Air and Cavitation in the Ink Supply: Diagnosis and Purging

A printhead that drops a band of nozzles, recovers after a purge, and then fails again in exactly the same place is usually not a printhead problem. On a single-pass ceramic or corrugated machine, the ink supply sits between a bulk tank and a very expensive piece of silicon, and most of what looks like nozzle failure starts upstream: dissolved air coming out of solution, a suction-side leak pulling air in, or a degassing membrane that has stopped doing its job.

This guide is written for maintenance engineers who need to separate those three faults quickly, purge the line properly, and decide which part — if any — actually has to be replaced.

Why Air Faults Get Misdiagnosed as Printhead Failures

Every nozzle on a Xaar 1003 head (1,000 nozzles at 360 npi) is fed from a shared ink chamber. If the ink arriving at that chamber carries free air, the air does not distribute evenly. It collects at the highest point of the local geometry and starves whichever nozzles sit underneath it. The result is a band of missing jets that can be one nozzle wide or twenty, that shifts when the head is purged, and that often returns in the same position once the machine is running again.

So the head gets replaced, the machine prints correctly for a shift, and the fault comes back. Nothing was fixed, because the head was never the source. Before any head is condemned, the ink path has to be excluded — and that is the entire purpose of the sequence below.

How the Ink Path Is Supposed to Behave

From Bulk Tank to Printhead

A typical industrial circuit runs bulk tank → supply pump → filter → degassing membrane → printhead inlet → return or drain. Each stage has one job that matters here. The pump has to move ink without creating low-pressure pockets. The filter has to stop particles without becoming a restriction. The membrane has to remove dissolved gas before the ink reaches the head. And the head inlet has to present ink at a stable, slightly negative meniscus pressure.

What the Degassing Membrane Actually Removes

A Liqui-Cel style membrane contactor is not a filter. It exposes the ink to a large membrane surface on one side and vacuum on the other. Dissolved gas crosses the membrane and is carried away by the vacuum stream. What it removes is gas that is dissolved in the ink — not free bubbles that are already travelling in the line. That distinction decides most diagnoses: if air is entering through a loose fitting downstream of the membrane, no amount of membrane performance will remove it.

Why Single-Pass Machines Are Less Forgiving

A scanning printer revisits the same area many times, so a short air slug often passes before it ruins a print. A single-pass machine gives each nozzle one chance per sheet. On a corrugated line running at production speed, two or three seconds of entrained air becomes metres of waste board. This is why the same air fault that is a nuisance on a scanning machine is a production stoppage on an SCB or single-pass ceramic line.

Three Different Faults, Three Different Signatures

Read the table as a filter, not a diagnosis. Most real events are a combination: a restricted filter raises vacuum at the pump inlet, which releases dissolved air, which then looks like a degassing failure.

Reading the Symptoms Before You Touch a Tool

Nozzle dropout that clears after a purge

If one purge restores the nozzles and the machine then runs clean for a while, the head is probably healthy and the fault is intermittent air. If the nozzles come back but the same ones fail within minutes, suspect continuous ingress rather than a trapped slug.

Foam or bubbles in the filter housing

A steady stream of small bubbles moving with the flow is a suction leak. A single large pocket sitting at the top of the housing that does not move is usually trapped air from a recent filter change that was never bled. Foam — many fine bubbles that persist — points to ink agitation or a degraded defoamer package in the ink, not necessarily to a leak.

Pump sound and gauge behaviour

A diaphragm pump in good condition has a dry, even pulse. A rattle or a metallic tick that follows the stroke rate is the classic cavitation sound. A gauge that swings with each stroke by more than a small margin suggests the pump is working against a restriction rather than against system pressure.

Ink weeping or dried crust at joints

On the pressure side, a leak shows as weeping or a dried ring. On the suction side it usually shows as nothing at all — which is exactly why suction leaks survive for months. Wipe every joint dry, run the machine, and look for the one joint that stays slightly wet.

Step-by-Step Diagnosis

Step 1 — Prove whether the fault follows the ink or the head

Where the machine allows it, move the suspect ink line to a different head position, or move the head to a different line. If the fault moves with the line, stop looking at the head. This single test saves more printheads than any other procedure in this article.

Step 2 — Static leak check on the suction side

With the pump stopped, dry every fitting, tube end and housing joint between the tank and the pump inlet. Start the pump and watch. A joint that becomes damp, or a clear tube that develops a continuous bubble train moving with the flow, is the leak. Where the machine supports it, close the line downstream and hold pressure to see whether the circuit bleeds down.

Step 3 — Filter housing, O-rings and filter seating

After any maintenance, this is the most common single cause of a new air fault. Check that the O-ring is not pinched or flattened, that the housing is torqued evenly, that the element is fully seated rather than sitting a few millimetres high, and that no air is trapped under the top of the housing. Refill slowly — pouring ink in fast is one of the reliable ways to create the problem you are trying to fix. If the trapped air in a continuous supply tube is your specific case, the short note on air trapped in an ink tube covers the cartridge-side version of the same issue.

Step 4 — Pump inlet restriction

A partially clogged filter raises vacuum at the pump inlet. That does two things at once: it promotes cavitation inside the pump, and it drops pressure enough for dissolved air to come out of solution as free bubbles. Compare the gauge reading before and after a filter change; if the reading improves, the filter was the root cause and the air was a symptom. Then inspect the pump for diaphragm fatigue, valve wear and debris held against an inlet valve seat.

Step 5 — Degassing membrane vacuum side

Confirm that vacuum is present and stable at the membrane’s vacuum port, measured against the membrane maker’s specified value rather than a guess — the target vacuum figure for the 3M Liqui-Cel 2×6 SP series contactor is not published on our product page and should be confirmed with our technical team or the membrane datasheet. Check for ink on the vacuum side, which means the membrane has been breached and the part is finished. Check the vacuum line itself for cracks. And check the membrane after a long shutdown: a contactor that has been allowed to dry out may need wetting or replacement before it degasses reliably again.

Step 6 — Printhead inlet, purge and meniscus

Inspect the inlet fitting and the short tube run into the head. Purge at the head using the machine’s own purge routine and the pressure the head manufacturer specifies. Never use shop compressed air to clear an ink line into a printhead; the pressure is unregulated and the air is not clean.

A Practical Purging and Re-Priming Sequence

1. Stop production and park the head at the capping station.
2. Isolate the affected line. Do not purge the whole machine to fix one head.
3. Fix the identified leak first. Purging a leaking line simply pulls more air in behind the slug you are removing.
4. Drain the segment downstream of the filter into a waste container and watch the bubble train. Keep draining until the bubbles stop.
5. Refill slowly, from the bottom of the circuit where the design allows it. Fast filling entrains air.
6. Run the pump at reduced speed with the head valve open, pushing the remaining slug to drain rather than into the head.
7. Close the drain, bring the line up to operating pressure, then purge at the head using the machine routine.
8. Print a nozzle test. If the same nozzles fail twice in the same position after a correct purge, stop. The air is being replaced as fast as you remove it, and the source is still open.

What Not to Do

• Do not raise pump pressure to “push the air through”. Higher flow lowers inlet pressure further, makes cavitation worse, and can damage the head.
• Do not purge with compressed air. Unregulated pressure and oil carry-over are both head killers.
• Do not top up a filter housing quickly from the top. You will trap the air you are trying to remove.
• Do not fit a new printhead before the supply is proven. The new head will fail the same way, and you will have spent the budget twice.
• Do not run with a known suction leak because “it still prints”. The leak pulls air continuously and dries ink at the joint, and the failure it causes will eventually be blamed on the head.

When the Part Is the Cause

Pump

The KNF PL10341 NF60 ink fluid pump is listed with a 0.6 L specification, and the PL9022 NFB60 is listed as a cyclic double ink pump with a 0.6 L specification. Flow rate, pressure range, voltage, wetted materials and diaphragm service life are not published on our product pages and need manual confirmation before you size a replacement against a machine requirement. Replace or rebuild when the rattle persists after a filter change, when the pump cannot hold pressure with the outlet closed, or when the diaphragm has reached the interval your maintenance plan sets. If you are choosing between the two models, the NF60 versus NFB60 comparison sets out the circuit differences.

Degassing Membrane

The 3M Liqui-Cel 2×6 SP series contactor is supplied under OEM references 8QA801380 and 9620530017 for Durst and Sacmi machines. Membrane area, connection size and achievable vacuum are not published on our product page and need manual confirmation. Replace the contactor when vacuum cannot be held at the port, when ink appears on the vacuum side, or when the dropout pattern returns after a complete and correct purge. The G638W replacement guide covers the physical swap and the wetting checks.

Filter

Replace on the interval your ink supplier specifies, and immediately after any air event that pushed debris downstream. A filter that has been run past its interval is the most common hidden cause of pump-inlet cavitation on high-utilisation machines.

What to Record So the Next Fault Is Faster

Keep a one-line entry for every air event: date, machine, head position, the nozzle numbers that dropped, how many purges it took to clear, filter hours, pump hours, membrane install date, ink batch and ambient temperature. Three or four entries are enough to show whether the fault is tied to a filter interval, a shift, a season, or a single line. That record is worth more than any single diagnostic step, because it tells you which of the three faults you are actually dealing with on your machine.

Frequently Asked Questions

How do I tell cavitation from air ingress?

Cavitation is generated inside the pump and is audible — a rattle or tick that follows the stroke. Air ingress is generated at a joint and is visible — bubbles or foam in the housing. If you can hear it, look at the pump inlet. If you can see it, look at the joints.

Can a degassing membrane cause nozzle dropout without any visible bubbles?

Yes, and this is the case most often misread as a weak printhead. Dissolved gas that is not removed comes out of solution at the nozzle plate, where the pressure drop is largest. There is nothing to see in the line, but the nozzle test shows scattered dropout that improves after a long purge and returns after idle time.

Is it safe to keep printing with a small air pocket in the line?

A small pocket that stays in one place and does not grow is often harmless. A pocket that travels with the flow, or that reforms within minutes of a purge, is not — it will reach the head, and on a single-pass machine it will reach it during production.

How often should the filter be changed to avoid cavitation?

Follow the interval your ink supplier specifies for your ink type and solids load. There is no universal figure, and a filter changed on a calendar interval rather than on differential pressure can still be restricted well before the date.

Does a new printhead fix an air problem?

Temporarily, and only because the new head starts with a clean, well-wetted ink chamber. The air source is unchanged, so the fault returns. Prove the supply first.

If you have worked through the six steps and the fault still follows the ink, send us the machine model, the head position and your nozzle test image. We supply the pumps, contactors, filters and printheads for these circuits and can tell you which part the symptom points to. Contact our technical team with the details and we will come back with a specific recommendation rather than a parts list.

Related: If the nozzles stay missing after a purge and the pattern is stable rather than scattered, work through Why Is My Ceramic Printhead Clogging? before replacing hardware.