Value A Concept in Screen Quality Evaluation

Evaluating the quality of a direct-emulsion stencil before it is on press is a difficult task. Pinholes and edge defects might be uncovered with illumination and magnification, but overall stencil quality and flatness are impossible to measure. After many months of research, a test has been established that can evaluate the relationship between the emulsion and the mesh. We call this relationship the Rz value, and it may provide the screen-printing industry with a concrete method of judging the quality of direct emulsion stencils before printing.

Mesh Equalization

The Rz value is a numerical reference to the mesh/emulsion equalization on the screen. Mesh equalization is the process of filling the mesh openings with emulsion and building up a thick enough emulsion layer to equalize (or smooth out) the surface structure of the woven mesh. In other words, mesh equalization means achieving a smooth emulsion coating on the uneven surface of the mesh. Mesh equalization is essential for sharp, sawtooth- free printing.

The screen must be smooth on the substrate side to ensure good contact between the stencil edges (image shoulder) and the substrate. This contact prevents the ink from running under the image edge and causing sawtooth prints. Since direct emulsions are water-based systems with a maximum of 50% solids, the dried coating will be at least 50% thinner than the wet coating. As the water evaporates during the drying process, the emulsions shrink in the mesh openings and creates a concave surface (Figure 1).

This concave form can prevent good contact (or gasket-like seal) between the substrate and the screen. Although a gap may exist between the stencil and the substrate at some points, if this gap is too large, ink will run under the printing shoulder and create the sawtooth effect or smear the image, both of which cause uneven line edges in the print. Such gaps or uneven screen surfaces cannot be detected with a normal magnifying glass or microscope.

It is possible to view this uneven surface with an electron microscope (Figure 2); however, this equipment is prohibitively expensive and not appropriate for normal production conditions. Currently, the only practical means to evaluate the screen is to print with it and judge the resulting printed-image quality.

Measuring Mesh Equalization

Measuring the surface flatness of a material is a common production technique in other industries and is often used as a quality-control specification. Until now, however, this technology has not been applied to screens coated with emulsions. We have found that it is possible to measure the “roughness” of the techniques and test instruments used in other industries.

Not only does the resulting test data offer insight into how the stencil will print, but it also provides information on the characteristics of the emulsion. The measuring instrument. Our research indicates that a roughness meter (Figure 3) can be used to evaluate the surface quality of a stencil.

The Feinpruef Perthometer M4P (Feinpruef Corp., PO Box 7547, Charlotte, NC 28217. Telephone: 704-525-7128) measures the stencil surface by automatically pulling a probe over a specified distance (4 mm) of the stencil to establish the profile of the coating (Figure 4).   

The measuring method. The screen is placed on a stable surface with the substrate side up. It is essential that no vibration occurs during measurement as this movement will show up in the final results. Although you don’t need to test on a granite slab, we do not recommend testing in areas where vibration is likely. In order to further stabilize the mesh/stencil, place a small piece of glass under the portion of the stencil to be measured.

This will prevent the probe from pushing down on poorly tensioned mesh and skewing the results. Position the probe so that it is at a 22.5° angle to the threads. This ensures that the probe will travel over the lowest (middle of the mesh openings) and highest (knuckles of the fabric) points of the surface. If the probe travels parallel to the threads of the mesh, the results will contain false values. The meter pulls the probe over the specified length of the stencil surface. The total length is divided into five sections and the maximum height differences in microns of all five sections are averaged to produce the Rz value.

Figure 5A illustrates a sample tape readout from the meter. You can see the minimum and maximum readings in each of the five sections. In each section, these two positive numbers are added together to establish the actual undulation of each section and thus the entire measure length. The following calculation reflects the computation: R(max) + R(min) = Rz R(max) = maximum reading for a section R(min) = minimum reading for a section Rz = value of mesh/emulsion equalization for a section The average of these five sections is then established as the Rz value for this particular emulsion/mesh combination (Figure 5B)

Interpreting The Results

The test will reveal a Rz value in microns. The higher the numeric value, the more undulation present in the stencil. Lower values, on the other hand, indicate a smoother surface. Obviously, the printer is looking for the lowest possible Rz value at the appropriate stencil thickness for the required ink deposit. Sample Rz measurements for typical screen-printing substrates are shown in Figure 6 for comparison purposes.

It is also possible to correlate emulsion thickness with the Rz value and establish data about different emulsions. If a sample emulsion is coated on the same screen under the exact same conditions, the Rz value and the emulsion buildup would be the same. Different emulsions, however, will produce different Rz values, even if the emulsion-over-mesh ratio is the same.

Since mesh equalization is, to a great extent, dependent on the solids content of the emulsion, the Rz value will vary at the same coating thicknesses with the different emulsions that are used. For example, a traditional, high grade diazo-sensitized emulsion with 27-28% solids might have an Rz value of approximately 9 microns. A diazo-sensitized photopolymer with 35-36% solids that is coated to the same wet thickness on the same mesh might have a Rz value of 7 microns. Obviously, the solids content of these emulsions has determined the difference in the total shrinkage and thus the lower Rz value for the higher-solids, photopolymer emulsion.

Rz Value Benefits

Until now, the screen maker and printer could only rely on the printed result as a reliable means of evaluating stencil quality. Yet, even this judgment is flawed because so many other factors affect the final print quality: squeegee pressure, substrate, squeegee durometer, squeegee angle, ink viscosity, mesh choice, etc. Even the perfect stencil can produce an unacceptable print if these other variables are not controlled. With the development of quality- control parameters and measuring techniques, however, it is necessary to start with the best stencil possible for the printing conditions.

The use of Rz values provides the screen maker or printer with quantifiable and repeatable stencil characteristics rather than subjective judgment. In addition to the quality of the stencil surface, the emulsion buildup can be measured on the wide range of mesh counts used in a typical screen printing plant, and this information can be recorded and used for future applications.

Our research indicates that it is possible to provide printers with the Rz values of various emulsions on specific mesh counts with the range of typical coating techniques, eliminating the need for the screen maker to test all the available products. Such information, if provided by the manufacturer, would help the screen maker evaluate the different emulsions and choose the right product for a job. It would also serve as a guide to the correct use of the emulsion to obtain the most desirable stencil thickness and Rz value.

The use of the Rz value could take much of the guesswork out of choosing the right emulsion and using it correctly. Extensive tests have shown that Rz values under 10 microns are required to achieve good print results on smooth surfaces, while values of 3-7 microns will provide a very good print on virtually any surface. (Please note that when printing on highly absorbent materials, such as textiles, the Rz value of the stencil material is of little significance.)

The Rz value can provide the printer with a new method of measuring stencil quality before the screen goes on press. In addition, it provides another means of qualifying emulsions and specific coating techniques.

Processing Variables

An important relationship exists between quality and economy in stencil making. The “perfect” stencil doesn’t necessarily imply the best one, if quality is tied to economy. Regardless of the desired quality/economy relationship, however, direct-emulsion coating must be standardized sufficiently in order to achieve repeatability - the most important factor in stencil making. It has always been difficult to determine what processing variables affect the repeatability of direct emulsion coating the most. To find out, we tested the influence that some of these variables have on the direct-emulsion coating. The test procedures we used were designed to evaluate three specific groups of variables:

• coating parameters: speed, technique, and trough fill level
• mesh parameters: mesh count, thread diameters, and mesh tension
• emulsion parameters: viscosity and solids content

Because manual coating techniques vary, we used an automatic coating machine for our tests in order to eliminate the human factor and ensure consistency from test to test. In all tests, except those evaluating emulsion variables, we used a diazo-sensitized photopolymer emulsion.

Coating Parameters

To evaluate the influence of coating speed on the stencil, we tested seven different speeds on four different mesh counts (Figure 7). All screens were coated 2-4 (print side-squeegee side), wet on wet, and measured after exposure, development, and drying. The coating trough had an edge diameter of 2.5 mm.

Figure 8 is a graph of the results from this test that shows a decreasing emulsion buildup, regardless of the mesh count, as the coating speed increases. However, the difference is only significant on the coarsest mesh count (195-50).

Obviously, meshes with the greatest percentage of open area are more sensitive to coating speed than finer mesh counts with less open area. These results indicate that you can fine tune the emulsion buildup on coarse meshes by varying the coating speed. Unintentional changes in the coating speed (which can occur with m According to our tests, the coating technique is the most influential variable of final coated-stencil quality.

As the coating thickness increases, mesh equalization improves and the Rz value decreases (i.e., the roughness of the stencil surface smooths out). Obviously, there is a maximum coating thickness where ink release from the stencil becomes difficult or impossible. This maximum thickness is dependent on the detail of the artwork. (Printing fine lines is possible with a emulsion buildup of 15 microns on a 120 threads/cm (305 threads/inch) mesh, but almost impossible with a buildup of more than 20 microns as the ink will not release properly from the stencil.)

Figure 9 illustrates the emulsion buildup and Rz value of various coating techniques on a 120 threads/cm, 37 micron mesh. We began the test with two coats on the print side and two on the squeegee side. For each successive test, we added an additional wet-on-wet coat to the squeegee side. While each coat adds 5 microns to the emulsion-over-mesh thickness on the print side, the Rz values do not change proportionately. For example, the Rz value of the 2-3 coating technique is 3 microns better than the 2-2, but the difference between the 2-5 and 2-6 techniques is only 1 micron, despite the one additional coat in each comparison.

This means that the emulsion buildup grows linearly, while the Rz value does not.  The third coating variable that we evaluated was the trough fill level. Aside from the actual construction of the trough (edge diameter and design), the fill level appears to contribute to the final stencil coating. Figure 4 represents the coating results of troughs with fill levels of 5 mm and 15 mm of emulsion (standing against the mesh during coating) on three different mesh counts. We used a 2-4 coating technique with a trough-edge diameter of 2.5 mm.

Figure 10 shows a clear change in emulsion thickness on meshes with the greatest open area (120 threads/cm, 32 micron). The flow-through properties of the mesh with the thinnest thread diameter mesh are better because of the larger mesh opening. The screen with the smallest mesh opening (120 threads/cm, 40 micron) seemed less affected by the trough fill level.

It is apparent from these results that when coating large screens of mesh with high percentages of open area, the fill level of the trough must be carefully controlled to avoid emulsion depletion during coating, which will cause uneven deposits. Although an automatic trough filling system, which controls the level after each coating movement, is a good tool for standardizing this process, filling the trough adequately (either manually or automatically) prior to each stroke prevents significant depletion of the emulsion.

Mesh Parameters

The choice of mesh is most often influenced by the specifications of the print order, ink color, or required ink deposit. Nevertheless, in most cases, the printer can choose among several mesh types that will offer the same printed result. We conducted evaluations to determine what influences the mesh count, thread diameter, and mesh tension would have on the final direct-emulsion coating.

To investigate the influence of the mesh count, we tested three mesh counts—100 threads/cm, 40 micron (255 threads/inch), 120 threads/cm, 37 micron (305 threads/inch, and 140 threads/cm, 37 micron (355 threads/inch) with different coating techniques (2-2, 2-3, and 2-7). Figure 11 shows that the emulsion buildup decreases as the mesh count increases. The emulsion buildup gains 7 microns per coating on the 100-40 mesh, 5 microns on the 120-37 mesh, and only 3 microns on the 140-37 mesh. In production, this means that good mesh equalization can be achieved quickly on coarse meshes, while fine meshes require additional coats. Figure 12 reveals the Rz values for these same meshes and coating techniques.

These examples further illustrate that fewer coatings are required on coarse meshes to achieve acceptable Rz values.  Mesh count is not the only variables, however. The actual thread diameter plays a significant role in the required emulsion coating. We used a single mesh count with three different thread diameters (and thus different percentages of open area).

Figure 13 illustrates that on the 120-32 mesh (S type), the emulsion has less resistance to flow through the fabric than on the 120-40 (HD type), which results in a higher buildup with fewer coatings. The buildup is double that of the 120-37 mesh which as 6% less open area. The buildup on the 120-40 mesh is even lower, and it was impossible to achieve acceptable Rz values on this mesh with fewer than 5 squeegee-side coatings. In practice, our results indicate that a better Rz value at a given buildup can always be achieved with the finest thread diameter (S type) in a mesh count. To test the effects of mesh tension, we stretched 120-37 mesh on screens to four different tension levels.

The coating techniques were also varied to determine what influence the quantity of emulsion buildup would have at the various tension levels. Figure 14 illustrates that we were unable to establish any significant differences in emulsion buildup due to mesh-tension levels. The 1-micron variances in both the buildup and Rz values are not statistically significant and may even be attributable to the measuring device. (It should be noted that all screens were tensioned evenly across the mesh and so the test was not designed to evaluate the influence of uneven tension levels on coating buildup.) In the future, testing different mesh counts will be necessary to clearly establish the role mesh tension plays in the emulsion buildup.

Emulsion Parameters

The specific characteristics of a direct emulsion (solids content and viscosity) are also important variables in establishing the amount of emulsion buildup on a mesh. The viscosity is established by the manufacturer and then adjusted depending upon the specific task (e.g., coarse meshes, automatic coating, etc.). Except for presensitized single-pot systems, most emulsions must be sensitized before use. This sensitizer is generally dissolved in water and added to the basic emulsion mixture, lowering the original viscosity.

The exact amount of water used to dissolve the sensitizer may vary, causing fluctuations in emulsion viscosity. To investigate the effects of viscosity changes, we thinned an emulsion, measured the viscosity, and calculated the emulsion buildup and Rz values of coated screens. As can be seen in Figure 15, even with high thinning percentages (by weight), neither the emulsion buildup nor the Rz value changed significantly, making it probable that the two effects are counteracting each other as follows:

• The thinning of the emulsion causes a decrease in the solids content and thus a lower viscosity.
• The lower viscosity changes the flow properties of the emulsion and a high buildup is achieved. This higher, wet buildup, however, reduces when dried because of the lower solids content.

These tests indicate that you can achieve good production results on most meshes regardless of the amount of thinning. Specific tests should be performed with each emulsion type since results may vary when viscosity is significantly altered. The solids content of an emulsion is the most important quality characteristic, but this depends on the type of solids used. Fillers provide a higher solids content but can cause poor edge definition and mesh bridging, in some cases, if the particle size is not carefully controlled by the manufacturer. High-quality solids are resins that improve edge definition, mesh bridging, and the chemical resistance characteristics of the emulsion. 

For this evaluation, we used three emulsions containing high-quality solids. Figure 16 lists the solids content, viscosity, buildup, and Rz values of the emulsions that we tested, while Figure 17 graphically presents the significant buildup that is achieved as the solids content is increased. It is apparent that mesh bridging, edge definition, and Rz value can be improved with the use of a high quality, high-solids-content emulsion.

Conclusions

Several conclusions can be reached from this preliminary investigation.

1. The degree of thinning and mesh tension have little effect on the actual buildup of emulsion on the screen.
2. The mesh type, coating technique, and solids content of the emulsion are the major parameters that influence the quality of the stencil. Any changes in these parameters will result in measurable differences in the coating quality.
3. Viscosity, coating speed, and trough fill level influence the quality of the coating; however, the actual significance of these effects depends on the specific mesh type.

How can you use this information in production to control the quality of direct-emulsion stencils? Our investigation revealed that the coating thickness and stencil quality on meshes with large open areas (such as S-type) are more likely to be affected by changes in coating technique, solids content, and coating speed than meshes with small open areas. The S-type meshes (with thin-diameter threads) also give a high emulsion buildup per coating, which provides good mesh equalization with only a few coating strokes.

On the other hand, it is almost impossible to adjust the emulsion buildup within fine tolerances unless the coating speed can be adjusted. We also learned that the solids content of the emulsion is very important when the printer needs to control mesh equalization (and the concave drying effect that occurs with low-solids emulsions).

If S-type meshes are coated with high-solids emulsions, good mesh equalization can be achieved with a very thin emulsion buildup because of the combination of thin thread diameter and large open areas that enhance emulsion flow. (Note: Thinner threads are not bent as much as the T- or HD-type mesh filaments at thread intersections, so the height differences in the mesh are lower and easier to equalize.)

The coating buildup and quality are influenced not only by emulsion viscosity but also by the type of mesh you use. High-viscosity emulsions are generally adjusted to provide stability in the wet coating for thick stencils and coarse meshes. If these same emulsions are used on finer mesh, the flow resistance through the mesh opening is very high. Medium-viscosity emulsions can be used on almost all kinds of meshes, but the number of wet-on-wet coatings is limited on coarse meshes.

If the emulsion buildup gets too high, the emulsion tends to flow into and through the openings, resulting in an irregular buildup. Low-viscosity emulsions can be used on fine meshes and ones with lower percentages of open area because the flow resistance is high. They should not, however, be used on coarse meshes.

Although this investigation is not complete, we can already see correlations within the data that could be incorporated into future developments in direct-emulsion technology. Controlling the emulsion buildup is becoming more critical, especially for screen printers using water-based and UV inks as well as those who need to print fine halftones and four-color process images.

Although the most recent developments (SBQphotopolymer and diazo-photopolymer systems) have made great strides in allowing screen printers to achieve mesh equalization at lower emulsion-buildup levels, the focal point will continue to be on the formulation of products that offer even better mesh-equalization properties. These and future investigations will help printers understand the emulsion-coating process and establish standards in production that will improve the quality of stencil making.

Reprinted with permission from ST Publications, first published in Screen Printing magazine, June and August 1990.