Thanks to Gord Neish for kindly allowing me to post his method of making replacement knobs. They are a particular challenge due to the narrow stalk that mounts the knob to the metal shaft. Gord addresses this problem with the use of a two part mold.

Producing The Two-Piece Mold
The material:
1. Smooth-On PMC-121/30 (dry)
2. Smooth-On Universal Mold Release
www.smoothon.com

1. Choose a container
The first consideration is to obtain a suitable mold container. The container must be large enough to accommodate the original knob(s), with enough space to accommodate a sufficient thickness of mold rubber. Overly large containers are simply a waste expensive mold rubber. The containers used here (purchased for $1.00 each at the dollar store) are large enough to accommodate 2 knobs; they are smooth plastic so mold release is easy. In addition to accommodating vacuum pumping, the containers also make the molds easy to store and keep clean. It should also be noted that multi-knob molds make color matched sets much easier to achieve.

2. The first mold rubber pour
Coat the interior of the mold container with mold release as per instructions and allow to dry. Mix and pour approximately ¼ inch layer of mold rubber in the container and allow sufficient time to set firm. You can proceed with set 3 as soon as rubber is firm; it does not have to be fully cured.

3. The second mold rubber pour
Apply mold release only to the knobs and not to the first pour of rubber as this layer of rubber must adhere to the first. At this point cover the rear hollow portion where the shaft meets the knob (not the shaft hole), I used a plastic washer attached with a small drop of super glue. The purpose is to eliminate the hollow rear portion of the knob to simplify casting. Mix enough mold rubber to cover a thickness equal to the knob front not including the shaft portion. Pour a small amount on top of the first pour and place the knobs face down working any bubbles out from underneath. Now pour the remaining rubber to a level flush with the rear of knob potion (this will be the split line). The shaft portion still extends above the rubber. Set aside to cure for 24 hours. Note, you may have to place weights on the knobs to keep them from floating on the rubber (I used steel ball bearings, but marbles should work).

4. Preparing the fill holes
To provide fill holes through the top of the mold, super glue two ¼ inch shafts (I used ¼ inch aluminum spacers) to the outside of the shaft portion of the knob. This will produce two holes into the mold, one for filling and one to let air out.

5. The final mold pour
Coat the container, knob and rubber with mold release. Take care that no spots are missed or you will be starting over. Also take care to make sure the inside of the shaft hole is well coated with mold release. Mix and pour mold rubber to a depth of at least ¼ inch above the knob. Allow to cure for 24 hours. Once cured remove the fill hole spacers, you should be able to break the super glue bond with a bit of side pressure. Remove the original knobs from the mold.

YOUR MOLD IS READY TO USE

Casting The Reproduction Knobs
The material:
1. Smooth-On Universal Mold Release (www.smoothon.com)
2. Polyester Resin and hardener
3. Powder tempera paint colorant

2. Preparing the resin
Measure enough resin to fill the molds and color it with powder tempera paint colorant. Once color is mixed add the resin hardener as per container instructions. You now have only a few minutes to compete the casting.

Making The Knob Springs
The material:
1. Large paper clips