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White With Envy

(February 2005) posted on Mon Feb 07, 2005

Is white ink a business necessity, a niche market, or both?

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By Peggy Middendorf

The molecule itself: The titanium dioxide particles that comprise
white ink are quite large, very dense, and abrasive. With
such a heavy molecule, it's difficult to keep it in solution (and
dispersed within that solution) long enough to spray it through
printheads. Part of this issue is not just the size of the particle,
but how much of the pigment is necessary to produce a good
white. To produce an opaque white, more pigment is required
than regular CMYK color, necessitating pigment loads up to four
to five times that of a normal color. More pigment in solution
compounds the problem of a heavy particle.
Because of the particle weight and the heavier pigment load,
the particles tend to settle quickly. Aellora Digital's Mario Carluccio
notes, "They will settle very quickly and form a layer that is
as solid as cement. This will permanently damage the inkjet
printhead and can occur within hours."
Most OEMs have solved this part of the puzzle by designing a
stirring mechanism. The Rho and Eastech printers, for instance,
have recirculating pumps and "stirrers/mixers" within the ink
system to eliminate the settling of the titanium-dioxide molecules.
Aellora says it has solved the problem by creating a suspension
and developing dispersion techniques that don't require
stirring mechanisms.
With this very abrasive and heavy ink going through printheads,
does running white ink translate into more frequent
printheads changes? No, says Durst's Christopher Howard: In
the eight months that the Rho machines have been printing
white, he says, Durst users have not expressed any problems. Of
note is that Durst's white-ink printheads carry the same warranties
as its CMYK printheads.

Opaqueness, workflow, and printer speed: Where white ink
is involved, these three seemingly disparate issues are interconnected.
Let's begin with the question, should UV-curable white
inks be opaque or transparent?

Different jobs require different solutions; when spot white is
required, customers may need either opaque or transparent inks,
while white as an underprint or overprint usually demands opacity.

Does that mean that two inksets are required, one opaque
and one transparent? At present, OEMs with white ink offer only
one inkset"?opaque. That simplifies one problem, but presents
another: How can customers achieve transparent white inks?