Excellence in stencilmaking yields screens that stand up to production wear and produce detailed, high-quality graphics. Find out how to regulate the instabilities in the process that can lead to less than desirable results.
By Ross Balfour
Screen mesh is composed of two parts. First are the threads, and we need enough of these to support all of the detail in our image. Second are the holes, the size and number of which, along with the stencil profile, control how much ink is laid down. The main factor that influences ink deposit is the mesh count of the fabric, or how many threads per inch, particularly when working with 305 threads/in. or less. The mesh count becomes less important once we get above 305 threads/in. Here, the actual thread diameter and weaving construction—plain or twill—becomes the dominant factor in determining ink deposit. Obviously, the higher the mesh count, the finer the detail that can be supported on the screen. However, the fact that threads are in the way at all does place limitations on what we can realistically screen print (Figure 4).
As far as fine detail is concerned, there’s a minimum size of opening in the stencil that will consistently allow ink to pass, regardless of where it sits on the weave of the mesh. Once the size of the detail on the screen, whether fine lines or halftone dots, becomes narrower than one mesh opening plus one and a half thread diameters, then it can be obscured by passing over the threads and the knuckles of the weave where the threads cross.
Choosing mesh with a thinner thread diameter can help squeeze out a little more detail, but at the cost of producing a more fragile screen. Mesh woven from thicker threads produces a more robust screen you can use at a higher tension level for better registration with multicolor printing, and it provides better adhesion at the shadow end of a halftone range—or for holding fine lines with reverse printing. Once the small specks or strings of stencil that have to block the flow of ink and differentiate between shadow tones or delineate text become smaller than two mesh openings plus one and a half thread diameters, they may only adhere to one or two threads and lack sufficient adhesion to withstand the rigors of processing, never mind printing.
As an example, with halftones, the line count determines the tonal range that can consistently be printed on any particular mesh count. As the line count increases, the smaller dots enable viewing from a closer distance without the individual dots themselves being visible. However, increasing the line count effectively decreases the range of tones that can be held before highlights experience moiré—and then cease to print—and separation between midtones and shadows is lost as everything collapses to a solid print. This phenomenon is illustrated for a 380-thread/in. mesh in Figure 5. If you set a target in which you try to print from 10% in the highlights up to 85% in the shadows for a print with good separation between all the tones of the halftone range, then each mesh will have a limit on how high the line count of the halftone can be (Figure 6).
A perfectly prepared stencil is, in fact, capable of resolving finer detail than it is physically possible to print, because of the intervening influence of the mesh. However, in order to make the perfect stencil, you have many screens ahead to expose, obstacles to be overcome, and variables to control.
Ross Balfour is research and development manager for Saatichem, a division of the global screenmaking product manufacturer, Saati Group (Como, Italy). Balfour directs the research and development of emulsions, screen-preparation products, and screen-reclaiming products from Saatichem’s facility in Chicago, IL. He is a graduate of The Royal Society of Chemistry in London and has more than 20 years of experience in the printing industry.
Editor’s note: This article was adapted from SaatiPrint’s Tech Tips for Screen Printers Handbook.
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