27 May 2019
May 27, 2019

MESH MATTERS

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Patrick Brunner discusses screen printing mesh geometry and selection

Screen printing mesh is usually not specifically in the focus of the screen printer. It is assumed that the screen printing mesh does the job without hesitation. However, in order to achieve the greatest possible process reliability in printing practice, it is advisable to take a closer look at the parameters around the mesh for screen printing.

Every screen printer knows that it is advantageous to use a coarser mesh type for high opaque printing areas and a fine mesh type for fine details. If there are still printing problems, it is usually sufficient to adjust the ink setting. Nevertheless, the limits regarding resolution of fine details, the maximal printable particle size or the achievement of a particular ink deposit are set by the screen printing mesh type. For this reason, we believe that taking into account the factors which lead to an optimal mesh selection will help to avoid unpleasant surprises, thus allowing greater process reliability in printing.
advantageous to use a coarser mesh type for high opaque printing areas and a fine mesh type for fine details. If there are still printing problems, it is usually sufficient to adjust the ink setting. Nevertheless, the limits regarding resolution of fine details, the maximal printable particle size or the achievement of a particular ink deposit are set by the screen printing mesh type. For this reason, we believe that taking into account the factors which lead to an optimal mesh selection will help to avoid unpleasant surprises, thus allowing greater process reliability in printing.

1. REQUIREMENTS FOR SCREEN PRINTING MESH

From a screen printing mesh, we expect a good stencil carrier, which reflects the applied printed image without interference even at high print runs. The screen printing mesh should ensure optimum ink release, so the printed ink layer thicknesses should be constant according to the mesh type within a print run. Fine lines and halftone printing require continuous precision to achieve a smooth image. High mechanical strengths are necessary in order to ensure a constant print
image and exact registration in multi-colour printing, even over long print runs. High chemical resistance is required in order to keep the screen printing mesh resistant to a wide variety of ink/paste systems or when multiple times of coating and stripping are desired

2. MESH GEOMETRY

The base of the mesh geometry is the number of threads and the thread diameter. The number of threads is given in threads / cm or threads / inch and the nominal thread diameter of the unwoven thread in microns – see Figure 1. From these basic elements, all other screen printing relevant values result such as mesh thickness (microns), mesh opening (microns) and open area (%) – see Figures 2, 3 & 4. The mesh opening limits the printable particle size, this should be at most one third of a mesh opening, preferably one fifth (Figure 5). Smallest printable dot: The combination of thread diameter and mesh opening limits the fineness of the smallest printable dot. This should not fall below a minimum of two threads and one mesh opening – see Figures 6 & 7. Finest printable line: It becomes more difficult when the finest printable line has to be defined, as this also depends on the ratio between yarn diameter and mesh opening.

Finest printable line: It becomes more difficult when the finest printable line has to be defined, as this also depends on the ratio between yarn diameter and mesh opening. Because lines follow often almost parallel to the threads, it is recommended to include at least two threads and two mesh openings for the finest printable line – see Figure 8.

The constancy of the line thickness also depends heavily on the coating quality of the stencil, on the resolution of the used emulsion and the printing template. The theoretical ink volume is calculated from the open area and the mesh thickness. The theoretical ink volume can be used to calculate the approximate ink consumption (+/ -10 %), and it is also helpful as a comparison between the different types of meshes in terms of ink deposit. The theoretical ink volume in cm³ / m² corresponds approximately to the wet ink deposit in microns – see Figure 9.

All these mesh parameters can be read from the article lists of the mesh manufacturers, so they do not need to be calculated, except for the ink consumption. If you look at the ink volumes in these technical lists, you will discover that they range in fine gradations from 5 to 323cm³/m². This is also a reason why plastic bottles or tubes are often decorated by screen printing. For example, Sefar PCF 120/305-34Y can be used for high dense sharp edged base layers or Sefar PCF 180/460-27Y for fine characters or halftones. A Sefar PET 1500 32/83-100 may be used for relief print on labels for wine bottles. Opaque and dimension stable ink
layers for dials are offered by Sefar PME 120/305-35Y. Why different yarn diameters with the same number of threads? (See Figure 10.) The answer to this question arises from the priority requirement of the print job. It is advisable, for example, to select the mesh with the thinnest threads for finest lines to avoid blockage or loss of line thickness in the event of parallelism between the thread and the line to be printed. When printing halftones, the mesh with the average thread diameter is more likely to be favoured, since this has a balanced ratio between yarn and open area. The thread surface area allows optimal

Figure 10.

anchoring of the dots in the three-quarter tonal values, without [adversely] affecting the ink release in the highlights. Mesh types with thick threads and small mesh openings guarantee high tension values and high durability. Also, ink consumption can be significantly reduced.

3. WHITE OR DYED MESH?

White/undyed mesh has the disadvantage that UVA light – in which the photosensitivity of the screen-printing emulsions is located – is reflected by the thread surface, which leads to undercut the film positive in the copy. This undercutting causes quality losses in the edge sharpness of the stencil. Therefore, white mesh cannot be recommended if sharp edges or fine details are required. Yellow dyed mesh converts the UVA radiation to yellow light; the UVA light is absorbed by the thread, as yellow is almost at the other end of the visible colour spectrum. (See Figure 11.) The yellow-reflecting light does not affect the emulsion, so there is hardly any UV-A light scattering, which results in sharp printing edges. The only drawback of dyed mesh is the almost doubling of the exposure time.

Figure 11.

4. MESH ELONGATION

A precision mesh geometry can only be maintained in combination with a balanced low elongation. This allows a reliable stencil making process and optimises the reproducibility of the ink deposit as well as the dimension stability in print. The Sefar mesh range offers product lines with different stretching characteristics. For additional support, see Sefar’s ‘Mesh Selection’ leaflet or contact your local Sefar mesh supplier.

Patrick Brunner is Product Manager at Sefar

Source: Special printing worldwide Digital, Screen & Pad Technology