Does acrylic shrink when heated?

Key facts

• Cast sheet: about 1.5% typical shrink when heated unframed; 2% maximum residual shrinkage (ACRYLITE GP, ASTM D702, after heating above 300 °F).
• Extruded sheet: up to 3% in the extrusion direction for 1/8 in. and thicker (ACRYLITE FF); thinner sheet can shrink more. Shrink across the sheet is much smaller.
• When it happens: above the softening range, roughly 210 °F and up, and mostly at forming temperatures (about 300 to 380 °F).
• At room temperature: acrylic doesn't shrink with age; it only moves with temperature and humidity.
• Formed parts remember: reheating a formed cast part above about 210 °F can make it creep back toward flat.

Why does acrylic shrink when you heat it?

Making sheet stretches the long polymer chains a little and freezes them in place as the sheet cools. When you heat it past its softening point, the chains relax back toward a coiled shape and the sheet pulls in. Cast sheet is made by polymerizing liquid between glass plates, so the strain is small and fairly even in both directions. Extruded sheet is pushed through a die and pulled through rollers, which stretches it mostly in one direction, so it shrinks mostly in that direction. ACRYLITE's test for "residual shrinkage" (ASTM D702) measures length and width before and after heating above 300 °F, and puts cast sheet at 2%.

How much does cast vs extruded acrylic shrink?

On a 24 in. part, 2% is about 1/2 in. That matters when you heat a whole blank for drape forming or vacuum forming. It matters much less for a line bend, where only a narrow strip along the bend line gets hot.

Does line bending shrink acrylic?

Much less than forming does. Only a narrow band along the bend line is heated, so the flat legs of the part keep their cut size. The heated strip can still pull in slightly, and on extruded sheet the amount depends on whether the bend runs along or across the extrusion direction. For parts with tight overall dimensions, plan to bend first and trim after, or ask us to. See how to bend acrylic and minimum bend radius.

How do you allow for shrinkage when thermoforming?

1. Clamp the sheet in a frame. ACRYLITE notes the shrink figures are for sheet heated without a frame. Clamped sheet can't pull in, so it stretches into the mold instead. Use a continuous frame or many clamps so it doesn't deform between them.
2. Cut blanks oversized and trim after forming. Trimming after forming is standard practice and lets you hold final dimensions.
3. Know the extrusion direction. Extruded sheet is usually marked on the masking. Line it up the same way on every part so they shrink alike.
4. Test one blank first at your actual heating time and temperature, then adjust.
5. Choose cast sheet for deep or precise forms, where its even, lower shrink is easier to plan around.

For the forming side, read can you thermoform acrylic and can you heat-gun acrylic.

Will a formed acrylic part go back to flat if it gets hot?

It can. ACRYLITE notes that heating a formed cast part above about 210 °F (99 °C) may make it revert toward its original flat shape. That's the same "memory" that causes shrinkage. Extruded sheet shows very little memory after forming. In everyday use you'll never get near that temperature, but it's the reason formed parts should be kept away from heaters, lamps, and hot car dashboards. For safe service temperatures, see acrylic heat resistance.

Questions

Sources

• ACRYLITE: Cell cast sheet (GP) physical properties
• ACRYLITE: Premium (FF) physical properties for extruded sheet
• Trinseo: PLEXIGLAS acrylic sheet fabrication manual