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Exposing Direct Emulsions: Accentuate The Positive
An emulsion exposed to UV light long enough becomes hardened, water-resistant, and firmly bonded to the mesh in the screen. If you place an obstruction between the light source and some parts of the screen, those areas remain soft and can later be washed away using nothing more potent than tap water. When we print with that screen, those open areas become channels that allow our ink to flow onto the substrate. The job of creating those channels by blocking UV light falls to the positive.
What is a positive?
The obvious difference between them is that a negative reverses the light and dark areas of the original artwork but a positive duplicates the original artwork exactly. What’s black in the original is black in the positive. The only change is that white in the original becomes the transparent areas in the positive.
What makes a good film positive?
The positive’s ability to block or admit light is expressed in terms of its density. This is usually expressed as a logarithm. Even if you have no idea what a logarithm is, it still gives you a numerical value that allows you to compare the relative density of one positive with another. A positive’s background transparency, or area of minimum density, is identified as Dmin. Ideally, the Dmin will be a very low number, less than 0.1, if possible.
Dmax represents the area of greatest opacity, which should be as high as possible, 3.0 or above. The difference between Dmin and Dmax is the dynamic range of the positive. Good positives have high dynamic ranges with a low number for transparency (Dmin) and a high number for opacity (Dmax).
All of these terms have been borrowed from photography, and may seem a bit too technical for someone who just wants to print a simple one-color sign. For simple jobs an experienced screenprinter can make an educated guess about how well a positive will perform just by holding it up against a light source. But what you really need to know is not how good a job a positive does at blocking visible light, but how well it blocks UV light. The only way to know for sure is to check the positive with a transmission densitometer, which can be calibrated to measure opacity to UV radiation. Transmission densitometers are not cheap. Costs range anywhere from $600 to $2,000, but one can easily pay for itself in the time and money it can save. If you can spot a bad positive before you put a lot of time and effort into making a useless screen, the savings add up quickly.
A transmission densitometer can also help you when exposing different types of positives or positives of different densities. Knowing the density of the positive before you make your exposure can allow you to adjust the exposure times accordingly. The lower the positive’s Dmax reading, the shorter you’ll need to make your exposure to prevent the thin opaque areas from burning in. The higher the Dmin of the positive, the longer you’ll need to make your exposures to give the solid areas of the screen time to harden.
A transmission densitometer is all but essential for a printer doing halftone work, because in addition to measuring density, a densitometer can read the tonal range of the halftone dots. Knowing the tonal range allows a screenprinter to estimate how well he can reproduce that particular halftone, areas where he is likely to encounter problems, and even the mesh count he should be using.
Tip: Make sure you specify a transmission densitometer if you’re shopping for a device to measure positive densities. Reflective densitometers are also used in printing to evaluate ink color in finished prints. Only a transmission densitometer can tell you what you need to know about the opacity of your positives.
How positives are made
Cameras and Imagesetters
Thermal imagesetters have become very popular because they eliminate the need for an additional processing stage and the chemicals associated with developing photosensitive film. They are often called dry-imaging systems, because not only do they not require photoprocessing chemicals, but also they don’t need inks or dyes or anything else that's wet. The film emerges from imagesetter ready to go right onto the screen.
Imagesetters, however, cost so much that only larger screenprinting operations can afford to own one. Smaller and medium-sized printers simply send their computer files to a service bureau and order individual film positives as they need them. They know it’s time to start thinking about buying their own in-house imagesetter when their annual cost for film starts to mount into the tens of thousands. At this point they will probably need film faster than an outside source can turn it around anyway.
At one time masking films were cut by hand, but most signmakers now use a vinyl-cutting plotter. In fact, using a plotter, you can cut pretty elaborate positives in a fairly short amount of time. Of course you still have to weed them, but because the films themselves are so inexpensive, they become economical, especially for exposing very large screens. Even large screenprinting shops that normally use film positives will occasionally switch to masking films for screens printing large areas of solid color.
Inkjet and Laser Positives
Problems with Toner Positives
If the positive is not sufficiently dense, the light begins to erode away the fine details. Some may disappear entirely. As the exposure continues, light begins to work its way around the solids, undermining the edges where the toner deposit may be thinner. This creates the ragged edges known as sawtoothing. If the exposure continues long enough, the light can even burn right through the middle of the solid areas, causing those parts of the stencil to begin filling in.
Another problem with vellums and acetates is that some of them are far from transparent. When a positive has a relatively high Dmin rating, the emulsion may not harden sufficiently unless exposure times are increased. As exposure times lengthen, the amount of UV pouring onto the dark areas is also increased, adding to the chance of undercutting.
Another problem comes up when you need larger positives. Most computer printers cannot accept sheets larger than 8 ½ by 11 inches, although some printers commonly used for positives will accept sheets of up to 11” X 17”. Still, if you need a positive larger than those sizes, you have to assemble them by taping individual sheets of vellum or acetate together. Where two sheets overlap, the positive becomes even less transparent, so exposure times have to be extended even more.
Despite such problems, vellum and acetate positives have become steadily more popular. Quick and easy to produce, convenient, and very low in cost, their appeal is hard to resist. Many screenprinters who work with them regularly swear by them, and have developed a host of tricks for making them more opaque. Some reheat their laser prints to darken the toner; others spray them with artists’ spray fixative. Sometimes all that’s required is switching to a less powerful setting on the exposure unit or reducing exposure times. Others have simply gone out and bought better printers. Epson, for example, produces several models widely used for making screen positives.
Right-reading, emulsion up
When a positive is in the right-reading position it is oriented so that it matches the orientation of the original artwork. (Text should be readable.) Right-reading, emulsion up means that with the positive in this position, the emulsion is on the side of the transparent film closest to you. If you are not sure, try scratching the surface of the positive in an out of the way spot with the tip of an Exacto knife. If the tip is touching emulsion it should leave a scratch mark. (You can fill in the tiny mark later with an opaquing pen.)
It is important that the emulsion is uppermost in a right-reading positive because during exposure it has to come into direct contact with the emulsion on the print side of your screen. If the emulsion were on the other side of the positive, light would be able to leak in between the intervening layer of transparent material causing undercutting.
TIP: Right reading, emulsion up. If you send your work out, it’s important to specify that your positives must be right reading, emulsion up. Unless the service bureau is experienced in dealing with the special requirements of screenprinters, they may not automatically get it right.
A positive approach to exposure
Other time savers include double checking to make sure the positive is positioned correctly in the screen and that its emulsion side is in direct contact with the emulsion on the screen’s print side.
The final problem-solver is insuring good contact between the positive and your screen. But contact is really the job of the vacuum frame. Next time, we’ll take a closer look at what the vacuum frame does. We’ll also take a look at some of the different types of exposure units available and what you need to know before you go shopping for one.
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