Acrylic has become an incredibly popular material, from the sign and POS (point-of-sale) industries to medical devices and outdoor furniture. Acrylics have found their way into CNC routing operations due to its strength, ease of handling, transparency, and flexibility in use. But what exactly is acrylic, how does it compare to other plastics and materials, and what is the best way to route?
Acrylic is a high-grade, clear plastic, made primarily from petroleum. Acrylics are synthetic plastics derived from natural gas, prepared from acrylic acids. The most common acrylic plastic is polymethyl methacrylate (PMMA), which is sold under various brand names such as Plexiglas, Lucite, Perspex, and Crystallite. There are two main kinds of acrylics: extruded and cell-cast.
Extruded acrylic is produced by a process in which liquid plastic is pushed through rollers, which press it into sheets as it cools. The sheets which are produced from this production process are softer than cast acrylic, can scratch easier, and may contain impurities. Even so, extruded acrylic is still generally considered to be good quality and is usually the more common type made available on the market. Extruded acrylic has a more automated production process and thus is usually less expensive than cell-cast acrylic.
In cell casting, single sheets are made by pressing the liquid plastic between pieces of a mold, often made of glass. This mold is then taken through a gradual heating process with the resulting sheet being much stronger than extruded acrylic. This strength makes it far more suitable for aquariums, awards, and other products that require greater strength as well as shaping or machining of the final product. Cell-cast acrylic also tends to be more scratch resistant than extruded. To make cell cast acrylic, it is necessary to assemble and disassemble individual molds for each sheet of acrylic to be made, which in turn increases the manpower needed and overall cost.
Acrylic has a number of highly valuable characteristics:
- Lighter than glass by half.
- Acrylic sheet has from 6 to 17 times greater impact resistance than ordinary glass in thicknesses of .125″ to .250″.
- Offers good insulting properties.
- Colourless acrylic is as transparent as the finest optical glass, with a total light transmittance of 92%.
- Acrylic is the most weather resistant transparent material; it will not yellow with direct sunlight.
- Low water absorption.
- Scratches and imperfections can be buffed out.
When routing acrylic, it is important to pay attention to manufacturer’s guidelines when selecting the appropriate tooling. For example, spiral cutters tend to produce better finishes at smaller diameters, while straight flute bits are better for larger diameters. It is also important to use the appropriate RPM to speed ration, as using the wrong machine parameters can creating too much heat causing the acrylic to melt and craze.
Tips for Cutting Acrylic
When cutting acrylic, the same depth as the cutting bit diameter is recommended for 1/8” or smaller. As the tool diameter gets bigger, the bits get become stronger and it is possible to cut deeper than a 1:1 ratio. A favorite programming method is to use multiple depths of cut when cutter breakage is an issue and then to take a final clean-up pass of .015” for the entire material thickness. This gives a premium edge finish while preventing broken tools in the smaller diameters. It is a common concern that taking finish passes in small parts will cause the parts to move once they have been cut away from the scrap, particularly in intricate parts like letters. The best solution is to use the multiple depth pass/single finish pass method described above, but to not cut through the paper masking on the bottom side of acrylic sheets. This allows the vacuum to continue holding the parts, while the .015” finish pass will not typically tear the parts off of the masking.