Film positives for screen printing will give you the best results when your films are dark and opaque. The film positive must be dark enough to block light from your exposure unit during the screen exposure process. If your film positives are not dark enough then light will creep in and begin to harden your emulsion. If that happens then you can lose details in your print (like halftones) or overexpose your screens altogether. Overexposing your screens will make the emulsion "hard" to wash out and develop the screen- minor overexposing and you may lose 5 % to 25% halftones- major overexposing and your design will not wash out or develop at all.
We also use Accurip with Photoshop or Illustrator to print our film positives. Accurip is a third party software that tells the printer to print in all black ink, how much ink to lay down on the films to make them dark and opaque and to print "fades or gradients" as halftones. Need help with color separations? Check out the color separation page.