Why white UV ink settles, and what to do about it

White is the channel that makes UV printing worth buying — it is what lets the same artwork work on a clear acrylic sheet and a black phone case. It is also the channel that blocks, settles and goes patchy while cyan behaves perfectly. There is one number behind all of that.

The number that explains everything: 4.2 to 1

White UV ink gets its opacity from titanium dioxide. Titanium dioxide has a density of about 4.23 g/cm³ (CRC Handbook, via PubChem). The liquid carrying it — an acrylate monomer such as HDDA, a standard UV inkjet vehicle — sits at about 1.01 g/cm³.

The white pigment is roughly 4.2 times denser than the fluid holding it up. Cyan, magenta and yellow use organic pigments that are close to the density of the vehicle. They stay suspended. White is trying to sink from the moment it is made.

That is the entire mechanism. Not ink quality, not brand, not the weather. Gravity acting on a density difference of four to one.

Why white is also the hardest colour to cure

The same pigment causes a second, separate problem. GEW, who make curing systems, note that UV formulations containing titanium dioxide “tend to cure better with gallium doped lamps”, because “titanium dioxide additives generally do not absorb light above 380 nm.”

White pigment is opaque — that is its job — so it scatters and blocks the very light meant to cure the ink underneath it. Research presented at RadTech describes the compounding effect in pigmented systems: “light penetration is significantly reduced”, which in turn worsens oxygen inhibition at the surface. Two mechanisms stacking, not one.

Practically: a white underbase is the layer most likely to be under-cured, and under-cure is what you feel later as poor adhesion. If white jobs are the ones failing your tape test, read the adhesion article and how curing actually works.

Does your head move the ink, or does it just sit there?

This is where a maintenance fact turns into a buying criterion. Some printheads circulate ink continuously through the nozzle chamber. Others hold a static column of it.

Read what the manufacturers say they built recirculation for:

• Ricoh MH5421F — recirculation benefits “rapid-setting inks (pigment, white, metallic materials)” and provides “self-recovery from jetting failure.”
• Ricoh TH6310F — “reduces the risk of missing jets due to nozzle dryness and ink particle sedimentation.”
• Konica KM1024i — “capable of ink circulation, making it possible to also use ink prone to precipitation.”
• Konica KM1800i — “suitable for use with recirculating ink systems for example for white or metallic inks.”
• Toshiba CF1 / CF3 — “Ink is recirculated through all channels, preventing sedimentation and reducing nozzle blockages drastically”; “Suitable for jetting ink with larger particle size and higher gravity”; “Stable start-up after long downtime.”
• Kyocera KJ4B-EX600-RC — “inhibiting ink sedimentation.”

Six manufacturers, independently, describing the same feature and naming white and sedimentation as the reason it exists.

Now the part that matters when you are choosing a machine: the Ricoh Gen5 and the Epson i3200 — the two most common heads in this market — do not recirculate. That does not make them bad heads; they are excellent heads. It means that on those machines, everything recirculation would have done for your white ink has to be done by the daily routine instead. Which head a given machine takes is on its specification page, and the printhead guide explains the families.

The daily routine for white

Roland publishes this as an instruction rather than a suggestion: shake the white and metallic inks “once at the start of daily operations”, because “if they are allowed to stand, the precipitates in them may solidify and cause clogging of the print heads or other malfunction.”

Note the word solidify. Settled pigment that stays settled stops being a suspension problem and becomes a solid one — and a solid in an ink line is no longer something agitation fixes.

What happens if white sits for a week

The same two rules that govern any idle UV machine apply hardest to the white channel. Roland: “We recommend using the machine at least once a week. If left unused for a long period of time, the ink may settle, resulting in unstable discharge.” And: “Switch the sub power on at least every 2 weeks… If the machine is not used for a long period of time, print heads may be damaged.”

A shop that runs white every day rarely has white problems. A shop that runs a white job once a month has them constantly. If your white work is genuinely occasional, that is worth saying out loud when you choose a machine — it argues for a recirculating head, or for a disciplined weekly run even when there is no job.

The full idle-period checklist is in the maintenance schedule.

White in the RIP is not a colour

One last thing that catches people. White is not managed like CMYK. It is a spot channel, switched on and positioned by the RIP — underprint beneath the colour on dark or clear material, overprint behind it for a back-lit or reverse-printed job, and sometimes both.

The two settings that cause most reprints are the choke — pulling the white slightly inside the colour edge so a hair of misregistration does not show as a white halo — and whether the white layer prints in the same pass or a separate one. Both are RIP-specific and both are worth having your operator demonstrate at a demo, on your own artwork, rather than discovering on a live job.

What we are not telling you

You will read that white ink costs two to three times CMYK. It may well. We found no price evidence for it, so we are not repeating it — ask your ink supplier for a same-brand, same-size quote for white and for cyan and you will have a real number for your own payback calculation.

Likewise, published figures for TiO₂ particle size in white inkjet ink, and claims that white has a shorter shelf life than colour, could not be verified against any manufacturer datasheet. Plausible, widely repeated, unsourced.

Sources

• CRC Handbook of Chemistry and Physics via PubChem CID 26042 — titanium dioxide density
• US DOE PAC database via PubChem CID 25644 — HDDA monomer density
• GEW — titanium dioxide absorption above 380 nm, gallium-doped lamps
• Hoyle, RadTech conference proceedings — light penetration and oxygen inhibition in pigmented systems
• Ricoh, Konica Minolta, Toshiba Tec and Kyocera printhead datasheets — ink recirculation and sedimentation
• Roland DG user documentation — daily agitation of white and metallic ink, idle-period rules
• Digital Output — titanium dioxide density in white ink (trade press corroboration)