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Want To Try Screenprinting? Knife-Cut Films Can Shave Your Start-Up Costs
By Bill Stephens
Over the course of the past decade the sign making industry has witnessed the arrival of a host of many new technologies. In the midst of the wow factor they generate itís easy to overlook the profit potential of venerable technologies like screenprinting.
Virtually throughout its history, screenprinting attracted newcomers because of its low start-up costs. New technologies tend to have one thing in common: they can be pricey to get into. However, with only a modest investment in equipment and materials a small screenprinting operation can be up and running. Itís impossible to estimate the number of T-shirt shops that had the family garage as their first business address. Screenprinting also happens to be arguably the most flexible printing method ever invented, allowing small entrepreneurs to venture into a number of diverse markets. You can screenprint on both two- and three- dimensional materials and substrates that range from textiles to glass to plastic specialty-advertising items.
One of the most successful users of screenprint technology has been the small sign shop. Whenever large numbers of identical signs are required, screenprinting becomes a viable option. There is simply no more economical way of producing real estate signs, traffic signs, and election signs.
Many screenprinters start out without being able to make their own screens, subcontracting that task to graphic shops or other screenprinters. However, anyone with any intention of staying in the screenprinting business soon realizes that he or she had better develop the ability to make their own screens. Thatís when the investment starts to mount: Exposure units, coating troughs, darkrooms, wash and reclaiming tanks, pressure washers.
Still, if your shop is currently producing cut vinyl signs, thereís a way to sidestep much of this expense and produce screen stencils by using your vinyl cutter. The key is a class of screen printing stencil materials known variously as knife-cut or hand-cut films. These films date from the early days of the screenprinting industry, when they were laboriously cut by hand. Hand-cut films have mostly been by-passed by newer technologies, but many old-timers in the industry can still recall how it felt to hold an Exacto knife for hours at a time.
However films still offer many unique advantages compared to other stencil-making methods. First of all, theyíre relatively cheap, and some larger screenprinting shops still use them for screens that will print large areas of solid color. One of their limitations is, of course, that they are limited in their ability to reproduce fine detail. You can only print a shape thatís big enough to cut. Still, if your plotter/cutter can cut it, thereís really no reason why you canít make a successful screenprinting stencil out of it.
WHAT ARE KNIFE-CUT FILMS
Masking film works by blocking UV light from a screen coated by a photoreactive emulsion during exposure to a high UV output light source. Essentially, masking films create mechanically made positives.
Stencil films, on the other hand, become the actual stencil. After being cut and weeded, they are bonded to the mesh by being partially dissolved by either water or a chemical solvent and being pressed into the mesh. Weeding is the process of picking up and discarding unneeded bits of emulsion. When dry, the screen can go straight onto the press. No exposure unit required. Because of this, they have been extremely popular with startup screenprinters.
Neither type of knife-cut film, masking or stencil, is photoreactive. This is important to keep in mind because films coated with photosensitive emulsions, known as direct and indirect films, are in common use in the screenprinting industry. Be sure to specify ďknife-cutĒ films if you want the type you can cut on your plotter.
It gets still more complicated because there are two different types of stencil film, one designed to be used with water-based inks and a second type for use with solvent-based inks. Water will dissolve the emulsion on the film for solvent-based inks and lacquer thinner is used to dissolve the film intended for use with water-based inks. So, the first step in choosing a film is to decide what kind of ink you will be using to print the job and select the type of film that will stand up to it. Itís important to get this right. If you use the wrong type of film, your stencil will fall apart long before the end of the print run.
Both masking films and stencil films can easily be cut on a vinyl cutter. Some are even available in perforated versions for cutters that require them. Generally, thicker films are preferred for use on vinyl cutters. You simply feed the film into the plotter the same way you would a sheet of vinyl. The only precaution you need to take is to back off the pressure on the blade. Experienced hand cutters used no more pressure than the weight of the knife itself resting on the film. The blade should penetrate only the emulsion layer, leaving the clear polyester intact. Remember, film is considerably more delicate than vinyl. Itís quite easy to cut right through the backing plastic.
If you make a mistake, you can often correct it by simply replacing the bit of film you removed in error. Some film is touted as being ďself-healingĒ which wonít help if you pull off a large piece of film, but may help to close up the occasional knife cut that went a bit too far. This, of course, will be far less likely to occur if your plotter is doing the cutting.
Some film manufacturers produce special weeding tools, but an Exacto knife with a dull blade often works just as well. If youíre presently making cut vinyl signs you may already have your favorite weeding tools, although you may have to get used to using a lighter touch.
ADHERING STENCIL FILM
New screen mesh should be roughened or slightly abraded to give the film a more secure grip. The best way to do this is by scrubbing the mesh with a commercial product designed specifically for this purpose. Trying to cut corners by using a household cleanser for this job can leave behind residues that can interfere with the bond between mesh and film. Now the screen still needs to be degreased.
Every screen should be degreased whenever you intend to apply a stencil material of any kind to its surface. Make it a rule of the shop and youíll sidestep one of the most common causes of premature stencil breakdown. When the screen has been degreased, set it aside to dry.
Position the film, emulsion up on a raised surface thatís larger than the film and smaller than the inside dimension of the screen. Lower the screen onto the film. Now, depending on the type of film youíre using, go over the squeegee side of the mesh with a clean cloth saturated with either water or adhering fluid. Little pressure is needed. Try to keep your strokes in one direction and definitely donít rub.
The next step is to get rid of the excess fluid. Water can be absorbed by blotting the surface with several sheets of newsprint. Excess adhering fluid can be taken up by drawing a clean, dry cloth over the surface of the mesh. The actual application process will vary depending on the type of film and the manufacturer. Your best bet is to follow the instructions provided by the filmís manufacturer as closely as possible.
When the screen is dry, the clear polyester backing sheet can be peeled away. Pull gently and keep the free edge of the film close to the bottom of the screen rather than tugging it away at a sharp right angle. If the emulsion is still slightly soft, it could easily be pulled right off the mesh.
Your screen is almost ready, but before you can start to print you have to seal the open mesh between the edge of the stencil and the screen frame. Otherwise, ink could leak around the edges making a mess of both you and your substrate. The ideal product for filling in around the edges is screen filler, or blockout. Be aware, however, that you have to use a screen filler that will stand up to the type of ink youíll be printing with. Using water-based blockout when youíre printing with water-based ink is definitely going to lead to problems.
MASKING FILMS ≠ THE POSITIVE APPROACH
One of the most well known masking films in existence is Rubylith, manufactured by Ulano Corporation. This film consists of a red emulsion layer on a clear polyester sheet. In 2007 Ulano discontinued a similar product called Amberlith that had been used by more than a generation of screenprinters, a clear sign that film has been bypassed by the mainstream of screenprint technology. Thereís no immediate danger of Rubylith being sidelined since itís widely used by the offset printing industry.
HOW MASKING FILMS WORK
Next, this image is taped to the bottom, or print side of a dry emulsion-coated screen. Be sure that the red or amber layer comes into direct contact with the emulsion. This insures the sharpest reproduction.
This if further insured by forcing the positive against the emulsion. In a standard exposure unit, this pressure is applied by a rubber blanket connected to a vacuum pump. The rubber is flexible enough to wraps around the frame of the screen and still press tightly against the mesh and the attached positive, forcing them against a piece of clear plate glass. Makeshift exposures have been done by placing the screen on a sheet of plate glass suspended over a UV light source. Weights are used to force the positive and the screen against the glass. Whatever the setup, good contact between positive and emulsion is essential to getting a good exposure.
It is important to keep in mind that screen emulsions react only to UV light, not visible light. The most intense light source in the world can produce a poor exposure if it contains little or no UV light. Light energy is measured by wavelengths using a unit of measure known as a nanometer, in other words, a billionth of a meter. The wavelengths for visible light fall between 400 and 700 nanometers; most screenprinting emulsions react to UV in the 350 to 420 range. This means that the light thatís actually doing the work of exposing your screens is all but invisible.
At the same time, UV light can penetrate materials that may appear to be opaque to the human eye. The failure of many makeshift positives, including many made from various types of vinyl sign material, can be traced to this fact. On the other hand, you can easily see through Rubylith, but it excels at blocking UV radiation. In fact it does much better than most clear film or vellum positives produced by laser printers. While you can see through masking films their ability to block UV light remains unsurpassed.
FILM AND YOUR BOTTOM LINE
While the use of masking films requires investing in an exposure unit and the other equipment and supplies necessary for applying and exposing a photoreactive emulsion they nevertheless provide good reliable positives. Those positives can be rolled up and stored. With care, they can be used to reprint recurring jobs well into the future. A single positive can also serve to shoot backup screens to provide a little insurance against screen failure when you have a critical job on the go.
As we have seen, by making your trusty vinyl cutter serve double duty, you already have everything you need to produce your own screenprinting stencils. Knife-cut films may be dated technology, but they still produce excellent printable screens at a remarkably low cost. So, look to the past for a way to cut yourself a slice of the screenprinting pie.
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