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Thin Is In

(August 2012) posted on Tue Aug 21, 2012

The role of thread diameter on a variety of printing characteristics

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By Art Dobie

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 application of shear stress is ceased.

The viscosity reduction of most printing inks during the print stroke can vary based on a number of factors, including the specific ink formulation, and the amount of shear stress applied to the ink during the print stroke. Some materials shear thin more than others. Some inks may shear thin very little or not at all. Depending on the how much the printed ink flows, some inks are more prone to leaving evidence of printing in the way of mesh marks or imprints of the mesh filaments in the surface profile of the ink.


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