Jim Hingst
In the sign industry, the acrylic sheet product family is divided into three major categories—cell cast, extruded, and continuous cast. And each manufacturing process is unique.
The cell casting process produces acrylic sheet with the best optical clarity among the three. The cell cast method is a very traditional way to make acrylic, but it’s also a very costly process.
Variations in gauge with cell cast are common. Nevertheless, for custom thickness, custom finishes, and custom colors along with the highest quality materials, cell cast sheet has no equal.
An advantage of the extruded process is that the extrusion maintains much tighter tolerances of sheet thickness than in the casting process. More importantly, this is a high-speed production process, making it a very cost-effective way to manufacture acrylic sheet.
But a disadvantage of extruded sheet is that, because it’s pressed between the metal rollers, it induces some mechanical stress. This can result in shrinkage in machine direction.
The continuous cast process produces sheet that exhibits very good optical clarity, consistent gauge, and minimal shrinkage during thermoforming.
Both extrusion and the continuous cast processes are very economical production methods compared to cell cast acrylic. In the continuous cast process, the raw materials costs are lower for the resin syrup versus the pellets used in extrusion.
The equipment cost of the extrusion equipment, however, is significantly lower than the investment required for continuous cast equipment. As a result, extrusion is the more common production equipment used.
Removing the Surface Protection Masking
Before decorating acrylic sheet, you must remove the paper or low-density polyethylene surface protection masking. To minimize the amount of contamination that can collect on the surface of the sheet and protect it from accidental scratching, don’t remove the masking until you are ready to use it.
Surface protection masking on plastic sheet can often become difficult to remove—especially if the plastic sheet has been stored for a long time and has been subjected to the pressure of sheets stacked one on top of another.
Under no circumstance should you store masked acrylic sheet outside. The effects of sunlight on the masking can crystallize its adhesive, often making its removal impossible.
If the masking gets wet, a milky white residue might remain on the sheet. Sometimes you can remove this residue by merely wiping the surface with a soft damp rag.
If that doesn’t work, try to clean the residue with Rapid Prep™. Then thoroughly rinse and dry the surface after cleaning.
If the paper masking is next to impossible to remove, don’t soak the paper with water or application fluid. This trick may work when removing stubborn application tape from applied vinyl graphics, but when removing masking from plastic sheet, the likely result is some type of adhesive residue.
Instead soak the protective paper masking with a solvent to loosen the grip of the masking’s adhesive. Be sure to only use a solvent approved by the plastic sheet manufacturer.
After waiting for at least ten minutes for the solvent to work its magic, attempt to remove the masking. If at first you don’t succeed in this procedure, try, try again.
In some cases, after removing the protective paper masking, an adhesive residue remains. In many cases, you can roll the adhesive off of the surface with the pressure of your thumb.
If this doesn’t work, saturate a clean cloth with naptha and rub it on the adhesive. After the adhesive is removed, wash the surface with detergent and clean warm water. As an alternative to naphtha, try cleaning with isopropyl alcohol (IPA) and rinsing with clean water.
Static Electricity
Most plastics accumulate a static electric charge on the surface of the sheet. When you tear the masking from the sheet, even more static is generated.
If you don’t neutralize this charge, the static will attract dust and dirt like a magnet, which can show up in your paint job. Spray painting a surface with a static charge can also result in a blotchy finish, which will stick out like the proverbial sore thumb, when the sign face is illuminated.
Static electricity creates problems in decorating with most any kind of plastic, and acrylic is no exception. Electric charges on the face of the acrylic sheet can cause an uneven deposit of paint. In screen printing, it can cause spider webbing. Static charges can also create a magnetic attraction between an acrylic sheet and vinyl graphics, complicating vinyl application.
One way to get static under control is to control your shop environment. Maintaining relative humidity at 60 to 70 percent can greatly reduce static related problems.
Sign makers use a couple different techniques to kill or neutralize the static charge. One common practice is to wipe the surface with a damp, lint-free rag. Others wipe the surface with a damp chamois moistened with a solution of water and isopropyl alcohol at a ratio of 9:1.
Dry any residual moisture with a hot-air gun. Another practice is to kill the charge with an ionizing air gun. Don’t use anti-static cleaners or dryer sheets, which can contaminate the substrate, potentially causing paint adhesion problems.
Surface Preparation
After removing the masking, it’s time to prep the sheet. Don’t use window cleaners or strong solvent to clean the sheet. Cleaning the surface with a dry rag is also verboten. Dry rags potentially could scratch the plastic as well as generate a static charge.
Regardless of how you decorate acrylic, you must clean any contamination off of the surface. Failure to properly prep the sheet prior to processing often results in graphics failure (poor paint or ink adhesion, inconsistent coatings, and peeling vinyl films).
Don’t clean acrylic with solvents such a toluene, xylene, or lacquer thinner (which can craze the sheet). Be very careful in using any solvent stronger than alcohol for cleaning. Strong solvents will subject plastic sheet to stresses that often result in crazing and eventually cracking.
If the surface has grease or tar, try naphtha or kerosene to remove it. If you have to use one of these stronger solvents, be sure that the sheet is completely dry before painting.
Joe Balabuszko of the Earl Mich Company recommends filling a bucket with clean water and a mild, non-abrasive detergent. If you use a dishwashing detergent as a cleaner, don’t use the type with hand moisturizers (which could cause paint adhesion problems).
After soaking a cellulose sponge in the water-detergent mixture and lightly wringing out the sponge, wipe the surface down. Then finally wipe the surface with a sponge dampened in clean water.
According to Balabuszko, this process ensures the static is neutralized and the surface of the sheet is absolutely clean. Balabuszko warns against using commercial anti-static cleaners. Some cleaners can leave a residue, which can interfere with good paint adhesion.
Not much looks worse on a painted substrate than dirt or other residue on the sheet. To avoid this problem, always keep your work environment clean and, most importantly, thoroughly clean the sheet prior to painting. When the weather is hot and the humidity low, you may consider dampening your shop floors to keep the dust down.
Although many acrylic materials are scratch-resistant, it is possible to scratch it, so be careful in handling and cleaning the sheet. Avoid using brushes, squeegees, or other cleaning devices that could scratch the sheet’s surface.
After washing the surface, rinse with clean water and dry with a soft cloth to prevent water spotting.
Painting
The extraordinary clarity of acrylic sheet (and its tremendous protection from the degrading effects of UV light) makes it an excellent substrate for backlit signs. Acrylic is typically painted in reverse on the second surface of the sheet.
Paint must be thinned to the right viscosity for spray painting. To ensure chemical compatibility, always use the reducers that the paint manufacturer makes for their paints. (Note: Adding too much reducer to the mix can reduce the exterior durability of the paint.)
Always thin paints according to the paint manufacturer’s recommendations, as well. Some thinners can craze acrylic sheet, while too much thinner can result in shortening the outdoor durability of the paint job.
Over-thinning the paint also lowers its viscosity, which can result in too much paint laid down on the surface and the paint sagging.
Given a choice between a mild thinner and one with strong solvents (such as toluene or benzene), always use the milder one.
If you have questions about mixing the paint, ask the manufacturer for direction before going into production. And always test the paint and the acrylic sheet for compatibility.
Before painting, stir the paint well to disperse the pigment. Failure to thoroughly mix the ingredients can result in the paint not drying properly.
In spray painting plastic sheet, apply very thin coats. Heavy coatings can result in the paint sagging and a high concentration of solvent (which can craze the sheet). Too-thin coats can result in a slight texture, which will give the paint finish a dull or matte appearance.
Keep the nozzle of the spray gun about twelve inches from the surface. In spraying, move the spray gun in a straight line from left to right (horizontally). You can also vary your spraying movement by painting a coat vertically or from top to bottom. Varying the direction of the spraying motion will ensure that you achieve a uniform coating of paint.
Keep the rate at which you spray at a steady pace. Continue spraying until the pattern is off of the substrate. Never stop a spraying pass on the surface of the sheet. “Build the color up slowly, spraying a number of light coats of paint,” says Balabuszko. “Spraying heavy coats of paint often results in a splotchy appearance.
“First paint top to bottom; then bottom to top. Next paint from right to left; then left to right. Between each coat, wait about five minutes for the solvents to flash off.”
Balabuszko recommends that you spray paint with some backlighting. This allows you the opportunity to check your work as you’re painting to see whether you’re spraying the paint evenly. You’ll also be able to view the transmitted color, which tells you how the sign will appear when it’s backlit.
You can also paint acrylic on the first surface of the sheet. If you do, spray a clear coating to protect the painted graphics. This clear coating will also provide protection from abrasion during cleaning.
When painting on the second surface, spray a thin layer of white mixed with clear, after the graphics have been painted. This protects the painted graphics and diffuses the lighting in the sign box (minimizing any hot spots and helping disguise any pinholes).
Digital and Screen Printing
For high-volume jobs decorating acrylic sign faces, screen printing with either solvent or UV-curable inks is an economical solution.
In selecting an ink system, ask your screen print supply distributor for a recommendation. The ink manufacturers’ technical bulletins will give you printing recommendations.
The type of mesh, squeegee, stencil, and thinners are a few of the variables that will determine the amount of ink deposited on the sheet, which affect the appearance of the sign (when illuminated).
According to Laura Maybaum, market segment manager at Nazdar, many shops have avoided reaching untapped markets by printing onto acrylic because of the difficulties in this type of printing—such as variations in the acrylic substrate from one manufacturer to another and selecting the right ink for the acrylic product used. “Acrylic is inherently hard, which makes it difficult for the ink to bite into the surface,” she adds. “Variations from one batch of acrylic sheet to another can also result in adhesion problems.”
Additives used to modify the physical properties of the acrylic can inhibit ink adhesion. That’s why, while an ink may work beautifully on one acrylic product, it can fail miserably on another acrylic sheet.
The chemistry of acrylic is so complex that testing the raw materials prior to production is an absolute necessity. When you find a combination of an ink that works with a particular series of acrylic using a certain profile, stick with that winning combination.
Maybaum suggests using a UV thermoforming ink. These inks adhere to a wider range of acrylic sheets than general-purpose inks and provide the flexibility required for secondary operations (such as cutting, bending, and thermoforming).
The tradeoffs when using UV thermoforming screen inks include their higher price per gallon and the higher energy output required to cure the ink. “Thermoforming inks cure in two steps,” says Maybaum. “The initial UV curing hardens the surface of the ink. The ink fully cures in the second step, when subjected to the high heat of the thermoforming process.”
Prior to thermoforming, these inks are very soft. So handle the acrylic sheets with tender-loving care to prevent scratching.
In some cases, a general purpose ink may provide adequate results, especially when the job requires either bending or forming. But the risk of adhesion failure when using these inks is much higher. For this reason, carefully monitor the performance of the ink during production.
In selecting a screen printing ink, the printer must weigh the risk of failure against higher material cost. Recognizing the problems that printers have had with ink adhesion, Plaskolite has developed a special acrylic polymer, which improves the adhesion of UV-curable inks without the use of an adhesion promoter prior to printing. Optix® DA (for Digital Acrylic Sheet) eliminates one step in production, reducing raw material and labor costs.
Choosing a Vinyl
When decorating acrylic sheet, selecting the right vinyl film for the job depends on a variety of factors: durability, budget, customer expectations, requirements of the application, compatibility of the adhesive on the vinyl film with the acrylic sheet, etc.
Vinyl films usually stick to acrylic without a problem, but there are exceptions here.
Additives in the acrylic sheet can inhibit adhesion, causing failure of the graphics. Even if the film adheres well initially, lubricants and plasticizers in the sheet can eventually weaken the adhesive bond, causing the vinyl to peel and delaminate from the substrate.
Test adhesion immediately following application and then twenty-four hours later. If the adhesion drops dramatically a day after applying the film, something in the acrylic sheet has adversely interacted with the vinyl film’s adhesive.
For internally illuminated signage, both cast and calendered vinyl films are used. For less demanding interior applications (such as menu boards), a calendered translucent vinyl suffices. For long-term, outdoor backlit applications, a cast vinyl film is your best choice.
In selecting translucent films, consider the temperature of the sign box. An internally illuminated sign box can reach over 150°F. Subjected to prolonged high temperatures, many calendered films can discolor, shrink, and crack.
Also evaluate the appearance of the film during the day and when the signage is illuminated. The appearance of some calendered films may be acceptable during the day, but inconsistencies in color and film gauge readily appear when the sign box lights up. Variations in film gauge can yield a blotchy appearance when the sign is illuminated.
Vinyl Application
Even if a plastic sheet includes protective masking, you must clean the sign face before applying the graphics. Some professional decal installers only apply vinyl dry.
In most cases, I agree that dry application is the way to go. However applying a translucent vinyl to a plastic sign face is the exception to the rule because the sign face tends to attract vinyl like a magnet. For this reason, I believe wet applications serve such jobs better.
You can make your own application fluid by mixing 1/2 teaspoon of a dishwashing detergent, 1/2 teaspoon of isopropyl alcohol, and 20 ounces of water. If you make your own concoction, don’t use dishwashing liquid that contains moisturizing lotions; these will contaminate the vinyl’s adhesive and can create adhesion failure.
The much better alternative is to use a commercial application fluid (such as Rapid Tac™). The companies that make application fluid know what they’re doing. All of those ingredients are precisely measured. And their products are routinely checked for quality assurance.
I have a few rules on using application fluids that probably differ from what the makers will tell you:
Rule #1: Use the least amount of application fluid to accomplish the task. Lightly mist the surface with the fluid; excess can create unwanted residue.
Rule #2: If using a kraft paper-type liner, be careful not to get the release liner wet. Chunks of the siliconization layer can flake off of the liner and contaminate the adhesive. When the sign is illuminated, these areas appear as dark blotches.
When applying graphics, many installers start in the center. From that starting point, apply the graphics on one side and then work to the other outside edge.
Now this isn’t the only method of application. The key is to find a technique that works for you. When you find it, continue to use it. If an application technique isn’t working, find one that will.
However you decide to tackle the application, apply enough squeegee pressure to force any application fluid from under the vinyl and always use overlapping strokes.
If the graphic is covered with an application tape, after removing it, re-squeegee the entire graphic using a squeegee covered with a low-friction sleeve. The sleeve will protect the vinyl from scratches. More importantly, re-squeegeeing will ensure that any edges you may have pulled away from the surface when you removed the application tape are secure.
When working with translucent films, avoid seams if you can. When translucent films overlap, the seam will be noticeable when the sign box is illuminated. Of course, overlaps aren’t always avoidable but keep them to a minimum.
According to Chuck Bules, technical service manager at Arlon, seams shouldn’t exceed 1/16-inch for cast film. “Shrinkage of calendered film is about twice that of cast, so the overlap should be 1/8-inch,” he says. “Abutting films can shrink and leave a noticeable gap when the sign is lit.”
The film’s light and color transmission can vary from roll to roll—sometimes, even within a single roll. Therefore overlap films from the same roll or lot number.
Even then, color variations and inconsistencies can occur within the same roll. For this reason, if a film needs to be seamed, take the time to match colors to ensure a consistent appearance.
Top photo courtesy of Ability Plastics.
