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Printing Halftones (Part Two): Two Critical Factors in Halftone Stencils
By Bill Stephens
The first step in applying any type of stencil material to mesh is to make sure that the mesh is free of oil and dirt. We insure this through the process known as degreasing. It may seem simple and rather basic, but its importance really can't be overemphasized. Degreasing helps the stencil stays securely fastened to the mesh. Even mesh that appears clean should be degreased, especially brand new mesh, which usually comes with a coating of oils picked up during the manufacturing process.
The only time you can safely skip degreasing is if the screen has just been dehazed. Dehazing is not the same as stencil removal, although both processes customarily require assistance from the blasting spray of a pressure washer. Dehazing chemicals are harsh caustics designed to burn away shadowy outlines, known as ghost images, formed from trace ink pigments left over from earlier print jobs. Locked within the knuckles of the mesh, where mesh threads overlap; these fragments can interfere with the process of making a new stencil. Dehazers are powerful enough to not only bust these ghosts but take care of any grease and oil clinging to the mesh as well.
If screens are taken directly from the dehazing tank to a clean, dust-free coating room, there's really no need to degrease them as well. But screens that have been sitting around the shop, even if only for a day or two, should probably be degreased. A human hand merely brushing against the mesh can leave behind an invisible coating of oil that may cause problems later on.
Fortunately, degreasing is easy and takes very little time. You simply apply a commercial degreaser to the screen, scrub it into the mesh with a soft-bristled brush, and follow up with a thorough rinsing with water. Some printers, in a misguided attempt to save a few pennies, will try to degrease their screens with off-the-shelf cleaning products from the local grocery store. Undoubtedly some will work, but others can even introduce contaminants to the mesh. On the other hand, if you stick with a product especially designed for degreasing screens, you're taking no chances. You also need to wash away all traces of the degreaser itself, so make sure you rinse your screens thoroughly. Make it a habit to rinse your screens a minute or two longer than necessary.
First, let's take a look at stencil thickness and how it affects imprints. If you read the preceding article in this series, you know that the diameter of the mesh threads is the most important factor in controlling the thickness of the ink deposit. However, the stencil will add a certain amount of thickness, which means the stencil also has an effect on ink deposit. Stencil thickness also affects a number of other factors, like print definition and the transfer of ink from the screen to the substrate.
In halftone printing we generally require lighter ink deposits, so we prefer to use the thinnest possible stencils. Light ink deposits mean sharper imprints; heavy ink deposits tend to spread out on the substrate resulting in fuzzy edges. When printing with UV inks, the thinnest possible deposits are required. UV light has to be able to penetrate the entire ink layer. If the ink deposit is too thick, the UV light can't make it the whole way through and the ink never completely cures. This means UV printers require thin stencils and meshes with the finest thread diameters.
Normally, we think of screens as two-dimensional objects. But stencils add a third dimension -- depth. The openings in the stencil become containers for ink, enough to make a single imprint. The thickness of the stencil determines the depth of these containers, which means just how much ink they can hold. The flood stroke of the squeegee fills these containers with ink, and because of the special viscosity of screenprinting ink, it remains suspended there until something pushes it out. That force arrives in the form of a pressure wave of ink, driven along by the squeegee's print stroke. This pressure wave of ink develops just in front of the squeegee blade, and it develops enough force to push the ink out of the stencil openings and onto the surface of the substrate.
Beyond its role in creating this pressure wave, the squeegee has nothing to do with whether or not the ink adheres to the substrate. This depends on the adherent properties of the ink, not on any downward force applied to the squeegee. So, if the ink isn't sticking, pushing down harder on the squeegee is the solution to the problem.
Different inks have different adhesive properties, but as a general rule, when ink comes into simultaneous contact with two surfaces, it tends to want to adhere to the one that offers the largest surface area. This is what allows ink to pass from the stencil container to the substrate.
Normally, the largest available surface is the substrate, but if the stencil is very thick and some of the openings relatively small, the ink may want to stay in the stencil rather than moving over to the substrate. The problem often shows up as gaps and voids in the prints. The first reaction is to assume it's a problem with the ink, but the real problem is the thickness of the stencil, and the only real solution is to reshoot the screen and make sure to use a thinner coating of emulsion.
A thick stencil creates tall, narrow containers, difficult to fill and difficult to empty. When filled, they hold too much ink. If it does manage to get onto the substrate, it lands there in a column, which inevitably spreads outward thanks to gravity. The result is poor print definition. In halftones, this means dot gain.
Overly thin stencils can also cause problems. The knuckles of the mesh have to be completely covered with stencil material. Otherwise they create an irregular surface that may have difficulty forming an ink-tight seal with the surface of the substrate. Ink can leak out through these gaps creating imprints with fuzzy edges. A stencil of the proper thickness not only covers the high points where mesh threads overlap it also fills in the openings, a leveling out process know as mesh equalization.
Shops that do a lot of halftone work or other types of finely detailed printing recognize the critical role that stencil thickness plays in the printing process. They way they insure that their stencils are of the optimum thickness is by measuring them with a stencil thickness gauge, an electronic measuring device.
A stencil thickness gauge costs around $800. While this may well be beyond your means, you can take steps to control stencil thickness through your choice of stencil materials and by standardizing your stencilmaking process. Standardized practices help eliminate variables. And when you get good results, it helps if you know how to reproduce what you did right.
Direct emulsions undoubtedly allow the greatest variation in stencil thickness, so many printers switch to films for halftone works. Films have consistent thickness built right in. Whatever stencil material you prefer, you need to make sure there is a built-up layer of emulsion on the print side of the screen. This layer of emulsion sits on top of the mesh threads and forms the perfect gasket for creating a seal with the substrate. It shouldn't be very thick, only equal to about 10 percent of the total stencil thickness.
When using film, maintaining this emulsion layer is easy since film is applied from the print side and much of the emulsion simply stays there. Direct emulsion stencils achieve this same effect through coating technique. The last passes of the coating trough are always made on the squeegee side of the screen, which has the effect of forcing liquid emulsion through the mesh to the print side. This build-up of emulsion is referred to as EOM or Emulsion Over Mesh.
Although the term could be applied equally to either side of the mesh, it is almost exclusively used to refer to the print side. With film, EOM thickness depends mostly on the thickness of the emulsion layer and only a tiny bit on the application technique. With direct emulsions, EOM depends only partially on inherent qualities of the emulsion, like viscosity and solids content, and a great deal on coating technique. One of the advantages of direct emulsion is that they give the stencil maker the ability to fine-tune the EOM thickness. For example, thickness can be added by face coating, the technique of applying additional emulsion coats on top of already dry emulsion.
Rz or stencil smoothness
The stencil's surface has such a profound effect on print quality that in the late 1980s the well-known emulsion manufacturer Kiwo developed a way of measuring it. The relative smoothness or roughness of a surface is measured according to a scale called the Rz value. Readings are taken using a highly sensitive instrument called an Rz meter or profilometer, which is capable of measuring differences far too small to be seen with the naked eye. The profilometer's probe is drawn across the stencil to record the difference in height between the microscopic peaks and valleys found on the surface. The Rz value actually represents an average of these differences.
The higher the Rz value the rougher the surface. Ideal halftone stencils will usually have an Rz value fewer than 10 microns. While your stencil's Rz factor can make a big difference to your printing results, profilometers like stencil thickness gauges, are expensive. Few smaller shops have them. But if you feel you can't get along without one, be sure to focus your search only on profilometers designed for measuring screenprinting stencils. They are also widely used in other industries and come in a number of different types.
Even if you never set eyes on an Rz meter, you can still take steps to keep your stencil's Rz values low. For example, with direct emulsions, additional face coatings will significantly reduce Rz value. Films tend to have some of the lowest Rz values, which is another reason they offer an attractive alternative for halftone work. We will go into films and direct emulsions in greater depth in the next article.
If you're curious about the Rz values you're achieving with your current stencilmaking practices, you might try asking some of your screenprinting suppliers if they have an Rz meter available. Ask them if they wouldn't mind checking out a few of your screens. A few minutes with an Rz meter can tell you which stencil materials and application methods produce the lowest Rz readings. Then, simply make that method as your standard practice.
The other thing to remember is that the surface of the substrate also has an Rz value. So, the quality of the seal formed as the squeegee makes its print stroke actually depends on the combined Rz values of stencil and substrate. The surfaces of textiles, for example, tend to be so rough that the Rz value of the stencil is irrelevant. Other surfaces can be so smooth that stencils with a low Rz value have a tendency to adhere to them.
Next time we'll continue our series with a close-up look at stencil material, application techniques, and a few thoughts on how to achieve perfect exposures.
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