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Ink Management and Stencil Processing

(November 2001) posted on Mon Nov 05, 2001

Problems and solutions

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By Carol Swift, Peter Kiddell

Companies that are slightly larger or more sophisticated should mix colors from color-matching ink systems, the most economical and practical option. You must mix by weight since the pigments used for each base color have a different specific gravity. A 50/50 mix by volume would give you a different and unpredictable color than 50/50 by weight, which is why ink manufacturers specify their mixing ratios by weight.

Working out mileage (the amount of ink you need for a job), can be challenging but there are methodologies available from ink suppliers and trade organizations. Most ink-management software programs will automatically provide estimates for required ink volume when the user provides certain job specifications. Additionally, they often will allow you to work ink leftovers into new formulations to avoid waste. Many also support color-measurement tools, such as spectrophotometers and store data on color matching.

When you color match based on actual measured color values, you are at the highest level of ink and color management. But at this stage, you must have high levels of skill and be extremely organized.

One type of equipment that is becoming increasingly popular to assist in ink management is ink mixing and dispensing systems. To make full use of these systems, however, you need to be a large user of one ink type. P-O-P and CD printers often fall into this category since ink manufacturers offer inks that are designed specifically for their applications.

Fully automatic mixing and dispensing systems generally consist of a computer-driven system with a graphical user interface, a database of recipes, and a storage system for the inks. Recipes are generally based on common matching systems such as Pantone. No matter what mixing system or technique you use, inks should always be press ready after mixing. Press ready means that no other ingredients are necessary to make the ink perform properly on the press. If an ink needs modifiers, these must be added by appropriate weight as the color is being mixed in the inkroom. If you leave it to the press operator, you will likely be back to the squirt, glug, and dollop system of measurement.

Ink is one of the greatest variables in screen printing. Tighten up the management of ink, and you are bound to increase profitability.


Is your stencil dry enough?

We have taken you into the nightmare world of the stencilmaking area before. Bowed frames, baggy mesh, and water everywhere. The moisture is insidious. You need water to reclaim mesh, degrease the mesh, and develop the image. However, you do not want it in the emulsion just before it is exposed. A "dried" emulsion that still contains high moisture content will not react fully to the UV light used for stencil exposure. If there is water present in the emulsion, the film positive will stick to the emulsion or the emulsion will stick to the glass of the exposure unit.

Later, when you wash out the image, some of the emulsion outside the image area will also wash away. And when it's time to print with the stencil, it will break down wherever there was water present during exposure.

You are also likely to find excessive pinholes. The sensitizer in the emulsion reacting with the water rather than the emulsion itself leads to all of these problems. And the problems only become more pronounced if you are using a fast-exposing emulsion, such as the variety typically chosen for projection exposure.

So what do you do? First, check the moisture content of the dried emulsion prior to exposure. You can do this with a contact moisture meter. These are inexpensive and very easy to use. The moisture reading should be less than 4% to create a robust stencil. Anything above 6% will make it impossible to expose and cure the emulsion fully, no matter how long you allow for exposure.

If you cannot get moisture content below 6% you have a serious fault in your process. The possible causes include high humidity in the screenmaking area, which itself may be related to weather, overspray from cleaning and reclaiming booths, and similar wet conditions. You need to isolate the drying and exposure process from water, so it should be separate from cleaning/reclaiming areas. And if you are in a high-humidity region, use a dehumidifier. These measures will help you get stencils that perform the way you expect.


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