Screen printing

Whereas with methods such as offset or gravure printing, two consecutive steps are required – i.e. inking up the printing form and then printing, with screen printing the ink is transferred directly to the printing stock using a doctor blade and a screen. The screen acts as a stencil. A printing screen is made by applying a light-sensitive film to a fine screen fabric (made, for example, from natural or synthetic fibers) in a darkroom, drying it and then exposing and developing an image using the contact process. The exposed sections of the film set and the unexposed sections are washed away. This creates screen areas that are permeable to ink and areas that are impermeable (photo stencil). The printing resolution is largely determined by the fineness of the screen, but the finer the screen, the less ink can be applied for each printing step.

With screen printing, we mainly use solvent-based inks for our films. They have the advantage that the film surface swells reversibly through the solvent, anchoring the ink very effectively.

Screen printing allows more pigment to be applied in a single printing step than any other method – up to a thickness of 0.1 millimeters. Due to the high pigment content and the thickness of the layer than can be applied during each printing step, the print image created is extremely opaque with a high color density. This is important for applications using backlit technology on printed films. In automotive engineering, backlit technology is used to create film components with a day and night design such as dials and heating/ventilation panels. In the field of IT, items such as cell phone keypads and housings, pressure-sensitive mats and membrane switches are backlit in this way.

With all these applications, it is mostly symbols and masks that are printed. The masks stay in black and form the black surround or “background” to set off symbols and make them stand out. Take, for example, the bright figures that stand out clearly on a dial against a black background. This 4-color printing method enables halftones and color and brightness gradations to be reproduced. One exception is components created using backlit technology where the distance between the film surface and the illuminant (an LED, for example) varies so greatly due to the component geometry that the brightness of the transmitted light is non-uniform. In such cases, a gradation screen is printed onto the back of the film to compensate for these differences in brightness.