Well, you'll only get halftoning when you print to a PostScript printer via a PostScript printer driver. Are you trying to do the screen printing yourself or are you giving the file to a screen printer? You'd be best off talking to the people who are doing the actual screen printing because you'd need to find out the LPI of their equipment and you'd need to find out about their dot gain. Depening upon the gradient levels you have placed in your image (i.e., say going from 100% to 0% of a color) some of it just won't print due to the limitations of the equipment. No one's going to know the equipment better (and its limitations) than your screen printer/SB - so you'll really want to talk to them.