Clogged nozzles on a UV printer: nozzle check, cleaning, prevention

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Clogged nozzles are the most common maintenance topic in UV printing. Unlike normal ink, UV ink does not dry in the air, but it cures under UV light. And once it has cured, it can hardly be dissolved again. Prevention is therefore more important here than with almost any other printer.

Reading the nozzle check

The nozzle check prints a pattern of fine lines for each colour channel. Each line stands for one nozzle.

It is best to print the test on a material on which all colours are clearly visible. White only shows on a dark surface or on transparent film in front of a dark background.

Using cleaning cycles correctly

Almost all machines have several cleaning levels, for example light, medium and strong. With each cleaning, ink is sucked or pushed through the nozzles and the nozzle plate is wiped.

1. Start with a light or normal cleaning
2. Then print another nozzle check
3. Only move to the next level if needed
4. After two to three unsuccessful cleanings, take a break instead of carrying on

Strong cleanings use a lot of ink and fill the waste ink tank. eufyMake, for example, recommends doing up to three normal cleanings and using the deep clean only when really necessary. Sometimes a clog also clears if the machine rests for a while and is then cleaned again.

Manual cleaning: only following the manufacturer’s instructions

If the automatic cycles do not help, what remains is manual cleaning of the nozzle plate, wiper and capping station. Caution is called for here:

• Use only the cleaning fluid approved by the manufacturer. The wrong products can damage the nozzle plate or seals.
• Never rub the nozzle plate with pressure; work only gently with lint-free swabs or foam sticks, as described in the instructions.
• Do not let any cleaning fluid get into the ink. It can disturb curing.
• For print heads under warranty, ask support first.

A damaged print head is the most expensive spare part on the machine. Better one cleaning too few than a wrong one.

White ink: the main cause

Most clogs affect the white. White UV ink contains titanium dioxide, a heavy pigment that settles quickly. A thick sediment then forms at the bottom of cartridges, lines and dampers and clogs the nozzles.

• Shake white ink regularly according to the manufacturer’s instructions. For longer breaks, Mimaki recommends shaking white and silver ink at least once a week.
• Leave machines with circulation switched on for white so that it can work
• Before the actual print, print a small white area on waste material

Varnish is also more viscous than colour ink and more prone to clogging.

Stray light: UV light cures ink on the nozzle plate

The UV lamp sits right next to the print head. If its light is reflected back from the object or the print table, it can cure the ink on the nozzle plate. This happens above all with:

• reflective surfaces such as mirror acrylic, chrome or polished metal
• light or shiny jigs and exposed table surfaces
• a large distance between head and object

Trade media and manufacturers therefore recommend dark, matt jigs, as little exposed table surface as possible and a small print gap. As a guide, Mimaki gives a distance of under 1.5 mm between the lowest and highest point. Fix reflective parts on a dark, flat base beforehand. More on jigs under Positioning precisely.

Sunlight also contains UV. A UV printer does not belong by the window.

Downtime and holidays

UV printers do not like long breaks. The ink settles, residue collects at the capping station, and stray light can cure ink.

• Leave the machine connected to power and switched on if the manufacturer provides for this. Many machines then clean and circulate automatically.
• Run a nozzle check or a small print at least once a week
• Before a holiday, clean the capping station and wiper, and check the ink levels and waste ink tank
• For longer breaks, follow the manufacturer’s storage instructions. Some machines have a dedicated mode in which the lines are flushed.

Room climate

UV ink reacts to temperature. In cold rooms it becomes more viscous, and in warm rooms or in the sun it ages faster. eufyMake, for example, gives 18 to 22 °C as a favourable range. Dust settles on the nozzle plate and deflects droplets. A clean room without draughts and with a stable temperature helps noticeably. For storage, ink manufacturers usually give a range of about 5 to 30 °C; the exact values are in the data sheet.

For business use

A daily routine takes five minutes and saves misprints: nozzle check in the morning, print white through briefly, check the capping station regularly. Include cleaning ink and wear parts such as wipers and caps in your running costs. If you only print sporadically, you often use more ink for cleaning than for jobs.

Quick checklist

1. Nozzle check printed and read to see which channel is affected?
2. Started with a light cleaning and tested again afterwards?
3. White ink shaken or circulation active?
4. No reflective surfaces or light jigs under the head?
5. Capping station and wiper clean?
6. Machine not by the window, room not too cold or too warm?
7. Printed briefly at regular intervals during longer downtime?
8. After several unsuccessful cleanings: break, then manufacturer’s instructions or support.

Continue with: Maintenance of UV printers and Streaks in UV printing.