Acrylic laser cutting is mostly a heat-management problem, and the sheet rarely complains until the parts do. A shop in Toronto cut forty 12 mm cast acrylic brackets on a 100 W laser and stacked them for assembly the next morning. Nothing fitted. The edge was frosted, the measured width was 0.4 mm narrow at the top of each cut, and the parts had been nested tight enough to heat-soak the sheet between passes.
Why an acrylic laser cutting edge goes white
A white rim is not a sign of insufficient power. It is a sign of too much heat in too little time. The beam vaporises the polymer faster than the assist gas can clear the melt, so the kerf wall re-solidifies as a frosted layer of roughly 0.1 to 0.3 mm.
Three settings cause it, alone or together: power set high enough to cut the thickest part on the nest, speed set low to compensate, and a focal point pushed below the top surface so the beam is already diverging when it enters the material.
- 3 mm: 40 to 60 W, 12 to 18 mm per second, focus at surface
- 5 mm: 60 to 80 W, 8 to 12 mm per second, focus 0.5 mm below surface
- 8 mm: 80 to 100 W, 4 to 6 mm per second, two passes if the edge matters
- 12 mm: 100 to 130 W, 1.5 to 2.5 mm per second, nitrogen assist preferred
- 15 mm and above: expect a frosted edge and plan a finishing pass
These are working ranges, not recipes. Every tube ages differently, so the first sheet of any run should be a test square cut at three speed settings.
Kerf, taper and the fit that follows
A laser removes material, so every cut is narrower than the drawing by half the kerf on each side. At 0.2 mm kerf that is 0.1 mm per edge, and on a 30 mm slot the difference is enough to turn a sliding fit into an interference fit.
Taper is the second effect: the top of the cut is typically 0.1 to 0.2 mm wider than the bottom on thick sheet, because the beam diverges as it travels. If the part is measured at the top and assembled at the bottom, it will bind.
Both are solved by cutting a coupon and measuring it, not by adjusting the drawing until the numbers look right. Offset the drawing by the measured kerf and specify which face is the datum.
Assist gas and the edge colour
Compressed air is cheap and leaves a slightly warm tint on cast acrylic at thicknesses above 8 mm. Nitrogen gives a cleaner, cooler edge on thick sections and costs more per hour. On 3 to 5 mm signage work the difference is invisible; on 12 mm furniture parts it is obvious.
Nesting without heat soak
Acrylic conducts heat poorly and stores it locally. Parts nested 2 mm apart share a hot zone, and the second cut startles the first part. Leave 5 to 8 mm between parts on sheet above 8 mm, and never nest a thin margin along the sheet edge, which distorts first.
Cutting order matters too: work from the centre outward, or the sheet relaxes mid-job and the beam loses focus relative to the surface.
For sheet that has already been stressed by previous operations, the fabrication notes in the cutting and polishing guide cover the annealing and cleaning steps that reduce cracking.
When routing beats the laser
Above 15 mm, on parts that need a polished or bevelled edge, and on any cut where the kerf tolerance is tighter than 0.1 mm, routing wins. It leaves a machined edge that can be flame-polished and it holds a dimension without a coupon test.
The comparison that matters is cost per finished part, not cost per metre of cut. A laser part that needs a second finishing operation is usually the dearer one. The CNC machining note sets out the tolerance and edge options that follow.
Send your cut file and thickness list to oversales@rpplexiglass.com; we will confirm kerf, edge finish and nesting before the first sheet is cut.
Frequently Asked Questions
Why is my laser-cut acrylic edge white?
Usually too much power at too low a speed, or a focal point set below the surface. The material vaporises unevenly and leaves a frosted rim. Cutting in two passes at moderate power gives a cleaner edge.
What kerf should I allow for?
Between 0.15 and 0.3 mm depending on lens and thickness. For a slot fit, cut a test pair first: a nominal 0.2 mm allowance on each side is normal for 5 mm cast acrylic.
Can a laser cut 20 mm acrylic?
It can, badly. Above roughly 15 mm the edge quality collapses and the cut time becomes uneconomic. Routing or sawing with a polished edge is the better route at that thickness.