Printing and Finishing - The Graphic Design: Reference and Specification - page 141

Screen printing: A squeegee is pulled across a silk screen to which a stencil has been
applied, forcing the ink onto the surface of the paper or substrate. More labor intensive
than offset, screen printing is used for printing on surfaces not accommodated on an
offset press, such as fabric, industrial papers, acrylic, and metal. The process can be
done by hand or by machine. Halftone screens need to be coarse, ranging from 65 to 85
lpi. (See Chapter 4, Imaging and Color, for more information on halftone screens.) Often
used for printing signage, bottles, garments and other unusual shapes and surfaces.
Screen printing involves forcing ink through a mesh screen onto a substrate beneath the screen. The stenciled area on the
screen acts as a mask, preventing ink from passing through the nonimage areas of the mesh.
Flexography: Web presses are fitted with rubber or soft plastic plates with a raised
impression for printing on unusual surfaces not appropriate for offset printing. Substrates
include kraft paper, tissue, vinyl, and other types of plastic. Flexography is often used for
printing toilet tissue, bread wrappers, plastic bags, and other types of packaging as well
as cartons, shopping, and grocery bags.
Digital: Often referred to as “on-demand" printing because of its quick turnaround time,
digital printing is quickly becoming a viable alternative to traditional offset printing. It is
particularly useful for printing short-run (under 1,000) four-color jobs where set-up time
and charges would make traditional four-color offset cost prohibitive. The digital process
differs from offset in that it is a toner-on-paper process, rather than an ink-on-paper pro-
cess. Because the color sits on top of the paper, it can flake off at folds or spines. Digital
printing also places limitations on sheet sizes, with many presses limited to paper sizes
of 11" × 17" (279.4 × 431.8 mm) or less, as well as the types of papers that can be
used. When using a digital process, check to see what size and basis weight restrictions
may be involved.