Screen Printing

SCREEN PRINTING
Silkscreening is one of the oldest and simplest methods of printing that is available. It also requires the least amount of technical equipment of any process. Although little equipment is necessary, the printing process can still be made more efficient and precise with the innovations in the silkscreen industry that have occurred over the last 50 years.

Silkscreening is essentially stencil printing. The process is one of squeezing ink through a screen that has been blocked, or masked, to allow the ink to pass through in selected areas. By placing the stencil on a screen, open and closed areas can be more detailed than would be possible with a simple stencil. Also, a simple stencil deteriorates more readily than a silkscreen, which can be used, potentially, for hundreds of printings.

Traditionally, the screens were made from silk, and the surface design was cut from a lacquer sheet and transferred to the screen. Then, with the use of a squeegee, the ink was forced through the areas left open. Consequently, the image was transferred to the paper, or fabric, or object, that was to be printed. A squeegee is essentially a rubber straight edge that allows a clean and even application of the ink.

Over the last 50 years a great many improvements have become available for the screen printer. Different materials are used for the screen: polyester fabrics, woven from strands, have replaced the less reliable and more expensive silks. Images are transferred to the screen with photographic processes. Squeegees have been developed with rubber edges that can be chosen from different hardnesses for different intensities. Inks have been developed that dry faster, apply more easily, clean up with water, and mix to more exact colors. Machines have been developed that make multi-color printing a one step process.

The basis of the silkscreen industry is still, as it has always been, essentially a home industry. Any space can be temporarily converted into a silkscreen studio. Very little is required. Printed T-shirts and baseball caps have become their own industry, and the ability to advertise any immediate event or issue on T-shirts has further developed the use of silkscreen. Silkscreen printing on T-shirts can be a quick response to a situation or issue.

The primary requirements of a silkscreen studio are a table to print on, a space to let the printed items dry, and a sink to wash up afterward. All the improvements that have been developed rely upon these basic requirements.

Contemporary screens are rarely made with silk. Silk has become too expensive, and polyester equivalents are used most commonly. The polyester strands that make up the fabric for printing are either simple woven strands or multitudes of strands twisted together like threads and woven into a fabric. The fabric is woven with different specifications. The holes within the weaving vary in size, and the weight of the threads used in the weaving vary. Consequently, fabrics can be chosen for a wide variety of specific applications. Different meshes work best with certain inks, or certain kinds of material being printed, or different kinds of detail in the imagery being printed. Some polyester filaments are easier to clean; others are designed to hold up under long printing runs.

The frames that silkscreen fabrics are stretched across have become more reliable. Treated hardwood frames no longer warp and bend as easily as they once did. Commercially available screens can be prepared from photo positive films and provided to the silkscreen printer ready to use.

Although many silkscreen printers prefer to have their screens stretched professionally with closely calibrated machines, there are still many printers who prefer to stretch their own screens and apply the image to the screen themselves.

Depending upon the material to be printed and the ink that will be used, a polyester mesh is chosen. Essentially, a lower mesh count is used for fabrics. Higher mesh counts allow less ink to pass through but create a more exacting image. (In this catalog there is a chart that details the characteristics of different silkscreen fabrics.) The material used in the silkscreen can be bought in rolls. Pre-cut hardwood frames are also commercially available.

Once the material is stretched across the frame, pulled evenly tight and firmly stapled with the staples spaced about an inch apart, the screen is ready for the image.

There are several ways to create an image. Lacquer film can be cut to create the image and is then applied to the screen with a careful and gentle application of lacquer thinner. The lacquer image is adhered to the outside of the screen. (If the film is attached to the inside, the pull of the squeegee would quickly destroy the image.) The image will print in reverse. There are available blockouts that can be used to create an image with direct application. They are brushed onto the outside of the screen. It is difficult to create a specific image using blockouts. The blockouts are a thick liquid that is difficult to use with any accuracy. They are best used to simply create specific areas where the colors do not print for variations in the background, or to print simple color areas. Where the blockout is applied, the inks will not go through the screen.

The most common way to create a screen is with photo emulsion. This process uses a photographic film positive. Sometimes, a double photocopy on film can be used instead of a photographic film positive from a printshop or graphics house. The photo emulsion is applied across the front of the screen in a thin and smooth surface. It is light sensitive. When it dries, a photo positive film, backwards, in full size, is placed against the screen and exposed to a light source. Since it is light sensitive, the emulsion should be kept in darkness until it is being exposed. Exposure can be accomplished with a large light bulb as the light source. After a few minutes of exposure, the screen is washed in water carefully, and the areas that have been covered -- that is, the areas behind the image -- will wash from the mesh. Adhering to the mesh will be a thin covering of the screen where the ink will not pass through. A simple light box, large enough for the screen, is the most effective means of exposing the photo emulsion. After this process, the screen is ready for printing.

The amount of pressure that is applied while running the squeegee across the surface of the screen will influence the amount of ink forced through the screen. A more rigid squeegee will help to create a more precise, defined image. Softer squeegees make thicker applications of ink and more intense color areas when precise and exact detail is less a part of an image than large color areas.

While the movement of the squeegee is still the essential basis of printing, machines have been developed that simplify color registration and exact repeats. T-shirt printing machines are available, still requiring a minimum of space, that hold the different screens used in four-color printing and ensure that each printed shirt will have the image repeated exactly. Printing color areas that are exactly registered with each other is the trickiest part of screen printing. Multicolor printing is done with one color at a time, and each screen must be situated so that it prints precisely in relation to the other colors printed.

Also, all the screens are situated in the machinery so that set-up time is condensed into a single set of activity. This is where the T-shirt printing machines prove their value. The T-shirt machines are oriented toward the small home industry rather than the one-time printers who print a specific project without plans to continue as a business. For the one-time printer, the kitchen table and the clothesline dryer are still the basic essentials of the industry.

The necessary materials and associated machinery for any silkscreen project undertaken as a cottage industry or to develop and replenish industrial requirements are available through this catalog.

Any questions can be answered or problems given solutions by the appropriate department staff at Holden's Screen Supply. Phone or email info@holdensscreen.com for price quotes and information.