Intermittent release of material that has built up on the nozzle bore, injector tip or HVOF barrel, which breaks loose and lands on the part as large semi-molten nodules called spits. Spits are weak spots and surface lumps that may need grinding out or a strip and respray. Common causes are fines in the powder, worn hardware, injection set too close to the plasma, and running too hot.
Gun's spitting every couple of minutes, I can hear it pop, stop and clean the injector.
Ask whether the spitting started after a powder lot change or a hardware change; that one question finds the cause most of the time.
Also heard as
- spits
- spit
- nodules
Related terms
Nozzle
In a plasma gun, the water-cooled copper anode the arc attaches to; in HVOF, the passage that shapes the jet.
Fines
Powder particles smaller than the specified size range, which cause buildup, spitting and poor flow.
Barrel
The long tube on an HVOF or detonation gun where particles are heated and accelerated before exiting.
Running hot
Booth talk for parameters that are overheating the particles or part; running cold is the opposite.