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Computer-to-Screen & Film Technologies for Screen Printing
By Johnny Shell, Vice President, Technical Services, SGIA
Research and development in new technologies is growing as screen printing continues to compete with alternative imaging technologies. As the technologies continue to advance and payback times become shorter, many screen printers are integrating some type of digital device to generate films and image screens.
Higher D-max measurements indicate greater opacity to light. So, a D-max of 4.50 has a greater ability to block light than a D-max of 4.00. The minimum D-max value recommended for screen printing purposes is 3.5 D. However, lower values may provide suitable results for non-critical work or short production runs. If the D-max value falls below 3.0 D, light will pass through and prematurely expose the emulsion in areas that were not intended. Some technologies used to generate films, such as film imagesetters, can achieve a D-max of 6.0 D.
The D-min value tells us how well the film allows light to pass through the clear areas, and is dependent on the base film material, processing procedures and the technology used. Acceptable D-min values for screen printing are usually below .05 D. Higher values will ultimately block light in clear areas of the film and may cause premature stencil breakdown, pinholes and other related problems associated with underexposed stencils. The difference between the D-min and D-max values is referred to as the dynamic range of the positive. Good film positives have high dynamic range values, with a low number for transparency (D-min) and a high number for opacity (D-max).
Another important metric to consider when choosing a device is its resolution, usually expressed as dots per inch (dpi). This standard of measurement indicates the addressable resolution of an output device or printer. It properly refers to the spots of light or energy used by imagesetters and digital-direct exposure systems; or dots of ink or toner used by an inkjet or laser printer, to output your text and graphics. So, a 600-dpi-by-600-dpi device is capable of imaging 600 dots horizontally and 600 dots vertically in one square inch. The ability to reproduce fine detail diminishes as dpi values become smaller. Additionally for many devices, imaging speeds fall as resolution increases. The importance of a focused inkjet dot or light spot is a considerable factor in producing a stencil image with a crisp edge. This is especially true with newer technologies that use light to image a screen.
A raster image processor (RIP) is another consideration when shopping for a technology solution. A RIP is a software application that is used to convert the information contained in your image to the format required by the optical or print engine in a device. Not all devices require a RIP, and actually can print halftones to some degree. However, if you regularly print halftone or fine-detailed images, a RIP is almost a must. They provide the necessary features and controls in order to achieve optimum results.
Film Positive Technology Choices
The first masking film introduced to the graphic arts industry was called Rubylith® and invented by Ulano Corp. Masking film is a knife-cut film (usually red or yellow) that is coated onto a clear, polyester backing material. It can be hand-cut but is now more commonly cut by a computer plotter device. While hand-cut stencils were replaced with digital pre-press, computer plotters can still quickly and easily cut text and solid color areas.
Unfortunately, masking films do not allow for the reproduction of fine halftones, and the additional cost to remove the excess material (weeding) can be labor intensive, time consuming and costly. Masking films cost about $2.40 per square meter (24 cents per square foot), making them the most economical choice among the family of films. Cutting can be done with a plotter device, which is commonly used for cutting vinyl letters in the sign industry. These are also economical devices with prices for a 66-cm (26-inch) wide plotter at approximately $2,600.
In order to print halftones with a laser printer, you'll need a RIP or a PostScript driver supplied by the manufacturer. The PostScript driver will simulate the halftone settings from your graphics application. However, if you have stringent requirements for halftone work this may not be the best approach.
Often, if there is heavy coverage, regions of the page can be "starved" of toner, rendering them lighter than they should be. This can be improved by using aerosol sprays - available from many screen suppliers - that enhance the quality of toner-generated positives by moderately increasing the density. Another drawback to laser is the relatively high D-min values (>1.0 D) caused by the foggy nature of the film material in non-image areas.
A new laser printer can be purchased for $2,000 to $4,000, but used printers can be found for much less. If you print halftone images, be sure the printer is PostScript compatible. Laser printers do offer a solution if your customers are more concerned with price than superb prints. However, if you intend to print photorealistic or fine-halftone images greater than 85 lines per inch (lpi), laser printing often won't provide the necessary detail, registration accuracy or film characteristics.
Inkjet printing's popularity has soared as a choice to generate film positives. It is an attractive technology choice because of its low start-up costs, lack of chemical processing and user-friendly software. Inkjet printing provides acceptable resolution for the garment decorator or shop producing low line-count halftones while producing the necessary densities (D-max 4.6 and D-min .07) with the aid of RIP software.
Without a RIP, inkjet printers simply will not produce the required density. Inkjet printing's disadvantages include: Its film and ink costs; slowness in producing a film piece; inability to produce extremely high-resolution images, compared with a film imagesetter; and potential for excessive ink waste and lost production time, which are caused by issues with the print head. Still, if properly maintained and operated on a frequent basis, inkjet devices offer a viable choice.
Several desktop models, costing several hundred dollars, are available from Epson to print film positives. Epson's R1800 and R2400 each produce 5760 dpi by 1440 dpi and have a maximum print size of 33 cm by 111 cm (13 inches by 44 inches). The Epson 4800 can print up to 43-cm (17-inch) wide, with a maximum resolution of 2880 dpi by 1440 dpi.
If you require films wider than 43 cm (17 inches), there are several wide-format inkjet devices available to meet your screen printing needs. MacDermid Autotype offers its Quadra Aspect systems in three sizes: 137 cm, 162 cm and 188 cm (54 inches, 64 inches and 74 inches). Mimaki offers the JV22-160. Epson 7800 and 9800 devices also can be used for generating films, but require the use of Fourth-party RIP software.
One of the biggest advantages for using thermal technology is having only one consumable for the printer (film), unlike other systems that use ink, toner and chemicals in addition to film. Plus, the image on the film never runs, smears or smudges, and is weather resistant, which means that water and extreme temperatures do not severely affect these variables compared with other technologies.
Additionally, thermal imaging cost is not based on a percent of print coverage like other printing methods. Printing a full black page will be the same cost as a page that has only 5 percent coverage. Thermal devices also render accurate registration in multi-color work. Their only inconvenience is the thermal head's sensitivity and individual nibs that are arranged perpendicular to the media travel direction. If one or more nibs become damaged, the resulting film may show unwanted lines where the material did not receive adequate heat. Damaged nibs are not repairable and, depending on the severity that is apparent in the film, may require the entire head to be replaced.
Thermal imagesetters are available from OYO Instruments LP in widths from 36 cm to 137 cm (14.25 inches to 54 inches), with prices ranging from $10,000 to $85,000. Film costs for thermal imagesetters average $21 per square meter ($1.95 per square foot).
As an example, a 150-cm-by-100-cm (59-inch-by-39-inch) final image size would only require a film size of 30-cm-by-20-cm (11-inch-by-7.87-inch) if using five times (500 percent) the magnification. The film positive used must have sufficient clarity and density with line counts ranging from 233 lpi to 300 lpi. Projection optics are available for 10 times and five times the magnification.
For emulsion choices, manufacturers have developed direct projection emulsions, which have been optimized for direct-projection systems. Exposure times will vary depending on the projector being used. Most manufacturers recommend calculating exposure by using a step-and-repeat exposure test with a test image containing various tonal ranges in order to optimize exposure.
CtS systems are also accurate and repeatable. Because the artwork is imaged directly to the coated screen, a vacuum blanket is not required during exposure, which removes any possibility of film distortion. Since there is no film, exposure time decreases and the amounts of light scatter and stencil undercutting are greatly reduced. Film dust and scratches also aren't a headache for the screen maker.
The four primary imaging technologies used in CtS systems are inkjet, thermal, Digital Light Processing (DLP) and laser. Each has its advantages and disadvantages, but there is not doubt our industry is moving heavily toward CtS in general. What appears to be cutting-edge today will be commonplace tomorrow, as more companies adopt these technological advantages to increase their margins and reduce costs.
Inkjet employs the use of conventional, water-based ink or solid wax, which is jetted directly to a coated screen and exposed. Devices using conventional ink typically utilize Epson print heads, and can reach resolutions of up to 1440 dpi. They are a cost-effective alternative to solid wax systems, which have slightly higher consumable costs. Since the ink is water-based, environmental conditions play a crucial role as day-to-day fluctuations in humidity can make repeatability a bit more challenging. On the upside, print head replacement in water-based systems is relatively inexpensive when compared with wax. Those prices range from a few hundred dollars to $1,000 or more.
Solid wax systems have a maximum resolution of 1300 dpi and employ the use of water-soluble wax tablets, which are heated in the CtS device. The molten wax is jetted onto a coated screen where it solidifies, producing great detail at densities at or above 4.5 D-max. Imaging speed will vary based on the selected resolution and number of nozzles in the print head, but printing speeds of 1-square-meter (10.75-square-feet) in a few minutes are not uncommon.
One of the biggest disadvantages in using a wax system is the cost to replace the print head. Prices can be as high as $5,000 per print head, which, depending on the use and maintenance, could mean an annual replacement. The cost of the wax used in these systems should also be taken into consideration.
Inkjet CtS system prices range from $75,000 to more than $300,000, depending on the frame size, print head configuration and consumables used with the system. If these figures send a shock through your spine, consider how much your company spends on film each year; an inkjet system may pay for itself in a very short time. Several hundred early adopters of these systems have converted from traditional film - with costs of about $10 per square meter ($1 per square foot) - to inkjet, and are now spending as little as 5 cents per screen in consumables. Inkjet devices are offered by CST GmbH (Colour Scanner Technology), Lüscher AG, KIWO Inc., Richmond Graphic Products Inc. and PlanetB Technologies Inc.
The ribbon cost is around $7 per square meter (65 cents per square foot). The Diablo utilizes OYO's patented 45.72-cm (18-inch) wide, 1200-dpi thermal print head, which can image at a speed of 38 linear cm (15 linear inches) per minute. It reaches a maximum image size of 45 cm by 76 cm (18 inches by 30 inches) on screen sizes up to 63.5 cm by 91 cm (25 inches by 36 inches). OYO plans to incorporate its larger 91-cm (36-inch) and 137-cm (54-inch) heads in future versions of the Diablo to meet the demands of large-format printing.
Digital Direct Exposure Systems
Digital Light Processing (DLP) systems are based on an optical semiconductor chip called a Digital Micromirror Device (DMD), which was developed by Texas Instruments in 1987. The DMD chip is covered with nearly 1 million tiny hinged mirrors, which can be quickly repositioned to reflect light to the emulsion. The chip reflects light emitted from a UV-exposure lamp to expose the image on a coated screen.
Thus, the screen is directly exposed with the reflected light from the chip without the use of masking materials such as wax, ink and film. These systems offer greater flexibility regarding acceptable choices in emulsion as compared with laser systems, which have much tighter requirements of the emulsion used. Using a standard emulsion is much easier than with a laser system.
DMD technology does have disadvantages worth consideration. DMD devices negatively expose stencil areas. With other systems, the positive is imaged onto the screen or film. With DMD, we are "imaging" the negative space with reflected light in order to expose the screen. This may require additional screen filler or increased processing times to fully expose the stencil material from edge to edge.
Furthermore, a consistent emulsion thickness is an absolute must to guarantee the emulsion is fully hardened. The light sources distance to the stencil surface also is a factor. Even precision-stretched screens do not have flat surfaces, which can cause fluctuation in the distance to the light source while being exposed. Some devices include a head-distance monitor that adjusts the head distance to the frame's mesh plane.
As we have covered, advantages in the laser system include the cost elimination of generating films as well as handling and storage issues. However, one drawback to these systems is the somewhat stringent emulsion requirements for their use. The technology is most attractive to those who usually print the same substrate with the same ink on a daily basis.
CD and DVD manufacturers are prime candidates because of the consistency of the materials used. On the other hand, a commercial screen printer who may print several dozen substrates using several ink systems would find it difficult to incorporate a laser system. However, if this technology is to be a part of our industry's future, this issue will most definitely be addressed by manufacturers.
Conclusion There are many considerations when choosing a system to generate films or image screens. Obviously, consumables and the initial cost are the most important. However, it's important to factor the time and labor savings each system offers. A higher initial cost may be quickly offset by the accuracy and repeatability the system offers, not to mention the improvements in workflow and savings in consumables. Suffice it to say, the future holds new and exciting developments for the screen printer. Advancements in digital technology will obviously continue, and can only benefit the screen printer's needs.
This article appeared in the SGIA Journal, 1st Quarter 2008 Issue and is reprinted with permission. Copyright 2008 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.
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