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Mesh Matters

The level of detail you can print, the quantity of ink you lay down during the print stroke, and the ability to reproduce an image accurately and consistently are but a few of the print characteristics that are influenced by your screen. Here you

As the foundation of the screen-printing process, screen mesh deserves some careful study if you want to optimize the quality of your prints. Understanding the makeup of screens, their physical characteristics, and their performance attributes will help you make the right decisions about which mesh to select for specific applications and how to use the fabric to get the best results. The following discussion reviews key points to keep in mind when choosing screen mesh for your jobs.

 

Mesh basics

Screen fabrics are woven from monofilament or multifilament yarns, most commonly polyester, although nylon and steel-wire meshes also are available. Monofilament fabrics are woven from single-filament threads. This thread construction provides predictable and controllable performance and has made monofilament polyester the dominant fabric used in screen-printing applications.

Multifilament fabrics are woven from threads that are made of multiple filaments twisted tightly together. These fabrics account for only a small portion of today’s market because the structure of the yarn does not lend itself to precision thread diameters and edge definition. Although the rough thread surface promotes stencil adhesion, the multifilament yarns are more prone to severe ghosting and are more difficult to reclaim.

Screen mesh usually is offered in standard white or dyed yellow colors. Un-dyed polyester threads reflect and scatter light during the exposure process, thereby causing stencil exposure beyond the image edges defined by the positive image, a condition referred to as undercutting. The result can be dot gain or loss, poor image edge definition, and decreased image resolution, particularly in images with higher halftone line counts and finer details. Yellow dyed mesh, on the other hand, absorbs the UV wavelengths of light that expose the stencil and prevents light scattering, which preserves image resolution and edge definition. The drawback to yellow-dyed mesh is that it requires longer exposure times during stencil production.

Screen fabrics have warp threads, which are the threads that run the entire length of the roll or bolt of fabric, as well as weft threads, which are woven through the warp threads and run from one selvage edge (edge of the roll or bolt) to the other. The threads june be woven in two different ways. The most basic weave is plain weave, which follows a one-over/one-under pattern in which a weft thread alternately goes over one warp thread and under the next (Figure 1). A plain weave creates a square mesh opening and the most predictable printed-image quality. New fabrics with extremely narrow thread diameters have made it possible to manufacture a wide range of plain-weave mesh counts.

The other fabric type is twill-weave mesh. A twill weave produces a diagonal or twill line across the fabric face and is produced by weaving the weft thread over two warp threads and then under one in a repeating pattern (Figure 2). The twill weave structure is less complicated for producing high thread-count meshes because it provides greater fabric strength. However, the fabric pattern created by some twill meshes can lead to unpredictable ink deposits and moiré, so it is used much less frequently than plain-woven fabric.

After mesh is woven, it june undergo a calendering process in which the fabric is passed through a set of calendering rollers to flatten one or both sides of the fabric. This reduces both the mesh-opening size and the overall thickness of the woven fabric. Calendered mesh is used in specialized applications where a very thin, measureable ink deposit is critical.

 

Mesh by the numbers

Mesh is manufactured to metric specifications, and the woven mesh count represents the number of threads per centimeter. In the US, however, mesh fabrics are typically referenced by their nominal mesh count, which is the calculated equivalent number of threads per inch. For example, a mesh woven at 120 threads/cm has a nominal thread count of 305 threads. This is the nearest approximation of the calculated count (120 threads/cm x 2.54 cm/in. = 304.8 threads/in.).

Thread diameter refers to the thickness of the threads used to weave the mesh (Figure 3). Thread diameters very widely, from thick yarns are used to weave coarse meshes (e.g., 24-thread/in. fabrics with 250-micron thread diameter) to thin yarns used to weave high mesh counts with workable mesh openings (e.g., 305 threads/in. with 40-micron thread diameter).

The mesh opening is the open area that is bounded on four sides by woven threads. The width of a mesh opening is measured in microns. Technical diagrams of mesh typically depict the open as perfectly square. But achieving a square opening is impossible when using rounded threads. The weaving and finishing processes create mesh openings that are less regular than a perfect square. Mesh openings in plain-weave fabrics tend to be more consistent than those in twill weaves. The size of the openings is an important parameter to consider when determining printability with certain inks, especially those that contain large and coarse pigments or metallic particles.

The open area of a mesh fabric is a percentage value that represents the area of the mesh covered by mesh openings versus thread. The percentage is generally calculated based on one square meter of the fabric. Two fabrics with differing thread diameters can have the same mesh count, but they will have different percentages of open area. This is important to consider when selecting fabric for a job with specific ink-deposit requirements.

For example, a mesh with 305 threads/in. and 40-micron thread diameter has an open area of 25%, while a mesh with 305 threads/in. and 35-micron thread diameter has an open area of 33% . Comparing the fabrics reveals that a reduction in thread diameter of only 5 microns (12.5%) results in an increase in open area of 32%. For a job requiring a heavier ink deposit, the fabric with 35-micron threads is clearly the better choice.

The measurement of the woven mesh’s cross section is fabric thickness. It is used to help calculate the theoretical ink volume that each open cell of the fabric will hold, the wet ink-deposit thickness that a mesh can be expected to deliver (Figure 4), and the total volume of ink required for a particular job. Though it might seem logical to assume that the fabric thickness is the sum of the two thread diameters, this is not the case. Weaving and finishing operations affect the total fabric thickness measurement, which is always smaller than the sum of two thread diameters. For example, a 305-thread/in. fabric with 40-micron thread diameter might have a total thickness of 63 microns.

 

Tensile strength and yield point

A screen fabric’s tensile strength is its ability to be stretched without breaking. Tensile strength usually is measured as the amount of force (N/cm) required to cause thread breakage. Low-elongation mesh fabrics possess higher tensile strength that conventional fabrics.

The yield point is the point at which the polyester becomes deformed and can no longer regain its former tension level. It occurs well before the fabric’s breaking point. Regardless of the amount of tension applied to the fabric, it will not hold tension any longer after its yield point has been reached. The objective of tensioning is to reach the optimum tension level that is below the yield point. Mesh manufacturers will provide you with information about the yield points of their fabrics as well as guidelines for optimizing tension levels.

 

Fabric behavior under tension

The minute you apply tension to screen fabric, it goes from a static to a dynamic state. As tension increases, the mesh parameters and physical characteristics of the polyester begin to change. Understanding mesh behavior under tension not only will help you to select the right mesh, but it will also influence how you use it.

Once tension is placed on the screen fabric, parameters begin to change. Some change in proportion to the amount of tension, and others are only minimally affected. It is impractical to predict the specific changes that happen in every situation at every tension level, but in general, the following things occur when mesh is tensioned:

• The size of the mesh opening increases.

• The mesh count (threads/in.) decreases.

• The fabric thickness decreases.

• The diameter of threads remains virtually unchanged.

Determining the degree to which these parameters change in your shop is matter of testing. By standardizing tensioning procedures and consistently stretching screens of the same thread count and thread diameter to the same tension level, you’ll be able to assess and document the effects of tension on ink-deposit thickness and image quality and make appropriate adjustments to get the results you want.

 

Work hardening

You june have heard it said that work hardening mesh extends its useful life and helps it retain tension longer. This is due to the changes in molecular alignment that take place in the polyester fabric as it is continually worked by the squeegee and floodbar during printing. Essentially, work hardening is like exercise for your screen fabric.

Low-elongation fabric is woven from threads with a molecular alignment that is different from that of conventional monofilament polyester. It requires less work hardening to achieve optimal performance quickly. Such fabrics also maintain more consistent tension in the warp and weft directions and can be stretched in both directions simultaneously with no compromise in performance.

 

Screen fabric selection

Choosing the right mesh is a key factor in the profitability of every screen-printing job. The wrong mesh might not hold the right level of tension or june yield an unacceptable ink deposit. It could contribute to moiré, leave mesh marks, or not support the stencil’s fine lines and halftone dots. All these factors create downtime and the need to remake screens, and they necessitate that you rethink your choice of screen fabric.

Before you can even think about the mesh, you need to take an inventory of the job parameters, employee skill levels, and shop environment. All these factors have a tremendous impact on the screen fabric that will fit your shop’s production practices and your customers’ needs.

Substrate Your first decision in producing a job is the choice of substrate. All other raw material you’ll use in production stems from this first choice, including the ink you’ll print with, the level of detail in the artwork, press settings, and even screen fabric. The mesh requirements will differ greatly depending on whether you’re printing polyester sheets, T-shirts, corrugated board, or ceramic materials.

Artwork In line work, the width of the narrowest lines and the registration tolerances among colors establish important parameters for fabric selection. In process-color printing, the smallest element in the artwork is the shadow dot, which can be lost through a mesh opening or behind a thread if it is not considered during fabric selection. As screen rulings (lines/in.) go higher, the printable tonal range gets smaller because that 5 or 10% shadow dot simply cannot be resolved or printed with the available screenmaking materials.

The two mesh criteria that are the most critical in process-color printing are mesh count and thread diameter. The mesh-to-dot ratio refers to the mesh count versus the halftone line count and is typically used to minimize the occurrence of moiré patterns (Figure 5). Though moiré can never be completely eliminated, higher mesh-to-dot ratios can reduce the incidence of global moiré. A minimum ratio of 3.5:1 is recommended, but results improve significantly at a ratio of 5:1 or higher. So at 3.5:1, a 65-line/in. halftone would require the use of a 230-thread/in. fabric (3.5 threads/line x 65 lines/in. = 227.5 threads/in.), and, at a 5:1 ratio, a 75-line/in. halftone would require a 380-thread/in. screen (5 threads/line x 75 lines/in. = 375 threads/in.).

In addition to mesh count, thread diameter is an important factor because the threads can block small dots or whole rows of dots, creating a visible moiré pattern. This is commonly seen in highlight dots and with high halftone line counts. For this type of fine-detail printing, first determine the mesh-to-dot ratio and then select the finest thread diameter available in that mesh count. Some experimentation june be needed with differing line counts, tonal ranges, and screen angles.

Many charts and mathematical formulas are available to help determine the smallest imageable element on any screen mesh, but there is a simple visual test you can perform to see whether your artwork can be reproduced on a given type of mesh. Place the film positive over the mesh and view both the mesh and positive under a 30x or 60x microscope. Look for signs that the image will either be blocked by the mesh threads or too small to be supported by the mesh.

Ink characteristics Frequently, ink selection is predetermined by the substrate or post-printing performance requirements, as in the case of electronic parts and membrane switches. The nature of the ink required should be taken into account when selecting a mesh.

For instance, when printing with metallic inks, the mesh opening must be at least three to four times the width of the metallic particle size or the ink will not completely transfer to the substrate. Very narrow lines or fine halftone dots would not be advisable in this situation because the mesh required to print them might not provide an acceptable mesh opening.

Stencil selection Rather than influencing which mesh you select for a job, the stencil system you use should be matched to the chosen screen fabric. Keep in mind that the resolution and edge quality characteristics of emulsions and stencil films vary from product to product. You should look for a stencil material that will provide the needed resolution and edge definition with a percent solids content that will be compatible with your mesh count. Very high solid content emulsions will often leave too much emulsion on fine mesh counts, leading to difficulties in printing a complete image. The best choice of stencil material is one the provides the optimum stencil thickness (emulsion-over-mesh ratio) with a smooth stencil surface (low Rz).

 

Focus on the fabric

Pairing the right mesh with the job at hand means smoother production and better print quality. By combining the guidelines presented here with your own workplace practices and experimentation, you’ll be able to identify the best mesh for every application and use it to its full potential. 

 

 

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