ScreenprintingToday: The Basics

SCREENS & STENCILS

By AndyMacDougall
©1999MacDougall Screen Printing Ltd.

All rights reserved.
Reprinted with permission

The most important single component of the screen printing process is the screen. This consists of the frame, usually wood or aluminum, and the mesh, a precision woven fabric stretched and attached to the frame. Originally the meshes were silk (hence the name 'silk' screen printing, or the Greek root 'seri' -silk) which gives us serigraph, serigraphy, serigraphics) but in modern usage the screens are made of polyester or metal.

The stencil is a thin material either coated or otherwise stuck to the mesh that carries the image to be printed as an opening in the mesh that ink can be pushed through with a squeegee.

THE FRAME

A good frame has the following characteristics:

• It has to be slim enough to fit in the clamping device of the press system
• Strong enough to withstand the pressures of the stretched fabric
• It should be constructed so water and cleaning do not affect it or interfere with the mesh tension or adhesion.

Frames in general can be divided into 3 types:

WOOD - The original, and still the easiest and cheapest to make. Wood frames are still used heavily in the garment industry, hobby & art printing for small jobs, and in non-critical large format work. Early screen printing with wood frames used staples or cord-in-groove for mounting and stretching the mesh. This does not allow adequate or even tensioning, and is not recommended, because of the problems slack screens create during printing.

METAL - Thin profile aluminum or steel frames are superior to wood, due to their resistance to warping or bowing, their resistance to water, their light weight, and longer usable service life. They are most commonly found in print operations with automated printing equipment or where quality printing is a requirement, and are manufactured to fit common press sizes, or as custom creations to fit specific jobs or formats. Mesh is prestretched and glued to the frame using a special stretching device.

RE-TENSIONABLE - There are a number of systems, notably 'Newman' roller frames and 'Diamond Chase', where the mesh is attached and tightened on the frame itself. It replaces the need for a screen stretching device, which is usually only found in larger shops or as a service from suppliers. It allows a screen which has lost tension after initial use to be re-tensioned, and it gives the individual printer or shop the ability to create a new screen in minutes. This is especially useful in production situations.

THE MESH

Meshes are available in many different types to suit many different applications. Fabric printing requires a coarse weave to allow more ink to pass through. Half-tone or fine detail printing requires a tighter weave to hold stencil detail. Electronic and specialized printing requires a metal or polyester/metal hybrid that can be heated electrically to facilitate ink flow. All meshes have to be able to withstand tensioning, stand up to squeegee action, and be impervious to water and reclaiming chemicals. Monofilament polyester has replaced silk as the most widely used fabric in screen printing today.

MESH COUNT

This is the term used to describe the number of threads per inch. Meshes range from 40 threads per inch (very coarse, with large openings between threads) up to 400 or more threads per inch. European measurement is threads per cm.

WHAT MESH DO I USE?

The printer needs to select the proper mesh for the individual job. Fabric printers might use 80 or 110 for applications where they need to lay down light coloured inks on dark fabrics. As the mesh count rises, the amount of ink flow and deposit decreases. In graphic applications on non-porous materials, less ink is required for coverage, and higher mesh counts of 250-300+allow finer images and details to be printed. Some inks have specific characteristics which put limitations on mesh counts. UV inks require thin ink deposits for proper curing, and won't dry in the screen, so mesh counts above 305 are used. Ceramics printing require the coarse particles of glass frit which make up the ink to pass through the mesh openings, so meshes in the 225-250 range are used. Most manufacturers and suppliers can recommend a specific mesh count that works best with their products.

MESH TENSION

This is probably the MOST IMPORTANT PART OF THE WHOLE SCREEN PRINTING PROCESS, AND CAUSES 95% OF ALL "PRINTING" PROBLEMS EXPERIENCED BY PRINTERS. IRONICALLY, MOST BEGINNERS TRY TO SAVE MONEY BY TENSIONING THEIR HOME MADE FRAMES THEMSELVES.

A hand stretched and stapled screen can only be tensioned to 1 or 2 newtons, unevenly. Compare that to some printers using 40. It is a safe bet the print quality is reflected in the mesh tension. CAN I SAY ENOUGH ABOUT THIS? DO YOU DRIVE AROUND WITH UNDER-INFLATED TIRES? RETENSIONABLE SCREENS IN THE SMALLER SIZES ARE CHEAP! DO YOURSELF A FAVOUR IF YOU HAVE BEEN USING HAND STRETCHED SCREENS!

Limitations on stretching

• If tighter is better, then why not stretch the screens to 50-100 newtons?
• First, the mesh can only take so much tension, and then it rips, delaminates from the frame, or is easily torn during printing.
• The other factor is the frame's ability to resist bowing or warping because of the tension. This problem increases as the size of the frame increases.
• Some meshes (high tension, low elongation) have been developed to counteract the first problem and allow higher tensioned screens, but it is always best to follow the manufacturer's or suppliers' recommendations as to proper mesh tension.

Screen Care

• Once properly tensioned, the mesh should be chemically scrubbed and degreased before use, to allow the stencil to adhere properly.
• When not being used for printing, screens should be stored to avoid being accidentally ripped or damaged. During printing, avoid contact with knives or other sharp objects.
• If screens are properly cared for, they can be used over and over, sometimes for years and thousands of impressions, before requiring replacement.
• The two main reasons for replacement are blocked openings which will not come out, and ripping or delamination of the mesh.
• It's always a good idea to mark the mesh count and age of the screen on the side, and institute a rotation and replacement system to ensure that screens are always in good shape for printing.

THE STENCIL

A good stencil carries an exact duplicate of the image on the film. It should be strong and resistant to inks and the motion of the squeegee and floodbar during printing. The underside of a good stencil should be smooth enough to form a 'gasket' with the substrate, which insures a sharp blur-free image during the print stroke. The stencil must also retain a certain flexibility to resist de-lamination from the mesh during printing.

HAND MADE STENCILS

Knife cut water-based or lacquer-based films, paper stencils, or images painted directly on the screen, are not used in most screen shops anymore. Before the adoption of easy-to-use photostencils in the 1960's & 1970's, when most graphics tended to be bold large signage and equipment was scarce, hand made stencils were common in many printing shops. Now they are rare. There are a number of reasons why:

• They are limited in their ability to reproduce typestyles, or modern commercial graphics. When cut by hand it does not look professional.
• They can't reproduce photographic images, or complicated designs.
• If something happens during the adhesion process or during the print run, the image has to be redone from scratch.
• In the case of fine art serigraphs, hand cut images or brush effects are just as easily rendered on rubylith or mylar, and then exposed. This allows previewing and preregistration of colours, and a chance to do it over again if aren't right. (See previous)
• The main reason for using handcut stencils is lack of an exposing system.

PHOTOSTENCILS

Photostencils are the industry standard, and are available in 3 different types, depending on application:

1. Direct Emulsion, which is coated on the screen with a scoopcoater, and then dried. Once dried and cured, the whole screen is exposed, then washed out with water. This is the cheapest, most economical emulsion, and is available in a number of different types, i.e. general purpose, water resistant, dual-cure for both solvent or waterbase inks, and one-pot polymer based, for high resolution/high speed/low light. Direct emulsions are tough and cleanable compared to other types, but require more preparation time, and a lightproof storage area for unexposed screens. Large shops have automated coaters to regulate and control stencil thickness and quality.
2. Indirect stencil films come on a roll with a very thin clear backing. The stencil is exposed, developed with peroxide, washed out with water, then adhered to the screen.('Novastar' uses no developer) These stencils are quick to use, require a smaller exposing system than direct emulsion, and are good for fine detail printing. Their main drawbacks are an inability to withstand cleaning, lack of durability, unsuitability for waterbase inks, and their high cost compared to direct emulsion.
3. Capillary type stencils are a hybrid of the previous two. They come in a roll like an indirect, but are then adhered unexposed to a wet screen and dried. The entire screen is then exposed like direct emulsion. Cap stencils provide the fine detail resolution of indirect, but are much stronger due to their bonding (capillary action) with the mesh. They are also sold in various microns (measurement of thickness) and produce a precise, even, consistent stencil in a predetermined thickness. This is important in electronics printing, UV, or other applications where ink film thickness is critical. They can be top-coated with direct emulsion before exposure for extra durability.

TIPS & TIDBITS

FRAME SIZE

• Proper frame size is determined by the printed image. The squeegee should have a run-on and run-off area of a minimum 4 inches to the inside of the frame, and side clearance of at least 3 inches from either side of the squeegee to the inside of the frame. An image 11'x15' would require a minimum frame size of 17"x23" I.D. for a problem-free print. Usually, the more space around the print, the easier it is to get a good result. Tightly jammed images on screens too small end up causing production problems.

HALFTONES When printing halftone images, the rule of thumb for determining correct mesh is 3.5 x line count of the halftone equals the minimum mesh count. For example, a 65 line half-tone image would require a 230 mesh screen. The higher the mesh count, the better the reproduction of the image on the stencil and the print and the less chance of moire patterns. Yellow meshes also ensure more exact dot reproduction.

SAWTOOTHING AND SHARP EDGES

• Coarse meshes will cause sawtoothing on the printed image. This is a result of the stencil being unable to bridge the gap between threads. Cappilex and Indirect, because their edge is actually below the mesh, print sharper lines. Photopolymar based emulsions are sharper than diazo or regular emulsions.

OTHER MESH CONSIDERATIONS

• Monofilament meshes clean and reclaim better than multifilament
• Yellow meshes will reduce light bounce from white threads during exposure. White mesh exposes faster than yellow.
• Meshes with the designation 6xx, 10xx, etc., are using the old silk measuring language, which does not readily translate into threads per inch. Under no circumstances use silk.

RIPS & TEARS

• Avoid pressing against the mesh along the frame and glue joint, as this can cause the mesh to delaminate from the frame. The adhesives are very strong side to side, but will give way when pulled up. (shear vs. tear strength)
• Pinholes or small rips in mesh can be carefully taped on both sides. Thin gold mylar decal material makes good patch material, and stencil placement and care can allow the printer to work around a hole and extend the useful life of the screen.
• Don't print over sharp edges of plastic or paper, or you will wear out the stencil, and the screen mesh. Tape over wear points underneath.

MESH OPENING VS. INK FLOW

• The amount of opening between the threads in a screen determines the ink flow through the screen. As mesh counts get higher, the thread diameter also comes into play. New mesh technology has produced finer threads, allowing more opening, even in the higher mesh counts.

CLEANING & DEGREASING

• Whatever the stencil system, the screen must be cleaned and degreased before each use. Failure to do a proper degrease can result in the stencil delaminating or breaking down during the print run. Failure to clean the screen before reuse leaves ghost images and blockages that will "haunt" your next stencil and cause problems in future prints.

CARE AND FEEDING OF YOUR SCOOP COATER

• Scoop coaters, used for coating direct emulsion on screens, must be kept free of dirt and nicks in the metal along the coating edge. Flaws will show on the stencil, and can affect print quality, especially on half-tones.
• It is necessary to experiment with coating techniques to determine a proper combination of coats. Coarse meshes require additional wet-on-dry coats to ensure proper mesh bridging and stencil thickness.

STENCIL BREAKDOWN

The number one problem with stencil breakdown when printing waterbase inks through dual-cure direct emulsions is an improperly dried and cured stencil. Stencils should be force dried with heat and air so that all moisture is removed. Use a temperature/humidity gauge to monitor the screen. Follow the manufacturer's guidelines for optimum temperature and humidity. In general, they require drying to a humidity level less than 50% for at least 3 hours to chemically harden.

So now you know all about screens and mesh and stencils, and you've got the best looking film positive in the world ready to go, what's next? Next we expose ourselves in CHAPTER 4 of Screenprinting Today.