Cutting and engraving acrylic
Acrylic is the material a CO₂ laser is best at. Nothing else gives you a cut edge that comes off the machine already polished, with no finishing at all. But it is also the material where the wrong choice of sheet, or too much air, quietly costs you that advantage.
Cast or extruded? This decides everything else
Section titled “Cast or extruded? This decides everything else”Two manufacturing processes, sold side by side, often not clearly labelled, and they behave completely differently under a laser.
| Cast acrylic (GS) | Extruded acrylic (XT) | |
|---|---|---|
| Engraves | Bright frosted white, high contrast | Barely marks; stays almost clear |
| Cut edge | Glossy, flame-polished | Glossy, and usually slightly better than cast |
| Thickness | Varies more across the sheet | Very consistent |
| Cuts | Needs a little more power | Cuts faster at lower power |
| Price | More expensive | Cheaper |
The short version:
- Engraving? Use cast. Extruded acrylic will not give you the frosted white that makes engraved acrylic worth looking at. This is the single most common disappointment with acrylic, and it is a material choice, not a settings problem.
- Cutting only, especially thin? Extruded is fine, cheaper and very consistent in thickness, which matters if you are cutting press-fit parts.
Getting a polished edge
Section titled “Getting a polished edge”The glossy edge comes from the material melting and re-flowing as it is cut. It is a gentle thermal process, and anything that cools the edge too fast will interrupt it and leave a frosted or matte finish instead.
What helps:
- Turn the air assist down. High-pressure air chills the molten edge and frosts it. Use just enough flow to clear the smoke and keep the flame down. See how much air assist do you need?
- One pass, not several. A single pass at the right speed and power melts and flows the edge. Multiple fast passes chew at it and leave steps and a duller finish.
- Get the focus right. Focus into the material rather than on the surface when cutting thicker sheet. See determining focal length.
- Clean optics. A hazy lens spreads the beam and widens the melt zone, which shows on acrylic more than on any other material. See lens cleaning.
Frosted, cloudy or rough edges
Section titled “Frosted, cloudy or rough edges”Work through these in order:
- Too much air assist. The most common cause by a distance.
- Focus off. The beam is not at its narrowest, so the cut is wider and less controlled.
- Dirty lens or mirrors. Scattered energy, uncontrolled melt.
- Multiple passes instead of one clean pass.
- The sheet itself. Low-grade acrylic with inconsistent composition will not polish well whatever you do. If everything else checks out, try a sheet from a different supplier before chasing settings further.
Cracking and crazing after cutting
Section titled “Cracking and crazing after cutting”It often appears hours or days after the job, when the parts are already assembled.
Acrylic sheet carries internal stress from manufacturing, and the heat of the cut concentrates it around the edge. Add mechanical stress (a tight press-fit joint, an over-tightened screw) or a solvent, and fine cracks spread out from the edge.
How to avoid it:
- Do not force joints. Design a light friction fit, not a hammer fit. See kerf and kerf compensation.
- Do not over-tighten fixings through acrylic, and use a washer to spread the load.
- Be careful with solvents and cleaners near a cut edge, including some glass cleaners and alcohols. They are a classic trigger for crazing.
- Extruded acrylic is generally more prone to it than cast.
- Annealing (a controlled slow warm and cool) relieves the stress, and is worth it for parts that must not fail. It is an oven process, not something the laser can do.
Masking
Section titled “Masking”Most acrylic arrives with a protective film or paper on both faces.
- Leave it on for cutting. It protects the surface from smoke residue and handling marks, and on acrylic the residue is fine but it is still easier not to have it.
- For engraving, it depends. Engraving through the mask gives clean surroundings but can leave the engraved area slightly less crisp. Many people remove the mask on the engraved face and leave it on the other.
- Paper masking behaves differently from plastic film. Paper is more absorbent and tends to burn at the edges; film can melt into the cut. Which you have affects both.
- Remove masking promptly after the job. Adhesive left on under warm conditions gets much harder to remove.
Acrylic produces less visible smoke than wood, which misleads people into thinking it needs less extraction. It does not; the vapour is not something to breathe, and it will condense on your optics. Extraction runs for acrylic exactly as it does for anything else: laser fumes and why extraction is not optional.
Watch out for acrylic look-alikes
Section titled “Watch out for acrylic look-alikes”Clear rigid sheet is not necessarily acrylic. Polycarbonate looks very similar, is often sold alongside it, and must not be cut on a CO₂ laser: it absorbs poorly, discolours, catches fire and leaves a yellow scorched edge. PVC sheet is far more dangerous still.
If you cannot confirm what a sheet is, do not put it in the machine: which materials can you cut with a CO₂ laser.
On a BRM machine
Section titled “On a BRM machine”Starting settings are in the example parameter tables: cutting, 100 W and engraving, 100 W. They are also in the BRM material library for LightBurn.
Treat all of those as starting points. Acrylic varies between suppliers and batches, and the method for dialling a material in from a starting point is in how can I find the right parameters for my material?
Reflection off the bed onto the underside of clear acrylic is a common cause of a marked back face: what can you do about reflection on your material?