Stencilmaking in the Age of CTS
Computer-to-screen imaging systems have helped screen printing to remain relevant as inkjet technology has progressed, but they require a different mindset in stencilmaking for best results.
Digital technology, specifically inkjet printing, was forecast by many to be the death knell of screen printing when the first systems were first showcased at the SGIA Expo in the early 1990s. While inkjet has taken much of the large-format graphics market and DTG printers are extending their reach into textiles, inkjet has not forced screen printing into obsolescence. In fact, the two disciplines seem to complement each other, giving screen printers more choices and variety of services to sell.
The screen-printing industry also embraced digital prepress technology many years ago with the introduction of computer-to-screen (CTS) imaging – systems that image screens directly from a digital file – as a way to advance the process. CTS systems save time and money by eliminating the need to output, cut, tag, transport, repair, and store film positives, not to mention the cost of the film itself. They also eliminate the need for a vacuum frame and glass, saving not just time but also largely overcoming imaging problems such as light scatter and pinholing. Moreover, precision image placement through a CTS system significantly reduces registration time on the press.
Two types of CTS technologies are currently used: inkjet masking and digital direct (or maskless) exposure. Combined with the recent surge in LED exposure systems, CTS technology has given screen printers many exciting new options for imaging and exposing their screens. Looking at the benefits and limitations of these technologies will help clarify which may be the best choice for your business.
Understanding and Assessing Options
Masked CTS systems use inkjet technology to apply an opaque ink or wax onto a coated screen, forming the UV mask traditionally created by a film positive. The screens are then exposed conventionally. Maskless systems instead image and expose in a single step utilizing a digital light processor (DLP) that scans the surface of the coated screen, exposing the negative non-image areas while allowing the positive image to wash out during development.
Masked systems have a lower initial cost, but add an on-going consumables expense in the ink or wax. They may also not provide as high a resolution as maskless systems, but they are compatible with a larger range of emulsions, especially when used in conjunction with static (or stationary) exposure systems. Generally, T-shirt and graphics printers working in smaller formats have been the primary base for masked systems, while maskless models have been aimed mainly at large-format graphics printers.
Masked CTS Systems
The printheads in masked CTS units are designed for either water-based or wax-based inks. Typically, the heads in water-based systems are less expensive, but the inks may not be compatible with all types of emulsions. It depends on the hydrophobic, hydrophilic, and surface tension properties of the emulsion – physical properties that alter how the inks behave when adhering to the emulsion surface. Since water-based inks don’t dry as quickly as wax, they may flow on contact with the screen and pool in the emulsion valleys formed by the mesh openings, possibly causing coverage variations. The higher the Rz value (surface roughness) of the screen, the more pronounced these variations are likely to be. Edge definition may be adversely affected by moisture in the ink coming into contact with unexposed emulsion, which can result in incomplete crosslinking at this critical area of the stencil. Water-based inks tend to cause more satellite dots as well.
Wax-based inks are also called “phase change,” which means they are converted from a solid to a liquid in order to be jetted and begin to change back to a solid in “flight.” By the time the ink lands on the screen, it’s solid. The chemical properties of the emulsion have no effect on these inks, as they “freeze” on contact with the screen.
Screens with higher Rz values (typical for coarser meshes) have little to no impact on the shape, size, or density of the deposited wax-based ink. This explains why wax-based systems can image screens in the vertical position while all water-based systems must image screens horizontally to help prevent the ink from running after contact.
At first glance, the biggest limitation of masked CTS systems is their limited resolution capability compared to film imagesetters, but typically, users report that the printed resolution is comparable to results achieved with higher-resolution films. This is because film positives cannot conform perfectly to the surface contours of the emulsion during exposure. Even with ample vacuum drawdown, light leaks into the gap between the emulsion and film.
Maskless CTS Systems
Like masked inkjet devices, these systems employ x-y plotting mechanisms. But instead of an inkjet head, they use a UV light source that selectively exposes the screen, controlling the light output through a digital micromirror device (DMD) with over 800,000 mirrors. Each micromirror represents a pixel; light can be directed onto the screen to expose that pixel or, through an angle change in the mirror, directed away from it as the head travels across the image areas of the screen.
Maskless systems are equipped with either conventional UV, LED, or laser light sources. In North America, mercury vapor and LED exposure lamps are most common. Conventional UV exposure units typically deliver high resolutions (up to 2400 dpi) and a wide spectral output between 350 and 420 nanometers, but the bulbs degrade with age; therefore, it is recommended to replace them at approximately 600 hours of use. LED units are also capable of high resolutions, up to 2540 dpi depending on the application requirements. Unlike conventional bulbs, LEDs output a very narrow UV spectrum, and therefore some devices employ LEDs with two different wavelengths. LEDs do not degrade over time and provide consistent UV output over the very long life (5000 hours and up) of the bulb.
Exposure Calibration with Maskless CTS
While many believe the purchase price is the biggest hurdle to implementing a maskless CTS unit, the bigger challenges are byproducts of two fundamental facts about the technology: It doesn’t use a UV mask and it relies on super-fast exposing emulsions to meet screen throughput expectations. Both of these lead to unintended consequences that can make it difficult to determine the optimal exposure time.
By eliminating the UV mask that clearly defines the image and non-image areas of the screen, maskless systems introduce a potential problem with stray light shining on unprotected emulsion outside the intended exposure area. Although the irradiance (or intensity) of this incidental light is significantly weaker, it is still enough to partially crosslink emulsion, especially fast-exposing products with the highest light sensitivity, which are being used by the vast majority of printers with CTS systems today.
How does stray light affect your ability to determine the correct exposure time? If fine details in the image have inadvertently been exposed to light, they can be difficult to wash out when developing the screen, especially if the water pressure isn’t sufficient. The person washing the screen will likely conclude – incorrectly – that the emulsion was overexposed and will then reduce the exposure time in an effort to open up the finer positive details in the image. The problem is that this sacrifices negative details, which may more easily wash off the screen. The durability of the stencil is also compromised. The emulsion will only be fully cured on the substrate side of the screen, while the emulsion closer to the squeegee side will be undercured, jeopardizing its chemical and mechanical resistance. As a colleague once told me when describing this phenomenon, “It appears as if the emulsion is overexposed and underexposed at the same time.”
The solution requires establishing optimum exposures based on the emulsion’s ink, chemical, and mechanical resistance (level/depth of cure), and then adjusting artwork files to compensate for tonal compression and lost details. Unfortunately, customers struggle to dial in this aspect of their screenmaking process in part because they often establish exposure times based on desired speed and resolution.
As with traditional light sources, an exposure calculator or a similar test based on a series of bracketed exposures should be used to determine the completeness of cure, which is critical to stencil durability. In these tests, solid sections of emulsion are exposed at successively slower speeds to determine the hardness of the emulsion across a range of exposure times. The processed stencil will show color variations from one step to the next before reaching optimum exposure. The step where the stencil first becomes darkest with no observable color change in subsequent steps indicates where the emulsion is adequately cured through its coating thickness.
Note that the emulsion in the top photo at the left keeps getting darker until 255 millimeters per second, which for this screen would be the correct level of exposure. (With maskless CTS systems, the higher number represents less exposure, as these values represent the speed of the exposure lamp, not time under a static light source.) In the lower photo, you’ll notice smudges that came from rubbing a finger on the squeegee side of the emulsion to further gauge the stencil durability. On this screen, the only step that cured the emulsion adequately was 200 millimeters per second.
Once you have completed this test, image another screen with a halftone test pattern using the exposure setting that you determined will give you the optimum emulsion hardness. Print that screen on press and measure the halftone values to determine the level of adjustment required in the artwork file to obtain the desired resolution in the print.
One other variable to watch: Many printers buy automatic screen washout systems when they invest in direct exposing CTS units to further streamline the process. Unfortunately, some models lack sufficient water pressure to adequately remove partially exposed emulsion left in screens, which adds to the confusion over calibrating exposure settings. A printer I know developed two identical screens that had been imaged on a maskless CTS exposure system, developing one with a handheld pressure washer and the other on an automatic washout system. The automatic unit failed to open halftone dots in the 3 to 10 percent range, while the handheld pressure washer opened them with ease. If you’re in the market for an automatic washer, ask about the maximum available water pressure and consider conducting the test above before buying.
High-pressure screen development can be beneficial even when using a slower emulsion on a masked imaging system. Using a wax-based CTS imaging system, I once imaged two 305-mesh screens with a 65-lpi halftone test pattern, each consisting of 10 boxes (one each for halftone dots ranging from 1 to 100 percent). I rinsed the first screen out with a hose-end sprayer and was able to open up highlight dots down to 7 percent. I used a 1700-psi pressure washer on the second screen with a 45-degree spray angle at approximately 12 inches from the screen and was able to open up dots down to 3 percent.
CTS and Emulsion Selection
The advantages of CTS screen imaging are significant and clear, both in cost reductions and time savings. Simply eliminating the need to clean the glass and wait for the vacuum to draw down has saved users a tremendous amount of time and all but eliminated pinholes. Just doing away with the glass can make exposures as much as 45 percent faster, as the glass prevents about 45 percent of the UV transmission from getting to the emulsion. (Consultant and longtime Screen Printing columnist Mark Coudray has tested iron-free tempered glass and found that it blocks only 15 percent of the UV transmission.) Jobs get to press much more quickly, with immensely faster setups.
What may not be as clear are the potential unintended consequences of the super-fast exposing emulsions that many shops use on their CTS systems. Printers should consider that in order to increase their exposure speed and screen throughput, they may also be compromising the resolution, resistance, and repeatability of their stencils. With an optimal exposure, the screen will have all of these characteristics – missing the mark, especially with an underexposed, soft stencil, will not.
Of the three types of emulsions, SBQ-photopolymer products have the least amount of exposure latitude. They are even less forgiving when underexposed. The fastest, most highly reactive SBQ emulsions sometimes give up resolution for exposure time, as the close-ups of the screens at right show.
Super-fast exposing SBQs require tighter process control in the screen department as well. Measuring and controlling key variables such as stencil thickness becomes critically important. Printers get lulled into a false sense of security – especially shops that have automatic coating machines. Although coating machines are capable of very precise, repeatable results, most printers do not realize that stencil thicknesses can vary due to two little known variables.
Consistency is the key to controlling stencil thickness and many variables come into play such as coating angle, speed, and pressure, but none are more important than coating trough fill level and changes in emulsion viscosity. Full troughs produce thicker stencils than ones with a low volume of emulsion. Unless you have a coating machine with automatic trough filling (and who does?), make sure you top off your emulsion early and often, especially when using very fast exposing SBQs.
The measurements at left were taken from 110/80-mesh screens that were coated on a shop’s automatic coating machine, with the level of emulsion in the trough the only variable. Such large variations in stencil thickness and surface roughness can make stencil quality very difficult to control when using super-fast emulsions. With their narrow exposure latitude, inconsistencies in resolution, edge definition, and mesh bridging will occur with only slight changes in stencil thickness.
Coating troughs and open containers of emulsion can also cause the viscosity and solids content of the emulsion to change from the manufacturer’s specifications as water evaporates from the formulation. The dryer the environment, the faster this occurs. Often, screens are coated and dried in the same room, which is equipped with heaters, dehumidifiers, and fans to expedite screen drying. These arid conditions cause emulsions to thicken quickly. The rule of thumb is to “coat in a swamp and dry in a desert,” though too few printers know or follow it. Again, the changes to the final stencil will be more pronounced with a super-fast emulsion.
CTS and LED Exposure
Sales of LED-curing units have exploded in recent years. They have a number of advantages over metal-halide systems:
• Exposure speeds (primarily with SBQ emulsions) that match those of high-wattage metal halide systems;
• Utility savings due to low power consumption;
• Much longer bulb life;
• And more efficiency (producing much less heat).
As the trend continues toward dynamic (moving) digital CTS exposure equipment, we’ll see increasing demand for faster emulsions. Emulsions that were once considered fast – with, say, 30-second exposure times on a static light source – are now considered slow due to the very short duration of exposure required by scanning-type light sources.
Though the demand for faster throughput will increase, switching to super-fast emulsions may not help, as many believe. They may allow for 8-second exposure times, but they can’t be developed that quickly. The bottleneck may not be exposure time, but rather screen development. Unless shops are equipped with multiple washout booths and employees to match, they won’t be able to keep up with the exposure unit. A presoak developing tank will help reduce the time required for the final washout, but when we’re talking about 8-second exposure times, let’s be realistic.
Understanding and Implementing Choices
Both textile and graphic screen printers are embracing digital technologies and screen department automation at a rapid pace. In pursuit of customer satisfaction, printers are pushing themselves to offer faster throughput and job turnaround, and advancements in screenmaking technology are helping shops get jobs to press faster and more efficiently.
Printers have so many imaging and exposing options (not to mention emulsions) to choose from that it can cause confusion. Today, textile printers purchase the vast majority of masked inkjet CTS systems, while graphics printers purchase the majority of the maskless direct exposure CTS units due to their larger format and higher resolution capability. But, look for manufacturers of direct exposure CTS equipment to market smaller machines to better compete for the textile market, while inkjet CTS manufacturers do the reverse and offer larger models designed to appeal to graphics printers. Also look for further advancements in LED curing, as manufacturers address the disadvantages of multipoint system configurations by improving light collimation through the use of parabolic lenses.
While CTS and LED technologies bring exciting changes to our industry, they require printers to have a comprehensive understanding of the fundamental principles of screenmaking. This knowledge, along with thorough measurement and precise control of screenmaking variables, will enable printers to realize the benefits of these new technologies without a long, steep learning curve.
This article has been excerpted from Dennings's white paper, “Stencil Making in the Age of Digital & LED: Market Trends & SWOT Analysis.” Download the complete white paper here.
Read more from our April/May 2016 issue.