Learn about frame options for oversized jobs and find out how to properly select, tension, coat, and expose jumbo screens for optimum performance on press.
Pick up a magazine about commercial printing or visit an industry trade show and you're left with the impression that large-format screen printing is increasingly being challenged by inkjet systems. It is true that digital-imaging technology is evolving quickly, offering the ability to decorate a variety of substrates with long-lasting and durable inks and encroaching on areas once solely the preserve of screen printing. However, advances in printing materials and innovations in screenmaking technology also are helping to maintain the competitive edge that screen printing enjoys over competing methods of decorating. Screen printing is a supremely versatile and established method of decorating, and it's still the best, or only, choice for many applications. Large-format screen printing particularly excels in medium- to high-volume production of display graphics, textiles, decorated glass, and other industrial and specialty applications. Screen printing dominates these markets because large-format screen-printing presses typically output 6000-30,000 sq ft/hr, compared to the 100-2000 sq ft/hr production rates of most inkjet printers. The ability to print conductive, abrasive, and other particulate and specialty inks is another reason screen printing remains competitive. Finally, a wide variety of ink colors is available, including colors that correspond to the Pantone Matching System, fluorescents, metallics, and custom colors created in each shop's own ink kitchen. In order to take advantage of screen printing's ability to efficiently produce great quantities of high-quality large-format graphics, you must first develop a solid understanding of large-format screenmaking. This article will explore the procedures and processing techniques required to produce effective screens for printing large-format images of 50 x 80 in. (1250 x 2000 mm) and larger. It also will highlight areas where technological advances have improved productivity in the screenmaking process and address various alternatives for stencilmaking--some of which take advantage of digital-imaging technology. Large-format screen frames Due to their size and weight, large-format frames create handling problems for screen printers. However, with a little help, these frames can be managed with ease. For some, the preferred method of frame transportation is to drag the frame around on its side or corner. Unfortunately, sliding the frames around on the floor not only causes dings and gouges to the frames and the floor, but it allows the frame and screen to pick up dust, dirt, and whatever else is lying about. A simple jarring can cause tension loss on a stretched screen, or worse, a half-hour reprimand from the boss. That's why the use of wheeled dollies or carts is truly the preferred method for transporting large frames. Keep in mind that a large-format frame can hold several hundreds of dollars worth of screen mesh, and the fabric is only a careless misstep away from popping. So even with the assistance of a cart, it is still wise to use two people to move the frames and minimize the potential for damage. In addition to handling issues, you also have to consider the workflow you set up for processing large screens. Shop layouts that are designed to make the best use of space don't always make sense when working with frames of this size. Your focus should instead be keeping the distance of travel down to a minimum. The shorter the distance a screen moves, the less chance you have to damage it. Let's face it--many screen shops have limited space and even less time to organize. But a lot can be accomplished with a little ingenuity. A third consideration is strength vs. weight. Frame manufacturers always try to strike a balance between the two. The key concern for you, the printer, is to eliminate, or at least minimize, screen deflection (the inward bowing of longer frame edges when tensioned mesh is attached). This deflection leads to an unpredictable decrease in mesh-tension level, uneven tension across the screen, unreliable ink-transfer characteristics, and other problems. To get past these problems, you have three options: using oversized frame profiles, pre-bowed frames, or reinforced frames. An oversized profile makes it possible to have a frame that provides zero deflection with edge lengths greater than 120 in. But how many presses can handle a frame with an oversized profile of 3 x 6 in.? Before you opt for oversized frame profiles, make sure they're compatible with your equipment. The next alternative is to use prebowed frames. Some frame manufacturers offer frames with their longer edges prebowed outward slightly, which helps them resist deflection and maintain the correct screen tension when the mesh is affixed. Alternately, you can prebow the frames yourself at the time of mesh tensioning, provided that your tensioning equipment supports this function. In this situation, you are prebowing the frame inward. Once the mesh is affixed, the frame either maintains its prebowed shape and screen-tension remains stable, or the frame resumes its square profile, increasing the mesh tension. The final option is to use reinforced frames (Figure 1). These are more expensive than the other alternatives, but they are becoming more popular among screen printers who want the added strength without additional weight. You'll have to judge the pros and cons of each option to make the decision that is right for your shop. A quick phone call to your supplier can help minimize any confusion that you have and lead you to an effective solution more quickly. Stretching techniques Stretching techniques have evolved in recent years. The newer generations of pneumatic clamps have made stretching large-format screens an easier proposition, and features like tensioning-clamp arms that support longer travel distances and above-frame tensioning deliver higher, more consistent, tension levels. Two main tensioning techniques are typically employed in large-format screenmaking. All other techniques are essentially variations of these two methods. The oldest and most trusted is staged tensioning. Staged tensioning can be done on any type of frame or stretching device. This method produces the most stable screen, but the process is time consuming. In the fast-paced world of production, the added time it takes to stage tension is usually not worth it for a difference of 2-3 Newtons. But if time is not an issue, this may be your best bet for large-format tensioning. Rapid tensioning is Saati's preferred method. Rapid tensioning is best done on pneumatic stretchers, but some mechanical stretchers also will work. This method involves the practice of the fabric to the desired tension or slightly higher (by 2-3 Newtons) within the first 30 seconds to shock stabilize the fabric. The tension is held from 10-45 min, depending on the shop's conditions. The fabric won't stabilize completely in less than 10 min, and any more than 45 min really won't provide any major benefits. The newest technique is pulsed tensioning, a hybrid of the two methods previously discussed. It offers some of the benefits of stage tensioning, such as stability, along with the speed of rapid tensioning. To pulse tension a screen, you use rapid tensioning to take the screen to a tension level below its targeted final tension and hold it there for a few seconds before releasing. You then retension to a slightly higher level and release again. The procedure is repeated until you reach the target tension level. This method isn't our favorite, but it is an effective way to stretch large-format screens. You'll need a pneumatic stretcher to get the benefits out of this method, and you should try to prevent the fabric from coming in contact with the frame until it is time to glue. This reduces the sawing action between the frame and mesh. We do not recommend pulse tensioning for multiple-shift operations in which different personnel stretch screens, because repeatability will become a problem unless strict procedures are followed to ensure consistency. Regardless of the stretching method chosen, always refer to the tension levels specified by the manufacturers, but keep in mind that frames of this size should not be tensioned to the maximum tension listed. Most mesh manufacturers put a range of acceptable tensions in their literature. These are only guidelines for stretching, and most of the figures provided are for 60-in.-wide mesh. We suggest reducing the maximum specified tension by up to 15% for fabrics more than 80 in. wide. Fabric selection Selecting the right mesh for large-format printing is an important step. Plain-weave, low-elongation screens are best suited for the job and help increase the stability and overall life span of the tensioned mesh. Twill-weave fabrics, which typically have less elongation and therefore give the illusion of strength, may be acceptable in shops where the travel distance of stretching clamps is limited or when the detail of the printed image isn't an issue. However, using premium mesh material will help ensure quality and consistency in a time when more and more customers are looking for high resolution in large-format graphics. Mesh manufacturers are helping screen printers meet these demands for high resolution by producing fabrics with finer threads. A few years ago, finding a 31-micron thread in widths greater than 60 in. was rare. Now, manufacturers are looking toward 100 in. and wider for the finer threads. Not surprisingly, there are challenges posed by fine, wide fabrics. The finer the thread and the wider the fabric, the weaker and less stable the fabric can become. However, mesh manufacturers are constantly developing and testing new thread types to fortify strength and stability. At this point, 27-micron fabric is not available in widths greater than 65 in. in any serious quantity from any manufacturer, but its arrival is not too far off in the future. Other thread diameters are being woven in widths of 145 in. on a regular basis. Specialty meshes also help keep screen printing a desirable decorating method for demanding jobs. For example, the automotive-glass market favors a variable-mesh-count fabric for printing defroster lines on windshields. Another type of fabric features a unique pretreatment with a structure that alleviates the need for degreasing and abrading prior to stencilmaking. These pretreated meshes, which are produced in different forms by a variety of manufacturers, help eliminate excessive screen handling and promote stencil adhesion. This quality is especially useful with stencils that are prone to underexposure, such as those produced with projection-exposure equipment. We still recommend degreasing the fabric after each reclaiming cycle, but the mesh's treatment will allow you to capture details that normally would fall off the screen, even after several uses. The treatment is a post-weaving process, so it can be done to any mesh count, any color, and most screen widths. Stencilmaking The sheer size of large-format screens makes the main steps in working with large-format stencils--mesh preparation, coating, exposure, and reclaiming--a challenge. However, many solutions are available to help simplify the process. For example, manual screen-making tasks have been largely replaced in many shops by automated equipment. This is particularly true for screen coating, where automatic coating machines have been in common use for many years. Let's examine each step in turn and discuss techniques and tools you can use to simplify the process of large-format screenmaking. Mesh preparation Polyester mesh is a precisely engineered synthetic fabric. The material exhibits excellent barrier properties and moisture resistance, which is why it's used for soda bottles and packaging films for demanding applications. It is, by definition, an inert plastic with poor adhesion properties. Direct emulsions behave like water-based paints. They're not attracted to plastics, and they display poor adhesion characteristics. To produce high-quality stencils that support long printing life, you must properly prepare the screens so that the emulsion will adhere well to the mesh. Good adhesion is especially important when using abrasive or aggressive water-based inks, such as those used in glass or textile printing. It also is important to prevent stencil damage when part of an image is masked off with tape or blocked out and then uncovered for printing at a later stage. Mesh has to be cleaned to eliminate oils or contamination that can prevent an emulsion from wetting its surface. The fabric also needs to be treated with a wetting agent. This raises the dyne level, or surface energy, of the polyester mesh to an acceptable level so that emulsion will want to adhere to it. This concept is familiar to anyone who prints on low-energy plastics, such as polypropylene. These substrates need surface treatment up to a minimum dyne level to guarantee optimal ink adhesion. A correctly degreased and prepped screen will display an even and unbroken film of water on its surface after rinsing. Exposure latitude and resolution are maximized when stencil adhesion is improved. Emulsion coating For consistent results, we highly recommended that you use an automatic screen coater (Figure 2) on large-format screens. It is impossible to guarantee an even coating thickness at these sizes with manual coating techniques. Besides, if your arms are long enough and you can balance the weight of a fully loaded wide-format coating trough, then you might be better off working in a circus! You should use an emulsion that has a high solids content. This reduces shrinkage during drying and enables you to use simple coating techniques to produce high-quality stencils. Coating machines also are available with built-in dryers that function reasonably well when large-format screens with high mesh counts are used. Screens with medium to low mesh counts hold too much wet emulsion to be quickly and effectively dried on the coater. Drying Screen drying should always be carried out in a segregated, clean, dry, and safe-lit area and protected from contamination until screens are dry enough for exposure. Warm, dehumidified air in the 100°F (40°C) range should be circulated freely to remove moisture from the drying area. Photopolymer emulsions are affected less by residual moisture and high temperatures during drying and storage. They can be exposed perfectly, even if they're not completely dry. However, diazo and dual-cure emulsions must be dried below 4% moisture content before they can be exposed properly. You can use a contact gauge to precisely measure residual moisture in the emulsion. Exposure Technological advances in stencilmaking have challenged conventional, large-format imagesetter film, and several competing methods offer improvements in efficiency, as well as cost savings. Computer-to-screen imaging systems (Figure 3) bring the efficiency of a digital workflow to screenmaking. Film is eliminated, and the exposure process is streamlined. Screen handling also is simplified because a large-format vacuum frame is no longer necessary. Finally, the time spent retouching pinholes is greatly reduced because there are no films or glass surfaces to attract dust. While large-format inkjet printers aren't the same as computer-to-screen systems, their output quality is often close enough for shops that are used to waiting for outsourced film. Many screen printers use large-format inkjets to produce oversized film positives in house (Figure 4). Projection exposure units (Figure 5) drastically cut film costs. At a 10x enlargement, which is typical for large-format screenmaking, the image on the film is only 1% of the area of the final print. Special emulsions keep exposure times in the single-minute range, and screen handling is again simplified because no vacuum frame is needed. Regardless of how you expose your screens, it is important that exposure time is sufficient to fully harden the emulsion and produce a durable stencil that resists chemicals and abrasion and offers good adhesion. You can identify the optimum exposure time for diazo and dual-cure emulsions by using a color-change exposure calculator. This method indicates correct exposure as the end point in a color-change process--for example, the yellow diazo is bleached during exposure, which changes the color of the exposed stencil. Typically, the color change is from green to blue or from brown to purple. An alternative is the 21-step grayscale exposure calculator, which can be used for photopolymer emulsions, as well as diazo and dual-cure types. Optimum exposure is determined as the time required for hardening seven grayscale steps after the screen is fully processed. Exposure latitude is a big issue when exposing large-format stencils. To maximize exposure latitude when exposing very large screens, it is important that you select an emulsion that supports high resolution, optimize the coating process to reduce any variation in coating thickness, and maximize stencil adhesion by properly preparing the mesh. Developing To guarantee optimum resolution, stencils should always be developed with a high-pressure, finely divided, spray of water applied to both sides of the screen. This is easy to do with small screens, but it is more difficult with large-format screens because of handling and space requirements. An effective solution is to use automatic washout equipment that also develops the stencil. Space requirements are minimized and, with some designs, over-spray is contained in the developing tower. The best manual alternative is to use a pressure washer with a minimum of 1000 psi and 3-5 gal/ min flow rate set on a wide fan spray. Reclaiming Manual cleaning of large-format screens requires that particular attention be paid to providing workers with the right tools to ensure efficiency and safety. Essentials include long-handled brushes that can reach to the far corners of the screen, as well as protective gear--glasses, gloves, boots, and so on. Separate brushes should be used for each step to prevent cross-contamination between chemical containers. Do not underestimate the importance of a good pressure washer. Buy the biggest and best you can afford. You won't be sorry. Automatic screen-reclaiming equipment can do most of the hard work and minimize worker exposure to reclaiming chemicals. Modular or inline units systematically remove inks and stencils, and they can degrease screens with good results. Ghost-image removal is still a challenge that the machines have not completely mastered. So even if you use an automatic system, there is still a good chance that you'll have to do some cleaning by hand. On the big screen Large-format screenmaking is a great response to the unending call to print bigger and better images on a diverse range of substrates. By embracing the continuing developments in materials and technology, you'll keep your business profitable and help ensure that screen printing remains at the forefront of wide-format imaging.
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