The screen printing process is distinctly different from most imaging processes typically associated with graphic arts. First, the printing plate is actually porous, formed from a woven mesh of synthetic fabric threads or wires (or, at least in one case, a nonwoven electroformed metal matrix), which is then combined with a selective masking material (commonly referred to as Because the coating material flows under pressure and through this mesh or substrate before being deposited on the substrate, the resulting coating is much thicker The thickness of the material on the substrate - jet printing.
For this and other reasons, the screen printing process has many practical applications in areas of industrial manufacturing where few or no other printing media are available.
The basic process steps are as follows (see also Figure 24.1). The woven mesh (or matrix) is secured to a rigid frame of aluminum or steel. In most applications, the frame forms a rectangular plan. Variations are possible, however, including cylindrical screens fixed and sealed at both ends. In the case of mesh, either synthetic polyester monofilaments or stainless steel wires, tension is applied simultaneously in opposite directions to obtain a semi-rigid planar surface. This stretched printing screen then performs three distinct functions: (a) metering the fluid coating (or ink) that flows through it under pressure, (b) providing a surface for shearing during transfer to the substrate. Columns of viscous coating material formed during (c) provide support for the imaging element (template).
The transfer of ink or coating is initiated by applying pressure on the screen with a flexible plastic blade (squeegee). Due to the flexibility of the blade material and its physical contour, hydraulic action is caused by forces applied in two directions. The blade is pressed into the screen, and its inherent flexibility allows it to make direct contact with the substrate, enabling ink transfer. The squeegee also sweeps horizontally, applying the ink or coating as it moves and causing the column of material to shear as the printing screen bounces after the squeegee passes.
Figure 24.1 Ink transfer for screen printing
The combination of horizontal printing stroke and vertical (downward) pressure, effected by the squeegee, forces the coated ink into the ink "wells" formed by the intersecting areas of the woven web. (Similar holes, albeit in different shapes, are created in the matrix of electroformed metal printing screens.) The elasticity of the printing screen then allows temporary contact with the substrate and subsequent ink or coating transfer.
