Dot gain.

InkSeps, a maker of simulated-process separation software, defines it as halftone dots enlarging after they are printed on the shirt or other substrate: in screen printing the ink goes through the screen's dots and then expands. Because simulated-process colour is built from separations made of numeric grey densities, the growth shifts the colours; its example of 50% black and 50% orange meant to give a brown prints as a much darker brown. The UltraSeps user guide makes the same point from the RIP side: its custom colour settings file must be loaded, or the channels render wrongly and adjustments cannot be made accurately because dot gain is not compensated for. Neither page gives a gain figure for a press, so the amount for a given ink, mesh and press has to come from your printer. A halftone that looks right on screen can print darker than the file.