Printing on Specialty Substrates
Managing variables associated with the substrates you use is critical to quality. Find out how to work with some popular, but picky, materials.
Printers can control many variables in the printing process; however, the customer often predetermines which substrate fits the purpose. Therefore, the more you know about all types of substrates and hot markets for certain types of substrates, the more prepared you will be to print on whatever comes your way. This article discusses printing on plastic, metal, wood, and glass and offers valuable insight from printers about production considerations, growing markets for certain substrates, and disasters to avoid.
You can use a wide variety of plastics for graphics and industrial applications. Thermoset and thermoplastic substrates are commonly classified by their physical properties with several minor classifications. These two plastic types perform very differently during the printing process.
Thermoset plastics undergo chemical and physical changes when manufactured. They cure, set, or harden to the final shape and, once the chemical cross-linking occurs, cannot be remolded. Ink systems for thermoset plastics rely on mechanical adhesion to the surface whereby the ink adheres to surface fissures in the plastic. Common thermoset plastics include aminos (melamines and ureas), casein, epoxies, phenolics, polyesters (alkyds), silicones, and urethanes.
Thermoplastics can be reshaped again and again. Their shape is not fixed, as is the case with thermosets; however, repeated exposure to the high temperatures associated with reforming eventually will cause material degradation. The following represents the most popular thermoplastics and applications:
ABS (acrylonitrile butadiene styrene) ABS can be injection molded, extruded, calendered, blow molded, and vacuum formed. ABS is used in outdoor point-of-sale signs, home appliances, plastic trash cans, and other rough-wear items. Because it has good high-impact strength, as well as heat and chemical resistance, it fits many rugged applications.
Acrylic General-purpose displays, point-of-sale and countertop signage, backlit signs and graphics, aircraft canopies, and windshields are made from acrylic. Care must be taken with solvents coming in contact with the edges of the acrylic material to avoid cracked or crazed edges. Acrylics can be heated and bent into various shapes for use in displays and brochure holders.
Polycarbonate This materials is useful for vandal-proof sheeting for signage, aircraft interior panels, safety helmets, visors, and backlit sign faces.
Polyethylene High- and low-density blow-molded containers—including high-density polyethylene (HDPE) packaging material, which allows safe handling of hazardous materials—and P.E.T. (polyethylene terephthalate) offer good barrier properties against oxygen and carbon dioxide. Polyethylene’s low surface energy necessitates pretreament or ink modification to facilitate ink adhesion.
Polypropylene Plastic bottles, corrugated-plastic sheeting, real-estate signs, storage boxes, and many other temporary indoor or outdoor point-of-sale/point-of-purchase displays are made from it. Low surface energy necessitates pretreament or ink modification to facilitate ink adhesion.
PVC (polyvinylchloride) PVC is highly popular because of its durability and strength. Display graphics, ID cards, containers, and medical supplies are a few examples of applications suited to PVC.
Inks for thermoplastics establish a chemical bond on the substrate’s surface, in addition to the mechanical bond created when the ink dries or cures. Solvent-based, multipurpose inks make adhesion on a majority of thermoplastics much easier.
Ink fails to adhere adequately to substrates that have a surface energy that is lower than the surface tension of the ink printed on the material. Surface energy is expressed in dynes. Using a dyne pen will help you determine the substrate’s level of surface energy. The objective is to test the surface of the substrate with one of several pens in a set until the ink begins to flow out and not bead like water on a waxed surface. You can change the ink’s wetting properties by using a special modifier or adjust the substrate’s surface energy by treating the plastic with an air-plasma (corona) system before going to press.
UV ink can also be used with thermoplastics as the curing action of the acrylic monomer forms a hard, mechanical bond that is useful when vacuum-forming screen-printed thermoplastics. Whatever the ink system, communicate with the manufacturer to ensure proper curing parameters.
Vinyls will leach out plasticizers, especially when encapsulated in an airtight package or subjected to prolonged heat. Plasticizer is a contaminant and will cause fish eyes, blurred printing, or ink delamination during and after drying. Wiping the surface of cast vinyl with 70% isopropyl alcohol just before printing usually removes most surface plasticizer.
Metal is an extremely durable substrate but needs a high degree of preparation. Some of the more popular metal products that are screen printed include member cards, discount cards, nameplates for desks, offices and restaurant signs, awards, clock faces, dials, appliances, point-of-sale graphics, and outdoor signage.
Uncoated steel provides a cost-effective substrate (common choices are between 24 and 30 gauge). The biggest disadvantages of uncoated steel are the lack of corrosion resistance, its weight, and the danger associated with sharp edges. Consider the weight of the material with regard to on-press pickup, conveyer, and delivery systems as well as transporting it around the facility. Metal, especially steel, also has thermal conductivity, which will affect curing and handling. The condition of the edge can affect screen life, registration, and handling. Material flatness or deflection can affect registration and press-feed and take-off systems. Metal substrates also require special equipment for cutting, forming, embossing, packing, and shipping.
Cleaning plain, uncoated steel before printing is absolutely necessary. A phosphate wash removes manufacturing salts and lightly etches the surface to promote protective-coating adhesion. Steel is commonly painted or powder coated to protect it from rapidly oxidizing in outdoor applications (the most common place for steel installations).
Painting or coating is accomplished by spraying, dipping, roller coating, or powder coating the metal. Powder coating as the name implies, refers to a thin powder (typically polyester) that covers the metal when it is subjected to an electrical charge. The coating has the opposite electrical charge and when the powder is sprayed onto the steel, it is covered with the powder. The powder coating is then heated to 350-450˚F for 15-20 minutes. The polyester coating melts onto the surface and aggressively adheres as it cools. The result is a hard, durable, and environmentally protected metal surface. Solvent-based thermoset inks are typically used to fuse the ink into the hard surface of the powder coating. UV inks are also used, but working closely with your ink manufacturer is vital in selecting the right ink.
Compared to steel, aluminum provides a better strength-to-weight ratio, has better corrosion resistance, and provides excellent formability. Typical sign materials range from 0.020-0.060 in. thick and are typically 3XXX series aluminum with H12-14 temper, a measure of hardness. Aluminum is often used for jobs that require a lightweight, rust-resistant metal.
Aluminum comes in coils or precut sheets with a coated, printable surface on one or two sides. Uncoated is designated as clear/clear. One side coated is designated as white/clear. Aluminum can be found in different colors either through powder coatings or enamel coatings or organic/inorganic coloring agents for anodized aluminum. If it has been coated with enamel, it has usually gone through a process called conversion coating or passivation, which prepares the surface for coating. In many cases a thermoset acrylic enamel designed for exterior applications is used. White-coated aluminum can go through the oven only one time before it yellows. Powder-coated aluminum, as with steel, can be heat processed several times without yellowing.
Anodized aluminum is done through a controlled natural oxidation process when aluminum is exposed to the atmosphere. Electricity and chemicals are used in conjunction to produce a hard, transparent surface that is integral with the base aluminum. The anodizing process includes a cleaning and etching pre-treatment and the buildup of the anodic film that is built on the aluminum itself. The hard, porous film can then be colored by organic and inorganic coloring agents. Later in the process, a hot water bath treatment causes the pores to swell shut, locking in the colorant.
Concerns for steel and aluminum include water stains on the surface of metals from when water condenses or gets trapped between sheets. This can cause white rust in the case of galvanized steel and marks on the anodized aluminum. The metals can stick together when exposed to large amounts of humidity, causing surface damage. Coating softness and excessive heat can also cause blocking, either on the enamel or powder coating when improperly cured. Wear gloves to prevent the transfer of oil from hands. Make sure to square all stock edges before setting up registration for a multicolor job, and keep stock as flat as possible to ensure uniform ink application.
You can print onto hard woods, soft woods, particle board, and veneers that are properly prepared for the application. All woods absorb moisture at different rates. Treating wood with a sanding sealer before printing allows the ink to set on the surface. Softer woods require more sealer applied in multiple coats prior to printing. Most commercial wood-sanding sealers suffice. Apply it by spraying, painting, or roller coating.
Particle board is seldom printed as a decorative piece unless it’s covered with a veneer. Veneers are typically roller coated and cured with a UV clear coat. Enamel ink is most often used for decorative printing on veneers. The end result is only as good as the compatibility between the ink and the wood sealer; therefore, test before you print.
Drying ink on a wood surface is especially troublesome when using a heated conveyor dryer, because the wood fibers dry and contract too rapidly, which results in cracks. Air drying is safer, because solvent inks dry quickly while the wood releases its moisture naturally over an extended period of time. You must also consider grain direction when printing onto wood. A lightly sanded and sealed wood surface shows a very definite grain pattern after printing. You can virtually eliminate this pattern by thoroughly sealing the wood.
Several ink formulations are available for decorating glass. Ceramic inks contain finely powdered glass or frit and inorganic pigments and are fired at a temperature exceeding 1112°F. Enamel inks have oil-based media that disperse the inorganic pigments, glass frit, and additives and can be printed through steel, polyester, or nylon mesh. Oils burn off before the pigments, and the frit fire into the glass or ceramic. The whole cycle takes 1-1.5 hours, during which the temperature rises to 1112-1202°F for 10 minutes before the glass is allowed cool. The disadvantage of this method is that multicolor printing is unrealistic. Previously printed colors can be lifted off by subsequent screens.
The same is true for two-part epoxy inks designed for printing on glass. But, for single-color applications, ceramic inks are favored by beverage and cosmetics companies because of the visual impact of screen-printed images and presence of ink thickness.
Screen printing with ceramic inks and firing those enamels into the glass in a lehr can produce lasting, quality images; however, the process is slow and expensive for decorating beverage or cosmetics glass containers. A lehr uses substantial energy, and the temperature variations within it can cause color variations. The ink system also imposes design restrictions, because the resolution of printed images is limited by the minimum mesh size required by ceramic enamels.
Thermoplastic (TP) inks, also called hot-melt inks, are solid at room temperature and require heat to become printable. The inks are pre-heated from 149-167°F, and the molten enamel is then poured into a metal screen that is heated via an electric current. The molten ink acts like conventional ink as it passes through the screen, but once it strikes the colder glass, it solidifies and returns to a waxy state. TP inks are nicely suited for use on automatic, multicolor presses. Once printed, however, the multicolor image is somewhat fragile and still requires the organic pigments and resins to be fired at 392°F in a lehr.
The big advantage of fired inks is that they fuse to, and become part of, the substrate. Metals such as gold, silver, palladium, and platinum can also be used as pigments and applied to glass and ceramics for decorative or industrial applications. The major disadvantage is the complexity and cost of operating gas-fired lehrs.
The method of printing UV inks is the same as printing onto plastics and board. Organic pigments and resins are cured by UV radiation. UV curing uses far less energy than a lehr and the ink cures instantly. Adhesion is excellent on pre-treated glass and
good on untreated glass. UV ink is not as tough as fired enamels, but it is an acceptable alternative for many applications.
The most effective method of applying complex, multicolor designs is to print onto transfer paper and apply the graphic either as a water-slide or heat-applied transfer. Image quality is second to none, and you can use many colors. Four-color process requires a white background, which may be printed.
Automotive applications are significant in glass printing. Shades and filters are printed on the windshield edges, as are electrical circuits (defrosters) and antennae. Architectural glass is another area where printing is popular. Screen-printed effects can generate significant added value to buildings. Even internal partitions imaged with exotic designs can transform the working environment.
Johnny Shell, SGIA’s vice president of technical services, directs and coordinates the activities for the Association’s Technical Services department. He also teaches SGIA workshops, writes technical articles for the SGIA Journal, and conducts seminars.
Bradley Nameplate Corp.
Jim Bradley, president
We print on metal, but we primarily print labels on polymers. Our main product is Lexan panels. We have been in business since 1976. In any given quarter, we do work for about 260 different high-tech manufacturing companies (Figure 1). Approximately 20% of our product is shipped to about 90 different overseas locations.
You need to print on a clean surface. Do a tape test after you print to make sure the ink doesn’t fall off. This is true for metal and polymer substrates. In addition to UL- and CSA-approved constructions, which are tested by UL annually, I always have printed samples (applied to magnetic sign material) of our printing on the roof of my van with a duplicate set in a cabinet in the shop. Every three months, I compare the two sets. Then I run my car through the car wash, leave it in the sun, drive it around, and that tests survivability. My car roof is my final test.
We will print signage for anyone making a product; however, we primarily print Lexan panels. We do flexo printing and hot stamping, as well as screen printing. Our customers are electronics firms. We print indoor and outdoor signage for industrial companies because they are the safest bet for us. Retail companies do not have the economic wherewithal. Big chains don’t always pay their bills, either. But people who manufacture are our bread and butter, making up 90%.
Most metal is cut with a shear. Chipping becomes a problem when your ink bleeds off of an edge. Sometimes getting up in the morning is a disaster.
Mike McDaniel, technical manager, screen-printing department
When printing on polycarbonate and polyester, make sure you have a good process for handling the material during manufacturing and transporting to prevent denting or scratching. Control and document ink thickness accurately so that it remains consistent from run to run. Document mesh correctly, such as plain weave vs. twill weave, thread or wire diameter, and mesh count. Control ink-curing parameters to ensure good cure and consistent color. Have good traceability, including substrate-lot numbers, ink-lot numbers, and expiration dates. Make sure you have a non-conforming ink and substrate area isolated from your controlled inventory.
Cleanliness is critical. Schedule tasks for cleaning the inside of dryers, and wear hair covers and gowns to prevent contamination. Incorporate good static-electricity controls, along with a controlled environment with humidifiers and air conditioning.
Establish good press checks prior to running the job to check for ink cure and adhesion. Have good control of pre-matching inks in your ink lab prior to printing, and have all controls in place to ensure inks matched in the ink lab are the same when printed on the press.
For printing on polycarbonate and polyester substrates, our best markets are automotive, medical (Figure 2), appliance, controllers (Figure 3), aerospace, solar, and RFID.
Poor screen stretching, improper curing, or poor imaging of the screens can lead to image distortion. Poor curing, wrong mesh count, or incorrect choice of ink can cause ink delamination. Careless handling leads to scratching and denting. Improper curing and excessively thick ink deposits result in ink fracturing during embossing and can cause actuation failures. Finally, improper curing can create unexpected color shifts.
KDM P.O.P. Solutions Group
Tom Kissel, manager of purchasing
In printing on Luan board substrate (Figure 4), find a supplier that understands the actual use of the material and the application for the P-O-P market. Our supplier picked out the flattest board he had and then sanded the edges to reduce the risk of damaging our screens. You have to keep an eye on board quality and surface flatness, as problems in these areas can lead to ripped screens, dot loss, and image loss. The best markets for this material are old-fashioned restaurants, hardware stores, and building-supply companies.
We also print on polyethylene (Figure 5). In printing this substrate, use films that lay flat without rolling, apply proper vacuum, and use static eliminators. The best markets for this substrate are retail environments and fast-food chains. The best application is outdoor advertising.
Problems with this substrate come up when printing with UV inks. The material is very unstable when exposed to heat. It expands after heating and then contracts—but not always to its original size. Sometimes polyethylene is difficult to run on a single-color press.
With polystyrene (HIPS) there are no trade secrets. This substrate is one of the easiest and fastest running substrates for screen printing. It’s easy to handle with a rigid, flat, smooth surface. Markets for polystyrene are endless. We use this material in all of our business segments in printing, plastic fabrication, environment, and retail applications.
A few things to watch for are static electricity and bruised product (which shows scuff marks on the styrene). The material can warp when the manufacturer bands it while it’s still warm.
West Orange, NJ
Joe Shondel, president
We mainly print on pressure-sensitive vinyls, but we also print on drywall, wood, and most surfaces. On wood we must control the absorption rate along with the print speed. We also need to know how many printing passes are necessary for opacity or brilliance.
Drywall has a paper coating, so we have to print that quickly. Then we have to add a number of passes to get the color we need. We did this for a museum (Figure 6) in Manhattan. We work with architects here for wall graphics (Figure 7). We produce a test print and then decide to add passes until we see the desired effect. Printing on metal requires surface preparation to remove oils and residue. We clean surfaces scrupulously before printing.
There are variables in printing. Under or over curing is an important one. You need enough UV energy (measured in joules/cm2) to cure the ink properly. It’s a technology now, not an art.
Frank Torlucci, VP of manufacturing and production manager, says the market for fleet graphics is the best for Selecto-Flash (Figure 8), The company also uses pressure-sensitive substrates to produce signage and decals for windows, doors, and walls.
“We’re printing on vinyl and use screen or digital printing with conventional solvent-based or UV inks. However, we do a lot of things on other specialty substrates, such as styrene, foam core, plastic core, and cardboard,” he says. “At present, we don’t use water-based inks—only solvent or UV.”
A common thing would be using the wrong inks or clear coats that would fade before seven years. Ours last over the long haul, but we’re careful in selecting them. Another problem might be adhesion failure caused by applying a substrate to a dirty truck. Using a substrate in the wrong climate could be another disaster.
La Crosse, WI
Pat Tully, senior applications consultant
When printing on polycarbonate, be sure to choose the correct type of UV bulb. Printing with UV ink systems requires the right UV bulb to maintain the integrity of critical colors without shifting the color readings or degrading the substrate on multiple pass jobs. There are better bulbs for curing—mercury or iron, for example—but on multiple-pass jobs or parts with critical colors, gallium bulbs work well. We will not use the mercury or iron bulbs on grays or other critical colors or on jobs with more than five colors. Polycarbonate can become brittle and/or yellow when over exposed to certain UV wavelengths, so consult your ink supplier for a recommendation on
the best way to cure a particular ink system.
Any market that can use the high-ly aesthetic virtues of a polycarbonate overlay is our best market. Polycarbonate is used in the medical, automotive, appliance, and OEM markets with great success. Velvet-textured polycarbonate is a very popular substrate for its durable, anti-glare, smudge-resistant surface finish. Applications include decorative logos, warning and safety decals, and low-use (low number of actuations) customer interfaces and control panels.
Not fully curing your inks, solvent or UV, can cause headaches for production down the line. Applying adhesives too soon after printing UV inks can cause issues with delamination as the ink and the adhesive interact. We recommend waiting approximately 24 hours before applying an adhesive.
Temple Hills, MD
Keith Prichard, general manager
We do screen and digital printing for banners, P-O-P displays, transit advertising, and a wide variety of projects. One growing area is printing on polycarbonate (Figure 9). When printing on polycarbonate, one thing to keep in mind is how it is used—decorative displays, panel fronts for electronic equipment, etc. Nine times out of ten, the image is subsurface printed and then laminated with adhesive. Pick your inks wisely, and let your ink manufacturer in on what type of adhesive you use.
You must be sure the inks you use can adhere well enough to the substrate so they will not give way when you apply the adhesive. In other words, if you pull on the panel and the polycarbonate lifts off—leaving ink and adhesive behind—you have a disaster! Unfortunately, the problem is hard to detect immediately. This type of failure occurs over time. The adhesive laminate sometimes breaks down the adhesive properties of the ink. You have to work closely with the adhesive and ink manufacturer to prevent this problem.
Selecting a matte finish for the subsurface-printed side of the polycarbonate sometimes helps prevent the problem. It gives more tooth for the adhesive to grab. Solvent inks tend to adhere better than UV inks, but solvent inks are more difficult to work in this application. Environmental concerns are also a problem.
OEMs—companies that once used metal for long-term durability in markings and control panels—present a large market for polycarbonate substrates. Polycarbonate gives the customer a wide variety of surface finishes. It is less expensive than metal substrates to mass-produce.