Putting Projection Exposure in Focus
A discussion of how the technology works, its capabilities and limitations, and the experiences of several printing companies that have adopted projection exposure.
In recent years, graphics screen-printing companies have been giving a great deal of attention to direct digital-printing technology for large-format graphics. Among the benefits of digital printing that attract screen shops, one of the most appealing is that this technology eliminates the costs of producing film positives, a particularly important consideration when short print runs are involved.
But when the number of short-run jobs escalates beyond the capacity of a shop's digital-printing equipment, it may be time to re-examine the inefficiencies of screen-printing prepress, especially those that relate to screenmaking. For many companies, the best way of breaking the bottleneck in screenmaking is a well-established, but often under-appreciated, technology called projection exposure.
Projection exposure is a mature technology that, while not completely eliminating film and chemistry consumption, reduces the need for prepress materials so drastically that the cost structure of screenmaking for large-format printing is completely rebalanced. With the ability to support both rapid exposure and high-volume screenmaking needs, the technology is ideal for large-format printers that require fast turnaround on a continuous stream of screens for both short- and long-run jobs. The discussion that follows will explain what makes projection exposure so ideal.
What is projection exposure?
Projection exposure, as the name implies, involves projecting an image onto an emulsion-coated screen in order to produce the stencil, as shown in Figure 1. The image is projected from a miniature positive, one that is a fraction of the final image size on the stencil. The process uses light from a high-intensity exposure lamp, which is concentrated with a lens, passed through the positive, then enlarged and focused with another lens assembly onto the coated screen (Figure 2). Think of a projection-exposure system as an oversized slide projector--one with the approximate size and weight of a small car.
The substantial cost savings projection exposure provides over conventional screenmaking methods relates directly to the very small film-positive size required. When we project and enlarge a positive image by a factor of ten, the image area required on the original positive is only 1% of the final printed image size. Even a modest enlargement of 333% requires a film positive that is only 10% of the final image area. This represents material savings of 90% over the full-size film used in conventional vacuum-frame exposure.
Besides film savings, projection exposure also provides greater production efficiency through reducing frame handling. Since there is no vacuum frame involved during exposure, vacuum drawdown time is eliminated entirely and screens can be positioned for exposure more rapidly than with conventional exposure systems.
However--and it is a big however--in order to use projection exposure in daily production and maintain control over the quality of the final printed image, the entire screenmaking process must be fine tuned. Projection exposure leaves little tolerance for inconsistency, and variables need to be controlled to a tighter degree than when working with full-size film positives for 1:1 stencil exposure. Also, most projection-exposure jobs require specially formulated photo emulsions that expose very quickly. In the next section, we'll explore the equipment, materials, and the methods we must follow in order to realize the full benefit of this technology.
A closer look at projection exposure
Projection-exposure technology dates back to the time of carbon-arc lamps and dichromate-sensitized emulsions. During the early years, the process was quite limited and produced only basic, low-resolution images on stencils that were far from robust. However, over time, improvements in both emulsion chemistry and projection-exposure technology enabled the process to evolve substantially.
Today, projection exposure delivers imaged stencils that rival the quality of those produced using full-size film, even those created for process-color printing. With the latest exposure lamps, film-handling mechanisms, and lens systems for projection exposure, some large-format graphics printers are regularly producing stencils with 65-, 85- and even 100-line/ in. process-color halftones.
Screen sizes imaged by projection exposure typically range from 3 x 4 ft to 12 x 20 ft or more and require positive enlargement of 3-15x. For process-color printing, both image size and line count will vary with the magnification level. For example, to print a 36 x 48-in. display graphic using a 400% enlargement, we would require a film positive with an image area of 9 x 12 in. If the final line-count on the print is supposed to be 65 lines/in., then the small projection positives would be output at 260 lines/in. Small-format proofs can be generated directly from the miniature films as well.
Projection-exposure equipment tends to be rather bulky, both because of the heavy lens systems used and because the machinery is engineered to resist vibration and other conditions that could affect image registration and stencil quality. All systems incorporate some form of screen-holding and positioning components that are designed to optimize registration. Screen handling can be further streamlined with cassette loading systems and automated screen-transport devices. Today, many projection screen-exposure systems are used in conjunction with automatic coating, developing, drying, and transport systems to optimize production efficiency.
A complete projection-exposure unit typically costs several hundred thousand dollars. However, the payback on this investment can be rapid as the savings in film and film-processing chemistry begin to add up. The more large-format screens processed with the system, the faster the payback will be. For many users of the technology, the equipment pays for itself within the first year.
The main contributor to the cost of projection-exposure equipment is the lens systems on which these units rely. Highly specialized optics are required to focus the image over a flat screen without distorting the image in the corners. Additionally, the lens system must prevent diffraction of image detail due to dispersion of the different wavelengths of light produced by the exposure lamp.
Narrow-pass filters are employed to help limit the wavelength diffraction effect, but they also diminish the light intensity delivered to the screen. This becomes more of a concern at high enlargement sizes. Since the amount of light energy output from the projector remains constant, the intensity at the emulsion surface will decrease as the image area increases. So, if you compare a 300% and a 1500% enlargement, exposure intensity decreases by a factor of 25 from the smaller to the larger size. As a result, the larger image would need an exposure that is 25 times longer than the one required for the smaller image.
To keep exposure times manageable and realize productivity benefits from minimal screen handling, extremely sensitive photo emulsions have been developed for projection exposure. Some screen printers, particularly those working at lower enlargement sizes, are able to use conventional, fast-exposing dual-cure emulsions to produce their stencils. In the majority of cases, however, very fast exposing pure photopolymer (SBQ) emulsions are the stencil system of choice.
Refinements in SBQ technology have led to the development of photopolymers that display such high sensitivity to UV that they expose in as little as 5% of the time required for normal dual-cure varieties. These emulsions allow for short exposure times at high levels of magnification, but they are still able to reproduce very fine detail. In addition, SBQ photopolymer technology enables the formulation of a range of emulsions that can be used to produce stencils for printing with UV, solvent-based, and water-based inks.
Along with these specialized emulsions, projection exposure brings other variations to screenmaking that aren't common when working with full-sized positives. First, white mesh is commonly recommended because it shortens exposure time. Surprisingly, white mesh allows the production of stencils that are capable of holding very fine detail, equivalent to that obtained when using dyed mesh, but with only 50-60% of the exposure time. This is definitely not the case when exposing with full-sized film in a vacuum frame, where white mesh is vastly outperformed by dyed mesh when it comes to reproducing fine detail at the equivalent exposure time.
Another difference is that stencils need to be thin in order to maximize exposure latitude. Typically with projection exposure, latitude is very narrow compared to full-sized film. Thicker emulsion coatings also generally suffer from reduced exposure latitude. So when we combine a thick coating with the inherently lower latitude of projection exposure, the result can be poor stencil adhesion. In halftones, for example, we would have trouble obtaining good adhesion in shadow areas and also good detail in the highlights.
When stencils produced by projection exposure are optimized for printing the full tonal range, they will not win prizes for the best stencil profile and Rz value. Nevertheless, correct and consistent stencil thickness remains important because it results in a reproducible exposure from screen to screen and constancy over the whole screen area. To optimize consistency with the thin emulsion coatings required for projection exposure, the screen-coating process must be carefully controlled. Using automatic screen coaters is highly recommended in order to minimize variables.
To determine optimum exposure, the best approach is to use a 21-step continuous-tone sensitivity guide. It can either be taped directly to the coated screen or stripped into the miniature film positive. Correct exposure times are reached when 7 of the 21 steps remain on the stencil after processing. Generally, when producing screens with very fine detail, working in the range between 5-7 steps is considered acceptable.
Projection exposure in action
Reduced film costs are only part of the appeal of projection exposure. Many companies also use the technology to enhance the efficiency and productivity of their screen rooms, with some producing as many as 150-200 large-format screens per day. Several of the companies that are already taking advantage of the prepress economics and screenmaking productivity benefits of projection exposure agreed to share their experiences for this article.
One is the John Evans Co., Salt Lake City, UT, a company that specializes in multisheet billboard printing. Printing process-color billboards requires a lot of screens, typically 40 per job, and the screenmaking challenges this poses become even more exaggerated when run lengths are short. However, with projection-exposure equipment, the company is more than able to satisfy the demand.
The company uses 84 x 138-in. screens with 158-thread/in. white mesh that has been coated with a fast-exposing photopolymer emulsion suitable for printing solvent-based inks. Projecting the stencil image at a magnification of 1400%, the company is able to achieve a complete exposure in approximately 1 min per screen.
According to the company's production manager, Frank Ventrello, projection exposure is essential in enabling this kind of productivity level. Because no vacuum frame is involved, he says screen handling is minimized and no time is wasted waiting for vacuum drawdown. Image placement is controlled from the projector.
Minimal handling, coupled with the fast exposure, substantially reduces screen turnaround. For John Evans Co., the turnaround is so fast that every screen in the company's inventory is generally reclaimed and reprocessed at least once each day.
Ventrello explains that holding fine detail is less important for the company than maintaining absolute control over dot size. The dots may be large when printing an 8-line/in. halftone, but they have to be controlled to ensure there is no color break as the image transitions from panel to panel. Consistent coating thickness is also essential since the narrow exposure latitude of projection technology will change dot sizes if the stencil-exposure level is incorrect. Altered dot size would stand out like a sore thumb when the final image is assembled, Ventrello says.
Another company that has seen benefits from projection exposure is IntegraColor, Mesquite, TX. Rather than large-format work, IntegraColor focuses on mid-format, high-quality, process-color display graphics (Figure 3). Its specialty is producing 100-line/in. process-color halftones for retail display, most measuring 36 x 48 in.
According to Mike Miller, production manager for IntegraColor, prepress control and imagesetter quality are critical factors in assuring high-quality output from the company's projection-exposure equipment. Miller explains that setting the correct gray level and optimizing the output resolution of the imagesetter are important to avoid hard breaks or banding when printing gradients and other areas of tonal transition. The company applies a proprietary calibration curve to all its imagesetter output, which en-sures a wide and reproducible tonal range of 5-95% in the final print.
The screenmaking process at IntegraColor relies upon the light sensitivity and high-resolution capabilities of fast photopolymer emulsions designed specifically for projection exposure. The company produces around 50 screens/ day, some used for print runs as large as 30,000 pieces.
Flag printing is yet another large-format application that benefits from projection exposure (Figure 4). Traditional methods of stencilmaking for this dye-printing application rely upon slow exposing, but very water-resistant, emulsions. This is because the standard equipment used for printing flags employs a rolling steel rod, rather than a polyurethane squeegee, to cause ink transfer. Powered by moving magnets under the print table, the steel rod provides high enough pressure to drive the dye solution evenly through the fabric so that the flag design can be viewed from both sides. However, this printing process is also extremely abrasive to screens.
Bill Goralchuk, production manager of Flags Unlimited, Barrie, ON, Canada, explains the role of projection exposure in his operation. He says that his company uses coated 158-thread/in. mesh that has been exposed for 2-3 min. with a projection-exposure system. For halftones up to 20 lines/in. the company uses 305-thread/in. mesh that has been exposed for 70-90 sec. A specially formulated photopolymer emulsion is used to produce a very water-resistant stencil. Image enlargement of 1000% is normally used to produce screens for runs that typically average about 500 pieces. For longer jobs, screens are generally post-exposed with a regular exposure lamp to improve durability. Post-exposing this type of photopolymer emulsion results in an impressive increase in water resistance, but without creating a difficult-to-reclaim stencil as is the result when using a chemical catalyst to harden the emulsion.
One of the early adopters of projection exposure in North America for use in producing large-format process-color display graphics (Figure 5) is Middleton Graphics, Markham, ON, Canada. John Meredith, screen department supervisor, estimates that since it first added the technology in 1994, the company has used it to produce approximately 50,000 screens. He says the quality of Middleton's final printed graphics is consistent, regardless of whether its employees expose stencils using full-sized film in vacuum-exposure equipment or miniature film in the projection system. According to Meredith, controlling the quality of the original film used for projection is the key to the high-quality results his company achieves.
Unlike most project-exposure equipment users, Middleton Graphics doesn't use a high-speed SBQ photopolymer emulsion. Instead, it relies on a fast exposing dual-cure emulsion. Meredith explains that this emulsion provides an optimum combination of exposure time and finished screen quality for the types of applications the company prints. Most of Middleton's jobs involve enlarging positives 400-700%. With process color, it achieves 50-line/in. half-tones on images up to 48 x 78 in., and 40-line/in. halftones on jobs between 78-120 in. wide.
Projecting for profit
Projection exposure offers large-format screen printers greater control over costs and tremendous gains in productivity. This proven method not only avoids the screen-handling issues and film-usage expenses associated with full-sized film exposure, it outpaces all other large-format stencilmaking alternatives, particularly when it's configured as part of an automated screenroom workflow. Using projection exposure with tightly controlled prepress variables leads to unbeatable screen-preparation speeds. Such streamlined screenmaking procedures recast the screen-printing process as a viable alternative to direct digital imaging for short-run jobs.