Laser cutting acrylic: how the beam cuts PMMA

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  How a CO2 laser actually cuts laser cutting acrylic
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  Cast vs extruded PMMA under the beam
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  What the cut edge looks like, and when it goes wrong
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  Where acrylic lasering stops working
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  Drawing rules that keep acrylic parts clean
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  After the cut: bonding, finishing and tolerance stack-up
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  Laser cutting acrylic compared with other acrylic processes
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  Starting parameters for laser cutting acrylic on a CO2 machine

A CO2 laser emits at 10.6 μm, and PMMA absorbs almost all of that energy within a few micrometers of the surface. The sheet does not melt through in one go. Material heats past its glass transition temperature near 105 °C, then depolymerizes back toward methyl methacrylate monomer, and the assist gas pushes the vapor out of the kerf. That is why a properly tuned cut leaves a glossy edge rather than a burnt one.

The kerf is narrow. On 3 mm cast acrylic a typical kerf measures 0.2–0.4 mm, which is small enough that most designers ignore it. It is not zero, though. If you nest a press-fit joint and cut both halves on the same machine, the kerf cancels out. If you cut one half on the laser and the other on a router, the fit will be loose.

Heat has to go somewhere. The beam removes material, but the surrounding sheet still sees a heat-affected zone of roughly 0.1–0.3 mm. On thin sheet that zone is invisible. On 10 mm sheet it shows up as a slight taper: the top of the wall is wider than the bottom because the beam defocuses as it travels down.

Thickness drives everything. A 60 W tube handles 3 mm at a comfortable feed. A 150 W tube pushes through 10 mm, and 20 mm is possible on high-power industrial machines with a longer focal length. Every step up in thickness costs disproportionately more time, so the cost curve is not linear.

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  AbsorptionPMMA absorbs 10.6 μm strongly; polycarbonate and PVC behave very differently.
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  Kerf0.2–0.4 mm on 3 mm sheet; account for it in tight fits.
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  HAZRoughly 0.1–0.3 mm of softened material beside the cut.
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  TaperGrows with thickness; visible above about 6 mm.

Cast acrylic is polymerized in a mold from monomer. Extruded acrylic is pushed through a die as a continuous sheet. The chemistry is the same, but the molecular weight is not, and the laser notices. Cast sheet cuts to a cleaner, more polished edge and machines better afterward. Extruded sheet cuts faster and costs less per square meter.

Extruded sheet has a known weakness at the laser. Because it is stretched during production, the sheet carries internal stress. Cutting releases that stress, and thin extruded parts can bow slightly after cooling. The effect is small on 2 mm sheet and obvious on a long, narrow 5 mm strip. Cast sheet stays flat.

Thickness tolerance matters more than most people expect. Cast acrylic is commonly sold in ±10% thickness tolerance, so a nominal 5 mm sheet may arrive at 4.5 mm or 5.5 mm. If your design has a slot that a sheet edge slides into, cut the slot from the actual sheet, not from the nominal number.

Color changes absorption slightly. Clear, white, black and translucent tints all cut on the same settings within a small window, but opaque white and dense black absorb more energy and may need a slower feed or a second pass. Transparent colors behave almost like clear.

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  CastCleaner edge, no stress bowing, better for visible parts.
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  ExtrudedCheaper and faster, but thin strips can warp after cutting.
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  Tolerance±10% on thickness is normal; measure before you cut fits.
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  ColorOpaque white and black may need slower feed or a second pass.

A correct laser cut on cast acrylic leaves a flame-polished edge. It is glossy, smooth and slightly rounded at the top. Most people consider it a finished surface and skip any further work. If the edge is matte and grey, the feed is too slow and the material is sitting in the beam too long. If the edge shows fine vertical striations, the feed is too fast or the power is fluctuating.

Bubbles in the edge are the most common complaint. They come from trapped gas, usually when the assist gas pressure is too low or the focus point sits too deep in the sheet. Increasing air or nitrogen pressure and raising the focus to just above the surface usually clears them. On 8 mm and thicker sheet, a small amount of edge bubbling is hard to eliminate entirely.

Micro-cracks appear when the sheet is cooled too fast after cutting. This is rare with acrylic, but it happens when cold shop air blows directly across a hot cut. Let parts cool in still air rather than next to an open door in winter.

Laser marking is a separate operation and follows different rules. Our laser marking and engraving has a minimum character height of 1.5 mm. Below that, letters fill in and become unreadable on acrylic.

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  Glossy edgeCorrect power and feed; usually no post-processing needed.
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  Matte or grey edgeFeed too slow; the material is overheating.
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  Vertical striationsFeed too fast or unstable power output.
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  Edge bubblesRaise assist gas pressure, lift the focus point.

Thickness is the first wall. Above roughly 20 mm, the laser kerf becomes badly tapered and the cut speed drops to the point where routing is faster and cheaper. If your part is 25 mm thick, a router with a polished cutter will beat the laser on both cost and edge quality.

Holes have a minimum size. A hole smaller than about 1.5 times the sheet thickness is difficult to cut cleanly, because the beam cannot clear the molten material from a small circle. On 5 mm sheet, keep holes at 8 mm diameter or larger unless you are willing to accept a rougher bore.

Structural parts are not a laser job. Acrylic has low impact resistance and stress-cracks at sharp internal corners. A laser-cut corner is a sharp internal corner unless you design a radius. For anything load-bearing, add at least 1 mm corner radius and consider switching to a machined part in a tougher plastic.

Tolerances differ from machining. A laser holds roughly ±0.1 mm on thin sheet and looser on thick sheet, mostly from taper. If the drawing calls for ±0.005 mm, that is a CNC machining tolerance and no laser will meet it.

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  Above 20 mmTaper and slow speed; routing wins.
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  Small holesKeep hole diameter at 1.5× sheet thickness or more.
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  CornersAdd a 1 mm radius; sharp corners stress-crack.
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  Tolerance±0.1 mm on thin sheet, looser as thickness grows.

Give the sheet a relief path. Acrylic expands about 0.07 mm per meter per degree Celsius, which is roughly seven times the expansion of steel. A large panel bolted on all four sides will bow or crack on a warm day. Use slotted holes or oversized clearance so the panel can move.

Keep the cut path away from the edge. Cutting within 2 mm of the sheet edge often produces chipping or a ragged edge, because there is not enough material to carry heat away. Leave a margin of at least 3 mm whenever the sheet size allows.

Think about how the part will be held. Small parts drop through the bed and can be scratched by the fall. A sacrificial backing sheet or a honeycomb table with a paper mask solves this. Leave the protective film on during cutting and peel it after.

Plan the assembly before you cut. If two laser-cut parts will be solvent-bonded, cut both in the same run and on the same sheet orientation. Kerf and thickness variation then match, and joints close without gaps.

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  Thermal expansionAbout 0.07 mm/m/°C; use slotted holes on large panels.
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  Edge marginKeep cuts 3 mm from the sheet edge where possible.
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  MaskingLeave the protective film on; peel after cutting.
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  BondingCut mating parts in the same run for matching kerf.

Solvent bonding is the standard way to turn flat laser-cut acrylic into a box or housing. A thin solvent such as dichloromethane wicks into the joint by capillary action and welds the surfaces. It only works if the gap is small, which is another reason to cut mating parts together. A gap over about 0.1 mm will not draw solvent and the joint stays weak.

Annealing removes the internal stress that cutting and machining leave behind. A typical cycle for acrylic is a slow ramp to 80 °C, a hold of one hour per 3 mm of thickness, then a controlled cool of no more than 10 °C per hour. Skipping this step is the main reason laser-cut acrylic parts crack weeks after assembly.

If a part needs an optically clear edge, laser cutting is only the first step. The cut edge is glossy but not optically flat. Diamond polishing removes the taper and the rounding, and it is usually done after the part is cut to near-net shape. Ask for it only where the edge will be visible, because it adds cost.

For anything that must be load-bearing, optically clear on all faces, or held to ±0.005 mm, the process changes. We cut acrylic on the laser for flat profiles and switch to CNC machining when the drawing needs pockets, threads or tight tolerances. The same shop runs both, so the decision stays with the geometry rather than the supplier.

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  Solvent bondingNeeds a gap under about 0.1 mm to wick properly.
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  Annealing80 °C hold, slow cool; prevents delayed cracking.
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  Diamond polishingAdds optical clarity; use only on visible edges.
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  Process switchPockets, threads or ±0.005 mm go to CNC.