The printing tool in screen printing is a squeegee, a tool similar to the squeegee used in gravure printing. However, squeegee blades are more flexible and in most cases are composed of a high-density polyurethane elastomer, chosen for their characteristic durability and solvent resistance. Depending on the application, other elastomers and rubbers may also be used.
In flat screen printing, a squeegee sweeps intermittently and repeatedly across a printing screen, forcing a coating of fluid into the pores of the screen and then into contact with the substrate (by virtue of the flexing of the screen). This action necessarily involves forces applied both vertically (usually downwards) and horizontally; therefore, the frictional forces involved require the blades to be wear-resistant.
Because the blade is made of a flexible elastomer, it also bends, which creates an angle of attack with the mesh, typically about 80° from the horizontal.
A properly tensioned screen has a force of a few pounds per inch, usually expressed in Newtons per centimeter, which can resist the force of the scraper. Inks or paints have less force, depending on their viscosity, which contributes to hydraulic activity. Immediately after the drawdown blade passes in the horizontal direction, the screen begins to separate from the substrate, and the coating is sheared from the screen and flows onto the substrate for deposition.
Another tool, called a flood bar, is a common component of automatic flatbed screen printing machines. This thin, wide, and relatively flat metal strip is contacted by the printing screen between squeegee printing passes, filling the open mesh cells with coating material. However, the overflow stroke does not apply enough pressure to bend the screen into contact with the substrate. Therefore, transfer of the coating off the screen does not occur. The overflow stroke helps ensure uniformity in printing or coating applications.
