Laser Cutting Acrylic: Cast vs Extruded, Settings and Tips
CO2 lasers cut clear acrylic cleanly; typical 3 mm settings are roughly 6-10 mm/s at 70-90% on a 40 W machine, tested on scrap. A 445-455 nm diode laser cannot cut clear acrylic because the light passes through it, but may cut opaque black or dark sheet slowly.
Cutting acrylic with a CO2 laser is one of the most satisfying things a laser can do. The beam draws a line, the edge turns glass-clear on its own, and you peel the protective film off a sign that looks like it came from a shop. Then comes the other kind of day: the cut edge is frosted, the sheet cracks, a flame licks up from the bed. Let us make sure you only get the first kind.
What laser cutting acrylic needs
Acrylic (PMMA) absorbs the 10.6 micrometre CO2 wavelength very well, so CO2 lasers vaporise it into a polished edge. Diode lasers use 445-455 nm blue light that passes through clear acrylic and hardly heats it. The result:
| Laser | Clear acrylic | Black / dark opaque | Notes |
|---|---|---|---|
| 5-10 W diode | Cannot cut | Slow, 3 mm at best | Engraving works poorly on clear |
| 20-40 W diode | Cannot cut clear | Can cut thin dark sheet | Many passes, poor edge |
| 40 W CO2 | Cuts 3-6 mm | Cuts | Best value for acrylic |
| 100 W CO2 | Cuts 10-20 mm | Cuts | Fast, polished edges |
If you only own a diode, read can a diode laser cut acrylic before you buy any sheets.
Cast vs extruded acrylic
Both are PMMA, but they are made differently, and the laser notices.
| Property | Cast | Extruded |
|---|---|---|
| Engraving | Frosty white, high contrast | Smoother, lower contrast |
| Cut edge | Clean, often slightly frosted | Often glossier, flame-polished |
| Thickness tolerance | Looser | Tighter |
| Solvent gluing | Fair | Easy |
| Price | Usually higher | Usually lower |
Cast is the choice for engraved signs. Extruded is a good, cheap choice for cut-only parts. Full comparison: cast vs extruded acrylic.
Typical settings for 3 mm acrylic
These are typical starting points on a CO2 laser. Test on scrap.
| Laser | Cut speed | Power | Passes | Engrave |
|---|---|---|---|---|
| 40 W CO2 | 6-10 mm/s | 70-90% | 1 | 200-400 mm/s at 15-30% |
| 60 W CO2 | 10-18 mm/s | 70-90% | 1 | 250-450 mm/s at 12-25% |
| 100 W CO2 | 18-30 mm/s | 60-85% | 1 | 300-500 mm/s at 10-20% |
Thicker sheets need slower speeds, and 6 mm needs about half the speed of 3 mm. Laser engraving vs cutting covers the difference in more detail.
Step by step
- Leave the protective film on both sides (it keeps smoke off and reduces flare marks).
- Place the sheet on pins or slats instead of flat on a honeycomb, which reflects the beam and marks the underside.
- Focus on the top surface; for thick sheets, focus about a third into the material.
- Use low to moderate air assist; high air can cool the cut and frost the edge, no air risks flames.
- Cut; leave the lid closed until the smoke clears.
- Peel the film slowly; wipe with soapy water, not solvent.
Pro tip: for clear acrylic, engrave from the back in mirrored mode. The frosted design then shows through the glossy front like it was floating.
Common mistake: wiping cut acrylic with alcohol or cleaning sprays. Solvents can make stressed edges craze into a web of fine cracks. Use mild soap and water.
Problems and fixes
| Problem | Cause | Fix |
|---|---|---|
| Frosty cut edge | Too fast, too much air | Slow down, reduce air |
| Cracks near cuts | Internal stress, cold sheet | Warm to room temperature, use cast |
| Flames in the cut | No air, slow speed | Low air assist, faster speed |
| Bubbled, melted edge | Too slow or power too high | Raise speed, reduce power |
| Shiny marks underneath | Honeycomb reflection | Use pins or slats |
| Sheet melts together after cut | Residual heat | Cut small details first, let cool |
Safety
Acrylic fumes carry the monomer methyl methacrylate, with a strong sweet smell. Extract outside. Acrylic burns well, so never leave the machine running unattended and keep a fire extinguisher nearby; see laser cutter safety. Do not confuse acrylic with polycarbonate, which should not be cut: it chars and gives off harmful fumes. What materials can you laser cut lists the safe ones.
Colours, mirrors and special acrylic
Not all acrylic reacts the same way. Clear sheet engraves nicely and cuts to glass; coloured and black sheets cut similarly but show engraving differently (black engraves to grey-white, white engraves to light grey with little contrast). Mirror acrylic has a metallic backing layer: it cuts, but you must put the mirrored side down, and the coating can stop the beam from marking. Fluorescent and edge-lit acrylic lights up from the edge when you add LEDs, which is why makers love it for night lights.
| Type | Cuts on CO2 | Engraves | Tip |
|---|---|---|---|
| Clear cast | Yes | Frosty white | Engrave from the back |
| Black or dark | Yes, also on strong diodes | Light grey, very visible | Best match for dark diode work |
| White | Yes | Low contrast | Use for cut-only designs |
| Mirror | Yes | Removes the coating | Mirror side down |
| Frosted | Yes | Hardly visible | Cut only |
Edge finishing and gluing
A good CO2 cut leaves a nearly polished edge. If yours is a little rough, a quick pass of a flame polisher or a soft cloth on fine wet-and-dry (400, then 800, then 1500 grit) brings the shine back. Do not wave a lighter around on a thin part; it warps and blisters.
For joins, solvent cement fuses the surfaces and gives an almost invisible joint, especially on extruded sheet. Apply a thin line with a needle applicator, hold the parts for a minute, and give them a day to cure. Never use cyanoacrylate glue on clear acrylic you want to look good: it fogs the surface with a white bloom.
Common mistake: assembling parts while the edges are still hot or fresh, then gluing with solvent. Hot stressed edges can crack at the joint a day later. Let the parts rest and cool first.
Thickness, kerf and tabs
Acrylic cut kerf is typically about 0.1-0.25 mm, similar to plywood, which is small enough to ignore for loose parts but matters for press-fit joints. See how to measure kerf. For thicker sheets the cut face is slightly tapered, so measure the actual fit on a test joint before you cut a whole box.
Quick pre-flight checklist
| Check | Why it matters |
|---|---|
| Sheet is real acrylic (PMMA), not polycarbonate | Polycarbonate chars and gives off harmful fumes |
| Protective film is on, both sides | Less smoke on the surface |
| Exhaust is running and vented outside | Monomer fumes irritate eyes and lungs |
| Sheet is raised off the honeycomb | Avoids reflection marks |
| Test square cut first | Every batch behaves a little differently |
Next step: buy a small offcut of 3 mm cast and one of extruded acrylic, cut a 20 mm test square from each at the table speed, and compare edges in daylight.
Questions and answers
Can a diode laser cut acrylic?
A blue diode (445-455 nm) cannot cut clear acrylic because the light passes through it. It may cut thin opaque black or dark acrylic slowly.
Should I remove the protective film before laser cutting acrylic?
No. Leave it on to keep smoke off the surface. Peel it after cutting.
Why does my cut acrylic crack?
Internal stress, cold material or too much heat. Cast acrylic is usually less prone to it.
What speed should I cut 3 mm acrylic on a 40 W CO2?
Roughly 6-10 mm/s at 70-90% power, tested on scrap.
Is laser cutting acrylic safe indoors?
Only with proper extraction to the outside. The fumes irritate and acrylic can catch fire.