The role of thread diameter on a variety of printing characteristics
By Art Dobie
While recognizing that the ink transfer occurs at the moment of separation of the screen from the substrate, we also must contend with the common rheological properties of most printing inks to shear thin when squeegee motion is applied, and recover to rest viscosity when the shear stress is ceased. This suggests that it is best to have that separation of screen from substrate occur before the ink has completely returned to its higher idle viscosity. Considering, it would be most desirable to have the screen and substrate separate progressively and immediately behind the moving squeegee, rather than waiting for the entire print stroke to complete and then attempting to separate the two. At that point, the ink viscosity is increasing and it will be difficult to get the mesh to cleanly release from the print without leaving some print flaws such as voids and mesh marks.
Fortunately, screen-printing mesh is woven with yarn or wire having appropriate tensile strength, and can be tensioned to a reasonable tightness while still maintaining a small amount of elastic property. Assuming the screen frame is dimensionally stable enough to maintain the mesh tension level, we can artificially generate the necessary separation between the screen and substrate to allow for ink transfer by fixing the tensioned screen in a parallel and elevated position above the substrate, and deflecting the mesh down into a single line of contact with the substrate using the squeegee.
Mesh tension acts as a counter force against the downward squeegee deflection of the screen, allowing the screen to separate and lift up from the substrate directly behind the traveling squeegee line, much like peeling a piece of tape up from a table top. This relatively immediate screen lift or screen peel, allows the mesh to be removed from the print deposit before the ink can completely return to higher rest viscosity. The small amount of elastic property retained in the mesh permits this repetitive deflection action for continued use.
For separation of screen and substrate to occur, the counter force provided by mesh tension and off-contact gap must exceed the hold down force provided by ink tack. It is more desirable to increase counter force (when necessary) by using higher mesh tension rather than increasing the off-contact gap. Deflecting the screen into contact with the substrate from a higher distance has a greater chance of initiating dimensional distortion, and likely creating image registration concerns. Higher screen tension creates desirable screen peel activity while maintaining a lower off-contact distance to minimize image distortion.
Stronger, small filaments
The smaller diameters associated with advanced filament mesh types provide for increased open area percentages which aid in easier ink passage. The mesh thickness is noticeably thinner on these advanced mesh types as well, helping to maintain smoother ink film and stencil surface profiles. The resulting estimated ink-film thickness remains basically unchanged to ennsure opacity and color aren’t substantially altered.
The most significant parameter is tension value (Figure 6). It is here we can recognize the biggest benefit of these smaller, higher strength filaments. The tension capability of these new filament mesh types meets or exceeds that of its standard mesh counterparts, even though their diameter size is significantly smaller. These higher modulus filaments create a more stable mesh, helping to maintain dimensional accuracy while the higher tension capability combined with small diameter size and increase open area allow for better ink transfer. Thin is in, and you no longer need to settle for lower screen tension.
Art Dobie is technical marketing manager for Sefar, Inc., Depew, NY. He has been with Sefar for more than 30 years since receiving a bachelor’s degree in graphic communications, specializing in screen-printing technology, in 1980 from California University of Pennsylvania. Dobie is a Life Member and Fellow of the Society of the International Microelectronics and Packaging Society and was the 2006 recipient of the IMAPS John A. Wagnon Technical Achievement Award for outstanding technical contributions to screen-printing technology as related to microelectronics. He is a member of the SGIA’s Academy of Screen Printing Technology and received the 2010 Swormstedt Award for the best published article or technical paper written for any aspect of the screen-printing industry.
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