Water based Wonder

This new monthly column is intended to help explain some of the complex issues presented by the widening variety of ink technologies and their applications in wide-format inkjet printing. One very hot area is production flatbed printing aimed at the applications currently served largely by traditional analog printing.

WATER-BASED INKS ONTO PLASTIC BOARD?
The overwhelming majority of the flatbed printing systems offered today use UV-curable inkjet inks. A few use solvent or mild-solvent inks. Only one, the HP Scitex FB 6700, uses water-based inks.

The large, industrial flatbed digital inkjet press is aimed at the production printing of P.O.P./P.O.S. displays, exhibition stands and short-run packaging materials. It is a fully automated production printing system requiring only one operator and offering production speeds of up to 150 square meters per hour (1,615 square feet per hour).

The FB 6700 uses water-based inks, and, according to HP/Scitex, can not only print onto corrugated and compressed cardboard materials, but also onto polymer-based sheet materials such as foam board, polypropylene, foam PVC and other rigid substrates. So, how does HP Scitex achieve good results with water-based inks for these applications?

The secret is the combination of a pretreatment spray specifically designed for the uncoated substrate (one for paper-based packaging materials and another for vinyl surfaces), and a post-printing oven that bakes the board after printing.

Just prior to inkjet printing, the spray is applied. It controls the dot gain and bleed on paper or vinyl board while holding the colorant near the surface.

In combination with the ink and subsequent baking, the two materials provide an excellent color gamut at up to 600 dpi with good UV protection for up to two years (accelerated outdoors tests) and water and abrasion resistance. An inline laminator is available for increased durability and some packaging applications.

WHY WATER-BASED INKS?
So why water based inks and not UV-curable inks for this job?

The answer is easy: color, cost and ease of lamination. The ingredient cost for typical water-based pigmented ink is between 25 and 35 percent the cost of UV-curable inks.

Remember, water-based ink is mostly water, while UV-curable ink is all solids when cured and requires the use of some very expensive ingredients.

This enables a digital production press to produce printed corrugated sheet with an ink cost of about $1 per square meter. That’s less than half the cost of using UV-curing printers at equivalent ink coverage. The dried ink is a thin layer of pigment and binder and can give a larger color gamut, particularly in the reds, than typical UV-curable inks on the same substrate (see gamut chart).

For packaging applications, UV-curable inks may create an unacceptable amount of relief on the surface of the substrate and make lamination difficult.

From an environmental perspective, water-based inks offer further advantages. With the exception of a relatively small amount of glycol, similar to those used in desktop printer inks, the effluent from the press is only water.

With UV-curable inkjet printing, both ozone from the UV lamp and some un-reacted volatile materials from the ink are released and should be exhausted to avoid exceeding OSHA limits on exposure to these dangerous materials.

In addition, water-based inks and pretreatment materials can be handled easily, are non-hazardous, and are not flammable like solvent and light-solvent inks.

UV-curable inks must be handled very carefully so that they are not exposed to UV light prematurely. If spilled on the skin they can cause serious skin irritation. Some people also become hypersensitive to the ingredients in UV-curable ink, and react very severely to exposure to extremely small amounts in the air.

So, in these specific applications, the HP Scitex FB 6700 offers a very attractive production printing process with some significant advantages over UV-cure and solvent ink alternatives.

Keep in mind the limitations on the substrates that can be used and the applications for which it is not suited. The system does not print on glass, tiles or flexible materials. Use your UV-curing or solvent-based inkjet system for those.