Sawtoothing, Pin Holes & Breakdown - A Practical Guide to Prevention

Sawtoothing, Pin Holes & Breakdown
A Practical Guide to Prevention
By Geoff Baxter

It never ceases to amaze me that the most neglected item in nearly all screen printing facilities is the screen itself.

Many of us have come to accept poor screens as just one more unfortunate, unavoidable fact of life. These screen problems ruin garments and steal valuable production time, but they are not a necessary evil. Screen printers have developed the unfortunate habit of treating the symptom rather than eliminating the cause. Instead, we go for the quick fix. Loose screen? No problem. Just use three inches of off-contact.

As we discuss each of these common problems, we will break them down into control-point variables, seeking out and attacking the causes.

Sawtoothing/Stairstepping is the loss of image definition, especially on the edges of curves and circles. This problem is usually more prevalent in the coarser mesh counts (110 and lower), and it can be caused by:

1. Film positives of insufficient density. If even small quantities of light are transmitted through the blackened area of the film, light undercutting may result, adversely affecting the quality of the image. Films should read at least 2.0 on a standard transmission densitometer.
2. Insufficient Emulsion Layers. Emulsion should be thick enough to fill all of the open areas between the threads, leaving the garment side of the screen smooth to the touch. Even very small valleys between the threads can cause light undercutting, resulting in sawtoothing and poor edge definition.
3. Inferior Emulsion Quality. All photo screen emulsions are not created equal., As with most products, you get what you pay for. Most of the less expensive emulsions have low solids content. This means that a disproportionate percentage of the emulsion is water, and the emulsion is priced lower because water is cheap. As the high proportion of water evaporates, emulsion thickness is drastically reduced. This can result in more undesirable valleys. When coating screens with lower mesh count, you should use an emulsion with a solids content of 25% to 30% or more. Most dual-sensitized products fall into this range. The new SBQ photo polymer emulsions have solids contents as high as 40% to 50%, and are exceptionally well-suited for this application.

Pinholes are probably one of the most costly and troublesome problems. However, like sawtoothing, pinholes are not inevitable. They are usually the result of one or more of the following:

1. Poor housekeeping. It's important to keep the entire printing facility clean, and it's critical for screen coating and drying rooms to be free of all dust, dirt, lint, and other contaminates. To minimize airborne particles, the floors and walls should be sealed with enamel paint. The addition of an electrostatic air cleaner to these areas can also be beneficial.
2. Improper pre-treatment of mesh. The mesh must be free of oil and other contaminates in order to promote proper emulsion adhesion. There are several good screen degreasers on the market, and one should be used before each coating. It is important to use a product that is specifically formulated for this purpose. Avoid household detergents, which may contain dyes, fragrances, and abrasives that can introduce contaminates.
3. Inferior coating technique. Coating a screen too quickly can also create pinholes. If the scoop coater passes over the mesh too rapidly, bubbles can be churned into the emulsion. These bubbles are then transferred to the mesh where they burst, leaving pinholes. Coatings must be smooth and even.
4. Inappropriate drying technique. Avoid using fans on wet screens to speed drying. Fans pick up dust and other airborne contaminates and blow them onto the wet emulsion. Every loose particle can result in a pinhole.

Premature Breakdown: Screen breakdown is probably the single most common and costly problem in every print shop. Properly prepared screens can remain trouble-free for more than 25 000 impressions. On-press breakdown can be the result of one or more of the following:

1. Excessive humidity. All screen emulsions contain water, and a certain percentage of this water must be removed in order for cross-linking (polymerization) to take place. During exposure, excess humidity in the emulsion film will hinder the evaporation process and result in pinholes and premature breakdown. For optimum results, most emulsion manufacturers recommend a relative humidity around 40%. When necessary, dehumidifiers should be used to control humidity. The addition of an inexpensive hygrometer will help determine if, and to what extent, humidity is a problem.
2. Underexposure. One of the most common causes of screen deterioration is inadequate exposure. The emulsion coating must be subjected to a sufficient amount of actinic light to cause the molecules to cross-link . If the emulsion is underexposed, there may not be an adequate bond between the stencil and the mesh, reducing screen longevity. An exposure-calculation test should be run and logged for each mesh count used. Excessive slime on the inside of an exposed screen during washout is a sure sign of underexposure.
3. Improper emulsion selection. It's possible to use a high-quality emulsion and still experience breakdown problems. The emulsion's chemistry must be compatible with the chemistry of the ink. Plastisols are quite forgiving, and work well with most types of emulsion. However, water-based and solvent-based inks require specialized stencils for maximum lifespan.

These screen-related problems can be annoying, but the true consequence is not the annoyance, or even the cost of the lost screen. It is the easily avoidable loss of productivity and revenue.

When it comes to properly processing screens, I've lost count of the number of times I've heard the phrase, "But we just didn't have time." Yet the cost of time-saving shortcuts far exceeds the cost of doing it right the first time.

GLOSSARY

Actinic Light: Light that produces a chemical change in photographic emulsions.

Diffused Light Source: A multi-point light source like that found in fluorescent-tube exposure tables.

Dual Sensitized Emulsion: A screen emulsion formulated with standard Diazo and photo polymer.

Emulsion: A stable mixture of two or more immiscible liquids held in suspension by small percentages of emulsifiers. It also refers to the part of the stencil embedded in the fabric.

Nanometer: One billionth of a meter. Expressed here as a measure of electromagnetic waves.

Polymerization: A chemical reaction, usually carried out with a heat or light catalyst, in which a large number of simple molecules combine to form a macromolecule chain.

Relative Humidity: A measure of the amount of water vapor in the air, expressed as a percentage of the maximum amount the air could hold at a given temperature.

© 2004 Geoffrey Baxter Duplication without written consent of the author is prohibited.

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