Screen Printing

A form of printing in which a thick paste ink is forced through a stencil attached to a finely-woven mesh screen, transferring ink to the desired substrate in those areas not covered by the stencil. Screen printing, also known as screen process and serigraphy, is used to print almost any surface imaginable, from T-shirts and other textiles to posters to signs to tablecloths to shower curtains to leather goods.... At one time called silkscreening (and, more obscurely, "fabritecture" and "mitography"), as the screen through which the ink was forced was made of silk, the term is no longer in use in the screen printing industry.

Screen printing derives from older practices of stencil printing, such as the pochoir method of stencilling. Essentially, a design is cut or punched into a sheet of paper, metal, or other material, and ink is applied with a brush to the surface of the stencil, where it passes through the openings onto the desired surface. Stencil printing was popular in Japan until the early nineteenth century, and early forms of European stencil printing date back at least to A.D. 1500, in particular, playing cards. (Just about every printing method had its origins in the printing of playing cards, perhaps indicative of the emphasis that has always been—and probably always will be—placed on that particular form of diversion.) The screen process as we now know it was devised in Germany and Scotland in the late 1800s. In 1907, Englishman Samuel Simon created a process of screen printing utilizing a cut stencil of the intended design mounted on a finely-woven silk screen, a brush being used to force the ink through the screen in the cut-out areas of the stencil. In 1914, American John Pilsworth devised a system for the silkscreening of banners, and until the late 1930s screen printing was used primarily for commercial purposes, eventually becoming favored among artists. Screen printing today still retains the simplicity of earlier forms of it; a hand-cut or photographically-produced stencil is mounted on the bottom of a fine mesh screen, which has been stretched taut on a wooden frame. A fairly viscous ink (formerly paint) is placed on the screen, a glue is applied to the non-printing areas beyond the edge of the stencil, and a squeegee pulls the ink down over the stencil, where it prints the design onto the intended substrate. Despite its simplicity and compatibility with a virtually unlimited variety of substrates, screen printing cannot economically compete with offset lithography, gravure, or flexography in the high-speed, high-volume printing of books, newspapers, etc.

The screen printing apparatus contains several basic elements:

'Screen'. The printing screen is a fine mesh, composed of filaments of natural fibers such as silk or—more commonly—synthetic fibers such as polyester, nylon, etc. In some screen printing applications, a metal screen fabric is used. (See Metal Screen Fabric.) The fibers used for the mesh can either be monofilament or multifilament, which describes whether each fiber is composed of a single thread or several threads tightly braided together, respectively. Multifilaments provide a greater cross-sectional area and allow for a better adhesion of the stencil. They also retain their dimensional stability more easily, ensuring consistent ink transfer over the course of a print run. Monofilaments, although capable of facilitating ink passage through the mesh opening more readily, are smoother than multifilaments and consequently must often undergo roughening in order to properly adhere the stencil. (The surface roughness of a monofilment screen fabric necessary to adhere a stencil material is known as its tooth.)

The mesh number, or the number of mesh apertures in a given unit area or length of fabric, affects the image quality of the print. For fine line art, a finely-woven fabric will hold the stencil better, and pass a fine film of ink. A coarser fabric will pass a thicker layer of ink, and will fail to hold a stencil containing fine detail. Another consideration with respect to mesh number and the size of the mesh aperture is the type of ink to be used. Some inks contain large pigment particles, which may not be able to pass through a finely-woven fabric. An ink that will not dry quickly on the sceeen (causing a problem called clogging) is also necessary.

The screen fabric is stretched across a frame, which can be any of a variety of configurations. The only considerations of the printing frame is that it be large enough to hold the fabric (and accomodate the stencil), be sturdy enough the hold the fabric without warping, and be deep enough to hold the necessary quantity of ink. The frame, as a general rule, should be about 4 inches longer and wider than the stencil it is to accomodate. The amount of tension required varies by fabric type, and each fabric manufacturer has specific recommendations. As a general rule, polyester fabrics are stretched from 1:4% of their original dimensions, nylon from 4:7%, and silk from 3:4%. Mechanical stretching devices can be adjusted to stretch the fabric over the frame with the proper tension, and the fabric is then stapled securely into place. After attaching the fabric to the screen, it must be cleaned and degreased, after which point it is ready to accept a stencil.

'Stencils'. The stencil is a paper or, more commonly, a water-based or lacquer-based emulsion attached to a plastic backing sheet which can be either hand-cut or photographically prepared so as to provide openings in the image areas for the ink to pass, while remaining hard and durable in the non-image areas. Stencils prepared by hand are known as hand-cut stencils, while those produced photographically are called photostencils. Hand-cut stencils are prepared by cutting the emulsion from the backing sheet with a knife. The stencil is then adhered to the screen with water or a special adhesive material. The backing sheet is then removed. A photostencil is composed of a photosensitive emulsion applied to a plastic backing sheet (or to the screen fabric itself) and is exposed via ultraviolet light to a film positive, which hardens the emulsion in the non-image areas, leaving the unexposed image areas unhardened, and capable of being washed away during developing, creating image-area opening in the stencil. There are a variety of means of preparing photostencils. (See Photostencil.)

Once the stencil is applied to the screen fabric and the plastic backing sheet removed, the portions of the screen not covered by the stencil must be masked, either with a paper mask (which is effective only on short-run jobs) or with a liquid blockout solution. Once dry, the screen will now only transfer ink through the openings in the stencil. (See also Stencil.)

'Squeegee'. Printing is accomplished by means of placing a quantity of ink on top of the screen, and forcing the ink through the screen by means of a squeegee, essentially a flexible rubber or plastic blade attached to a handle. The shape, hardness, and chemical composition of the squeegee blade can vary by application. (See Squeegee.)

'Printing'. There are two basic means of screen printing: on-contact printing, in which the screen, stencil, and substrate are in direct contact with each other throughout the press run, and off-contact printing, in which the screen and stencil are only in contact with each other during a printing stroke, directly under the line of the squeegee and the train. A problem with on-contact printing is that the substrate occasionally sticks to the bottom of the screen, and needs to be peeled away after each impression. On occasion, adhesive tapes or vacuum-backed printing beds can be used to prevent sticking.

Prior to a printing stroke, the ink is placed in the inkwell or reservoir, on the top of the screen away from the image area of the stencil. The process of flooding—or applying a flood coat—involves coating the printing area of the screen with a film of ink without making an impression, which can either be accomplished by the squeegee (manually or automatically) or with automatic flood bars. When the substrate is properly positioned, the printing stroke can be made, which essentially involves holding the squeegee at the appropriate squeegee angle and drawing it across the image area with a uniform pressure, speed, and angle. (Since the blade is flexible, its angle—called the angle of attack—will be different than the squeegee angle.) The squeegee should not be stopped until it is well out of the image area of the printing screen.

In multi-color screen printing, like other printing processes, proper registration is important, and any of a variety of means can be employed to ensure that successive colors or images print where they are supposed to, from registration marks to register guides—called lays. Each color requires a separate stencil. Some substrates, especially fabrics, may stretch during printing, and its return to its normal dimensions after each impression may result in some degree of difficulty in replicating the stretch to achieve the proper color fit on successive passes.

After printing, in the process of reclaiming, the stencil and blockout solution can be removed, either with water or with a solvent. The screen must be completely cleaned of ink, blockout solution, and cleaning solvent before it can be reused for a successive print run.

'Inks and Substrates'. As was mentioned several times above, screen printing is suitable for printing on an almost infinite variety of substrates. The most important consideration is ensuring that the ink used is suitable with the surface, both in terms of chemical compatibility and the facilitation of proper drying. Screen printing commonly requires paste inks that are thick and able to print sharply through the screen. They must also perform well under the action of a squeegee. The solvents used should also not be overly volatile, as excessively early evaporation would cause the remaining ink components to clog the screen. Screen inks typically utilize a drying-oil vehicle, although ultraviolet curing inks and other forms of fast-drying inks are making strong inroads in screen printing. Often, in the decoration of fabrics, glassware, and ceramics, heat transfer printing is utilized, which involves screen printing the design onto a decal (in one of a variety of ways; see Decal), then transferring the design (which is composed of sublimable dyes) to the desired end substrate by means of exposing the decal to increased heat and pressure. (See Heat Transfer Printing.)

'Press Configurations'. The most fundamental screen printing press configuration involves what could be described as a clamshell press, in which the printing screen frame is hinged at the back or side to the printing bed, and the screen can be lowered onto the substrate in a manner reminiscent of a clamshell opening and closing. These types of presses are hand-operated, and as a result are only capable of a few impression per hour. Various types of automated screen presses exist. Some presses operature by means of a lever; these are still considered manual presses, but the action of the lever lowers the screen into position and moves the squeegee across it, requiring only one operation per impression. Semi-automatic presses automate the lowering of the screen and the motion of the squeegee, leaving the operator responsible only for the insertion, registration, and removal of the substrate. The automatic movement of the printing screen and squeegee may be set to make an impression at a fixed rate, or only after the operator hits a foot pedal. Some screen presses are also fully automated, from feeding to printing to delivery.

Some presses have been specially designed to print directly on three-dimensional objects (called geometric printing), such as bottles, cans, balls, etc., and several press designs have arisen to handle these objects, such as radius printing, for example. Wallpaper and other forms of continuous images are printed by rotary screen printing, or printing utilizing seamless screen cylinders. In these presses, the squeegee dwells inside the cylinder. For multi-color printing, a carousel press is commonly employed. A carousel unit has a printing bed which rotates around a central hub, carrying the substrate beneath a series of printing stations which successively lay down colors.

Screen printing has a strong commercial presence, and as press speeds increase (modern cylinder-based screen presses are capable of 4,000:6,000 impressions per hour) and ink-drying systems decrease the drying time of the inks, the true potential of the process—which in principle has changed very little from the days of silkscreening—can be realized.

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