear thermal expansion is about 0.00004 in. per inch per °F (0.000072 per °C, ASTM D696), roughly 3 times that of metals and 8 times glass. A 48 in. panel grows or shrinks about 0.002 in. per °F, or about 3/16 in. (0.19 in.) over a 100 °F swing, and it also swells up to about 0.6% in very humid air, so mount it with oversize holes, room in frames, and flexible adhesives.
How do you calculate acrylic expansion?
Change in length = length × coefficient × temperature change.
For acrylic, use about 0.00004 in./in./°F. In metric, about 0.07 mm per meter per °C. Makers' published values:
| Sheet | Coefficient | Test |
|---|---|---|
| Acrylite cast and extruded (FF) | 0.000040 in./in./°F (0.000072 /°C) | ASTM D696 |
| Plexiglas G | 0.000039 in./in./°F at 60 °F | ASTM E831 |
| Optix-L | 0.000035 in./in./°F | ASTM D696 |
| Röhm PLEXIGLAS GS and XT | 7 × 10⁻⁵ /K (0 to 50 °C) | DIN 53752-A |
Acrylite's own rule of thumb: a 48 in. panel moves about 0.002 in. for each °F, about 3/16 in. over a 100 °F swing.
How much does an acrylic panel move?
| Panel length | Temperature swing | Movement | Typical situation |
|---|---|---|---|
| 12 in. | 30 °F | about 0.014 in. | Indoor sign, heated and cooled building |
| 24 in. | 60 °F | about 0.058 in. | Sign near a sunny window |
| 36 in. | 100 °F | about 0.144 in. | Outdoor sign |
| 48 in. | 100 °F | about 0.19 in. | Outdoor window or panel |
| 96 in. | 120 °F | about 0.46 in. | Full-sheet greenhouse or patio panel |
Example in full: 48 in. × 0.00004 × 100 °F = 0.192 in. Nearly 3/16 in. on one panel.
Outdoors, use the real surface temperature range, not the weather forecast. Dark acrylic in direct sun can get much hotter than the air, and a winter night can be far below freezing. A swing of 100 °F or more is common in many climates. Do your own numbers with the thermal expansion calculator.
How does acrylic expansion compare with other materials?
| Material | Approx. in./in./°F | Acrylic moves about |
|---|---|---|
| Acrylic | 0.000040 | - |
| Polycarbonate | 0.0000375 | about the same |
| Aluminum | 0.000013 | 3 times as much |
| Steel | 0.0000065 | 6 times as much |
| Glass | 0.000005 | 8 times as much |
Acrylite puts it simply: acrylic expands about 3 times as much as metals and 8 times as much as glass. The other values above are typical handbook figures. That mismatch is why an acrylic panel screwed tight into an aluminum or wood frame cracks at the screws, or bows outward when it warms up.
How do you allow for expansion?
Oversize holes
Drill mounting holes larger than the screw so the panel can slide. A simple approach: work out the total movement of the panel, and make each hole at least that much larger than the screw, centered on the screw. On a 48 in. outdoor panel that moves about 0.19 in., holes roughly 3/16 to 1/4 in. bigger than the screw are reasonable. Use washers large enough to cover the hole, and tighten until snug, not tight, so the panel can move under the washer.
Röhm gives a simple formula that covers heat and humidity together: hole diameter = screw diameter + 5 mm per meter of panel length (about 3/16 in. per 39 in.), with holes at least 1.5 hole diameters from the edge.
An alternative on long panels is to fix the panel with a snug hole at the center and use slotted or oversize holes everywhere else, so it grows outward from the middle.
Frames and channels
- Leave a gap at each end of the panel inside the channel, with the channel deep enough that the panel stays engaged when it's shrunk in the cold. Plaskolite's glazing table cuts panels short of the opening by 1/8 in. up to 36 in., 1/4 in. for 36 to 60 in., 3/8 in. for 60 to 96 in., and 7/16 in. for 96 to 120 in.
- Measure and install in mild weather when you can. If you install on a cold day, the panel will only grow, so leave most of the gap for expansion.
- Use flexible glazing tape or silicone, not rigid glue. Acrylite says large sheets should be installed in frames, never fastened with bolts alone.
Adhesives
Rigid epoxy is fine on small parts but tends to fail on large ones as the materials move at different rates. Use flexible adhesives or tape for big panels. See gluing acrylic to other materials.
Does acrylic expand with humidity?
Yes, more than most people expect. Acrylite measures dimensional changes of 0.2%, 0.4%, and 0.6% at 60%, 80%, and 100% relative humidity. On a 48 in. panel, 0.2% is about 0.1 in., the same as a 50 °F temperature swing. Moisture moves slowly, so it shows up as seasonal growth rather than daily cycling, but it's another reason not to fit panels tight edge to edge. Röhm's 5 mm per meter allowance covers both effects.
We can drill it for you
Tell us how the panel will be mounted and we'll drill oversize or slotted holes where they belong. For outdoor jobs, read can acrylic be used outdoors and how to mount acrylic to a wall, then get an instant quote.
Questions
How much gap should I leave around an acrylic panel in a frame?
At least the panel's calculated movement, split between the two ends. For a 48 in. outdoor panel, about 1/8 in. at each end is a reasonable starting point, with a channel deep enough to keep the panel held when it shrinks.
Does thicker acrylic expand less?
No. Expansion depends on length and temperature change, not thickness. Thicker sheet does resist bowing better when it's restrained.
Do I need to worry about expansion indoors?
For small signs and parts, hardly at all. For large panels near sunny windows or heaters, yes: give them slightly oversize holes and don't clamp them hard.
What is acrylic's coefficient of thermal expansion in metric?
About 7 × 10⁻⁵ per °C (70 × 10⁻⁶ /K, or 0.07 mm per meter per °C). Acrylite lists 0.000072 m/m·°C and Röhm 7 × 10⁻⁵ /K for 0 to 50 °C, so a 1 m panel moves about 3.5 mm over a 50 °C swing.