How I did my tires

[Egon]

Again

[Roger Zimmermann]

It's not finished Egon! Fortunately, I have many pictures, I will not have too much to write...

[Egon]

I know, but so far and a picture tells a 1000 words, as they say.

[Roger Zimmermann]

With a sharp cutter, I took the excess paint away and I began to paint the letters. The choice of paint color is not important! I believe this process is well described in the Gerald's book, anyway, I took the method there. Now, the letters and dimensions are progressing slowly. In fact, the most important difficulty is that the paint does not stick very well on polished brass; by taking away a bit from paint it may happens that half the letter is gone...



Now, it's ready:



The fun with chemicals can begin. When I saw the price of the Dow Corning product, I searched for something less expensive than gold. I found a Swiss company offering a huge variety of products for modelling. I bought indeed 2 different products: a white one for the pattern and a transparent one as well as black paste for the final product. The white silicone rubber will also be used for the white wall, a bad decision as I will explain later.
The lower half of the mould is filled with plasticine, the junction is a smooth as possible. I used a piece of Plexiglas for this:



Then, the estimated needed quantity is mixed with a catalyst and poured into the mould:



As everybody knows, the process to mix the catalyst entrapped air; this is THE enemy. For that, I bought many years ago a desiccator or vacuum chamber; the vacuum is created either with water or with a manual air pump. I bought first a cheap plastic air pump; after a short time the handle broke. I repaired one time, twice, and so on...Finally I had to buy a metal one!:



As you see, with some vacuum, the air is coming out:



After some curing time, usually 12 hours, the silicone is cured and the plasticine can be removed. Usually, silicone sticks on itself, sometimes not. I had to do various tests and I came to the conclusion that a release agent was necessary. So, some was sprayed at the joint, and another batch of silicone poured over the cured one. The can was removed after 12 hours and I had a nice white drum:



And now, Ladies and Gentlemen, the great moment: failure or success?



It was a success! The next step can begin: after spraying some wax, some silicone is poured into one of the negative molds and, after curing , can be extracted:

[Roger Zimmermann]

After the first half positive pattern comes...the second one. If my vacuum chamber would be large enough, I could to both in one step. According to the picture, the second one is also done successfully:



The question will certainly arose: why the tires are not done with the first negative patterns? One reason is described by Gerald Wingrove: if the separating agent is not well applied, the silicone rubber may stick to the pattern and ruin it. Therefore, more steps are necessary.
People who knows the book from Gerald very well will probably notice that I'm now following exactly his method; for example, the way of pouring the black rubber is totally different. We will see that later.
Now that both positive patterns are ready, it's time to make the negative ones with polyester. It's better to do that outside as the smell of polyester is not appreciated by all people...Here, before the polyester is poured:



And after:



The brass parts over both positive halves is just there to add a little more weight; I was afraid that the patterns may "swim" in the polyester.

Now, the polyester has cured, it's time to remove the outer walls:



and extract the positive patterns:



I was lucky! no bulbs stayed trapped near the thread and the details are well rendered:



As you can see, the brass weights were embedded into the polyester moulds. Once washed, they are ready to accept the black silicone rubber. The product I bought is called Neukasil RTV 27 from Altropol, a German product. The next few steps are well adapted for this product; other silicone rubbers may require a different approach.
After I calculated the necessary weight, I poured the transparent silicone into a plastic cup:



Then, I added some black paste, the minimum possible:



Followed by a calculated amount of catalyst:



The whole must be well mixed with a lot entrapped air as "by-product". Then, it is poured into both forms, but, not completely full; you will see why:



The red plasticine is here as a dam.
The first try was to have both polyester moulds joined together with a central hole on top. With a funnel, I poured then the silicone after his passage into the vacuum chamber. Unfortunately, the curing time was exceeded and the top of the tire was...full of air. This event required another method of joining the tire; it's described below.
The patterns and cup still containing some silicone are placed into the vacuum chamber:



Now, it's time for hand's muscle building:



As you can see, the entrapped air let the silicone's volume grow; this is the reason why the pattern are not completely full:



When almost no more bubbles are coming up, it's time to let some hand's relax and take the elements out the vacuum chamber. To compensate for the air which is gone, more silicone is added, this time, more than necessary:



Now, it's waiting time! About two hours are elapsed between the addition of the catalyst and the moment when the silicone is no more flowing but not yet cured. This moment must be monitored and, with some experience, can be well catch. 5 minutes are allowed to join both halves together and press them to evacuate the excess silicone. Some weight is added on the polyester patterns during the remaining curing time, another benefit to have a solid pattern:

[Roger Zimmermann]

After 12 hours waiting time or overnight, it's time to see the result. Both patterns' halves must be separated with some strength because the pattern must go over the thread:



After a while, there is some gap:



And now, the first tire's half is released, time for a quick inspection. Up to now, there is no problem:




the second half can be extracted, requiring less force:



A nice tire is born!



It's now time to make the pattern for the white wall. Of course, this step must be planed into the main tire design. As far as I know, silicone rubber cannot be vulcanized nor painted. A solid element must be added to the tire; it's up to the tire maker to decide the best approach:



Just one step was necessary as there is neither thread nor other detail difficult to reproduce into the brass.

The end result:



As I noted during this tutorial, my choice of using a white silicone rubber was not clever: after some weeks and months, the white silicone in contact with the black one retracted itself; I had to enlarge the mould (also the one for the Avanti tires) to compensate for this behaviour. I suppose this problem would not have occurred if I had used the transparent silicone and mixed some white paste. As you see, each product can react in a strange manner. Now, you are ready to open a tire factory!

[Egon]

Beauuuuutiful

[spinellid82]

Wonderful Roger, thank you very much!

[Roger Zimmermann]

Thank you! I hope explanation are not too confuse. "my" mthod is right for a car which tires are not well seen, especially the thread. For a vehicle from the thirties or before, the tread would probably not be acceptable.

[BrassBuilder]

Thanks for posting this tutorial Roger! Very informative!
Mike

[Abhishek]

man you deserve a standing ovation for this .

[Roger Zimmermann]

Thank you Mike and Abhisheck! There are also other methods, some are more sophisticated...