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![]() Stretching Your Own Screens, Part III: Of Mesh and Meters
By Bill Stephens
The right mesh at the proper tension goes a long way toward insuring successful print jobs. Here's what you need to know when stretching your screens.
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In the first article in this series on stretching your own screens we considered some of the benefits and challenges in setting up your own screen-stretching operation. In the second, we focused on the screens themselves with a close-up look at frames. This time we're going to consider the second screen component: mesh, or fabric. And we'll also talk about the critical factor of mesh tension and how to measure it.
Mesh materials As for stainless steel mesh, if you intend to print signs you probably won't be using it. This is unfortunate in some ways, because stainless steel does offer outstanding strength and stability. However, it's mostly used for printing glass, ceramics, and electronic circuit boards. In sign printing, and indeed in almost all types of screenprinting, polyester dominates. Strong, durable, and flexible it can be used for printing with almost every kind of ink on every type of substrate, including those mentioned above. When you begin stretching your own screens, you will almost certainly find yourself working with polyester mesh. There are, of course, many different types of polyester mesh, and it's important to understand what makes them different and how those individual characteristics affect the printing process. The first major classification is thread type. Meshes can be woven using threads that are either monofilament or multifilament. Monofilament meshes are easily the most popular for general use. Monofilament threads consist of a single strand. Look at a piece monofilament fishing line for a good example of what that looks like. Multifilament threads, on the other hand, are made up of many tiny strands twisted together. Multifilament meshes are usually identified by the letters "xx" following the thread count.
Thread count and thread diameter Common thread counts range from less than 60 threads per inch (24 threads per centimeter) to over 420 (165 threads per centimeter). Not every mesh count is available from every manufacturer, but most will offer a range of thread counts. Thread count has a direct bearing on print quality. Generally, the higher a mesh's thread count the better it will be at holding finer details in prints. Thread diameter is another key mesh measurement. It is one of the most important factors in determining the strength of the mesh as well as ink deposit. Thicker mesh threads create heavier ink deposits, helpful when printing on rough-surfaced or porous substrates. Thread diameter is measured in microns, a unit in the metric system equal to a millionth of a meter. This figure can often be found printed on the selvedge of the mesh, immediately following the thread diameter. The two measurements are somewhat related since as thread counts go up, thread diameters tend to get smaller. Some popular mesh thread counts may be offered in two or three different versions, offering screenmakers the opportunity to choose the thread diameter that best suits their purpose. To get an idea of how the different thread diameters perform relative to one another, look in the specifications for "percentage of open area" and theoretical ink deposit. Remember that the mesh you are ordering may be available in more than one thread diameter. Get in the habit of always specifying the thread diameter you want when ordering mesh. Thread count and thread diameter can make a profound impact on your final print, so it is essential to record this information on every screen. Also, make sure these measurements appear on every roll and every cut off piece of mesh you intend to reuse in another screen.
Mesh color
Weaving pattern
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Ordering mesh
Mesh tension and how to measure it
Why you need a tension meter The problem is that as screens wear, mesh threads lose their elasticity, the ability to return to their original shape after being stretched, also sometimes called elongation or "loss of memory." Whatever term you prefer, it's an inevitable fact of the screenprinter's life that screens will lose tension over time. At some point, so much tension will be lost that the decline in print quality will become noticeable. Depending on the type of work you're doing this may go unnoticed for quite a while. But if you're doing jobs that involve close registration, like four-color process work, tension loss can bring production to a halt. With a tension meter you can spot potential problems well in advance of your print run. Typical prices range from about $250 to more than $500 dollars, a relatively modest investment considering what an under-tensioned screen may cost you in ruined substrate and wasted labor. Two different types of tension meters are available, mechanical or digital. The lower-priced meters tend to be mechanical. For most screen stretching, a lower-end mechanical meter may be all you need. But if you plan to do a lot of halftone printing and other demanding work, an electronic or digital meter has the potential to provide more accurate readings. Generally, more expensive tension meters will be capable of measuring a wider range of tensions and measuring them with greater accuracy. Tension is measured in newtons per square centimeter. The meter is placed directly on the mesh and the dial or display instantly displays the reading. What the meter is actually reading is the degree of deflection in the mesh caused by a known weight, which happens to be the weight of the meter. Even the least expensive meters should be able to measure tensions that range from seven to 50 newtons, more than adequate for meeting the tension specs of most mesh manufacturers. Advocates of super high mesh tensions will require a meter capable of reading in excess of 100 newtons. The other factor to consider is error tolerance. The cheapest tension meters may offer little more than a tolerance of +/- 2 newtons; adequate to meet the specs for most types of printing, but high-end meters may have an error tolerance of +/- 1 newton or less. Any meter must be regularly calibrated to maintain its accuracy. You can do this yourself by placing the meter on a hard, flat surface and moving the adjustment screws until the meter displays a reading of zero. Meters often come in a carrying case complete with a small piece of glass to provide a surface for calibration. In addition, the meter may have to be returned periodically to the manufacturer for recalibration. This means you really should think about having at least two tension meters on hand, one to serve as backup while the other is away for servicing.
Taking readings Readings should be taken from several different locations, generally the more the better. The minimum number of measurements required depends on the size of the frame, but you will want to take several readings of both warp (longways threads) and weft threads (threads that run edge to edge across the narrow dimension of the mesh). It's a good idea to take measurements as close as possible to the intended print area, each of the four corners and midpoints between them. With longer frames, take more readings from more positions. Your objective is a screen with uniform tension throughout the print area.
Care and handling In the next article in our screen-stretching series, we'll continue to focus on equipment with a closer look at screen-stretching systems.
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