THE BASICS: Acrylic Sheet

What are the differences between "Extruded," "Continuous Cast," and "Cell Cast" acrylic sheets?

That's a big question and the answer can get very technical. But to avoid putting everyone to sleep answering it, I'll stick to the primary differences. The obvious difference is that each is manufactured according to a different process.

Extruded Acrylic is manufactured on automated machinery which combines "pellets" of resin with acrylic monomer and catalyst, dispenses the mixture through a "die-block", runs it in semi-molten form through rollers which produce the final thickness, and finish or texture, and cuts and trims the sheets to their final size. Because the machinery is very expensive and requires a fair degree of effort to change rollers and set the die blocks, it is ideally suited for producing large volumes of commodity materials. A typical extrusion company may run as much as 40,000 lbs. of material in one color and thickness before they will begin manufacturing another product. At that volume, it is becomes the most economical way to produce a plastic sheet. At least in the United States.

Continuous Cast materials are also mass-produced, but don't utilize die-blocks or rollers. Essentially, big vats of Acrylic Monomer and catalyst are batch mixed and then poured onto highly polished stainless steel belts about 10'ft wide and 300'long and separated on the top and bottom by a space equal to the thickness of the material desired. These belts continuously convey material through a series of cooling and heating units to regulate the curing of the acrylic and saws at the end of the line cut the material "on the Fly" to it's final size. Because of the high cost of polishing these belts and the capital investment to build a continuous cast sheet line, the cost of materials is typically higher than extruded, but because it is again totally automated, still cheaper in general, than cell-cast acrylic.

Finally, "Cell Cast" sheets are produced by assembling a mold for each individual sheet, and then pouring the acrylic materials into the mold with a funnel or other device. These molds, typically made of two large glass plates separated by a gasket the thickness of the desired material, are then placed into curing tanks, followed by being placed in a post-curing oven. The molds are then cooled, disassembled, and the sheets removed and inspected prior to masking. It is by far the most labor-intensive method for producing acrylic sheets, and although the capital investment in equipment is lower than the previous two technologies, the investment in American laborers is much greater.

So, what does it all mean? Well, the different processes produce materials with different properties, which may effect the end use. Cell Cast acrylic is generally considered to have the best optical clarity of all the materials. It also has greater surface hardness and machines more cleanly than continuous Cast or Extruded. It comes in many more colors and thickness' and is easier to get custom colors and special effects, which is why we've chosen it as our method of manufacturing Acriglas ®. However, it also has the down side of having more thickness variation than the other methods, which may present difficulties in applications where thickness tolerance is very important i.e.: fitting into extrusions, thermoforming applications where walls must maintain uniform thickness, and assembly of complex structures.

Continuous Cast is the next best thing for clarity, and it has the benefit of maintaining uniform thickness as sheets and in thermoformed parts. It is the material of choice for most Skylight manufacturers. Also during thermoforming, it does not exhibit the differences in the amount of shrinkage that extruded materials exhibit. It has the down side of not being as hard as Cell Cast materials which means that it may show scratches more readily and does not machine as cleanly as Cell cast without adjustments to feed rates, bit or blade geometry's, and technique. Additionally, it is not offered in very many colors or thickness', making it somewhat limiting in design options when compared to cell cast.

But the industry workhorse would still be extruded sheet. It comes in a fair selection of colors and sizes, and advancements in extruder technology have allowed the entry of materials of up to one inch in thickness, without noticeable die lines in the sheet which in the past have been a barrier for these material. Extruded sheets are the most uniform in their thickness and because of the automation are also the most economical. They satisfy the needs of most acrylic applications because of the combination of cost and quality. The down side is that they are also the softest in the acrylic family. Fabricators complain of "gumming" during fabrication and more awareness is required to eliminate this as a problem. They also absorb fast drying solvents faster than cell cast and continuous cast, so complaints of joint failures or incomplete glue joints are not uncommon. While this is easily remedied by working wiser and changes to slightly slower drying solvents, many fabricators still expect this material to glue like cast sheets. Finally, this material exhibits the property of shrinking along the extruded direction and expanding across the extruded direction which may present difficulties during thermoforming if the former does not anticipate this in advance.

The bottom line is, understand what your application requires before you select a product to use. Not all clear acrylic is created equal for all applications, and the price should be the last consideration when planning your project, not the first. Most problems occur when purchasing people buy a "deal" on clear acrylic without fully understanding its intended end use. This unfortunately is not the fault of the Acrylic chosen, just the person choosing it unwisely.

Written by Dan Rustin, Marketing Manager, Acrilex, Inc., manufacturers of Acriglas® cast acrylic sheet.