Industry member working with digital inkjet sign printing, packaging mock-up and prepress proofing applications have long desired white and metallic inks that could truly match what is available using traditional and some other digital printing processes. This has been a difficult technical problem to solve for eco-solventⁱ inkjet system designers. In this article I will explore the reasons for the difficulty and shed some light on a new approach that is making this capability a reality for many print providers.

 

THE PROBLEM

The transparent color pigments used in nearly all inkjet inks are made up of very small particles with a densityⁱ similar to the fluid in which they are suspended. If the pigment particles are properly dispersed the pigments will remain in suspension for years without any agitation. They must be very small to transmit and not scatter the light. If they are not small enough the colors will be muddy and the color gamutⁱ very poor. This is a requirement for the production of process colorⁱ images.

 

On the other hand, unlike traditional translucentⁱ inkjet inks, pigments used for graphics in white and metallic inks must be opaqueⁱ; and the particles must be larger so they canⁱ effectively scatter the light—that’s what makes them work! It’s the ink’s ability to scatter light that makes them opaque. This has been achieved to a good degree in UV-curingⁱ print system, but so far has eluded solvent and eco-solvent type printing systems. If the particles are made small they become translucent.

 

An additional characteristic of pigment particles used in white inks—like the commonly used titanium dioxideⁱ pigment—is that they are very heavy and dense and settle more quickly in the water-like inkjet inks. They behave more like sand in water! So even the best dispersions and the best control of particle size cannot fix this settling problem in the way that was done with the color pigments.

 

White ink is often used to provide a white surface on which color inks can be printed to give good process color images. This may be needed when printing onto a transparent or colored substrateⁱ. To achieve the desired results the inks must have a large amount of pigment compared to the color inks—as much as five times as much. Even at this level it usually takes large drops or multiple passes with small drops to achieve the hiding power required to provide an opaque white surface capable of reflecting enough light to cover another color or provide a good reflector for transparent substrates. This higher loading exacerbates the problem of settling.

 

THE SOLUTION

One approach to solving this problem of ink settling is to provide agitation so that the pigments may be kept in suspension. This can be achieved in the ink tank with an agitator—but what about the printheadⁱ, damper or ink lines leading to the print head? The pigment will settle everywhere in the system if not agitated. If the printer manufacturer does not provide agitation, print reliability will suffer and the system will not be viable as a production printing device.

 

Some have addressed this issue is by flushing the lines, damper and printheads on a regular basis to clean out the settled pigment and maintain reliability. Reliability has still been a problem in some cases, and the process uses a lot of ink—and these inks are expensive.

 

Recently Roland DGA unveiled a new print system that employs both white and silver metallic inks. To address the ink settling issue the new SOLJET PRO III XC-540MT 54” inkjet printerⁱ/cutter incorporates what they call their Advanced Ink Circulation System. According to Roland they prevent the pigment reliability issues from occurring due to the settling of metallic and white pigment inks by providing a recirculation system for the inks. In addition the printer automatically flushes the dampers and printhead as needed without operator intervention. They recommend the ink cartridge be shaken in the morning to re-suspend the pigment particles in the cartridge and it remains viable for the day.

 

CONCLUSIONS