You pull a test print and there is a line of ink across the top edge. You lift the carriage cover and the bottom of the printhead is wet. Before you order a new head, do this instead. Wipe the nozzle plate dry with a lint free wipe, park the carriage on the capping station, and leave the machine idle for fifteen or twenty minutes. Then look again.
Wet again? Something upstream is still pushing ink through the head, and in most of the cases I have been called out to, the head was fine. Still dry, with the problem coming back only once the machine starts printing? Then nothing is leaking at all. That ink was sitting in the maintenance station, and the carriage is dragging the printhead through it on every pass.
Those two failures put the same mark on the same sheet of media and they cost very different amounts to fix. One is a rubber part you can change in ten minutes. The other is the single most expensive item on the machine. Here is the order I run them in, cheapest part first.
Two Failures That Wear the Same Face
The service documentation for Epson solvent machines splits a drop of ink on the media into exactly two causes, and that split holds up on Mimaki, Roland and Mutoh hardware just as well. The first cause is a build up of ink on the nose of the printhead. The second is a leak inside the ink delivery system right at the head. The manual names the parts behind the first cause as the cap, the pump, the wiper blade and the wiper blade cleaner. For the second it names the connection between the ink supply tube and the damper, and a damper that has failed on its own.
Read that list again and notice what is missing from it. The printhead is not on the list. A head that has genuinely cracked its nozzle plate will drip, and it does happen. But it is the last item on the list and the most expensive item on the list, so it should be the last thing you test, not the first thing you order.
There is a reason the maintenance parts cause ink on the nose so often. A cap top that does not seal lets the pump pull air instead of pulling ink, so the cleaning cycle removes almost nothing while still wetting the plate. A wiper that has hardened does not wipe, it smears, and it leaves a film that gravity then collects into a bead. Neither part touches the nozzle plate, and both of them leave it wet.
The workshop version of this check takes twenty minutes and no parts. Clean the nozzle plate until a dry wipe comes away clean. Park the carriage. Set a timer and walk away. Come back and press a dry wipe against the plate without wiping sideways, just touch it and look at the cloth.
Moisture on that wipe after a clean park means the ink supply is losing its hold on the meniscus. A dry wipe means the supply is holding, and the ink on your prints came from below the head, which is the maintenance station and the wiper. That single answer decides whether your next purchase is a thirty dollar rubber part or a head.
Read the Timing, Not the Stain
The stain tells you almost nothing. The timing tells you which subsystem to open. I sort the complaint into three buckets before I pick up a screwdriver, and the bucket is usually obvious on the first question.
Dripping while the machine sits idle. Nothing is moving, nothing is firing, and ink is still finding its way out. That is a back pressure failure. The ink column in the supply line is being pushed by gravity instead of being held back by vacuum. Look at damper condition, at the seal on the ink bottle or sub tank, and at anything in the supply chain that was moved, refilled or topped up in the last few days. On UV flatbeds the negative pressure set point and the tube that carries it are the usual suspects, because a loose pneumatic fitting leaks air and the vacuum never reaches the head.
Dripping during printing, worst at the ends of the travel. This one fools people because it looks like the head is over inking. It is not. The maintenance station left a film of ink on the nozzle plate, the head parks and picks up more of it, and the direction change at each end of the carriage stroke is where the film finally lets go. Print a narrow job in the middle of the media and watch where the drips land. If they cluster near the left and right edges of the image and leave the center clean, the maintenance station is your problem and the head is not.
Dripping right after a cleaning cycle. The pump pulled ink up through the head and it did not all get pulled away again. That is a cap that does not seal, a cap whose lip is crusted with dried ink, or a pump pulling too weakly to clear what it lifted. The head is soaked, not broken. Clean the cap lip with fluid and a swab, run one more cycle, and see whether the plate stays dry this time.
Write the bucket down before you order anything. A shop that buys a head for a timing problem spends the money and still has the same drip on Monday morning, and then it buys a cap top as well.
One Channel or Every Channel
Count the wet channels next. This is the fastest way to cut the suspect list in half and it costs nothing but a flashlight and a dry wipe.
One color wet with the rest dry is a local failure. The damper on that channel, the o ring where the tube meets that damper, or the manifold seat underneath it. A single channel that goes wet right after a refill is usually a damper that was pushed in at an angle, or one holding air and letting ink past its membrane. Pull that damper and hold it against a light. A swollen or collapsed membrane, a housing with a hairline crack, or sludge sitting in the bottom of the clear body all mean replace, and replace is cheap.
Every channel wet at once is a system failure, and no amount of damper swapping will fix it. That pattern points at shared hardware. A vacuum pump that cannot hold, a leaking fitting, a solenoid valve that does not close, a pressure sensor reading wrong, or a supply tank sitting too high above the head. On machines with a sub tank, a level set too high does exactly the same thing, because the head now sits below its own ink supply and gravity takes over from the regulator.
There is a middle pattern worth knowing. Two or three adjacent channels wet, the rest dry, usually means the damper manifold on one head, not the supply system. Wet on one head and dry on the head beside it means the problem is in that head's own ink path, and that narrows the search to a handful of parts.
Check tank height with a tape measure before you check anything electrical. The supply should sit at or near printhead height, not above it. Machines get moved, shelves get added, and a bottle that was level last year can be sitting on a new shelf this year. I have watched a whole afternoon of head swapping end when somebody put the bottle back on the floor.
After the Part Goes In
Replacing the part and closing the cover is where a lot of these jobs fail a second time, so give the repair a proper acceptance run instead of a hopeful nozzle check.
Prime the new damper before it goes on. A dry damper forces the head to pull against an air lock, and the first cleaning cycle after that will look exactly like the old fault coming back. Fill the damper most of the way with a syringe at the outlet, seat it straight and vertical, and feel for the firm seat rather than guessing at it. A damper leaning even slightly will pull air at the o ring, and you will be back under the cover in a week.
Leave the cap lip alone with tools. Clean it with fluid and a swab, look for swelling, hardening or a crust ring, and replace it if the rubber has lost its shape. A cap that has been scraped with a screwdriver will never seal again. If you need to confirm the seal before you commit, park the head, disconnect the waste line at the bottle, and pull gently with a syringe. Steady resistance is a seal that works. A draw that runs away with no resistance is a cap that has given up.
Then run a real job, not a nozzle check. A nozzle check proves the head fires at low demand. A meter of solid coverage at production speed proves the supply can keep up and that nothing is creeping out under load. Watch the first and last thirty centimeters of that run, because that is where a marginal maintenance station shows itself.
Keep the old part. Write the date, the machine hours and the channel on it with a marker and put it in the drawer. Eight months from now, when the same color goes wet again, that label tells you whether you are looking at normal wear or at a repeat of the same bad batch of dampers.