Screen Printing - August/September 2012 - (Page 14)

THIN IS IN THIS ARTICLE EXAMINES THE ROLE OF THREAD DIAMETER ON A VARIETY OF PRINTING CHARACTERISTICS. Art Dobie Sefar, Inc. ower numbers are always fancied when talking about age and price tags. We like our laptops, waist lines, and televisions when they’re thinner. It is no different when it comes to mesh-thread diameter. Filament diameter (thread or wire), in its intimate relationship with mesh count, dictates the mesh parameters that most define our print results. The association of filament diameter and mesh count directly determines mesh opening size, open area porosity, mesh thickness, and a handful of other significant mesh parameters. In turn, the combination of mesh thickness and open area at tension provide the dominant influence on the thickness of our printed wet ink film. L Influence of thread diameter on mesh parameters When mesh count remains unchanged, choosing a smaller filament diameter opens up the mesh and renders it much less obstructive to ink flow. This is readily apparent in the representative illustration in Figure 1 of 380-thread/in., low-elongation polyester mesh, incorporating three different thread diameters (27, 31, and 34 microns Ø). The increased open area of mesh types with reduced thread diameter at similar mesh count equates to less filament surface area within the print cavity. As ink passes through a screen’s print cavities, it is in direct contact with the threads. By reducing the thread surface area, mesh will release easier at the moment of ink transfer. Smaller diameter thread at the same mesh count creates higher open area by increasing the size of the mesh apertures. Larger mesh openings allow for easier transfer of higher viscosity inks, or inks that have relatively larger particle size or high solids content, often desired to achieve special effects and to help give the color some visual pop. 14 SCREENPRINTING Additionally, filament diameter size has the most influence on mesh thickness. When the diameter is reduced, overall mesh thickness becomes thinner. Larger thread diameter increases mesh thickness. As we can see in the overhead view in the top portion of Figure 2 showing similar mesh counts, the smallest filament diameter indeed provides the largest mesh aperture size, providing greater open area. The cross-sectional view of the mesh weaves shown in the lower half of Figure 2 depicts the reduction in mesh thickness resulting from thinner thread diameter. The red blocks represent the open columns in the mesh through which the ink must transfer. The taller, chimney-like column on the right is a result of a thicker, more closed mesh, due directly to a larger diameter at similar mesh count. The increased thread-surface area, taller mesh, and smaller opening size make for more difficult ink flow and transfer from screen to substrate. Influence of thread diameter on ink profile Mesh thickness, as a direct result of thread diameter, can have an impact on the surface profile of the printed ink film. In rheological terms, most printing inks are formulated to be pseudoplastic or shear-thinning. When a shear stress is applied to a pseudoplastic ink (stirring, or spreading with a squeegee, for example), the viscosity of that ink drops noticeably, making the ink more fluid during the period the shear is applied. This rheological property is used to assist the ink to flow in and out of the screen mesh easier during the print stroke. However, we all like to hold sharp detail in our prints, so we need this decrease in ink viscosity to stop, and in fact reverse course, when the print stroke and ink transfer are complete. Fortunately, part of the pseudoplastic materials formulation is that they return to their at-rest viscosity once

Table of Contents for the Digital Edition of Screen Printing - August/September 2012

Screen Printing - August/September 2012
Contents
Newsworthy
New Products
Strategies for Navigating the Regulatory Maze
New or Used?
Thin Is In
Color-Management Aids
Performance Equals Profits
A Look at Dye-Sub Printing for Garments
Shop Talk
Classifieds
US & Canadian Directory
Advertising Index
Editorial Insights

Screen Printing - August/September 2012