You changed every nozzle on the header, the line is back up, and two shifts later the same pale stripes are running down the panel. It is tempting to blame the supplier or assume the tips wore out already. In most plants the nozzles are fine — the streak is built into how the pattern was specified or laid out. This guide walks through the failure modes that survive a fresh set of nozzles, and how to pin down which one is yours.
A flat fan spray nozzle turns liquid into a thin, flat sheet. That sheet is the most common tool for coating, washing and rinsing a moving surface because it lays down an even film across a defined width. The catch is that “even” depends on how the sheet is shaped at its edges and how neighbouring sheets meet. Get either detail wrong and the defect shows up immediately, on brand-new tips.
What the Pattern Looks Like Up Close
Hold a flat fan against a wall and you see a rectangle-ish band: dense in the middle, thinning toward the two ends. The shape of that thinning is the single most ignored property in nozzle selection.
There are two families. Tapered edge fans fade gradually to nothing at the ends. Even (or “full”) edge fans hold near-constant density across almost the whole width and then stop abruptly. Both are legitimate; they just belong in different layouts. This distinction matters far more than the brand stamped on the body, and it is the first thing to check when a line streaks.
Flat fan spray nozzles, flat spray nozzles and the plural variants you will see in catalogs are all describing this same sheet-forming family. The words overlap in everyday use; what separates a good choice from a bad one is the edge profile and the spray angle, not the label.
Tapered Edge vs Even Edge: The Usual Culprit
When nozzles sit in a row and their patterns overlap, you want tapered edges. The thin end of one sheet lands in the thick middle of its neighbour, and the two blend into a continuous line. If instead you mount even-edge nozzles side by side, each one stops hard at its boundary — so between every pair you get a low-density seam, and directly under each nozzle a slightly heavier band. The result is exactly the repeating light-and-dark stripe operators call “banding.”
The reverse is also true. A single even-edge nozzle aimed at one spot, with no overlap to worry about, is perfectly happy. But mix the two types on one header — common when someone tops up a row with whatever was in the bin — and you have created a stripe that no amount of new nozzles will fix, because every new nozzle is also the wrong edge type.
If your streaks repeat at a regular spacing that matches your nozzle pitch, edge mismatch is the leading suspect.
Coverage Width Is Set by Angle and Distance, Not Pressure
A second, quieter cause of uneven coating is simply aiming the wrong width at the part. Coverage width follows basic geometry: it is twice the standoff distance multiplied by the tangent of half the spray angle. At a 300 mm standoff the standard angles give you roughly:
| Spray angle | Coverage at 300 mm |
|---|---|
| 15° | 80 mm |
| 25° | 130 mm |
| 40° | 220 mm |
| 65° | 380 mm |
| 80° | 500 mm |
| 110° | 860 mm |
| 145° | 1900 mm |
These figures assume the nozzle fires straight at a flat surface. The practical point is that widening the angle is the only way to cover more width from a fixed distance. Raising pressure does not widen the sheet — it only pushes more liquid and more impact through the same angle. Operators who “open the pump” to fix a gap at the edge are spending energy on a problem the geometry will not solve.
For coating duties a 65° or 80° flat fan spray nozzle is usually the starting point, because it spreads the film wide without throwing liquid past the part. Narrow 15–25° fans concentrate energy into a thin high-impact line that is better suited to cutting or descaling than to laying an even coat.
Overlap Is a Planned Number, Not a Guess
Even with the right edge type, the spacing between nozzles has to be deliberate. The rule of thumb for tapered-edge fans is to space them at about 40–50% of the pattern width at the work surface, so each sheet overlaps its neighbour substantially. Too little overlap and you get a dry seam between nozzles; too much and you double-dose the overlap zone and waste fluid.
The number that matters is the pattern width measured at the actual part distance, not the catalog width quoted at 300 mm. If your line runs the header at 200 mm the real width is smaller, and your spacing has to shrink with it. Build a quick cardboard test rig, fire each nozzle at a sheet of paper taped to the part, and read the true width before you commit to a layout. A spray nozzle flat fan that looks right on the data sheet can be badly wrong two inches closer to the belt.
Clogging Hides in One Nozzle
Worn tips are obvious; partial clogs are sneaky. A single nozzle in a ten-nozzle header running at 70% flow breaks the symmetry of the whole row. The stripe it causes often looks identical to an edge-mismatch stripe, which is why teams keep swapping tips and never fix it.
Flat spray nozzles resist clogging better when they have a larger free passage, but any small orifice will eventually pick up a chip or a dried chunk of coating. Two habits prevent most of this: strainers upstream of the smallest orifice, and a monthly pattern check where each nozzle is fired at a bare surface and compared to its neighbours. When one fan looks narrower or off-centre, cleaning or replacing that one unit restores the line — no full re-order required.
When Pressure Helps and When It Doesn’t
Pressure is the lever people reach for first, so it is worth being precise about what it does. Flow through a fixed orifice scales with the square root of pressure. A nozzle rated 4 L/min at 3 bar delivers about 5.7 L/min at 6 bar, not 8. Impact rises with it, which is good for cleaning but irrelevant for coverage width.
So: if your problem is “the coat is too thin,” more pressure helps, up to the point the pattern distorts or the pump complains. If your problem is “there is a gap at the edge,” pressure will not help — that is angle and distance. Fan spray nozzles and fan nozzles behave the same way; the physics does not care what you call them.
A Quick Checklist Before You Re-Order
Before you buy another box of nozzles, run through this on the line:
- Note the stripe spacing. Matches the nozzle pitch? → edge-type or overlap problem.
- Pull one nozzle and compare its fired pattern to a known-good unit. Narrow or off-centre? → partial clog.
- Measure the real standoff and compute the actual coverage width with the formula above. Gap at the edge? → wrong angle or spacing.
- Confirm every nozzle in the row is the same edge type. Mixed? → that is your stripe.
- Only then decide whether you need new tips at all.
Most “streaking with new nozzles” calls turn out to be a layout or maintenance issue, not a product defect. If you want a second set of eyes on your header geometry, send us your duty conditions — line width, standoff, fluid and target flow are enough for us to sanity-check the angle and overlap you have specified.