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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
The two most popular mesh materials in use today are polyester and stainless steel. In the past other materials were used, among them silk, which is why so many uninformed sources still insist on referring to screenprinting as "silkscreen printing." For many years, nylon also enjoyed considerable popularity as a screen material and it can still be purchased from some sources. Its appeal is limited because it generally can't compete with the performance of polyester or steel.

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
Another major factor that distinguishes meshes from one another is thread count (sometimes called mesh count). This is the number of threads per inch of fabric or threads per centimeter in countries using the metric system. Often this figure is printed directly on the mesh near the selvedge, the finished edge of the roll.

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
Another major factor in mesh is color. White is the default color, but mesh may also be dyed a different color, most often yellow or orange, to help cut down on light reflected from mesh threads when the screen is exposed. Stray light reflections can undercut the positive, eroding fine lines and creating saw-toothed edges. Colored mesh tends to be found only in the higher thread counts, because these exposure problems most often become an issue in finer-detailed printing.

Weaving pattern
The final major factor to consider is the mesh's weaving pattern. The most common is a simple over-and-under weave known as Plain Weave (PW). On the other hand, Twill Weaves (TW) are created using any one of a number of more complex weaving patterns. Twills tend to be thicker and stronger than plain weaves, even though the two may have identical thread counts. According to most mesh manufacturer specs, twills also produce thicker ink deposits, but a study carried out by the SPTF (Screen Printing Technical Foundation) claims just the opposite. Twills and plain weaves perform quite differently on the press, and care should be taken to specify whether you want a plain weave or twill when ordering mesh. The one time twills should definitely be avoided is when a screen will be used to print halftones. The complex twill weaves can interact with the halftone dot patterns and cause significant moiré problems (interference patterns).

Ordering mesh
When ordering mesh, careful planning can save you some money. Meshes with popular thread counts may be available in a number of different widths. Try to order the width that makes the minimum waste. Often the excess mesh trimmed from a large screen can be used to stretch smaller ones. Never order a width that corresponds exactly to the dimensions of your frame. You need to allow several extra inches to provide a gripping edge for your stretching mechanism, so some waste is inevitable, but with careful planning, it can be kept to a minimum. Don't be surprised if you end up keeping rolls of several different widths in inventory.

Mesh tension and how to measure it
Unless mesh is stretched, tensioned, and secured to a frame, it's useless for screenprinting. Without tension, screenprinting simply doesn't work. It depends on the ability of the mesh to stretch in response to the pressure of the squeegee and return to its original state once that pressure is released. When stretching screens, you will be building in that tension, and the more carefully you control it, the better your screens will be. With that in mind you can easily see how essential it is to be able to measure mesh tension accurately.

Why you need a tension meter
The most basic tool in the screen stretching toolbox is a tension meter, or tensiometer. Even if you never intend to stretch a single screen you should probably pick one up. With a tension meter you can make sure your screens are always at their best, especially important if you do technically demanding work like halftone printing.

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
To take accurate readings you need to read and follow the directions provided by the manufacturer of your meter. As a rule, the meter is placed directly on the mesh, the wide part of the base parallel to the direction of the tension being measured. It should be placed at least three inches (7.6 centimeters) away from the frame.

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
Meters have to be handled with care, although good ones are fairly rugged. Shocks and rough handling take their toll of the toughest tension meter, and once a tension meter can no longer provide accurate readings, it's useless. Most meters come in rugged padded cases, and you can prolong the life of your meter by putting it back in the case when not in use. But accidents are sometimes unavoidable, which is why comparing manufacturers' warranties is an essential part of shopping for a tension meter. Warranties vary considerably, so be sure you know what's covered and what isn't.

In the next article in our screen-stretching series, we'll continue to focus on equipment with a closer look at screen-stretching systems.