You need to use an ink jet film specifically design for screen printing film positives. You'll never be able to achieve the right opacity with this printer using regular transparencies.
We use:
Film and Supplies | Screen Printing | Victory Factory (around $1 per sheet for a 13" x 18"). For the Epson 1400, it's my preferred inkjet film to use.
If you're printing all spot colors, you can print without having to use a RIP using this film. Just set the brightness and contrast settings of the printer to the lowest settings (-25).
If you're printing halftones, the only way to do it is by using a RIP. We use AccuRip and it works great (about $450).