Originally Posted by LegendCJS 
To escott76: Can you tell me if you think pourable urethanes will work for a classroom situation? If it takes lots of time and skill to get right then that moves the job form the students to the TA. But if its an easily teachable operation then I guess the time goes into making the molds. How would you compare delrin as a mold material in relation to any other mold materials you may have experimented with? The molds for our application don't have to be particularly reusable, and it would be ideal if they were still workable with hand tools after the cnc stage- which leads me to think something like plaster is ideal. Remember these are not Mechanical engineering students, they are Electrical Engineering students, so they won't have even basic machine shop skills, and programming/ watching the cnc mill and using hand tools to fix up any mistakes in their molds will be about all that we want to allow them to do.
Thanks
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Read more about the specifics of the materials I was talking about at
www.smooth-on.com . A & B mixed together, generally by volume which is the easiest. Anywhere from 3 min to an hour and a half demold, with various times in between. You can pigment them as well.
Plaster isn't going to be the best stuff to machine. What will come off is a fine dust that you do not want to get into your ways, screws or spindle bearings. It is IMO a horrible material to use for machining purposes and will cause more trouble than it saves you in "cheapness". I use delrin because it gives good results, but there are some much cheaper recycled HDPE boards (search "plastic lumber" on McMaster) that I have been eyeing as a good cheap alternative. I keep meaning to order some, but I haven't had to place a McMaster order in a while. Delrin is workable by hand.
The other mold material that will work well and can be cheap in the long run is machineable wax. When you are done with a mold, remelt and reclaim it. No release agent required with a lot pourable resins, and it hardly wears tools at all. Absolutely hand workable after cutting.