A drooling nozzle gets blamed on the orifice more often than anything else. People fit a smaller bore, wind back the decompression, change the material, and the drool comes back. That is usually because the orifice was never the problem.
The mechanism
The nozzle's spherical nose seats against the concave seat in the sprue bushing. Under clamp pressure that contact is what seals the melt path. The melt channel runs down the middle of that sphere and exits at the orifice.
For the seal to work, two things have to be true: the two radii have to match, and the bore has to run true to the sphere it exits through. If the bore is off-centre relative to the spherical nose, the seat cannot contact evenly all the way round. Under pressure you get a crescent-shaped gap on one side, and melt creeps into it.
On the machine that shows up as drool between shots, flash at the sprue, a sprue that will not release cleanly, or stringing. None of it is fixed by changing the orifice diameter.
Three things to check before buying another nozzle
- Seat radius match. The nozzle SR and the bushing SR should be the same nominal, with the nozzle very slightly the smaller radius so it seats on a line contact rather than on the rim. A nozzle radius larger than the bushing radius contacts at the outer edge and seals on nothing.
- Concentricity of the bore to the sphere. Ask what the supplier holds. "It's fine" is not a number. This is the single dimension that is hardest to see on an inspection report and easiest to get wrong.
- Contact witness. Blue the nose, seat it against the bushing, and look at the ring. An even ring all the way round means you are sealing. A crescent means you have found your problem, and no orifice change will fix it.
Why this bites harder on replacement nozzles
An OEM nozzle is made to a drawing that specifies the relationship between the bore and the sphere. A replacement is very often turned from a measured sample — someone put the old nozzle on a surface plate, recorded every diameter and length, and drew what they measured.
A caliper records dimensions. It does not record relationships. So the replacement drawing carries the diameters faithfully and says nothing about concentricity, and the part that comes back checks good on every dimension and still drools.
If you are ordering a replacement nozzle from a measured sample, adding one line to the drawing — the concentricity of the bore to the spherical nose, and the tolerance on the seat radius — changes the outcome more than any other single thing you can write on it.
Related: hardness
Machine nozzles are normally quoted through-hardened rather than case-hardened, typically 48–52 HRC in H13 / SKD61 or 40CrMo. Through-hardening is right here because the part carries a thread, seats under clamp force, and cycles thermally — a hard case over a soft core would deform at the thread before the nose ever wore out. That is the opposite of the reasoning for an ejector pin, where the case is exactly what you want.
If a quotation says "hardened" without saying surface or through, ask which. The prices differ and so do the parts.