How do I cut acrylic or polycarbonate sheet without cracking it?

The two materials look alike and behave completely differently. Acrylic (Perspex, Plexiglas) is hard, glass-like and brittle, so it scores and snaps cleanly and cracks if it is stressed. Polycarbonate is tough and slightly flexible, so it will not score and snap but tolerates sawing and drilling much better. Knowing which sheet you have decides the method.

Safety

What you'll need

• Plastic scoring knife (optional)
• Straightedge and clamps
• Fine tooth count TCG saw blade or a plastics blade (optional)
• Backing board
• Eye protection
• Fine file or wet-and-dry paper

Step by step

1. Identify the material.Acrylic sounds hard and rings when tapped, snaps rather than bends, and a scored line breaks cleanly. Polycarbonate flexes noticeably, is far harder to break, and is what greenhouse and roofing sheet usually is. If a corner offcut bends without breaking, it is polycarbonate.
2. Leave the protective film on both faces.It stops the surface being scratched by the straightedge, by the saw shoe and by swarf, and it holds the surface at the cut line. Peel it only when the piece is finished and fitted.
3. For thin acrylic, score and snap.Clamp a straightedge on the line. Draw a plastic scoring knife along it under firm pressure five to ten times, cutting a groove roughly a third of the way through. Then lay the sheet with the score directly over the edge of a batten and press down evenly on both sides. It breaks along the line.
4. For thicker sheet, saw it — with the right blade.A fine tooth count blade with a triple-chip grind and, ideally, a negative or zero hook angle. A blade sold for laminate or for non-ferrous metal is the right kind. A coarse aggressive wood blade grabs, chips and cracks.
5. Support the sheet completely and clamp it.Sheet plastic that vibrates cracks. Lay it on a full backing board with the good face down, and clamp near the cut on both sides. Support both sides of the cut line as with any material.
6. Keep the feed moderate and constant.Too slow and the blade rubs, heat builds, the plastic melts and re-welds behind the blade, seizing the cut. Too fast and it chips. Aim for a steady rate that produces chips, not molten strings. Stopping mid-cut leaves a melted patch that is very hard to recover.
7. Use a jigsaw with a plastics blade for curves.A fine-toothed plastics blade, pendulum action off, low speed. Keep the shoe flat and taped to protect the surface. Cooling the cut with a damp cloth or a trickle of water helps in thick sheet.
8. Never let the offcut fall free on thin sheet.The weight of a large offcut swinging as it separates cracks the last few centimetres. Support the waste side right to the end.
9. Finish the edge.File or sand the edge with progressively finer wet-and-dry paper. On acrylic the edge can then be polished clear with metal polish, or flame-polished by a specialist. On polycarbonate, sanding smooth is enough.
10. Drill with care, if at all.Standard twist drills grab and crack acrylic. Use a drill bit ground for plastics, or blunt the leading edge of a standard bit slightly, run slowly, back the sheet with scrap, and keep holes well away from edges — at least twice the hole diameter.

Tips

• Acrylic expands and contracts with temperature far more than glass or timber. Drill oversize clearance holes for any fixing so the sheet can move, and never screw it down tight.
• A router with a straight two-flute bit gives the cleanest possible edge on acrylic, cutting rather than chopping — but only with light passes and a good extraction setup, as the swarf is stringy.
• Polycarbonate roofing sheet has a UV-protected face, usually marked on the film. Cut with that face up so you do not fit it upside down.
• For a one-off straight cut in thick sheet, a plastics supplier will usually cut to size on a panel saw for very little.

Common mistakes

• Using a coarse wood blade — Big aggressive teeth grab the brittle surface and start cracks that run away from the cut line.
• Removing the protective film before cutting — It is doing three jobs — scratch protection, chip support and a surface to mark on.
• Feeding too slowly to be careful — Slow feed means rubbing, which means heat, which melts the plastic and welds the kerf shut behind the blade.
• Snapping acrylic over an edge without a full-length support — Uneven pressure sends the break off the score line and across the sheet.

Questions people ask