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The Damaging Effects of Underexposure
The damage of underexposure can be hard to detect because it comes in small doses. You get used to working around these glitches on a day-to-day basis, but they tend to accumulate on the bottom line, the real costs showing up as ruined substrate and in higher outlays for equipment maintenance and labor.

Chronically underexposed screens are the inevitable byproduct of an inadequate exposure unit. The most obvious sign of an underexposed stencil is an excessive number of pinholes. These often get dismissed as a mere annoyance, but they can also cost you. First of all, it takes time to touch them up and somebody has to pay for that time. On occasion they turn up in areas where repairs simply aren’t possible and you have no option but to re-shoot the screen.

The effects of underexposure don't stop at the prepress stage. Because the emulsion in underexposed screens never properly hardens, the stencil can sometimes break down in the middle of a print run. Incredibly, some printers adapt to this state of affairs by shooting backup screens as insurance, although this adds considerably to both their costs and their workload. If they were to invest instead in a good exposure unit and learn to operate it properly, not only would they save these costs, but their screens would probably come off the press in good enough condition to print reorders, which could save even more time and money.

Underexposures continue to plague us even at the reclaiming tank. The very same underexposed stencil that broke down all too readily in the middle of the print run, now clings to the mesh with a death grip, even though your pressure washer is cranked up so high it’s blasting the mesh to ribbons. Only a few underexposed screens may actually be impossible to reclaim, but even the ones you succeed in reclaiming may suffer so much damage in the process that their life expectancy has been seriously reduced. That translates into screens that have to be re-stretched more often, adding to your equipment maintenance costs. Well-made screens should be capable of being reused many times and proper exposure is one factor that insures that they enjoy a long lifespan.

The Cause of Underexposure

Underexposures occur for two reasons:

1)The exposures were simply not long enough.

2)Your exposure unit isn’t producing enough UV radiation to completely expose the screen.

The most obvious solution is to extend exposure times, but even an exposure unit that may not be producing much UV, can generate heat. They can bake emulsions long before they generate sufficient UV to expose them. The other problem is that long exposure times can slow screen production to a crawl. Of course, screens can still be underexposed even with the finest equipment, but if you buy good equipment and learn to use it properly the odds of producing good exposures are definitely tipped in your favor.

What separates a good exposure unit from a bad one? Let’s try to answer that by taking a look at exactly what an exposure unit has to do.

Why Some Exposure Units Produce Better Results
The most important feature of any exposure unit is its ability to produce UV light. UV belongs to a special category of radiation known as actinic light, or light that has the power to effect chemical change. Actinic light can be found only in a fairly narrow slice of the electromagnetic spectrum. It includes a bit of the lower end of the visible light spectrum as well as the upper part of the UV band, the area known as UVA radiation. Direct emulsions react to UV light between 350 and 420 nanometers (billionths of a meter) in wavelength. To give you a bit of perspective: Visible light ranges from 400 to 700 nanometers and UV runs from 400 nanometers down to 100 nanometers.

Emulsions said to have a wide latitude respond to a light that ranges over a broad stretch of the actinic light range. This makes them ideal for printers who happen to be working with less than perfect exposure units. A wide latitude emulsion can produce good stencils even if the exposure unit’s output happens to be a bit off the mark. Other emulsions are not so forgiving. Man-made, SBQ photopolymer emulsions respond to a very narrow part of the UV range. If you’re planning on working with pure photopolymers, the output of your exposure unit had better be right on the money.

The best results come from closely matching your exposure unit’s output to the sensitivity of your emulsion. As a rule, diazo-sensitized emulsions react best to radiation in the lower reaches of the UVA band. Pure photopolymers seem to prefer radiation in the upper part of the range, just on the borderline with visible light. But why guess when you can get the exact information with a phone call to your emulsion manufacturer or a visit to his website? He’ll gladly specify the exact range of actinic light each emulsion is most sensitive to. Remember, he wants to see that you get good results with his product.

Of course you can’t judge an exposure unit’s output just by looking at it. First of all UV is invisible so you can’t even see the most effective part of the light it produces. It might appear to be very powerful, but what you’re seeing is mostly wasted energy. Its actual UV output might be next to nothing. The only way to be certain is to check the output with a radiometer, or at the very least, make some test exposures.

Tip:
Trying to judge a unit’s UV output by looking at it is not only futile, it can be damaging to your eyesight. When checking out exposure units make sure you wear eye protection.

Remember that output is largely dependent on the lamp or lamps in the unit and they can, and will eventually have to, be replaced. You need to know if the exposure unit will accept lamps that match the part of the spectrum your emulsion is most sensitive to. If you are looking at used equipment, you need to know how long the lamp has been in service. As a lamp ages its ability to produce UV deteriorates. It may still do a good job of producing visible light, but its ability to generate UV is a thing of the past. Make sure that there are replacement lamps available and that they match the exposure requirements of your particular emulsion. You also might want to check into price. Lamps can be costly.

It’s not enough to base your assessment on whether or not the unit will expose screens. Even the worst exposure units can generate at least some actinic light. The problem is that most of their output will be outside of the effective UV range. Keep in mind that even normal white light contains enough actinic light to expose screens, which is why unexposed screens have to be stored in the dark or under safelight conditions. This does not mean that you can use your shop lighting to successfully expose screens. Its actinic light component is far too weak to produce an effective stencil. The same cannot be said for the sun, which is such a great source of UV radiation that it has sometimes been used to expose screens. Despite its low cost, few screenprinters are tempted to switch to sunlight as an exposure medium, because it never seems to be there when you need it.

Types of Exposure Units
There are two basic components in every exposure system: a way of supporting the screen during exposure and a light source. In some exposure systems the holding mechanism and the light source are incorporated into a single unit. In other types they remain separate and can be moved independently.

The usual holding mechanism is a vacuum frame, which not only supports the screen, but also has the additional very important job of insuring good contact between the screen and the positive. Good exposures depend on the emulsion on the positive being tightly forced against the emulsion in the screen. In fact the quality of your exposures depend on the effectiveness of the vacuum frame almost as much as they depend on the output of the lamp.

Vacuum Frames
A vacuum frame has four basic parts: a glass plate, a flexible blanket of rubber or neoprene, a means of supporting them, and a vacuum pump. The rubber blanket is usually mounted on a metal frame that’s hinged on one side. The hinges allow it to be raised from the glass plate so that the screen can be inserted. The positive goes next to the glass, then the screen. The frame supporting the rubber blanket is then lowered and the vacuum pump switched on.

The time between switching on the vacuum pump and the point when all the air has been exhausted and the blanket pulled tightly down against the glass is referred to as drawdown. Because this can be a minor, but irritating, time waster, manufacturer’s ad copy often stresses that their unit offers rapid drawdown. This is actually a definite advantage, because it means the unit can expose more screens during the course of a day.

Blankets are made from rubber or a rubbery type of material known as neoprene because they have to be very flexible to create an airtight seal between the blanket and the glass. The blanket has to stretch enough to accommodate the raised edges of the screen frame while still delivering enough pressure to force the mesh firmly against the positive. The seal between the emulsion on the print side of the screen and the positive must be so tight that even light from a powerful exposure unit can't leak between them.

Another thing that helps cut down on light leakage is the color of the blanket itself. The underside is always black, which helps absorb stray light. Light that gets in between the emulsion and the positive can muddy the sharpness of the stencil. Sometimes the problem is not so much light coming directly from the exposure unit but reflected light, a problem known as light scatter.

White mesh contributes to light scatter, because like all white-colored objects, it tends to reflect light. The white threads actually carry the light along them for short distances, acting a bit like tiny fiber optics. These are the principal reasons that most white meshes are found in mesh counts of 200 threads per inch or under, where light scatter is less of an issue than in the higher mesh counts used for high-resolution print jobs.

An early effort to combat light scatter led to the development of colored meshes. Colored meshes do a better job of absorbing light, which means they can more accurately reproduce fine detail. But colored meshes not only reduce light scatter they can also absorb so much light that exposure times must be lengthened considerably.

In part III, we’ll continue our focus on exposure units, by taking a closer look at the lamps that generate UV output and consider the question of how we go about determining exposure times.