What it means: Air and paint mix inside the airbrush body before exiting
In an internal mix airbrush, compressed air and paint combine inside the airbrush body — specifically at the nozzle — before the mixture exits as a fine spray. This internal mixing produces far finer atomization and a smoother, more consistent spray pattern than external mix designs.
Virtually all professional-grade airbrushes are internal mix. The improved atomization enables realistic gradients, fine lines, smooth color transitions, and the control needed for illustration, portrait painting, scale modeling, and other demanding applications.
Tradeoff: Internal mix airbrushes are slightly more complex to clean (paint can dry inside the nozzle assembly) and more sensitive to paint viscosity — very thick paint can clog the nozzle. Proper thinning is important.
Related: External Mix · Nozzle · Atomization · Viscosity