There are two basic types of acrylic: cast acrylic and extruded acrylic. Cast acrylic is made by casting the poly (methyl methacrylate) resin into a mold or onto a moving steel belt to form sheets. Cast acrylic is ideal for laser engraving because of its premium optical clarity and because the material turns frosty white when laser engraved.
Extruded acrylic is made by forcing the poly (methyl methacrylate) resin between a set of steel dies to form a continuous sheet, which is then cut into individual sheets. Extruded acrylic is ideal for laser cutting because it forms a flame polished edge during the laser cutting process. Extruded acrylic is popularly used for certain applications such as producing display fixtures and, sometimes, for accents on acrylic awards, such as mirroring.
In general, cast acrylic products work best for engraving gifts and awards. Extruded acrylic works best for profile-cutting letters and special shapes where a smooth, polished edge is desired. In either case, though, keep in mind that different types or brands of acrylic sometimes engrave with different results, so most engravers find a variety they like and stick with it.
Acrylic Laser Engraving
Acrylic is unique in that it is highly sensitive to absorbing certain wavelengths of light. One highly absorbed wavelength of light energy is 10.6 microns which, coincidentally, is the exact same output frequency of a CO2 laser. When exposed to a laser beam, acrylic acts like a sponge and essentially soaks up the laser energy. Even low watt lasers—as low as 10 watts—can produce excellent engraved results (albeit slowly) as the laser vaporizes the material.
Acrylic products can be engraved on the front or the back so the engraving can be viewed through the acrylic. Engraving the back of an acrylic piece is often preferred because it gives the item a dimensional quality that really enhances the look of the material. If you do this, however, remember that the graphics and text should be reverse-reading to make them read correctly from the front. You can use the “mirror” function in CorelDRAW (or your engraving software) to easily accomplish this.
Tips for engraving
When setting up a job to be laser engraved, follow the old carpenter’s rule “measure twice, cut once,” especially when engraving expensive acrylic items. One way to check your layout skills is to apply a green-tinted polyester mask to the acrylic piece and use very low power, e.g. 5% power and 100% speed on a 30-watt laser system, to cut only the mask (not the acrylic). Once you are satisfied the image will engrave in the correct location, remove the mask and engrave at the correct power and speed settings for acrylic.
On some occasions, you may experience problems such as cracking, melting, or hazing which usually indicates that there is too much heat present. To remedy this, try decreasing the power and/or increasing the engraving speed. Optimum power and speed settings will depend on the acrylic you are engraving and your laser system. Figure 1 shows some general starting guidelines.