I wanted to cast pen barrels but use 10inch long brass tubes which I could then cut to pen barrel length requirements depending on the pen. I made one from 5/8" ID PVC tubing.
The saw kerf and pour holes were added after everything else was done, fitting, threading, etc.
The PVC tube is threaded 7/8-14 on both ends. The white blocks are nylon and are female threaded 7/8-14 to accept the PVC.
The short piece of PVC on the outside of the blocks is only there to make the blocks wider. If I had had 1 to 1-1/4" nylon I would not have need them. The are also threaded 7/8-14 to make them part of the nylon. They are lined with smaller glued in PVC which is thread 1/2"-20 to accept the "plugs" that plug the ends of the brass tubes to prevent resin from entering into the tube. Again if I had had thicker nylon I would have threaded it on onside with the 7/8-14 and on the other with the 1/2"-20 but you use what you have.
This pic is prior to the pen mode but shows all the components disassembled. It was taken just after my first cast. There is a casting inside the mold. The black on the ends are oring seals that are place between the PVC and the plug holders to prevent fouling the 7/8-14 threads.
I have made 3 castings with it in the pressure chamber I made for it and it has worked well. Had to make some minor changes to the plugs to prevent casting resin material into the 1/2"-20 female threads and had to add extensions, the metal pens, to the plugs to allow for easier loading of the tube but as a concept it worked well.
All comments are appreciated.