Printing screen geometries can vary widely due to differences in materials and manufacturing methods. Common in industrial applications is woven mesh, consisting of extruded polyester threads woven into a precise and regular pattern known as a "plain weave". For a given linear measure, the number of thread crossings or intersections, called the grid number, can be expressed in metric (European) or British (Japanese) increments, as produced. At 130 intersections (mesh) per cm or more, fabrics are usually woven in a twill weave, where one x-axis fiber crosses under and over every other y-axis fiber (rather than each y-axis fiber in a plain weave weave). The weaving creates thousands of roughly cubic holes formed by fiber intersections at each of the four corners. It is the length, width and depth of these pores that form the ink pool and have a major impact on the amount of coating deposited.
Thread diameter, although consistent in each woven fabric, is a variable that allows a wide choice of fabric thickness and therefore substantial control over the thickness of the applied coating. Available thread diameters range from just over 700 microns to just over 30 microns (0.0275 to 0.0012 inches). The thickness of the fabric, and therefore the depth of the inkwell, is approximately 1.85 times the diameter of the thread.
The woven mesh is tensioned to a predetermined level and fixed to the frame. For synthetic fabrics, the effect of tensioning can also slightly reduce the pore depth by reducing the fabric thickness by up to 3%. However, for steel wire, there is no such reduction.
Template materials, when present, partially or completely block the pores, thereby reducing the amount deposited, while increasing the depth of the pore walls, thereby increasing coating deposition.
Thus, the total volume of ink or coating deposited is determined by the fabric thickness (the area of the screen/stencil surrounding the image) minus the volume of the lines (or lines) within the image area.
The flow of paint through the mesh is also affected by its viscosity and the operation of the drawdown blade.
rotate screen
In principle, the cylindrical screen used in rotary screen printing acts like a flat screen in that the coating material flows through the open areas of the perforated thin printing "plate". However, this differs in the base configuration, which is cylindrical rather than flat. Ink or paint is pumped inside the cylinder, which is sealed at both ends by printheads. The squeegee is also located inside the drum.
The screen (in one variant) consists of a tube of seamless electroformed nickel mesh which then receives an emulsion coating for the production of the pattern template. Other methods of producing rotary printing screens can achieve similar results.
Rotary screen printing has the advantage of continuous operation, without the intermittent sweeps and return strokes of the printing blade as in lithographic printing. Thus, opportunities are offered to increase production speed, especially for substrates printed on web.
