Printing Inks

Inks find use in almost every aspect of human activity. Our inks are used for printing on cosmetic items, toys, mobile phone keypads, glasses, ceramics, metal, thermosets, pens, etc. which are indispensable in our daily life.

Solvent-based inks are predominant in the pad-printing industry. They dry very rapidly through solvent evaporation alone. Care must be taken when printing non-absorbent materials with these inks, as absorbency is required to give best adhesion. They are available in gloss and matte finishes and perform particularly well with various thermoplastic substrates. A simple way to tell if a solvent based ink is suitable for a particular plastic is to rub the solvent, used to dilute the ink, onto the substrate. If the solvent melts the surface of the plastic, chances are the ink will adhere very well.

In Oxidation-drying inks, the resin absorbs oxygen from the atmosphere and undergoes a polymerization process, producing a very tough, flexible, weather-resistant ink film. They have limited uses in pad-printing applications due to their slow drying speed, but they are excellent for printing onto metal and glass.

Two-part or Reactive (catalyst curing) inks, used extensively in pad printing, also contain resins capable of polymerization. However, the required catalyst is either blended into the ink by the manufacturer or supplied separately and mixed in by the printer when required. Either way, these inks have a restricted shelf life once the catalyst is added.

Two-part inks cure very rapidly when heated. They are generally printed on difficult substrates such as metals, some plastics and glass. They are particularly preferred when good chemical and abrasion resistance is required. Care must be taken when mixing the base ink with the catalyst. Manufacturers specify an exact weight to be added, so you must always weigh the components when mixing - no exceptions! Inaccurate mixing can give inconsistent adhesion and product-resistance characteristics.

Another important factor to be aware of with two-component inks is that after printing and prior to complete curing, the temperature of the printed ink film must not drop below 59F (15C). Should this occur, the ink will cease curing and cannot be restated. This may not be a problem if the curing may nearly be complete. However, if curing takes place in storage over a period of time, the ink film becomes vulnerable. A dry ink film is not necessarily cured. It takes time, temperature, or combination of both to affect a complete cure. With two-component inks, curing typically takes five days at 68F (20C) or 10 min at 212F (100C). This information is available on the ink's technical data sheet, which unfortunately, most users do not bother to read.

Baking inks need a certain minimum temperature and time to cure. The cure time varies inversely with the temperature: the higher the temperature, the shorter the drying time. The flexibility of the ink film is another factor to consider with these inks. Ink films that must retain their flexibility require lower temperatures, for a higher temperature can cause brittleness.

Sublimation inks involve a special process where a solid turns into a gas when heated. When these inks are applied to an appropriate surface and then heated to the specified temperature (approximately 392F or 200C), dyes in the ink sublime, the surface of the material becomes porous, and the dyes pass into the material. This actually changes the colour of the base material. Once the material has cooled, the ink is sealed into the surface. Pad printing is a suitable process for sublimation since the ink deposit must be kept to a minimum to prevent colour bleeding. Applications include keyboards and other areas where abrasion resistance is critical. Care must be taken when selecting colours, as certain inks are sensitive to UV light and fade very quickly. The range of colours is very limited, and matching Pantone colours is almost impossible. Another limitation is that the substrate colour must be lighter than that of the ink.

Ceramic and gas thermoplastic inks are used quite successfully in the pad-printing industry. These inks are similar to the ones used in screen printing in that, at ambient temperatures, the ink is solid (like candle wax). It becomes fluid when raised to 176F (80C), which is accomplished in the ink reservoir and clich.

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