Tackiness by degrees
In pad printing, controlling and maintaining the condition of the ink is key to a successful job. Often, other uncontrolled process variables may create what is perceived solely as an ink-related problem. All too often I have seen technicians struggle for hours to solve what they believed to be an ink problem while ignoring other potential problem-causing variables such as machine settings and environmental conditions. This frequently is due to a lack of understanding of the pad-printing process and the role of the ink in the process.
The first thing to understand is that the inks specifically designed for use in pad printing are different from conventional solvent-based, water-based, and UV-curable screen-printing inks. Although pad-printing and screen-printing inks have some similar characteristics, there are also important differences.
Unlike screen-printing inks, pad inks are designed to evaporate quickly, which is a crucial part of the pad-printing process. Also, pad-printing inks are applied in thin films, which causes them to be easily affected by variables such as temperature, humidity, and static.
All pad-printing inks consist of resins, pigments, a vehicle, and sometimes special additives. Resin (or binder) is the portion that forms the ink film. Typical resin systems include epoxy, enamel, vinyl, acrylic, and polyurethane. Pigments are colorants in the form of powders or dyes that are dispersed in the resin system, giving the ink its color and opacity. The vehicle is the solvent (or thinner) used to adjust the viscosity of the resin/pigment mixture and to give the ink its printing characteristics. Special additives will be covered in following sections.
In order for the pad-printing process to work successfully, the ink must be capable of rapidly changing its viscosity to become tacky. The components listed above all work together to produce that "tacky" quality.
Types of pad-printing inks
Several different types of ink are specifically formulated for pad printing, including one-component or "solvent evaporating," two-component or "chemically reactive," baking, oxidizing, and sublimation inks.
One-component inks, or "solvent- evaporating" inks cure by evaporation of a solvent. They do not always require the addition of a catalyst. (Some ink may be used with or without a catalyst.) One-component inks are available in matte and gloss finishes and work well with many plastic substrates.
Two-component inks, also referred to as "chemically reactive" inks, do require the addition of a catalyst prior to printing. The catalysts react chemically with the ink's resins to cure by polymerization. This chemical reaction takes place independent of solvent evaporation over a period of time referred to as the "pot life" of the ink.
When working with two-component inks, it is necessary to add the catalyst in a specific ratio, usually by weight. Adding too much catalyst may significantly reduce pot life, and adding too little catalyst may result in the ink not achieving maximum performance once it is cured. Basically, if the base ink and catalyst are not weighed when mixing, inconsistent adhesion and image durability will result.
Once the pot life has expired, the ink must be replaced. Although it may be tempting to add solvents to achieve a visually acceptable print after the pot life has expired, it is not recommended because the ink may not achieve an optimal level of performance.
Baking inks come in two forms: a standard two-component ink with a different catalyst, or a specially formulated ink designed for use on glass, ceramics, and some metals. As the name implies, baking inks must be cured at elevated temperatures. To determine cure time, remember the higher the temperature, the shorter the drying time. However, high temperatures can cause brittleness, so use a low temperature for inks that need to maintain their flexibility.
Oxidizing inks absorb oxygen from the surrounding environment to polymerize, without the addition of a catalyst. These inks are typically used on flexible, synthetic substrates such as tires and keypads. Due to their slow drying and curing time, the use of oxidizing inks is limited.
Sublimation inks involve a special process whereby the substrate is heated to become porous directly after printing. The dyes in the ink change to a gas when they contact the heated substrate, then they pass into the surface of the substrate actually changing the substrate's color. Once the substrate cools, the ink is permanently sealed into the surface.
The use of sublimation ink is common in computer keyboard printing and other applications where standard two-component inks may lack the required resistance to oils, perspiration, and abrasion. It is important to note that since sublimation inks actually change the color of the substrate, color matching is difficult, and the substrate must be lighter than the desired finished color because color changes to darker substrates may be less evident.
Specialty inks and additives
Other materials such as edible inks, silicone inks, lubricants, resists, conductive materials, adhesives, and specially formulated UV-curable inks may also be pad printed, although applications for these inks are limited. Again, the ability of these inks to rapidly become tacky will determine whether you succeed in efficiently transferring them to the substrate.
Also, there are numerous additives besides solvents and catalysts that may be combined with ink in order to adjust print characteristics and performance. Adhesion modifiers, matting powder, antistatic agents, and flow-control agents are a few examples. However, solvents have the most profound effect on printing characteristics.
The pad-printing process
Pad-printing ink is unique in its ability to rapidly become tacky. Other unique characteristics are found in the size and percentage of the pigments used in the ink formulation. The depth of the etched image for most pad-printing applications is 25 microns (0.001 in.) or less. For this reason, the pigments used for pad-printing inks are smaller than those used in most screen-printing inks. Additionally, since pad printing deposits on average approximately 20% of the ink film thickness that screen-printing inks deposit, pad printing-inks have a higher concentration of pigment in order to achieve acceptable opacity.
Since rapid changes in the tackiness of the ink are necessary, the solvents used for pad-printing inks evaporate faster than the solvents used in screen-printing inks. Inks such as UV-curable and water-based formulations either contain no solvents or have solvents too slow for use in the pad-printing process, making them very difficult or impossible to efficiently transfer.
Now that we've covered the types of pad-printing inks and the role of those inks in the pad-printing process, choosing and correctly preparing the ink is essential in producing a successful print.
Ink manufacturers can recommend which inks may adhere to a given substrate. I said, "may" because not all inks will adhere as recommended all of the time. In order to select an ink, you need to know several things, including exactly what your substrate is and whether or not that substrate requires any pretreatment. You also need to know what colors are required and the degree of abrasion, chemical, and weather resistance the ink must have, as well as its recommended drying and curing schedules.
Whether you're printing on a plastic, metal, glass, painted, or plated substrate, it is most helpful if you can determine what the chemical or trade name is. With the correct information, you and your ink supplier can eliminate a lot of guess work in the ink-selection process.
Once you have chosen the correct ink for your substrate, be sure that you prepare it exactly as recommended by the manufacturer. Most manufacturers provide a technical data sheet detailing which catalysts, additives, and solvents are compatible, as well as the correct ratios for mixing them. Most mixing is done by weight, not by volume, so a good scale is a necessity. I like to use a digital scale capable of weighing to within at least
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