Ink Management and Stencil Processing
Problems and solutions
The amount of ink that many printers waste because of bad organization must bring joy to ink suppliers and sadness to the printers' accountants. Poor ink management and the problems it creates often aren't recognized until a company experiences a change in management or the inkroom staff.
We saw an example of poor ink management in a company where we recently consulted. This company was annually discarding approximately $100,000 in ink. Disposing of this amount of ink as waste represented a severe financial blow to the company.
The problem developed over years for a variety of reasons. Inadequate stock rotation was one cause, while the inability to estimate ink needs properly (mixing 10 gallons when only 5 were needed) was another. Additionally, labels covered in ink made it difficult to find existing batches of mixed colors, and frequent mismatches led to more unusable ink piling up in storage.
The company used both solvent-based and UV inks. Solvent inks with lids tightly closed can have a shelf life of several years if they do not contain a catalyst. UV-curable inks, however, are nowhere near as stable. They must be stored between 50-77°F (10-25°C) and have a shelf life of around 12 months. Storing them at temperatures above 77°C shortens their shelf life. Dropping them below 10°C and approaching freezing can also damage the ink.
Once these problems were discovered, the company began rectifying the situation by checking its store of UV inks. Those that were undated or more than 12 months old were scheduled for disposal. The remaining inks were set aside to be cataloged.
Solvent-based inks were treated in a similar manner; but in their case, mixing dates and storage age were not as important as the condition of the ink. If the lid was not firmly secured, the inks were marked for disposal. There is no point trying to revive ink that has completely lost its solvent balance and formed a thick crust of dried material on the surface.
How can you avoid similar problems in your company? One way is to develop an ink-cataloging system. Create two screens with a 2 x 2-in. square-image area. Use one screen for your solvent-based inks, the other for UV. With a manual print table, print two samples of every ink you have in stock onto small index cards for filing. For each color, dry or cure the prints, label them with the sample date (or better, the date they were mixed) and ink type, then tape one of them to the storage container. You may need to print extra cards if the same color is stored in multiple containers--each must be labeled.
The second card you store in a filing system (one file set for each ink type). It's wise to include a reference to where the ink is stored on these cards. Then file the color samples in spectral order, starting with red and finishing in violet, black, gray shades, and white. You now know what ink type and color every container holds, how old it is, and where it can be found.
As for the ink containers themselves, they must be organized for easy location. Store UV inks with the oldest toward the front of your inkroom shelves. Again, solvent-based inks are not so critical in terms of date. If you can't identify the ink system or its original order date, it's probably best to dispose of it.
The way to avoid this nightmare, however, is to manage the inks correctly in the first place. Even if your shop and ink inventory is small, organized matching, mixing, and storage will save a great deal of money. Always mix your inks by weight, never by "dollop" or "glug." Stir the inks thoroughly and record on the container what it is, how much is contained within, and when you mixed it. Creating a color-swatch index card at this stage can help prevent you from having to do so later.
For very small shops, ink manufacturers can assist in color matching needs by premixing inks. The key to this approach is to inform the supplier what type and color of substrate you will be printing on to ensure that you will get a matched color after printing.
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.