[Greg]

Hello,

I've been experimenting with molding dog bone resistors and restuffing electrolytics. I have a 37-116, a 37-690 and a 16b I'd like to do this for. These are for a chassis from that other company.

This is why it takes me so long to do things...

Dog bones:
I molded them with urethane resin in silicone modes created from the originals.
I seem to end up with a tiny bubble or two. Those I fill in with resin "dust" and super glue. Can't tell after painting.
For the larger ones I created leads with bus wire of the same gauge as the originals' leads.
I painted them with Testor's. Is there something thinner? I suppose I could try thinning the Testor's. I tried acrylic paint and didn't like it.
The black one is unpainted.

Electrolytics:
The goal here was to be able to use the body as the negative terminal as originally done. The problem was soldering to aluminum.
I did find some aluminum solder (I think the flux is the magic element) that works.
I solder the negative wire to the inside of the can.
I used a brass rod and created a new terminal from a brass tube, attached a wire and used that as the positive terminal. I used the original plastic "insulator" epoxied into place to pass the rod through.
I'll use caulk to glue the tops back onto the cans (cut off where they were crimped in place).

Does anything look odd about these, except possible the thick paint on the resistors?

Thanks,
Greg

[rfeenstra]

Awesome! They look much better than my first attempt! Do you mold around the original and then cut the mold or mold it in 2 steps? What silicone did you use? Shore 60?

[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.

[Greggshere]

Those are gorgeous, great work Greg!

[Phlogiston]

You might be interested in this:


https://www.russoldradios.com/blog/stuff...ansoceanic


There are quite a few other posts on stuffing/casting parts.

[klondike98]

nice work gregg!

[EdHolland]

Very well done Sir

[radiorich]

Hello Greg,
Nice Job I bought some Alumilite resin and silcon molding material I need to make one those bigger Dog bone resistors for my Philco it's a 10k at 3watt .
I also need to make some smaller ones too.

Sincerely richard