SCREEN PRINTING
Feel the tension, watch the spread
Despite the constant evolution of digital printing technology, screen is still widely used in graphics and is crucial to many advanced industrial processes By Petter Kiddell
W
hen you compare the screen and digital processes at a fundamental level they both rely on the same
physical properties of liquids and solids - surface energy and surface tension. Of course viscosity and shear rate of the ink are key characteristics, but without the energy characteristics of a surface, structure and form would not exist..
Surface tension is the molecular force required for a
liquid to create a surface. Surface energy may therefore be defined as the excess energy at the surface of a material compared to the bulk. Both screen printing and digital printing rely on the relationship between liquids and solids being suitable for the processes to operate.
Tension explained Wetting of a surface occurs when the surface tension within a liquid that causes it to form into a ball is overcome by the adhesive forces of a solid.
Digital inkjet printing causes a liquid to be expelled through a hole, through the air and onto the solid substrate. Once the liquid impacts the solid it should
immediately wet the solid surface and spread across the surface by a predictable amount. Surface tension of the ink determines the conditions of the ink droplet as it leaves the orifice, how it wets the substrate and hence spreads on the substrate. It also governs what happens to the ink held in the orifice before the next pulse ejects it. If the surface tension is too low, it will drip out rather than be ejected. Of course there are other characteristics of the ink that determine its performance during the process, but the
16 SCREEN PROCESS AND DIGITAL IMAGING l January/February 2011
relationships of surface energy and surface tension are crucial.
With screen printing the mechanism is slightly different but not as different as you may think. Surface tension and surface energy are still vital to its operation. A mesh is actually a series of orifices. In one square centimetre of 120 threads per inch mesh there are 14,161 mesh openings and in one square metre 141,610,000. These mesh openings are filled with ink by the action of the flood coater and the squeegee. The squeegee also brings the ink filled mesh into contact with the substrate by closing the off contact distance. The ink that is in the mesh openings comes into contact with the substrate. If the surface tension of the ink is lower than the surface energy of the substrate it will wet the substrate. In this event there will be adhesive forces between the ink and the substrate that should be sufficient to draw ink out of the mesh openings when the mesh is pulled away by the mesh tension. If the surface energy of the solid is less than or close to the surface tension of the ink, the ink will stay in the mesh opening and not transfer onto the substrate.
This is a simple view of what occurs and there are many factors that influence the mechanism. Research by Swansea University and the team at MacDermid Autotype has demonstrated the complexity and beauty of the process. With both screen printing and digital printing, however, the characteristics of the ink have to be such that they will flow just enough to move through the process but then stabilise on the substrate.
Understanding ink
The much criticised ink manufacturers who supply inks for both techniques take great care
Visit us at
www.spdi-online.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32