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Laser engraving acrylic: cast or extruded, clear or dark

How to laser engrave acrylic: why cast sheet frosts white and extruded does not, why a diode laser passes through clear acrylic, and how to engrave the back.

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Two things decide how acrylic engraves: how the sheet was made and what kind of laser you have. Cast acrylic turns frosty white where a CO2 laser touches it, which is the look of engraved awards and edge-lit signs. Extruded acrylic engraves clear or dull grey and is better kept for cutting. A CO2 laser engraves any acrylic, clear included. A blue diode laser passes straight through clear acrylic and only marks sheet that is opaque or has been coated first.

Cast or extruded

Both are the same plastic, PMMA. They differ in how the sheet is formed, and they behave differently under the beam.

Cast acrylicExtruded acrylic
How it is madeLiquid acrylic poured between plates or into mouldsAcrylic pushed through a die to the thickness wanted
Engraving“it turns a frosty white color when engraved” (Epilog)“instead of a frosted look you will have a clear engraving” (Epilog). Trotec describes it as matt grey
CuttingCuts, but without a flame-polished edge (Epilog)It “will cut cleanly and smoothly and will have a flame-polished edge” (Epilog)
ThicknessWider tolerance, about ±15% (Trotec)More accurate
CostHigherLower
Trade lettersGSXT

Epilog’s summary is the one to remember: “cast acrylic is better for engraving, while extruded acrylic is better suited for laser cutting.”

You cannot tell them apart by looking. xTool’s guide notes that the two “look exactly the same”, so read the supplier’s description, and look for GS or XT in the product name. If you know a sheet is acrylic but not which kind, engrave a corner with a CO2 laser: frosty white means cast.

Which laser

CO2. xTool’s guide gives a CO2 laser’s wavelength as 10,600 nm, which is “readily absorbed by acrylics”, clear and coloured alike. It is the tool for clear acrylic.

Blue diode. xTool gives a diode laser’s wavelength as 450 nm, which it says “cannot be absorbed by clear acrylic and other transparent materials”. The light “passes through the clear acrylic, leaving it intact”. Dark and opaque acrylic absorbs it, so a diode laser can engrave and cut those.

To engrave clear acrylic with a diode, xTool’s method is to give the beam something to absorb: cover the surface with a masking layer that washes off with water, such as tempera paint, watercolour, chalk paint or dry-erase marker, and engrave through it.

There is a safety point in this. In xTool’s words, diode lasers “pass straight through clear acrylic”. A sheet that the beam passes straight through does not stop it, so do not treat clear acrylic as a shield for a blue laser, and follow the rules on enclosures and goggles in laser engraving tips.

Engraving the back

Most engraved clear acrylic is engraved from behind. Epilog explains that laser users generally “engrave acrylic on the back side to produce a look-through effect from the front”. The front stays smooth and glossy, and the engraving seems to float inside the sheet.

  1. Mirror the artwork. You are engraving the back, so the design must be flipped left to right or text will read backwards from the front.
  2. Peel the film from the back only. Epilog recommends “removing the protective adhesive paper from the back of the acrylic before engraving, and leaving the protective cover layer on the front to prevent scratching while handling”.
  3. Engrave fast and light. “Acrylics generally engrave well at a high speed and low power”, and “if your power is too high you’ll notice some distortion in the material” (Epilog).
  4. Clean with water. Trotec’s cleaning advice for acrylic is clean water or a mild cleaner on a damp cloth, wiped away from the engraving, then a dry cloth. Avoid alcohol and strong cleaners, which Trotec warns can make the material brittle.

Engraved acrylic is often lit. xTool describes the LED versions: when the lights shine through the acrylic, “the engraved areas catch the light”. xTool suggests sheet at least 3 mm thick so the design stands out.

Finding the settings

Start from the acrylic settings supplied with your machine and burn a test grid on an offcut of the same sheet: small squares, with power rising one way and speed the other. In LightBurn 2.1 the grid is under Laser Tools, then Material Test. You are looking for the square that is evenly white with crisp edges. Squares that look melted or distorted had too much power.

Two details from Trotec help keep the result clean:

  • Engrave only as deep as you need. A shallow engraving is sharper and leaves less residue.
  • Engrave from the bottom of the design to the top, so the extraction draws smoke away from the fresh engraving and not across it.

Photos on acrylic

A photo engraved on acrylic is made of dots, like any engraved photo, and the dots are white. On a clear sheet engraved from the back and viewed against a dark background, white dots are the light parts of the picture. That is the reverse of wood, where the dots are dark. Epilog’s photo guide says photos “are often inverted for black marble, granite, and clear acrylic engraved from the back”.

So for clear acrylic:

  1. Invert the photo, so that the laser fires in the light areas.
  2. Mirror it, because it goes on the back.
  3. Dither it at your laser’s resolution.
  4. Run a small test first. Epilog notes: “The same photo may require different dithering settings on wood, acrylic, anodized aluminum, or stone.”

Plotcaster does the first and third steps: turn on Invert under Photo, choose a dither, and export a PNG at your laser’s DPI. Mirror the photo in a photo editor before you load it. Dithering for laser engraving and what DPI for laser engraving photos cover the choices. For dark acrylic engraved from the front, use LightBurn’s rule: its documentation says to invert an image for a “material whose original surface is darker than the mark your laser will make on it”. No mirroring is needed on the front.

Mirrored acrylic

Trotec engraves its own mirrored acrylic sheet with a fibre laser, “which only engraves the mirror layer” and not the acrylic itself. Epilog’s advice, given for glass mirrors, is to engrave the reverse side, flip the artwork, and “start with a lower output and increase it as necessary”, since coatings vary in thickness. Find out from the supplier what the backing is made of before you laser it.

Safety

  • Do not leave it. Epilog: “Never leave your machine unattended when working with acrylic.” Acrylic “has been shown to be especially flammable when cut with the laser”.
  • Be sure it is acrylic. Glowforge warns: “A lot of plastics look the same and may not be laser-compatible.” Epilog names hydrogen chloride and vinyl chloride, “mostly found in PVC”, as hazardous to a laser, and Glowforge says vinyl gives off gases “that can hurt you”, so PVC must never be lasered. Epilog says polycarbonate discolours when cut. Buy from a supplier who states the material.
  • Extract the fumes, as with any material. Laser engraving tips covers ventilation, eyes and fire.

Questions

Why is my engraving clear, not white? The sheet is probably extruded. Cast acrylic is the one that frosts.

Can a diode laser engrave clear acrylic at all? Only through a coating that absorbs the beam, such as washable paint. Bare clear sheet lets the beam through.

Should the protective film stay on? Epilog takes it off the side being engraved and leaves it on the other. xTool notes that plastic film melts under the beam and spoils the design, so a plastic film must come off the engraved side.

Can a Cricut engrave acrylic? A Cricut Maker cuts a fine groove in it with a metal point, which is a different process. See the Cricut engraving tool.

Is engraved acrylic the same as etched acrylic? The laser makers’ pages use the two words for the same thing: a shallow frosted mark in the surface.

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