Advantages Mold Acrylic Into a Continuous Success

With benefits aplenty and an ever-growing catalog of applications, acrylic has become a juggernaut among thermoplastics.

BY KEVIN BASTIAN

When driving down a street or casually strolling through an overcrowded shopping mall, the average person rarely, if ever, takes the time to stop, look and devote attention to what goes into the makings of his or her surroundings. However, if this were to occur, that person would come across a surprising and overwhelming multitude of products made from acrylic.

Acrylic, or polymethyl methacrylate, sheet is a glassy thermoplastic used for cast and molded parts. Industries have found it to be a good material for applications that require high-impact resistance and good weatherability among various beneficial properties. With polycarbonate as its chief competitor, acrylic continues to venture into new territories with fresh developments and groundbreaking technologies on its back.

To construe the driving force behind the material's progress, Plastics Machining & Fabricating asked leading manufacturers the following:

1. What are some of the most common and unique uses for acrylic?

2. What do you consider to be the outstanding properties of acrylic?

3. What developments do you foresee in the next decade for acrylic?

4. What challenges does acrylic pose in regard to fabricating and machining?

Although responses varied from one representative to the next, all shared a favorable perception of the qualities and performance of acrylic.

Acrylic Sheet Manufacturing
Four basic methods exist in the realm of acrylic sheet manufacturing: cell cast, continuous cast, extrusion and continuously manufactured. "There's a right product for each application," said Grant LaFontaine, technical manager for sheet products for Cyro Industries in Orange, CT. "They all have their pluses and minuses."

Certain applications require different properties. Cell casting utilizes the chemical ingredients of acrylic such as methyl methacrylate monomer, pours them between two sheets of glass separated by a gasket and seals them. A heat process then causes a reaction to occur, producing a good surface quality and optics, equivalent to that of the glass used to seal it.

In explaining the benefits of cell casting, LaFontaine said, "When you view a material at a low angle, there's less distortion &endash; it looks much smoother. This makes it an ideal product for museum cases. From anywhere in the room, the museum wants you to see whatever's on display with an undistorted view. This is where cell casting works best."

The logic behind continuous cast acrylic lies simply in its name; chemicals are successively poured in one end of two highly-polished steel belts, where heating and cooling take place, and come out at the end as sheets. This process resembles cell casting except the glass sides are replaced by the turning belts. Larry Heinzelman, product manager for Aristech Acrylics LLC of Florence, KY, said he prefers to use the continuous casting method. "Continuous casting makes for higher molecular weight (than extrusion), resulting in better stress and craze resistance and harder surfaces which make the acrylic less likely to be scratched," Heinzelman said.

"After thermoforming a sheet, the plastic cools and will shrink. With continuous casting, the plastic will shrink the same in each direction, and it is consistent from batch to batch.

"In the sign industry, people are pre-screening sheets, painting them as flat sheets and thermoforming them. With continuous casting, in a year or so, if that person wanted the same sign, they don't have to mess with the screening process," Heinzelman added.

Extrusion, on the other hand, takes acrylic polymer in pellet form and turns it into a sheet. "Extruded is better in its ability to offer a tighter control on thickness due to its fabrication process, which is one sheet at a time," Heinzelman said.

"Shrinkage depends on thickness, the process being used and the individuals running it," LaFontaine said. "I can make a thinner sheet that doesn't shrink at all and then make a thick one that will."

Continuous manufacturing uses more high-tech equipment, operating knowledge and more sophistication for a higher end extrusion process that results in better optics and surface quality, LaFontaine added.

Top Properties
"The number one property of acrylic is optics," said Liz DiGiose, communications manager for Cyro Industries in Rockaway, NJ. "Acrylic transfers light the same as glass does, but it's lighter with more impact resistance."

In fact, acrylic maintains an outstanding luminous transmittance of 92 percent while carrying half the weight of glass. Its weather resistance is yet another of its most beneficial properties. Tom Branhan, general manager of Continental Acrylics Inc. in Compton, CA, noted that "acrylic in sheet form can withstand exposure to sun, weather resistance to extreme cold and sudden temperature changes."

DiGiose noted that acrylic's impact resistance is also an important feature because, if subjected to force beyond its limit, it does not shatter into small segments but rather large pieces. The measure of this resistance depends largely on the thickness chosen for the sheet.

Yet another vital resistance, according to DiGiose, is that of chemicals such as ammonia, sulfuric acid and hexane. Acrylic has also been FDA approved for contact with food.

Acrylic can be manufactured in up to 10,000 different shades while insulation, or thermal conductivity, of acrylic sports an overall coefficient of heat transfer approximately 10 percent lower than that of glass, making it a valuable choice for any buyer.

DiGiose also cautions that "it is essential that acrylic sheet has a consistent weight-average molecular weight below 2 million grams per mole" and that "acrylic sheet with a higher molecular weight does not have the desired formability, cementability, weatherability, coatability, combustability and purity characteristics."

Some forms of acrylic are specially made with unique properties such as UV filtering or enough impact resistance to combat the bullet of a .44-caliber handgun.

Applications
"The thing with acrylic is that the more you're exposed to it, you start to see it all over," said DiGiose. "Like when you go into a shopping mall, you'll see it everywhere. It's used in the mall directories, signage, displays and bins. This is because it's an affordable product with a lot of durability and flexible design capabilities as far as colors and clarity. There are limitless design capabilities."

Acrylic was initially commercialized in the early 1930s and first used in World War II for aircraft windshields and canopies, according to Heinzelman. Since then, the thermoplastic has taken on a mass of applications in various industries and continues to find its way into new avenues of utilization.

The list of acrylic uses is extensive. Manufacturers differ on what they say are the most common uses for acrylic as it is often fabricated for specific products or industries.

While John Gieber, plant manager of Ram Products in Sturgis, MI, cited replacement windows for glass applications as one of the most common uses of acrylic, John Hirsch, national sales manager for American Sheet Extrusion Corp. in Evansville, IN, named sanitaryware, such as tubs and sinks, as a frequently used application.

A consensus was reached, though, when considering outdoor signs and point-of-purchase display products as common applications. "In grocery stores, you'll see a lot of acrylic used in their displays such as salad bars," DiGiose said. "When considering consumers, there are a number of safety concerns, which is why acrylic is used. It has the impact resistance, light weight and optics needed for the displays."

Other common uses include bathroom stall partitions, skylights, greenhouses, picture frames, security and surveillance windows and mirrors, and spas. Some unique applications are painting canvases, furniture, photography aids in light diffusion, sight glasses, animal habitats, bus shelters and boat parts.

DiGiose cited a few internationally renowned acrylic applications that most people may not have been aware of: the famed Batmobile, Las Vegas signage and The Hard Rock Cafe's outdoor guitar.

Growing Trends
What many representatives are finding is that particular markets are beginning to lean toward the use of certain types of acrylic for their respective products. Susan Bruno, director of inside sales for Plaskolite in Columbus, OH, has noticed a growing trend with more coated acrylic sheet, higher-end acrylic replacing glass and "impact-resistant acrylic being used more in replacing polycarbonates in the market."

"What we're finding out in the industry is that more and more users have applications that require more impact resistance than the generic purpose acrylic offers," Heinzelman said regarding the future of acrylic. "They're now switching from general purpose to impact-modified acrylic."

Hirsch lent a similar view, saying, "There's been a growing trend in the need for components of composites that need some of the properties of acrylic and the properties of other materials."

Properties such as increased UV protection, abrasion and chemical resistance, and anti-static are currently being developed by engineers for future acrylic implementation, said DiGiose.

Research chemists are developing new technologies to improve acrylic. PMMA make-up including modified acrylic copolymers will improve the overall physical properties, Branhan predicted. With improvements, the automotive medical engineers can bring new product applications into these fields. Lighting fixtures, optical discs, appliances, housewares and personal accessories markets will soon see improved product items such as telephones, button lamps, tables and furniture parts, he continued.

"More and more acrylic-based products are also being used in the solid surface market (kitchen, bath, countertops)," Heinzelman said. "DuPont Corian has about 80 percent of the market. A few years ago, we introduced our own, but it requires a lot of tech service and marketing work." Heinzelman also remarked on the changing face of the spa market, noting that acrylic makes up 85 to 90 percent of those made in the United States and has replaced fiberglass as the primary spa component.

Fabricating Tips & Techniques
While acrylic features several advantages in its repertoire, it also presents some small obstacles for fabricators. Heinzelman cited the narrow coefficient of thermal expansion as one of acrylic's particular challenges. Acrylic sheets must be allowed enough room to expand or contract to avoid cracking and/or distortion, he noted.

Branhan added that "heated acrylic sheet has limits when it comes to thickness to be stamped, sheared and punched" and that safety equipment must be worn when routing, drilling or cutting.

Gieber also referred to the heat load and speed of tooling, and types of bits, tips and cutting equipment as specific challenges to acrylic's fabrication. "Acrylic is a thermoplastic and has a unique ability to be heated and formed in various shapes, depending on dye, mold and related equipment," he added.

Acrylic's thermoforming capabilities are often touted by industries as one of its many advantages. This is achieved at temperatures anywhere from 290F to 320F and preferably with a strip heater. Other means of fabrication -- cutting and machining, laser cutting, cementing, annealing -- are executed as easily as with other plastics.

"In all fabrication steps involving acrylic, it should be understood that you can only demand so much stress and strain on the polymer matrix," Gieber noted. "Whether it's extruded or cast will also play a major role in its ability to be heated, formed, etc."

Several helpful tips were contributed from those surveyed, most suggesting tool types and how to use them, precautions to be aware of and certain pitfalls to look out for during the fabrication process.

John Lee, director of business development for Polycast Technology Corp. in Stamford, CT, suggested using the same tools and processes as one would in fabricating aluminum, wood or metal. Milling machines, routers, grinders, thread cutters, engraving equipment, files and reamers can all be used with acrylic.

Branhan also touched on polishing, warning that "prolonged heating temperatures above 285C cause acrylic to depolymerize" and to use water when extinguishing burning acrylic.

Ulrich Miersch, president of Degussa Canada Ltd. of Burlington, Ontario, also suggested using buffing wheels and polishing wax.

Branhan offered additional pointers. "Cutting blades should be kept sharp and clean. Circular saws should be run at or about 8,000 to 12,000 linear feet per minute, teeth per inch 0.080 to 0.150 and sheet thickness blades should have six to eight teeth per inch," he suggested. "A speed of 18,000 to 24,000 RPMs should be used when routing. Elimination of sharp corners will reduce cracking."

Gieber added that "heat load may cause polymer breakdown. Router bits should be extremely well-sharpened, and clamps must be accurately set so as to not allow vibration, movement or misalignment during processing." He also warned against cutting large thicknesses or large stacks too fast as the material chips.

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