Printer's Guidebook, Part III

Screen Mesh, continued

7. Mesh count
A mesh count-the number of threads per inch-should be selected with ink deposit, ink flow and mesh strength in mind.

Mesh count is measured by counting the number of threads per inch or per centimeter. You can find the range of mesh counts your supplier sells in its product chart. Notice how some mesh counts are made of different thread diameters (see Variable 6: THREAD DIAMETER); this changes the percentage of open area and mesh thickness. Ink flow, ink deposit and stencil support are all considerations when determining mesh count. Mesh counts are typically only estimates, because of the limitations of the weaving process. They are different in both the warp-threads running the length of the bolt-and the weft-threads running across the bolt (remember this by thinking "weft and [w]right"). Inconsistencies in mesh counts are rarely important, though, even between mesh brands, because only actual testing under your shop conditions can be relied upon to determine actual ink deposit. Measuring mesh count For any screen-printing application, a mesh should be selected with ink deposit and ink flow in mind, but chosen according to both mesh count and thread diameter, the combination of which will define open area (see Variable 8: MESH OPENING). To measure a mesh count, use a commercial mesh tester. This can be a test-film-positive with a design that makes a distinctive moiré pattern that will indicate the count of the mesh being measured. A slower but more accurate method requires a linen loupe and something pointy to help you keep your place. You measure an inch then, through the loupe, count the threads with your pointer. After two or three tries, most people use the short-cut method of counting a quarter of an inch and multiplying by four to save time. The odd mesh count numbers (305, 330, 355) stem from the fact that mesh is woven in Europe and based on the metric system. An obvious question when troubleshooting a problem print on the press is: "What's the mesh count?" To often, nobody knows and we have to guess or try to find enough open area in the screen to count, which wastes time. To make matters worse, even if they write it on the mesh, many screen makers cover the mesh-count label with tape. Don't let them do it. Insist on visible mesh counts! Mesh count is probably the most often-used screen modification. Choosing a higher or lower mesh count usually creates the most measurable change in ink deposit or ink flow, but we should first consider mesh that will support our stencil before considering ink deposit and flow. Fineness of line When threads are far apart, they can't support thin or fine stencil lines. Small halftone dots are nothing but very short lines and must have something to support them or even casual cleaning of the stencil will knock them off the mesh. As everyone knows, you need to make a formal test with different line thicknesses and different dot sizes in order to calibrate your screen exposure. When you actually print a screen with this test pattern under your conditions, you will see how fine a line you can print with each different mesh count. Tension and tack Mesh tension is also effected by mesh count. Generally, the more threads of a given size there are, the stronger the mesh is, and the better it will be able to withstand high tension (see Variable 11: MESH TENSION). Along with support for the stencil, many printers use a method (that happens to be the opposite of litho printing) called "increased tack." This means that if you lower the mesh count as you work through the print order, you may enjoy better trapping and reduced ink build-up on the bottoms of your screens. By starting with a high mesh for background, light color and small area size, you reduce the amount of extra ink buildup on the bottoms of the screens. It is also a given that increasing the mesh count (choosing a higher one) will decrease ink deposit because of smaller open area and thinner mesh.

8. Mesh opening
The dimensions of the mesh openings affect stencil support, meter ink flow and contribute to the print's appearance.

The best mesh for screen printing is commonly woven out of monofilament polyester and it functions both as support for the stencil and to meter the amount of ink deposited on the substrate. The dimensions of the mesh count, percentage of open area and thread thickness all contribute to the appearance of the end product. You will find that accurate measurements of these mesh specifications are supplied by the manufacturer. Poor image registration, image elongation and uneven ink deposits can result from the improper selection or preparation of the mesh fabric. Mesh opening As a general rule, the dimension of the mesh opening should be at least three times larger than the pigment fineness of the ink. The mesh opening is measured with a microscope from the edge of one thread across the opening to the edge of the opposite thread, in microns (1 micron = 1 millionth of a meter, or 0.00003937"). High quality mesh is woven so the finished product offers a perfectly square opening, within tolerances. Since threads come in different diameters, you can have mesh openings that are the same in vastly different mesh counts. A fat thread in a low mesh count can have the same mesh opening as a thin thread in a higher mesh count. The threads will make the mesh thicknesses different. Percentage of open area Percentage of open area (POA) is more useful when calculating ink consumption. The percentage of open area, or aperture percentage, refers to the relationship between the thread area and the actual opening between four woven threads in the mesh. The open mesh area helps determine the ability of an ink to pass through the mesh, as well as the thickness of the ink deposit in the printed image. Textile printers don't use percentage of open area as much as those who print non-absorbent substrates such as glass, plastic or metal. The absorbent nature of our substrates provides great latitude, so the subtleties of choosing a mesh based on POA are not as significant.

9. Weave structure
Plain weave versus twill weave.

With plain weave, each thread goes under one perpendicular thread and over the next. With twill weave, a given thread follows an under-one/over-two pattern. Mesh counts higher than 305 may be woven with plain or twill patterns. Twill mesh typically deposits more ink than the same or even a lower mesh count of plain weave. You may find high mesh counts in either plain or twill weave (shown at right). Plain is recommended because of the unpredictable moire patterns twill can cause. Much more important to the textile printer, though, is the odd effect twill has on halftone dots and moiré patterns. Since twill is an irregular weave, it creates an irregular pattern that can show up more readily than that of a plain weave, especially in terms of edge definition. Use a microscope and look for weaving that is one-over/one-under, instead of one-over/two-under. Double check meshes over 305, just in case your supplier made a labeling mistake. Check your suppliers' catalogs and look for "PW" or "TW," or ratios such as 1:1 an 1:2 in the specifications. The reason it is important to correctly identify the weave structure is because we textile printers are much better served by plain weave. It will be more expensive, but buy it anyway.

1997 National Business Media, Inc.