Electronic Aids to Screen Exposure
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SignLab from CADlink


Electronic Aids to Screen Exposure

electronic exposure tools: digital radiometers --- devices that measure light intensity, and integrators --- exposure controls that eliminate the endless fussing with exposure times.

By Bill Stephens

In a previous article, we talked about how using exposure calculators and control scales can improve the accuracy of your screen exposures. Among the best features of these tools is that they cost so little that any screenprinter can afford them. But when your screenprinting operation starts to get busier and more demanding jobs start coming in, it may be time to take a look at a couple of tools that can take your control over exposures to the next level.

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  • These are the electronic exposure tools: digital radiometers --- devices that measure light intensity, and integrators --- exposure controls that eliminate the endless fussing with exposure times. Both come with price tags hovering in the several-hundred-dollar range, but they can also give even the busiest shop the ability to produce consistent, perfectly exposed screens.

    When discussing screen exposure variables printers often speak only of time and distance. It's as if they made all the difference. The truth is that the length of an exposure and the distance from the light source to screen are merely ways of controlling the amount of UV light that hits the screen. This is the key factor in any screen exposure. We need to get enough UV onto the screen to transfer the artwork to the stencil and harden the emulsion sufficiently to withstand the wear and tear of the printing process. When we begin to think of an exposure in terms of a specific quantity of UV we need special tools to help us measure and regulate radiation. These are digital radiometers and integrators.

    The factors that affect screen exposure
    So how much UV do we need to expose a screen? Actually, the amount varies. It depends on a number of factors like the color of the mesh, the thread count, and even the thickness of the mesh threads. The type of emulsion probably makes the greatest difference of all, but even the thickness of the coating can affect a screen exposure. The other key factor is the ability of our exposure unit to generate the right kind of UV. Not all output from an exposure unit and not even all UV radiation are useful for exposing screens. Photo-emulsions respond only to a relatively narrow slice of the UV band. Even within that small area, different types of emulsions will tend to favor radiation from different parts of the spectrum. We don't just need UV; we need enough of the right kind of UV.

    The place to start is to find out which area of the UV band your emulsion is most sensitive to. Then you can begin to think about how much UV you need. Fortunately, the information is usually easy to get. Emulsion manufacturers often have it up on their websites, although online exposure information may be limited to a chart giving suggested exposure times for different combinations of coatings, mesh types, and exposure units. Specific information about UV radiation may be a little harder to dig up. You may have to turn to the manufacturer's catalog or to a product information sheet. The sensitivity range of an emulsion will be given in wavelengths of light measured in nanometers. It may extend over fifty or more nanometers, wider still for some emulsions said to have wider exposure latitudes.

    Digital Radiometers
    So we know the kind of UV radiation we need. Now we need to find out how much we need. The manufacturer can give us some pretty specific information here, too. The quantity of UV needed is usually referred to as a dose. The dose required to expose a screen is usually given in millijoules per square centimeter (mj/cm2).

    To see how long an exposure it would take to generate a dose of that amount, we need to measure the intensity of our exposure lamp's output. A radiometer reads the intensity of the UV light being generated, sometimes referred to as radiant energy or irradiance. Radiometer readings are usually shown as milliwatts per square centimeter mW/cm².

    A radiometer measurement usually covers only about one second of an exposure lamp's output. To calculate the dose or total energy produced we have to multiply the radiometer's light intensity reading by a time factor, usually the exposure duration in seconds. A dose is merely intensity measured over time. There is an instrument called a dosimeter that measures doses directly. Basically, it does the math for you. But, don't worry, you don't have to run out and buy still another piece of equipment, many radiometers incorporate a dosimeter function and measuring a UV dose is as easy as flipping a switch.

    You can also use your radiometer to calculate exposure times. Simply divide the manufacturer's recommended dose by the light intensity reading from your radiometer and you'll get your exposure time in seconds.

    Exposure times based on radiometer readings are extremely accurate because a good radiometer will ignore all of your exposure lamp's output except for UV radiation within the sensitivity range of your emulsion. This means, of course, that when you go out to purchase a radiometer, you have to make sure it can measure radiation within those limits.

    The digital radiometer uses a photocell to sense light energy. Often the photocell is fitted with a filter that admits only radiation that falls within the effective range. In many digital radiometers the photocell is separate from the radiometer itself. The photocell is linked to the main unit by a cable, giving you the flexibility to place the light sensor directly in the path of the UV. It's usually best if you place the photocell close to the screen where it is in the best possible position to measure the intensity of the UV radiation on the surface of the emulsion.

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    One of the most beneficial uses for a digital radiometer is to check on the health of your exposure lamp. A lamp's UV output drops as the lamp ages. Unless exposure times are regularly adjusted to compensate for this reduced output, you will eventually be producing underexposed screens. Regular readings taken with a digital radiometer can let you know when and by how much to adjust your exposures.

    You can also use a digital radiometer to check on a screen exposure as it is taking place. Place the radiometer's sensor behind the screen that is on the side opposite your exposure lamp. When your exposure begins, your radiometer will give a reading of zero. All of the UV radiation is being absorbed by the energy-hungry sensitizer in the unexposed emulsion. However, as the exposure continues the sensitzer will gradually take on energy until it can hold no more. At that point it can no longer block the UV and as more and more UV penetrates into the emulsion, your radiometer readings will slowly rise. When all the sensitizer has been used up and UV can now pass through the emulsion coating virtually unhindered, your readings will level off.

    Despite its obvious usefulness, many screenprinters will never lay eyes on a digital radiometer. That is, unless the shop is printing with UV inks. In UV printing a digital radiometer is often regarded as a necessity for checking the output of the UV curing unit. Beware, though, in many cases the radiometers used to monitor curing units are not the same as those used to measure exposures. If you're thinking about buying one, prices for most digital radiometers start close to the $500 mark, although Chromaline offers a digital radiometer called the UV Minder, currently selling for $175.

    Why go to so much trouble over screen exposures?
    Screenprinters who do long print runs or who use mainly water-based inks might consider a digital radiometer a very practical investment. Underexposures are one of the principal causes of stencil failure, so wherever stencils will be subject to more than the normal amount of stress accurate exposures become vital. To understand why underexposures play havoc with the durability of stencils, we need to think about what happens during the exposure process.

    Exposure begins with the layer of emulsion closest to the light source, that is, on the print side of the screen. From there, it progresses slowly on through the emulsion coating as the sensitizer in the emulsion gradually loses its power to block UV. The last part of the emulsion coating to be exposed is the emulsion on the squeegee side of the screen.

    This layer may be the most important part of the entire emulsion coating when it comes to durability of the stencil. Not only does it play an essential part in locking the stencil to the threads of the mesh, it must withstand the abrading passes of the squeegee blade. If the emulsion on this side of the screen remains underexposed, a critical part of the stencil will forever be soft and weak.

    Proper exposure times can eliminate this potential vulnerability. Regular tests with an exposure calculator can help, as will regular measurements with a digital radiometer of the output in your exposure lamp. But when we get busy, exposure tests tend to be the last thing we think about. Long intervals may pass between tests during which the UV output of the exposure lamp may drop significantly. Without regular exposure tests, your exposure times are really based on nothing more than guesses. To put an end to the guesswork once and for all you really need to take control of the amount of UV striking your screen.

    Integrators
    This can be a lot easier than you think. In fact, thanks to a device called an integrator, it can be almost automatic. Like a digital radiometer, an integrator measures the intensity of UV radiation, and like the radiometer, it uses a filtered photocell as a sensor. But an integrator not only measures the intensity of UV output, it also acts like a dosimeter and keeps track of the total dose of radiation aimed at the screen. When the correct amount of UV has been produced, the integrator switches the exposure lamp off. The big difference between radiometers and integrators is that a digital radiometer is merely a measuring device, an integrator is a control.

    Integrators are not used as independent instruments. They are components of an exposure system. But they can be purchased separately. Although, according to Cornel Jurca, Customer Service Manager of Olec Corporation, most customers shopping for integrators simply want to replace or upgrade existing equipment. Integrators from Olec range in price from $390.00 to $895.00 USD. Jurca says integrators are most in demand for industrial processes where "repeatable exposures" are essential, such as in the manufacture of circuit boards.

    Overcoming Exposure Variables
    Integrators bring consistency to screen exposures. This can be especially important when screens have to match one another as closely as possible, as in four-color printing. Even if you expose your screens months apart, screens should be exactly the same. The exposures should be identical even if you made the first using a brand new exposure lamp and the second with a lamp approaching the end of its life. The integrator-controlled exposures are based only on one single factor: the amount of UV generated. Exposure times are adjusted automatically to keep the exposures consistent.

    The placement of the photocell, however, can make a pretty big difference. If the photocell is mounted on the exposure unit itself, the only thing it can measure is the output of the lamp. But if it's placed on the vacuum table close to the screen, the photocell can register the actual amount of UV light striking the screen.

    If the photocell is mounted closed to the screen, it can also compensate for variations in UV intensity due to distance. Distance can have a dramatic effect on output. For example, if your exposure unit somehow got positioned at twice its usual distance from the screen your exposure would have to be four times longer to compensate for the drop in intensity. If you were making a time-based exposure and failed to adjust your exposure time, you would end up with a seriously underexposed screen. But an integrator-equipped exposure unit would automatically compensate for the lower intensity by making the exposure longer. The actual amount of light striking the screen would never vary.

    Electronic UV measuring and control devices give us the ability to produce accurate and consistent screen exposures. Digital radiometers and integrators can be wonderful tools for maintaining the pace and consistency of screen making in busy shops, but are they for you? That really depends on the type of jobs you take on now or hope to take on in the future. If you plan to go looking for more complex work the consistency these devices bring to your work may give you the confidence you need to tackle bigger jobs. On the other hand, if your print runs are frequently interrupted by stencil breakdowns, stop and take a minute to add up the costs of screen failures. These electronic exposure helpers may begin to look like very wise investments.

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