Improve Print Results

In screen printing, some images we see are soft, beautiful and with the colors registered perfectly. Others are heavy and, well, just poor quality. The difference can be summarized by one word: options. Printing involves lots of options and within each, a range of choices. Here, we'll examine how all of the variables affect final print quality.

Mesh tension

Mesh can be stretched to different tension levels, but no matter what level, all mesh loses 25 percent of the initial tension in close to two hours. The mesh could be re-tensioned, but still, within about four more hours, another 15 percent of the tension will be lost. And so it goes. Mesh is like an elastic band—it stretches and moves if its elasticity is not worked out. When mesh moves during the print stroke, the image becomes enlarged. Colors may not register; half-tone dots and fine lines and details will grow in size and ink spreads across the bottom of the screen, requiring periodic cleaning. Controlling the amount of ink deposited becomes very difficult in low-tension scenarios, where ink is typically pushed into the fabric during printing rather than being cut off and laid on top of the fabric. This can result in more fl ash cycles, which equals more time and less money. To avoid all of the above and produce optimal results, mesh should be re-tensioned to the point where its elasticity is worked out.

Squeegee practices

The degree of the angle at which the squeegee transfers the ink through the screen determines how much ink is being deflected down toward the garment. We want just enough to provide the desired coverage without any extra ink. The trick here is to print over the entire image at the same optimal angle on every print stroke like a robot. Try selecting the angle, locking the wrists and printing by stepping back rather than by pulling the arms back. Next consideration is speed. Pulling a squeegee fast results in less ink being deposited; pulling it slowly puts more ink through the screen. Optimize the situation by avoiding extremes—try stepping back or with printing at the same speed. Squeegee pressure is the last of this trifecta. First, the squeegee blade should never bend. If the blade is bending, you do not know the pressure or angle of the blade where the blade is touching the mesh. When holding a squeegee in your hands, the function of your hands is to cradle the handle of the squeegee. Pressure is applied only with your fingertips on the blade. Your fingertips are very sensitive and, therefore, a better measure of how much pressure is being applied. By contrast, if the squeegee handle is being pulled with the palms of the hands, or worse, being pushed with all your upper body weight and strength, the sensitivity to pressure is lost. The outline of the image in the screen defines the size and dimension of the image. Excessive pressure can force excessive amounts of ink through the screen which either goes into the garment, producing a heavy print, or spreads to a larger dimension. The thickness of the ink deposit should be defined by the thickness of the mesh and emulsion or stencil only. Note that off-contact distance can influence squeegee pressure as well. When the off-contact distance is excessive, more pressure is required to close the distance. Excessive pressure will also cause the mesh to lose tension faster over the course of a print run. Thus, mesh deflection should be minimized.

The squeegee pressure on a very tight screen will cause the frame of the screen to bow downward, closing the off-contact distance and preventing ink from releasing from the screen as designed. The solution is simple: Cut a piece of cardboard out of a carton and tape the cardboard to the neck of the platen as a spacer. This trick makes it so the frame cannot bow under pressure, but the mesh can still deflect and release the desired quantity of ink. Further, squeegee sharpness plays a role in quality prints. The square edge of a new squeegee blade is sharp and will cut ink better than a dull blade. After each job, ink should immediately be removed from squeegees to prevent contamination and deterioration of the blade. The blade should then be polished back to the sharp state of a new squeegee blade. Running a squeegee repeatedly over a screen will round the edge of the blade. Squeegee blades can be polished using fine sand paper, such as 300 grit, on a flat board with bars on either side to support the squeegee in a vertical position. The bars need to be screwed into the board under the sandpaper. Draw the squeegee down the track several times before each use to keep the edge in peak shape.

Parallel platen and screen

If a uniform distance is not maintained between the screen and platen, the amount of ink released will not be constant. A first step to quality printing is to run a straight edge over the platen, front-to-back, leftto- right, looking for space between the straight edge and the platen. Warped platens will not produce consistent print results. Similarly, screens should be checked for flatness. Test it on a glass top (such as on an exposure unit) or on a table that is flat. Lay the screen on the glass. If the screen rocks when pushed on the four corners, the screen requires correction. Push down on the platen. The slightest movement indicates that the spacing between the screen and platen will change under the pressure of the squeegee. On presses with multiple platens, all platens must be level and at the same elevation to

get consistent print results. To check, position a fl ash dryer (that is turned off ) over a platen and tape a piece of paper to the fl ash dryer. The paper should just touch the platen. Rotate the platens to ensure all the platens just touch the paper.

Printability of ink

Ever notice how easily a soft hand (also known by terms such as base and extender base) can be stirred? Pull the stir stick out of the ink container and watch how the soft hand on the stir stick separates from the soft hand in the can. This is easy because there is no pigment in the soft hand. Repeat the test with process ink. Process ink has little pigment and is transparent when printed. The process ink should also be easy to stir. Then, try stirring an ink used on light-colored garments. This ink will have more pigment, but will most likely still be friendly. With a little stirring in the manufacturer's container, this ink is ready for printing. Then there are the athletic inks, polyester inks and other inks designed for dark garments. These inks are heavily pigmented and do not stir, and therefore print, as easily. When you draw the stir stick out of the can, the ink might draw out like taffy. Hence, in the screen, more squeegee pressure will be necessary to get the ink through the screen. There are many ways such unfriendly inks can achieve friendly status. One is simply to stir vigorously, such as with a 3/8" variable-speed hand drill using a spade drill bit. Note that the can will have to be secured and covered (or prepare to have ink all over the room… and all over you). The next step to making that ink more print friendly is blending; mix 80 percent of the unfriendly ink with 20 percent of an ink of the identical color, but from an ink series designed to print on the light-colored garments. Mix thoroughly, and you will find diluting the pigment makes the ink a lot more printable without losing opacity. Polyester mesh and emulsion (or capillary film) are very slippery materials. By contrast, pinch some "unfriendly" ink between two fingers. As you separate your fingers you will see the ink draw out—it is not slippery. During printing, the surface adhesion of ink-to-garment holds the ink in place as the slippery mesh and stencil snap off the surface of the garment. Excellence is achieved when sufficient ink is deposited on the surface of the garment to produce the required opacity and without ink being driven into the garment. Screen print professionals are known to use curable reducer, which can be a dangerous habit. The tendency is to pour curable reducer into the ink until it stirs easily.

This is a big mistake. Reducer reduces the cohesive factor to ink, making the ink settle into the garment rather than stand up on the surface. Some fabrics, synthetics, for example, might have dyes that migrate into the ink, turning what was white ink into pink or some shade of reminiscent of the garment. Curable reducer also can promote dot gain. The half-tone dot just cannot hold its shape. If the job is not process, two or three drops of curable reducer (per screen) can be added and thoroughly stirred into the ink so that the reducer is evenly dispersed throughout the quantity of ink before it is put in the screen. That will help the ink shear, or, in other words, cut. The shear of all inks should be checked before being put in the screen.

Capillary film

Top-quality results can be achieved with liquid emulsion. Capillary film should also be used under some circumstances, but it is fundamentally different from liquid emulsion. Liquid encapsulates the mesh. Since the thickness of the ink deposit is determined by the thread diameter of the mesh, the printed image might show the influence of the mesh. Capillary film is a dry sheet of stencil material coated to a plastic sheet that is applied to a screen with plain water. During printing, the capillary film is on the substrate side of the screen (at the "bottom" of the screen between the mesh and garment). Capillary film comes in different thicknesses and can be laminated to create thick stencils. With experience, the screen printer learns to associate thickness with opacity and feel of the print. The optimal thickness of capillary film produces the look and feel of the image you want. These options are your guideposts to better quality and more profitable printing. Each option requires careful selection in order to produce the best results for any given scenario. The more options you employ at optimal levels, the better products you will produce.