Acrylic and polycarbonate are highly visible plastics used for POP displays, store fixtures, signage and more. Shown is one example of an abrasion resistant acrylic sheet in use.

Acrylic and polycarbonate are two plastics which can be utilized in many similar applications due to their comparable characteristics, like clarity and durability. However, these two materials are not interchangeable in every application, as Plastics Machining & Fabricating found when interviewing nine industry experts about machining acrylic and polycarbonate.

Examined in the article will be the advantages/disadvantages, differences/similarities, limitations, common/uncommon uses, fabricating rules and future applications of acrylic and polycarbonate

What is Acrylic?
Acrylic, or polymethyl methacrylate, sheet is a glassy thermoplastic used for weatherability, ease of fabrication and light weight. Industries have found it to be a good material for applications that require high impact resistance and good weatherability. With polycarbonate as its chief competitor, acrylic continues to venture into new territories with fresh developments and groundbreaking technologies.

When taking a close look at some of the many common uses of acrylic, it doesn't take long to realize it is everywhere. One of its most popular markets is POP displays and store fixtures. It also is used nearly everywhere in shopping malls around the world in signage and displays. Due to the affordability of this product as well as its flexible design capabilities, acrylic is a perfect match for this market.

Acrylic was initially commercialized in the early 1930s and was first used in World War II for aircraft windshields and canopies. That application is still being used today, although with a much higher engineered piece of acrylic. It has also replaced fiberglass in many industries including the spa industry where it makes up 85 to 90 percent of the United States market.

Some other popular uses include: sound barriers, kitchen and bath countertops, bathroom stall partitions, furniture, signage, car accessories, skylights, aquariums, button lamps, telephones, musical instruments, bandit barriers for banks and limousine interiors.

What is Polycarbonate?
Polycarbonate is a thermoplastic that is known for its toughness, clarity, high heat deflection and molecular weight. It also brings to the table a combination of impact strength, superior dimensional stability, glass-like transparency, remarkable thermal resistance and good electrical properties. Developed commercially in 1957, it is one of the pioneering members of the family of engineering thermoplastics that was created to compete with die-cast metals.

Engineers looking for some of the above qualities to meet specification requirements can use polycarbonate for a broad range of applications.

Some of the common uses of polycarbonate include: business machines such as copiers and fax machines, signage, correctional facilities where bullet-proof sheet is needed, taxicab partition windows, machine guards, security barriers, glazing, as well as windows and bus enclosures.

Although manufactured in plate, rod and tube, polycarbonate in the sheet form is the most popular choice to use when a high quality product is desired. Polycarbonate can be injected, extruded or blow molded for the automotive, electronic, lighting or appliance industries to name a few. With a current growth rate of 10 percent a year, it is not hard to realize the popularity of this plastic.

Making a Choice
It is easy to know why these two transparent thermoplastics are comparable in many areas. Both offer ease in fabrication and are used throughout the signage, glazing and security window market for just that reason. Their durability, clarity and endless thermoforming capabilities put them one step above the competition when it comes to those industries. But they also bring varying qualities to the table as well, like weight and scratch resistance and weatherability.

"It's like night and day," said John Blum, marketing manager for Comco Plastics Inc., a division of Commercial Plastics & Supply, in comparing acrylic to polycarbonate. "Although they share some of the same excellent qualities, to a varying degree, they are very different. It comes down to what the fabricator is trying to accomplish because there is a right material to be used for each application."

With that, it seems that it is very important to know what needs to be accomplished in order to choose which plastic to use. With the addition of additives and coatings that can be added to sheet plastic, the limitations are nearly endless.

"The plastic industry is growing by leaps and bounds, and the main reason it has taken off has been the ability of the resin manufacturers to develop certain resins, teflon or alloys for example, to accomplish anything," said Global Plastics vice president Don Drew. "Now they have the opportunity to go after industries that in the past they couldn't because nothing was developed to help adjust plastics to meet the wear and conductivity levels needed. Today you just need a job big enough and an application for a custom compound to solve your problem by alloying or adding additives. If you need a plastic with good wear and conductivity to match a certain job, it can readily be made. It has become that easy."

With the ability to add special resins to a plastic, and the already numerous qualities of acrylic and polycarbonate, these two plastic materials can offer a lot to the industry.

"Both are excellent materials for clarity and durability," said Dan Rustin, marketing manager for Acrilex, "but they do have their own advantages and disadvantages in the machining process. Acrylic is a better all-purpose material in general. It cuts, polishes, thermoforms and machines easily and comes in many colors whereas polycarbonate offers a much, much higher impact resistance."

Bill Faulhaber, ICI Acrylic, agrees with Rustin that acrylic has much more to offer in terms of overall quality.

"No other thermoplastic material can match the clarity, light transmission, scratch, haze and fade resistance and weatherability of acrylic sheet," according to Faulhaber. "The rigidity and cementability of acrylic sheet makes it an attractive choice for construction of POP displays and skylights."

In addition to clarity, acrylic also offers vast color offerings, textures and patterns as well as ease in the thermoforming and fabricating process. According to many of those interviewed, acrylic can also be said to offer better light transmission and harder surface character which makes it less prone to scratching and easier to cut, machine and cement. The fact that it is lightweight and non-yellowing are other positive qualities of acrylic.

"Both acrylic and PC can be manufactured as thin film gauges which allows them both to be used in many situations," said Grant LaFontaine, sheet products technical service manager for CYRO Industries. "Continuously manufactured acrylic can go up to 1inch thick while acrylic manufactured by the cell cast method can reach thicknesses of many inches, even up to 1 foot. Both products have many great features."

LaFontaine went on to explain that acrylic has better light transmission, is more rigid and more weatherable with a harder surface character. "That makes it less prone to scratching and easier to cut, machine and cement."

Polycarbonate brings something else to the table according to Steve Sichterman, director of sales and marketing for Polygal, that can't be said for acrylic.

"The key issue and difference between the two is the fire ratings," said Steve Sichterman. "Polycarbonate is rated CC1, which means that it is a better fire-rated material than most other plastics. This is becoming required more often during construction to follow zoning guidelines. Most companies offer guarantees that the physical properties of polycarbonate will not change over time.

Future of acrylic and pc PM&F also queried the industry about what these two thermoplastics, acrylic and polycarbonate, have to offer the industry in the future. Here are some of their perspectives. • Tom DeMain, marketing manager for Elf Atochem North America Inc., Atoglas Division: "It still amazes me that we sell a flat piece of acrylic sheet and people are able to turn it into some form of artwork. That is what I enjoy the most. It isn't a limited material. I am not sure I can ever get used to seeing the endless advances in this field. Custom coatings and potential combinations of various polymers offer potential for future growth. Uses, such as Digital Video Disks, continue to be developed showing promise for continued growth of these products. Future growth is really limited only be the 'ideas' and 'visions' of the plastic fabricators and their customers. POP displays are currently a major market and the number one usage of acrylic. Signage is coming back a little stronger as well, and with 2000 fast approaching, there is going to be a perception and demand for stores to create a new image. I think that will do nothing but improve and increase the popularity of polycarbonate and acrylic." • John Blum, marketing manager for Comco Plastics: "Where glass or other non-plastic materials are being used currently, acrylic and polycarbonate could substitute for cost savings and longer wear. For example, boats, noise barriers for highways, fiber optic applications and laser filtering devices." • Susan Bruno, director of inside sales for Plaskolite: "Impact modified acrylic sheet has and will continue to become an important part in the marketplace. It offers excellent weatherability and versatility in thermoforming." • Janice Pravorne, account manager for Manchester Plastics: "In the future they are planning on using polycarbonate where it will be FDA approved so that it can be used for food packaging. The plastic that sandwiches sit on in displays must be approved by the FDA. Therefore, the store fixture must be a controlled substance manufactured in a certain environment where there isn't any dust."

Challenges Faced by the industry
"If polycarbonate isn't treated properly it will yellow," Sichterman added. "We co-extrude a UV stabilizer right into our polycarbonate sheet which prevents any change in physical properties. As far back as the 1970s there was a problem with fading due to lack of weatherability, but that has been resolved. Polycarbonate also offers cost efficiency in any project; initial and long-term cost efficiency.

"Some people can put a coating on their acrylic and say that they have a weatherable sheet, but that isn't exactly true. That coating can wear off, so the co-extruded stabilizer is the best choice and it will last longer as well," said Sichterman.

Many companies work with plastic in many different forms and therein lies a problem, according to Rustin.

"Most machining difficulties arise from a general lack of understanding on the part of the fabricator between cell cast, continuous cast and extruded acrylic," explained Rustin. "Cutting, polishing and thermoforming are all affected differently by these different forms of acrylic sheet. Polycarbonate on the other hand is only extruded so you don't run into the different machining rules or confusion."

Many of the experts mentioned that the manufacturing process and end product ultimately determine which plastic will be used.

"Acrylic does not shatter, it cracks," said Blum. "Polycarbonate however, is virtually unbreakable. If a person lived in a high vandalism area where kids were prone to throwing bricks at windows for fun, that person would probably want to go with polycarbonate due to its breakage resistance. Acrylic, on the other hand, has more weatherability, better clarity and is less prone to scratching. It also isn't going to break into a million pieces like glass, but it will crack. Polycarbonate will withstand such punishment."

One other major concern must be the overall cost to use each plastic.

"Obviously the market is expanding because all the major players in plastics are adding major volume," said Drew. "Polycarbonate is more costly across the board but it depends on the application and performance you are looking for whether you need to use polycarbonate. It offers superior mechanics and a higher ability to use in different environments. Acrylics are clearer and utilized more often in the POP market due to cost savings and color variances. But polycarbonates ability to handle dynamic loading, high-impact shock loads is much higher than acrylic."

Polycarbonate also has a strong advantage when faced with situations where impact resistance is important.

"Polycarbonate is heavier than acrylic which in certain applications, is what you want," explained Janice Pravorne, account manager for Manchester Plastics. "Polycarbonate can be, and is, used in race car and aircraft windows. You can't use acrylic in that situation because of the pounds of pressure per square inch. It won't allow it. Certain thicknesses of polycarbonate, 1 to 2 inches thick for example, have a bullet retardance, whereas acrylic will crack and doesn't have that opportunity. PC is more durable and also has a higher burn and breakage rating. You don't have that with acrylic. So you really can't say that one is better than the other. They both offer their own advantages."

A difficulty for both polycarbonate and acrylic is maintaining dimensional stability and close tolerances to receive good machine finishes. Wanting to keep the same look and keeping the same look can sometimes be difficult.

Blum mentioned that keeping tools sharp, workers experienced and the plant looking sharp can lend a hand to making the whole process go a little easier.

"All parts are made from blueprints and maintaining the dimensional stability can be a problem when machining because of the challenge to keep the product consistent. If you maintain that stability you don't have to worry about the material expanding unexpectedly."

One industry leader said he felt that one of the recurring problems with plastics had nothing to do with the acrylic or polycarbonate, but actually with those who are machining the materials.

"The biggest drawback in using polycarbonate is the inexperience of the normal machine shop," said Drew. "They are dealing with metals and are not recognizing the pitfalls of the resins. Sensitivity in certain applications is not the same using certain solvents. The coolant might work absolutely fine with one application but cracks another. The reason is because the resin attacks material depending on the stress level. If a resin is used on a simple surface cut, the result is a perfect look. If the same resin is used while drilling a piece of plastic and the part cracks they may wonder why. Well, there is more stress built up drilling a piece of plastic than surface cutting. The plastic is going to react differently, which sometimes might not be apparent. The stress riser is the result of not understanding that material in reference to machining. This is very true with all the thermoplastics."

Another aspect of machining acrylic that can cause problems is overheating. However, Faulhaber said that it is not necessarily an ongoing problem.

"That is only if there is moisture in the sheet," explained Faulhaber. That can happen to any plastic material and is usually a rare occasion, because that is normally something that can be controlled."

Where is the future of these two plastics going to lead the industry? Many of those interviewed feel that plastics are going to take the world by storm.

In the meantime, some manufacturers are faced with the problem, or pleasure, of enticing non-acrylic/polycarbonate users into switching from their current material.

"That may be the hardest thing to do. Some people don't want to change their ways because they are comfortable with their current material because they know what they can and can't do with that material," said Blum. "Talking a customer out of their existing material and into acrylic or polycarbonate can be difficult."

"Companies have a sense of comfort when staying with what is working, but if you let them know that something else is out there that is equal to, if not better than their current material choice and give them an option, it can't hurt," added Blum.

"The deciding factor when choosing which material is going to be used comes down to what it is going to be used for," added Blum. "Is the final product going to be exposed to where people can see it? Does the weight of your final product make a difference? How can acrylic or polycarbonate assist your process? We can get acrylic to look as smooth as glass if we want, but in some instances it doesn't have to. Polycarbonate can not be polished, but it can be chemically treated. There are ways to helping people see the advantages, but first you have to be given that opportunity."

Fabrication tips for acrylic

Acrylic sheet's ease of fabrication offers designers and fabricators freedom of design for making functional, attractive and long-lasting displays, store fixtures, signs and showcases among other objects. To realize acrylic sheet's full potential, fabricators should understand how to most effectively work with acrylic sheet through the following fabrication tips. These apply for all forms of acrylic sheet and some steps vary slightly for specialty products.

To begin with an overall caution, leave the masking on during all fabrication operations to prevent damaging the sheet surface.

Sawing
A conventional panel or table saw and a no-melt blade for plastics are ideal for sawing acrylic sheet. This kind of carbide-tipped blade has a triple-chip-grind tooth design, where every other tooth has a beveled cutting edge to help clear away chips. Teeth should have a clearance angle of 10 to 15 degrees and a rake angle of zero to five degrees. The proper blade angle scrapes away the material, instead of chipping it away. A less aggressive 10-inch, 80-tooth blade is recommended.

When cutting, the blade should protrude between 1Ú8 inch to 1Ú2 inch above the sheet surface. A relatively fast cutting rate, e.g. 50 fpm, minimizes frictional heat build-up between the acrylic sheet and blade. Too slow a feed will melt or burn the material. Too fast a feed will cause chipping. When exiting the material, slow the feed rate to prevent chipping or blowout of the exit edge.

A saw blade mist cooling system will yield a cleaner, smoother edge, and increase the life of the blade. For more intricate cuts, acrylic can be cut with a saber saw or jig saw.

Laser Cutting
Laser cutting produces the most intricate parts. More fabricators are turning to laser cutting as prices of laser equipment drop.

A special masking for laser cutting acrylic sheet such as TPA does not fuse to the product. If the sheet comes with paper or polyethylene masking, it is still recommended to leave it on while laser cutting to prevent damage. Since lasers leave highly-stressed edges, the sheet should be annealed to minimize the chance of crazing during the part's service life.

Scribing and Breaking
Scribing and breaking is perfect for straight-line cutting of short lengths of single acrylic sheets.

Lay the masked sheet on a flat surface and mark the line to be scribed. With the aid of a straight edge, pull the scribing tool or knife across one side of the sheet surface several times until removing one-third of the sheet's thickness. Lay the scribed line at the edge of the table with the scribed side facing up. Apply quick, steady pressure to the free end; the sheet will snap along the scribed line.

Drilling
Acrylic sheet drills cleanly with high-speed steel, modified-for-plastic twist drill bits. Unmodified metalworking drill bits will cause melting and other damage to the plastic. You can modify metalworking drill bits for plastic by grinding the tip angle to a sharp 60 to 90 degrees . The cutting edge must be ground "flat" to a 0- to4-degree rake angle, so it scrapes the acrylic rather than gouges it. A correctly-shaped bit operated at the proper speed will create two continuous spiral strips as the bit passes evenly through the acrylic.

The workpiece should be held firmly, preferably clamped to the worktable. Back the piece with acrylic or wood so the drill bit continues into solid material as it exits the bottom surface.

A circle cutter can be used for creating holes greater than 1 inch in diameter in acrylic sheet. Use a drill press with a cutting speed of about 400 rpm. Water or air to cool the cutter makes the hole walls smoother.

Routing
Routing produces a better edge than saw cutting. Router bits may consist of one to four flutes; single- and double-fluted bits are commonly used. A bit whose cutting diameter equals the shaft diameter produces the best routing results. The length of the cutting edge should not exceed three times the diameter of the tool.

Recommended speeds fall between 18,000 to 28,000 rpm. The higher rpm rate allows faster feed rates and a smoother finish. For maximum production, operate at a feed rate just below the chipping speed.

Vibration during routing needs to be minimized. Use clamping if possible. Even small vibrations can cause crazing and fractures in acrylic sheet.

Proper feed direction is essential for a smooth cut. Feed the router counterclockwise for external cuts; clockwise for inside edges.

Edge and Surface Finishing
Sanding, polishing. buffing and edge finishing give acrylic sheet an attractive, smooth transparent edge.

The edge of a saw cut must be finished. Scraping is the easiest edge finishing technique. A knife or almost any piece of metal with a sharp, flat edge can serve as a scraper. Special acrylic scraping tools are available.

Polishing, done on a stationary polishing head, creates the best finished edge. An 8-inch to 14-inch diameter bleached muslin buffing wheel with bias strips that give the wheel a pleated appearance is recommended. This design runs cooler than a stitched wheel and does a faster job. A medium polishing compound gives a good finish. For a high-luster finish, remove sanding marks with a fast cutting compound, then final buff with a high-luster compound.

When buffing edges, avoid excessive heat build-up. Too much heat can induce stress into the sheet and lead to crazing. Edge finishing machines are also commercially available.

Flame polishing with a hydrogen-oxygen torch, adjusted for a narrow, 3-inch-long flame, will produce a highly-polished, clean edge. Hold the torch at the angle and draw the flame along the edge of the sheet. If the first pass does not produce a completely polished edge, let the piece cool, then try a second pass. Annealing the part after flame polishing decreases the stress imparted and reduces the chance of crazing.

If scratches or machining marks are not too deep, the surface can be buffed without wet sanding. If the imperfections are too deep to be removed by polishing, wet sanding and then polishing will work.

Line Bending
Nichrome wire is the most commonly used to bend acrylic sheet up to 1Ú8-inch thick. To bend material thicker than 1Ú8 inch with nichrome wire, flip the sheet over halfway during the heating cycle. A tubular rod heater can heat sheet thicker than 1Ú8 inch in one cycle. Quartz tube heaters bend acrylic sheets 1Ú4-inch thick or greater.

When line bending, heat the material until it bends easily to the desired shape. Then secure it in a cooling jig to prevent the piece from springing back to its original shape. Cooling time normally equals heating time. Bend away from the heated side. Do not place the material directly on the heater, as overheating can cause bubbling or blistering. Too narrow a heating area can cause wrinkling. Too wide a heating area can cause a bow effect.

Thermoforming
The typical acrylic sheet forming temperature is 290F to 320F, depending on the amount of shaping (stretching) and the forming rate. An excessively fast forming rate will impart high stresses and decrease craze resistance. To minimize stresses, form at a moderate rate; distribute temperature evenly over the surface and thickness of the sheet. Heating time depends on material thickness and heating method. Higher forming temperatures are needed to achieve greater "draw" or increased definition. Excessive temperatures will produce pimples, blisters, shading changes and other damage.

Acrylic sheet shrinks slightly when heated to its thermoforming temperatures. After forming, cool the part to below 140F to 160F. Make sure the interior is cool as well as the surface, and cool uniformly on all sides to prevent stress.

Cementing
Acrylic sheet joins easily with many commercially available solvent cements. Before cementing, edges must be cleanly cut and/or properly machined, but not polished. Polishing will produce an edge with rounded corners that result in an unsightly, weak joint. A flame-polished edge may craze when it contacts solvent cement.

Capillary cementing is the most popular joining method. Join the pieces with masking tape or clamp the pieces in a form. Space the two edges with shims to leave space for the cement to enter. Apply cement with a needle-nosed solvent cement applicator. Let the cement soak into the edges for 45 to 60 seconds before removing the shims.

Dip or soak cementing is an alternative. Dip the edge of one piece into the solvent for 20 to 30 seconds. Remove the sheet from the cement and place the soaked edge on the other part to be jointed. Hold the parts together for about 30 seconds without applying pressure. After 20 seconds, apply enough pressure to squeeze out the air bubbles, but not enough to squeeze out the cement. When the pieces are joined, place the part in a jig or clamp to maintain firm contact for 10 to 30 minutes.

With solvent cements, wait three hours before subsequent processing. High strength is reached in 24 to 48 hours.

To join parts that are difficult to cement by capillary or soak solvent methods, apply viscous cements. Viscous cements fill small gaps and make strong, transparent joints where solvent cements cannot.

Painting and Silk Screening
Before painting, clean the surface of dirt, dust or grease with soap and water. (Avoid solvents such as ketones and alcohols.) A static-eliminating airgun neutralizes static charges on the surface of the sheet that attract dust. You can also remove dust by wiping the sheet with a damp, lint-free cloth or cleaning with soap and water.

An atomizing spray gun uniformly distributes the paint. Hold the gun 12 to 14 inches from the workpiece. Vary the direction of the spray, horizontally and vertically, to assure uniform coverage. When painting, protect from overspray by leaving the protective masking in place, by applying a peelable spray mask or by applying liquid maskant by spray gun.

Annealing
All fabrication induces stress in acrylic sheet. Annealing to relieve this stress is recommended to minimize crazing or cracking that can occur. Annealing can also increase bond strength by 50 percent.

Anneal parts in a forced air-circulating oven. Commercial ovens are designed for annealing and heating plastics; restaurant-type ovens also do the job. Heat the acrylic sheet to 180F, just below its deflection temperature and cool slowly. Heat for one hour per millimeter of thickness -- at least two hours total. Cooling time should be a minimum of two hours. For sheet thicknesses above 8mm, cooling time in hours should equal thickness in millimeters divided by four.

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