February '21

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5 2 G R A P H I C S P R O F E B R U A R Y 2 0 2 1 G R A P H I C S - P R O. C O M S I G N A G E & P R I N T I N G T H E D I G I T A L E Y E | S T E P H E N R O M A N I E L L O SCREEN PRINTING AND THE DIGITAL WORKFLOW AN OVERVIEW OF SCREEN PRINTING AND THE PREPARATION OF FILM POSITIVES Y ou are no doubt aware of the many methods for transferring ink to a substrate. The most common ones like offset lithography, inkjet printing, and laser printing are all used to create multi- ple identical images. Serigraphy is among these common printing methods and is arguably the most versatile. In this article, we'll look at the ins and outs of serigra- phy or in the parlance of the work-a-day world, screen printing. A BRIEF HISTORY The first evidence of screen printing ap- peared in China during the Song Dy- nasty (960–1279 AD). It spread to other Asian countries like Japan and Korea where it was developed and refined. Screen printing migrated to Western Europe from Asia sometime in the late 18th century and was slow to gain ac- ceptance until silk mesh became avail- able for trade from Asia. It was used sparingly in the 19th century and took hold a century later as a viable commer- cial form of printing. In 1910, a technique was developed to combine photo-reactive chemicals with the hardening traits of potassium, so- dium, or ammonium chromate and di- chromate chemicals with glues and gela- tin compounds to create emulsions, which led to the development of photo-imaged stencils. The universal acceptance of this method took decades due in part to the toxicity of the materials, though nowadays commercial screen printing uses emul- sions that are far safer, less toxic, and more versatile than the early materials. AUTOMATION Another important milestone worth mentioning is the introduction of the rotatable multicolor garment screen printing machine introduced in the 1960s. The invention of this machine was responsible for the mass commer- cial printing on T-shirts and other gar- ments that currently accounts for over half of the screen printing activity in the United States by machines automating the printing process and enabling full- scale production of single and multi- colored garments (Figure 1). WHERE'S THE SILK? Traditionally, the process was called silk- screen printing because silk was used as the mesh material. Silk was originally used before the invention of polymer because it is a fine mesh material that is resistant to solvent-based inks. These days there is not a lot of silk being used in commercial screen printing. Silk has been replaced by synthetic threads. The most popular meshes in gen- eral usage are made of polyester. Mesh materials made of nylon or even stain- less steel are frequently used for various special applications. Mesh sizes vary depending on required quality, the fin- ished design, and the specific substrate to which the design is printed. THE PROCESS A piece of mesh is tightly stretched over a frame, which is usually made of wood or aluminum. A stencil is applied that blocks off parts of the screen where ink is not wanted. Conversely, the open spaces that are not blocked by the sten- cil are where the ink will appear when printed to the substrate. An emulsion is scooped across the mesh. Once this emulsion has dried, it is exposed to ultraviolet light through a film positive with the design. The ex- Figure 1. A Vastex V200HD rotatable multicolor garment screen printing ma- chine automates the printing process and enables full-scale production of single- and multicolored garments. (All images courtesy Stephen Romaniello)

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