Film Positives Adhering to the Stencil
- Post by: Bill Hood
- June 30, 2021
- Comments off
The number one reason that inkjet film ink adheres to the stencil during the exposure process is that the ink and/or the stencil were not dry.
Many screenprinters use inkjet printers to produce film positives that are used to transfer the image from the computer to the stencil. While the inkjet films are less expensive than using a high-quality film from imagesetters and also produce a higher quality than laser printers, they are not without problems in the production of film positives meant for the screenprinting technologies.
Moisture is the Problem
Most Inkjet films use water-based ink, and unless this ink is thoroughly dry, it will stick to the dry emulsion during the exposure process. Even then, if the emulsion is not below 4-percent moisture level, the ink will absorb any moisture in the emulsion causing the ink to stick to the stencil. If either the ink or the emulsion is not completely dry, the film positive will be subject to damage and may be destroyed in the exposure process.
Most of the problems stem from misunderstandings that are perpetuated by uninformed individuals on message boards and social media. The solution of evaporation of moisture relies on an understanding of removing the moisture from the film and stencil. We cannot rely on time, temperature, or the relative humidity levels of the ambient air alone to remove the moisture. Quite simply the moisture must be removed from both the product and the ambient air before drying can occur. Both the inkjet ink and the emulsion are subject to absorbing moisture through capillary action.
If the ink is completely dry it will become wet again if moved into a moist air environment. Normal room air is usually at 72˚F (22˚C) and holds a percentage of moisture. The ink on the film may have been dry when taken from the air-conditioned room of the art department, but absorb moisture from the air at the exposure unit. This is just one more reason why many air-condition their screenmaking areas. Likewise, if the ink is dry and then placed onto an emulsion that contains moisture it will draw the moisture from the emulsion and become wet again. Unfortunately, humans are unable to properly determine the level of moisture that affects both the inkjet ink and the emulsion. To determine the moisture level a moisture meter is required.
Drying a stencil overnight does not ensure that the emulsion is dry. The emulsion will only be acclimated to the environment that it is placed in. If the room where it is stored will have a percentage of moisture, as all air does, then the emulsion will likely contain the same percentage of moisture as the ambient air. They may both appear dry to the touch, but not be dry enough to be processed. This can only be measured with a commercial moisture meter designed for use in the screenprinting technologies, such as the TQM Aqua-Meter available from manufacturers such as Murakami, Chromaline, and Saati, for example.
Moisture meters come in three basic types: inductive, capacitive, and high-frequency galvanizing or dielectric constant. The inductive meters are some of the most accurate available. However, they are not inherently suitable for screen printing because they use two sharp probe pins to gather data. A few electromagnetically inductive models do not use pins, but their repeatability is difficult because of the thin nature of what we are measuring.
The capacitive models can use pins or contact pads but are more easily fooled by high-moisture surroundings in the screen room or galvanic charge differences, which can be water-borne, mineral-based or ionic. A relatively accurate and more expensive system is the high-frequency galvanizing method (sometimes called dielectric constant type). It uses the galvanic charges, which are problematic with the capacitive models as a data source. Galvanic charges change as the water that connects the ions dissipates.
If your screen-room humidity is not well controlled, these tools can work well in place of extreme and expensive measures to control the relative humidity in large spaces. A smaller, well-controlled emulsion drying and staging area, along with moisture reading tools, can work wonders in trimming drying time from the process. Also, do yourself a favor and purchase a hygrometer to measure the relative humidity (Rh%) in your screenmaking areas. In addition, a dehumidifier or air-conditioned room will achieve the best drying conditions.
Inkjet Ink Moisture
Modern inkjet inks are water-based and need time to allow for the evaporation of the water in order to dry, just like an emulsion. And, like emulsion, the nonporous inkjet film can absorb moisture through capillary action, so if the emulsion is retaining any moisture, the ink can re-wet.
As stated, the moisture in a stencil is not apparent to humans, but when ink re-wets and begins to bleed the results will be apparent. Inkjet ink manufacturers have a need to balance the maintenance of the ink nozzle and the desire of the user to have an ink that dries fast after printing. The inkjet manufacturers add glycol or glycerol humectants to the water-based ink to prevent nozzle blockage, retard the drying time, and to suspend the pigments. These also slow the evaporation of the inkjet ink once printed.
Inkjet printers were originally designed to print color images onto papers and allow the ink to dry over a period of days or even weeks – in the instance of heavier papers. When the use of inkjet printers became common in the screenprinting technologies, there was an assumption that the drying time would be satisfactory. However, today, many screenprint artists will output the films and attempt to use them within minutes, creating the problem of the ink either not being dry enough or re-wetting when placed into contact with the damp emulsion.
The heat from the exposure unit will also affect the ink. If the ink is not completely dry, the moisture and glycol will turn to steam and create problems. The bottom line is that you need a film positive with completely dry inkjet ink and a thoroughly dry stencil or the moisture in either or the heat from the exposure unit may cause re-wetting or damage your positive when the ink is pulled away from the film.