I have never has the plastic screw type ever unscrew or open up on me. I have seen a recommended re-use of a max of 5 mentioned but i see no reason why with careful use and cleaning with the between the teeth brush or sticky part of a post-it note the cassettes shouldn't last a lot longer.

The alternative, if you have a mini-lab close by, is to get it to give you its used cassettes. It probably throws away dozens in a week. Attach the bulk load film to the little bit of residual film on the cassette and you can then probably afford to throw away the cassette after one use. It's free as well!

pentaxuser