[Greg]

Sorry - This got kind of verbose.

I got the molding materials from "Smooth-On". The company turns out to be local, but you can get it online or from Blick art supplies (some types), etc... The stuff I chose cures quickly so I can iterate through mistakes faster . It's not especially cheap either .

The silicone is 2 part, Mold Star 20T. It has a pot life of 6min. and a cure time of 30min. After mixing it I do see some bubbles, but have never had any problem with it, just luck I suppose. Something that has a longer cure time might allow more bubbles to escape. You can also put it in a vacuum chamber to degas it. Doesn't everybody have one?
A good thing about this stuff is that it's translucent so you can see the original inside when it comes time to cut the mold open.

The urethane is Smooth-Cast 325. It has a 2-1/2 minute pot time and a 10 min cure time (takes longer for small stuff). I got some of their pigment (not dye) for it. I dye them black like the original carbon rod. That way paint imperfections look "original". I seem to get a tiny bubble or two, usually on top where the "wire" goes around the "carbon rod". I was getting frustrated until I figured I could just fill the holes with some "urethane sanding dust" with super glue. It sands easily and I paint over it. The other thing is that the parts are so tiny it's difficult to mix up batches that small and get the right ratio (1:1) and to mix it evenly. So it's good to have multiple molds to fill. I ended up experimenting by casting knobs with the waste.

To make the molds:
- Remove paint from a good looking resistor specimen.
- Make a mold for the silicone mold out of foam board (hot glue). I find it works better the resistor isn't surrounded by too much silicone.
- Stick the leads of the resistor through the bottom of the box.
  -- You need to support the box while the silicone cures and while the urethane cures.
  -- I make it so the resistor is pretty much in the middle of the box (whole thing's like 3/4" think).
- Stick popsicle stick haves on each side near each end of the resistor. Don't touch the resistor. Use to open mold.
- Pour the silicone into the mold and let it cure.
- Slice the mold down the length of the resistor (easy if you can see) plus a little more on each end.
- Remove the original... Your done.

To make the resistors:
- If desired, solder thicker wire on to the leads to match original.
- Bend your resistor leads to fit through the original's holes left in the mold.
- Place your resister into the mold with room all around it. THIS IS REALLY, REALLY HARD.
- Mix your urethane and add pigment if you want.
- Pry the mold open with the sticks and pour in the urethane. I kind of hold it open for a little and close slowly. It might let bubbles out. But you have to work fast if making more than one.
- Let the stuff cure, pry the mold open and take the resistor out.

Here's a picture of one of the large resistor molds and a knob I made with the excess.