This calculator is the big exception in my collection, because it is electric, and usually I don't collect those machines at all. However, being a keyboard MADAS, as soon as I actually had carried it to the car from the seller in Northern France, I knew I was not going to sell it on, as was the original plan. MADAS machines just ooze sheer quality, and I'm a sucker for a well-built machine. So I cleaned it up a little, and started worrying about the octagonal replacement keys that were on the machine - you can see them here.

Now, since all the replaced keys were "1" keys, this gave me the idea to try and make a cast of one of the existing "1" keys, and replace them with a better facsimile of the originals - and that is really what this story is all about.

So how to go about that?

This is how a "1" key looks:

There are a few difficulties making a mold for this and casting it. The first one is that the shape is irregular - you would need to find a parting line that allows the key to be removed from the mold. Luckily, these days you can very easily make flexible silicone molds, which alleviate this problem. There is also a hole drilled through the key to accept the tiny screw that fixes it to the keyboard. It would have been easiest to simply fill this up with plasticine, so the silicone mold compound would not run through to the middle hole. Of course I forgot this.

Another issue is the number. The original keys have the number filled in flush with the key top. This "stuff" needs to be scratched out so the number will be molded in and can later also be filled. Like this:

The next thing you need is silicone mold compound, mold release spray, and the activator. This was bought from a local company (actually, I had some left over from a previous job).

Then you need a small cup, and you can (provided you stop up the holes in the side) just float the key in the liquid silicone until it hardens.

Now you need a "riser" - a path in the mold for excess casting compound to go. For this, I usually glue a toothpick (or two, or more) to the object I'm casting - add some mold separation compound, and cast the second half of the mold. Before you do this, make sure you cut two irregularly shaped wedges out of the top of the bottom cast (see picture above) - this will ensure that the top always goes back in the exact same position. Obviously, you start by filling the hole in the key as well as possible, making sure there are no air bubbles left in there, or the keytop won't fit the stem.

After the silicone hardens, you can take the top off the mold and remove the original key. You now have a MADAS-key shaped hole in your silicone mold. Note the wedges on the mold top, and the corresponding bits that have been cut away from the bottom part.

The next issue is colour. It would be possible to paint the keys, but that would wear off, so it is better to colour them in the mass of the casting compound. For this you need some colours, and for the cream-coloured keys you need some very judicious mixing of yellow, black, white and red. You can see the result in the small cup. Black is very easy, just add a small drop from the black tube.

Now you can start casting stuff - for that, you need these guys.

You can buy polyester molding compound, but that is transparent, difficult to colour, and quite brittle. I prefer this polyurethane compound, which is white, much stronger, and also hardens much more quickly. So you mix it up, colour it, and then you have about 2 minutes to get it into the mold without air bubbles. Air bubbles really are your enemy while casting. If you do not want to invest in a vacuum chamber to make them come out and pop, you just have to learn to live with them. This was my first cast - looks like crap, there is a large chunk missing from the side of the key where there was an air bubble. It looks really bad because I had already tried to repair some of the damage with some more casting compound, but it didn't work out too well.

So that means I need to be more careful to inject the resin into the mold really starting at the edges of the key. I use a syringe and needle for that.

This is the next attempt, in white, next to the black original.

You can see clearly now there are two defects in the mold, which will appear in all subsequent keys we cast- - there was a tiny bubble stuck in the middle of the "1", and there is a dent in the key top, which is probably also caused by a bubble, although it is somewhat beyond me how that is possible.

Here's a black key with the dent filled in with some new polyurethane next to the white one "as cast":

The back side is a lot more problematic - it is very difficult to get all the air bubbles out when pushing on the top of the mold (coating it in resin first helps somewhat) - so the backs of the keys look far worse than then front. But of course, you don't see much of them when the keys are mounted on the machine ...

Here are some replacement keys ... you can fill in the dent in the top with a little more coloured resin (when casting the next key) and scratch out the air bubble in the "1" :

Then we need to fill in the 1 with something that will stay there. In the past I have used latex paint, this time I decided to go for plasticine. Smear it on ...

...and wipe it off. Looks like a million dollars, doesn't it ?

And finally, the challenge of the day - please indicate the keys which have been replaced, without scrolling back up to check which ones they are. Not bad, eh ?

Finally, the usual portrait of the finished machine - first the cover:

Then the instructions:

Seller's plate, in Paris, France - the address still exists, but the entrance seems missing (?):

Seuls fabricants


Zürich, Suisse

Made in Switzerland

Industrie Suisse