MORE ON THE MANUFACTURING OF ACRYLIC SHEET
In our January/February 2000 issue, "The Basics" column discussed the differences between "Extruded", "Continuous Cast", and "Cell Cast" acrylic sheet. The article was written to briefly illustrate the different production methods used to manufacture these three types of sheet with particular reference to how it impacts their cost and fabrication. As a follow-up to that article, the following column provides a more detailed look at the manufacturing process and includes a recap of the key differences in the finished products. For reference, the original article can be found in the "Articles" section of our website at www.plasticsmag.com.
Acrylic sheet is commonly manufactured by one of three techniques. All offer unique benefits from cost and physical properties to variety of color and finish. Depending on the fabricators needs, one type may be more appropriate than another. This article is meant to provide a layman's explanation of the manufacturing methods and reiterate the differences between the end products. Volumes have been written on these processes and our intention is only to help differentiate them for you.
Extruded Acrylic Sheet
Extrusion is a continuous production method of manufacturing acrylic sheet. In the process, pellets of resins are fed into an extruder which heats them until they are a molten mass. This mass is then forced through a die as a molten sheet. The molten sheet is fed to calender rolls, the spacing of which determine the thickness of the sheet and in some cases the surface finish. The continuous band of sheet may then be cut or trimmed into its final size.
In the manufacture of acrylic resin there is not a 100% conversion of monomer to polymer. The monomer is the unpolymerized form of a compound, in this case methyl methacrylate, which is the major component of the polymer. The polymer is a chemical molecule formed by the successive addition/joining of monomer units to form a molecule of high molecular weight. In the case of acrylics this polymer molecule is a chain of perhaps 100-1000 units (monomer) in length. A few percent of monomer may remain in the resin if the manufacturer has not removed the major amount of it in the pellet formation. Further, resin absorbs water if it is exposed to humid air. However, the feed stock, or pellets going to the extruder, will usually contain a small amount of water, if it has not been dried thoroughly before being processed. In an effort to obtain high quality sheet with a high yield, some production lines extract, in early stages of the melting process, the monomer and water from the feed stock. This may give some observers the false impression that monomer is being added to the pellets rather than being removed.
As an aside, monomer which is left in the molten resin can cause bubbles or streaks in the extrudate. Monomer left in sheet can reduce the hardness, promote crazing when the sheet is fabricated, and in extreme cases lower physical properties such as tensile strength and modulus of elasticity (rigidity or stiffness). The severity of these phenomena will depend on the level of residual monomer.
The final product of extrusion exhibits much closer thickness tolerances than cast sheet. Because of the volume at which extruded sheet is produced, it is the most economical form you can buy. It is available in a fair selection of colors, finishes and sizes. Extruded acrylic sheet is prone to shrinking along the extruded line and expansion across it. This is of particular note if you plan to use it for thermoforming. Extruded material also has a tendency to gum during fabrication when the cutting is too fast (linear feed rate) because it has a lower molecular weight. It also may absorb fast drying solvent cements faster than cast or continuous cast material. This can result in joint failures and incomplete gluing. To remedy this, use slower drying cements and tooling designed for cutting extruded acrylic.
Continuous Cast Acrylic Sheet
Continuous Casting is also a mass production form for manufacturing acrylic sheet. The process involves the pouring of partially polymerized acrylic (somewhat less viscous than "Karo" syrup) between two highly polished stainless steel belts. The belts are separated by a space equal to the thickness of the sheet and the "syrup is retained by gaskets at the edge of the belts. The belts move through a series of cooling and heating units to regulate the curing and are cut "on the fly" to size at the end of the production line.
One further advantage is achieved by the partial polymerization of the material prior to casting. Some of the heat of polymerization, heat which is evolved as the liquid monomer is converted to a solid resin, is removed before the "syrup" is fed to the belt system. This contributes to the control of the continuous process.
The final discussion of the cost of the continuous cast sheet process merits a comment. The material cost for continuous cast vs. extruded sheet favors continuous casting. The cost of monomer is much less than that of polymer pellets. Further, the production cost per unit of product, lbs./hr./$ capital investment, will be less for extrusion. A statement as to which process is more economical will depend on the capacity and utility of the facilities being compared as well as the cost of materials and equipment.
In terms of competitive pricing between continuus cast and extruded acrylic, there are a dozen extruders of acrylic sheet and only two continuous cast manufacturers in the U.S. This will have obvious effects on competitive pricing.
Cell Cast Acrylic Sheet
Cell Casting historically has been carried out using any of three processes. The first is the water bath technique. Acrylic syrup is poured into a mold typically constructed from two tempered glass sheets separated to produce the desired thickness of the sheet and sealed with a gasket at the edge. The mold is submerged in a bath which maintains/controls a curing temperature and efficiently removes heat generated in the process when the monomer is converted to polymer. Note - Monomer is not used in cell casting. The viscosity of acrylic monomer must be raised somewhat to avoid leakage of the molds or cells.
The two other casting techniques currently used include the original process which involved placing the "molds" containing a "syrup" in a circulating air oven in which air at a controlled temperature passes at a moderately high velocity over the surface of the mold.
The third method is an advancement over the water bath process, was developed by the Polycast Company. It involves the use of a piece of equipment similar to a plate and frame filter press. Sections which serve as the mold for the sheet are alternately configured with sections through which water at a regulated temperature is circulated to promote the polymerization and cure of the sheet.
Cell Cast products should be subjected to a "post-cure" or "annealing" process. The sheet, as taken from the oven, bath or casting machine, will usually have a "high" residual monomer content. The polymerization process is not complete. Post-curing reduces the residual monomer content and serves to insure that no bubbling of the sheet occurs if the sheet is heated for thermoforming.
In casting sheet from syrup a change in density (specific gravity) of the ingredients occurs. A shrinkage of about 20% is experienced. In the cell casting process most shrinkage occurs in the thickness direction. The surface of the mold (tempered glass sheet) restrains the plastic sheet from shrinkage in the length-width direction. Hence, the annealing, in which the sheet is heated to its softening point, allows it to relax or shrink removing residual stress.
Which sheet is right for your fabrication job? Cell Cast products provide optical clarity, greater surface hardness and machine cleanly. They are offered in many colors, finishes, and thicknesses They do, however, have greater thickness variation making them less desirable for fitting into extrusions or thermoforming where uniform wall thickness must be maintained.
Continuous Cast acrylic offers good optical clarity, more uniform thickness and limited shrinkage during thermoforming. However, it is not as hard as Cell Cast material which means it is prone to show scratches and does not machine as cleanly without adjustments to feed rates and tool geometry. Being mass produced, it also does not offer as many color and thickness options.
Extruded sheet is by far the most uniform and usually the most economical. It is offered in a growing number of colors, finishes and thicknesses which should satisfying most acrylic applications.
Written by George Graf, Graf Enterprises, Inc. Mr. Graf has over 50 years working in the acrylic resin manufacturing and processing industry. His background includes 39 years with Du Pont and 15 years as a consultant to the industry. Recognized as an authority in the economics of plastic processing and converting; he holds patents for fabric reinforced resin constructions and acrylic polymer manufacture. Mr. Graf is now retired. If you wish to contact him you may do so care of this magazine.
For more information, contact the author via the contact information listed at the article header and footer.