Why daily care decides print quality
An inkjet label press fires millions of droplets a second through nozzles thinner than a hair. UV ink does not dry in the air, but dust, dried ink at the nozzle edge or a clogged filter is enough to bend or block a jet. The effects look like different problems, but they usually come from the same few causes.
- Missing lines and banding
- Ink dots or smears on the labels
Colour
- Colour drifts during the run
- Repeat orders that no longer match
Printheads
- Clogged or skewed nozzles
- Scratched nozzle plates and shorter head life
Production
- Unplanned stops and reprints
- Waste of label stock and ink
Before you start the press
- Room: check that temperature and humidity are within the range given for your press. Outside it, the ink viscosity changes, colours shift and the heads clog more easily.
- Compressed air: dry, oil-free and at the specified pressure. Water in the air line reaches the machine.
- Around the machine: clean and tidy, with no dust or loose material near the web path.
- Web path: nothing left on it – tape, blades, cloths. Anything that passes under the heads can scratch them.
- Print platform and rollers: free of ink, adhesive and paper scraps, so the material is not marked during printing.
- Waste ink tank: empty it before it is full, and dispose of the waste ink according to local rules.
- Ink system screen: check that the ink temperature and the negative pressure for each colour are within the values set by the manufacturer.
- Nozzle check: print a test pattern and look at every head. Clean at once any head with missing or skewed nozzles.
Cleaning the printheads at the end of the day
Ink left on the nozzle plate overnight is the most common cause of skewed jetting and clogged nozzles. Cleaning takes a few minutes and is done by hand, then by the machine.
- Dip a clean swab in the cleaning fluid specified for the ink. Never wipe the nozzle plate with a swab that is already dirty.
- Wipe the nozzle plate gently, in one direction only. Wiping back and forth damages the nozzle surface.
- Turn the swab and use the clean side for the next head.
- Use a separate swab for each colour. Mixing colours on one swab can block the channels inside the head.
- When all heads are wiped, run the automatic cleaning cycle to remove the remaining cleaning fluid.
- At the end of the shift, and whenever the press stands idle for more than about 15 minutes, move the heads to the capping position so the nozzles do not dry.
Use only the cleaning fluid approved by the manufacturer. Other solvents can attack the nozzle plate or react with the ink.
When corona treatment leaves deposits on the heads
Corona treatment improves ink adhesion on films, but after long use it can leave fine deposits on the nozzle plates, and the jets start to deviate. These deposits dissolve in pure water, not in the usual cleaning fluid.
- Close the ink valve of the head, so that no water is drawn into the ink tank during circulation.
- Wipe the nozzle plate with a swab dipped in clean, pure water, using the same one-direction method.
- Wipe again with a swab dipped in the cleaning fluid, to remove all water residue.
- Open the ink valve and run the automatic cleaning cycle.
Do not use water as a daily routine. Water mixed with UV ink can form particles that clog the head, so keep it for the cases when the jets deviate.
Every week and every two weeks
| Part | What to do | How often |
|---|---|---|
| LED UV lamps | Clean the lamp windows of dust and ink mist | Weekly |
| Antistatic bar | Remove the accumulated dust | Weekly |
| Dust removal unit | Tear off the used sheet of cleaning paper | Daily, more often with dusty materials |
| Cable chain and ink tubes | Check for twisted cables, worn tubes and leaking connections | Weekly |
| Lifting guides and lead screws | Grease with the specified rail grease | Every two weeks |
| Ink tray guides | Grease with the specified rail grease | Every two weeks |
| Head wiper blade | Replace: the rubber hardens and can scratch the heads | Every two weeks to monthly, original parts only |
Every month
| Part | What to do |
|---|---|
| Printhead ink tubes | Check for swelling, wear and leaks at the connections; replace at the first sign of damage |
| Cooling unit of the LED lamps | Check the coolant level and top up with the specified coolant only, never tap water |
| Printhead filters | Replace on schedule; white ink filters are changed more often than colour filters |
| Waste ink and cleaning materials | Restock swabs, lint-free cloths, gloves and approved cleaning fluid |
Every three months, six months and once a year
| Part | What to do | How often |
|---|---|---|
| Electrical cabinets | Check that the cooling fans run and that no dust has built up on the wiring | Every three months |
| Blower and air ducts | Clean the filter screen; check the duct connections for leaks | Every three months |
| Print unit alignment | Check the mechanical position of the print unit | Every three months |
| Lamp cooling circuit | Replace the coolant to prevent scale | Every three months |
| Main ink tanks | Check for sediment and clean them; let them dry completely before refilling | Every three months |
| Ink supply and circulation filters | Replace according to the manufacturer's interval, by time or by litres of ink used | Monthly to every six months |
| Degassing modules | Replace for colour and white | About every eight months |
| Ink and circulation pumps | Check the ink supply time and listen for unusual noise | Every six months |
| Vacuum pump | Check that the pressure is stable | Every six months |
| UV lamps | Replace the mercury UV lamp after the operating hours set by the manufacturer; LED lamps last much longer | By operating hours |
| Whole press | General service: all the points above, checked together | Once a year |
Why white ink needs more attention
White ink carries heavy pigment that settles when it stands still. Presses built for white keep it moving through a circulation circuit, at a controlled temperature and negative pressure, and degas it before it reaches the heads. The white circuit still needs more frequent filter changes than the colours, and the press should not stand switched off for long periods with white ink in the system.
Common mistakes
- Skipping the nozzle check: a missing line found at the end of a run means the whole run is reprinted.
- Wiping back and forth, or with one swab for all colours: the first scratches the nozzle plate, the second mixes inks inside the head.
- Non-original wipers and cleaning fluid: they cost less, but a scratched head costs far more.
- Tap water in the lamp cooling circuit: scale blocks the channels and the lamps overheat.
- Leaving the heads uncapped during breaks: the nozzles dry within minutes.
- Ignoring the room: a press in a hot, dry or dusty room clogs more often, whatever the maintenance.
Daily checklist
- Room temperature and humidity within range
- Compressed air dry and at pressure
- Web path, platform and rollers clean
- Waste ink tank emptied
- Ink temperature and negative pressure normal
- Nozzle check printed and every head clean
- Dust removal paper changed
- Heads wiped, cleaning cycle run and heads capped at the end of the day
Frequently asked questions
How often should the printheads of a UV label press be cleaned?
Wipe the nozzle plates by hand and run the automatic cleaning cycle at the end of every working day, and clean any head at once when the nozzle check shows missing or skewed nozzles. Capping the heads during breaks of more than about 15 minutes keeps them from drying.
Can I use any cleaning fluid on the printheads?
No. Use only the cleaning fluid approved for the ink and the heads. Other solvents can damage the nozzle plate or react with the UV ink. Pure water is used only when corona deposits make the jets deviate, followed by the approved cleaning fluid.
Why does white ink need more maintenance?
White ink carries heavy pigment that settles. It is kept moving in a circulation circuit and degassed, but its filters are replaced more often than those of the colours, and the press should not stand switched off for long periods with white ink in the system.
What causes missing lines on digitally printed labels?
Usually a clogged or skewed nozzle: dried ink on the nozzle plate, dust from the material, corona deposits or a clogged filter. Print a nozzle check, clean the affected head and check the filters if the problem returns.
Does corona treatment affect the printheads?
After long use it can leave fine deposits on the nozzle plates that make the jets deviate. Wipe them off with pure water, then with the approved cleaning fluid, and run the cleaning cycle. Do not use water every day.
Which parts of a label press are replaced on a schedule?
The head wiper blade, the printhead, circulation and supply filters, the degassing modules, the coolant of the lamp cooling circuit, the UV lamps and, over time, the ink pumps. Each has its own interval, given in the manufacturer's maintenance plan.
