Acrylic Tips &Tricks
by Mike Fruciano
The range of products and materials a laser system can engrave or cut continues to grow every year. Wood, plastic, and metals all have a certain appeal when they are laser engraved, but the look of acrylic always has classic good looks with outstanding detail and clarity.
The super fine details of photos and text a laser can produce on acrylic are mostly due to the scientific aspects of this man made material. Acrylic has a unique feature in that it is highly sensitive to absorbing certain wavelengths of light. One highly absorbed wavelength of light energy by acrylic is 10.6 micrometers, which coincidentally, is the exact same output frequency of a Co2 laser. Because of this, acrylic acts like a sponge for the laser energy. Laser systems with output as low as 10 watts can produce excellent, although slow, engraving results on acrylic.
Although the term acrylic is a generic name for a product that is made by many manufacturers, many times it is referred by its product name. Common product names for acrylic are Plexiglas®, Acrylite®, Perspex®, Lucite® as well as many others. Major manufacturers put their brand name on the protective paper and include such chemical giants as Dupont, Eastman, Cyro, and Mitsubishi. There are also a great amount of acrylic companies that focus on low cost. These acrylic products can be quickly identified by a generic brown protective paper, along with having engraving and cutting results that can vary greatly from the name brand products.
Types of Acrylic
For laser engraving and cutting we will focus on two types of acrylics and their applications: cast acrylic and extruded acrylic. Cast acrylic sheets are usually used to fabricate awards and plaques. When cast acrylic is laser engraved, it turns a frosty white color. Cast acrylic sheets are made from a liquid acrylic that is poured into molds that are up to 4 x 8 in size. These large sheets can be molded into several sizes, ranging from a thin 1/16 of an inch to a larger 1 and 2 inch thickness.
First, the plaques and awards are cut from the large cast sheets using saws and high power lasers. Next, each piece is beveled and edged for special effects. Finally, each plaque and award are colored by silk screening and special sprays. This is not an easy process and is certainly worth the price of the finished product from a quality manufacturer.
Along with plaques and awards, shapes like wedges and pyramids can also be cast, polished, and painted to create a truly unique award or gift.
The second type of commonly found acrylic is extruded acrylic. Extruded acrylic is formed into sheets by a large machine much like a printing press. Being a high volume manufacturing technique, this process allows sheets of extruded acrylic to be less expensive than cast acrylic. Since extruded acrylic has a different base material, it engraves clear, instead of frosty white, when laser engraved. Extruded acrylic will cut clean and smoothly and will have a flame polished glass-like edge quality.
The range of colors and textures of acrylic sheets have improved greatly over the years. First introduced as a replacement for glass, acrylic sheets were only available in clear. Today, the color range includes white, black, fluorescent and pastel colors.
When choosing a type of acrylic for your engraving job, remember, cast acrylic products work best for engraving gifts, awards and plaques and extruded acrylic works best for cutting letters and signs.
Acrylic awards, plaques and gifts can be engraved on the top viewing surface or on the back surface so the engraving can view through the acrylic. The latter is the most popular method for engraving acrylic.
When reverse engraving on the back side of the acrylic the graphic and text should be mirrored to make it right reading from the front. The mirror function is easily accomplished using the mirror tool in Corel DRAW. Usually this function has an icon displayed on the top of the page or it can be accomplished by pressing Alt + F9.
Forgetting to mirror the graphic is a common mistake and one I have certainly made more than I admit. As a check of my layout skills I will use a green tinted polyester mask over the top of the acrylic piece and engrave at very low power (5% power, 100% speed on a 30 watt laser system). After I am sure the graphic is correct and engraving in the right position, remove the mask and engrave at the right power for acrylic.
A common issue when laser engraving large fill areas in graphics or text is horizontal lines, which cause the engraving to look coarse. Enlarging the spot size of the focused laser can easily reduce these raster lines. The focus lens brings the beam into a cone shape which gets larger the more distance it has from the optimum focus point. By lowering the engraving table by .020 ", the spot size of the laser is increased slightly to overlap each of the raster strokes, creating smoother engraving. This focus adjustment is a simple and effective method to increase the engraving quality without increasing the engraving time of the job.
Understanding a few tricks of the trade can make acrylic cutting fast and first rate in quality. These tricks involve some print driver settings, preparation of the acrylic sheet and set up on the engraving table. Most folks think the laser is on constantly when cutting but in reality it is being pulsed at a high frequency up to 2000 times per second. Controlling this frequency of switching the laser on and off is referred to in the print driver as 'rate' or 'PPI / pulses per inch'. Adjusting the rate or PPI settings to maximum will achieve maximum pulse frequency of the laser when cutting. The higher rate of laser pulses will give the effect of a fine tooth saw blade, creating smoother cuts. Working in tandem with the power setting of the print driver can derive a melting and smoothing effect that can look as though the edge was flame polished.
Preparing the acrylic sheet for cutting can also have a dramatic effect on the final results. All acrylic sheets have a protective brown paper keeping the polished engraving surfaces free from scratches. This brown paper contains paraffin that can cause charring and excessive flame ups during cutting. Eliminate the problems caused by the factory mask by removing it and replacing it with a medium tack paper mask. The paper mask cuts clean and is easily removed after the cutting operation is complete.
Finally, since the laser energy does not absorb into the metal table of the laser system but reflects off of it, this can burn the bottom side of the acrylic. Creating an air space between the acrylic sheet and the table of the laser system can dissipate this reflection of the laser energy. Cutting tables were created for this reason and work well to reduce or eliminate this issue. The cutting table is a honeycomb grid that allows the beam to pass through and dissipate. Any method to raise the acrylic will be effective to create both an air space to dissipate the reflected beam and increase the airflow underneath for reduced flame ups.
Please remember that acrylic is flammable, and cutting this material is a controlled vaporization process. More than one laser system has gone up in flames from unsafe cutting of acrylic sheet. Never leave the system unattended while cutting acrylic and make sure you have a fire extinguisher nearby just in case. Your exhaust system should meet or exceed the specifications from the laser system manufacturer. Cutting acrylic can produce excellent results when done in a safe and approved manor.
Cleaning and Polishing Acrylic
The brilliance and quality of acrylic can really shine through when the piece has been cleaned after engraving. Sometimes the acrylic is damaged in the cleaning process so knowing the proper cleaning techniques is very important:
- Never use paper towels to clean acrylic, the fibers in the paper towel act like fine sandpaper to make the surface cloudy.
- Do not use window cleaner or any other alcohol or ammonia-based cleaners, fast drying cleaners can craze the top surface with fine cracks and even shatter thin acrylic.
- When dusting or cleaning acrylic use a lint free soft cloth, preferably one that is designed for cleaning acrylic. Anti static cloths are available that zap the static generated in the cleaning process and reduce the build up of dust after cleaning.
- By using an acrylic cleaner a bright and shiny finish will appear on the acrylic. These spray-on cleaners remove any smoke residue from engraving or cutting and leave a glossy surface that looks deep and lustrous.
- Special fine polishers are available to remove hairline scratches and abrasions that sometimes occur in handling the acrylic pieces during the engraving process. These special polishes work miracles on small areas but should be used carefully and according to the instructions for best results.