Help needed finding suitable silicon rubber for molding

Hello everyone,

I've been trying to make a good mold for replicating the Dino GT Daytona Campagnolo wheel:

http://farm4.static.flickr.com/3596/3624993598_31227149e9.jpg

I used Hobby Time RTV/NV silicon rubber and made at least 6 tries but due to the complexity of the part I always end up with air bubbles (only few of them in the last try).

They claim the silicon rubber has a 30 minutes pot life but in reality depending on the amount of catalyst (used as prescribed by the manufacturer) I've ended up with maximum 15 minutes.

Anyway even using less catalyst than suggested by the manufacturer (and getting the vulcanization time from 3 hours to 12 hours) I couldn't achieve a bubble free mold as air was inevitably trapped between the spokes.

Does anyone know about a product that has a very low viscosity that would allow me to achieve the near perfect result without the use of a vacuum machine?

Thanks in advance for your help.

The stuff I use of late would probably be good. When I go back down to my workshop later today, I'll post you the name and type.
In the worst case, I can send you a pot.

Thanks a lot Steven. I'm looking forward having the name of the stuff you use.

Thanks a lot Steven. I'm looking forward having the name of the stuff you use.

I use silicon from this manufacturer (http://silikonypolskie.pl/) (there is an UK version of the site when you hit the flag)

I use Gumosil WW and Gumosil M. Both of them flow very nicely and pick up the finest details (including my fingerprint in one case! :rofl:). I have NEVER had problems with bubbles in the silicon. Needless to say, I don't have a vacuum chamber; I just use the "bang it on the table" technique combined with the "pump it with a toothpick" technique ;)

HTH

have you tried "painting" a coat of silicon onto the face of the wheel and letting it partially cure before casting it proper?

I use silicon from this manufacturer (http://silikonypolskie.pl/) (there is an UK version of the site when you hit the flag)

I use Gumosil WW and Gumosil M. Both of them flow very nicely and pick up the finest details (including my fingerprint in one case! :rofl:). I have NEVER had problems with bubbles in the silicon. Needless to say, I don't have a vacuum chamber; I just use the "bang it on the table" technique combined with the "pump it with a toothpick" technique ;)

HTH

Thanks a lot Steven! :)

PS: I took a look to their site and they seem to have lot of interesting stuff plus decent prices.

have you tried "painting" a coat of silicon onto the face of the wheel and letting it partially cure before casting it proper?

I thought about it but I do not know if it wil work because of the very fine detail.

But I have some more of it left and I'll check it out later on a similar wheel prototype to see what it gives.

Thanks a lot Steven! :)

PS: I took a look to their site and they seem to have lot of interesting stuff plus decent prices.
Enjoy! ;)
let me know if I can help out

Well, by painting silicon, you would reduce the effect of any bubbles that do form on the face detail of the wheel.

It's what I do when I'm casting car bodies.

There's also something else you can look for that thickens the silicon so that you can press it into the wheel instead.
Link (http://www.tiranti.co.uk/subdivision_product_list.asp?Content=Thixo+Additiv e+-+Silicone+Rubber+-+Mouldmaking&Subcategory=51&Subdivision=178) from my local sculpting store of the product.

Enjoy! ;)
let me know if I can help out

You bet I will! :D ;)

Well, by painting silicon, you would reduce the effect of any bubbles that do form on the face detail of the wheel.

It's what I do when I'm casting car bodies.

There's also something else you can look for that thickens the silicon so that you can press it into the wheel instead.
Link (http://www.tiranti.co.uk/subdivision_product_list.asp?Content=Thixo+Additiv e+-+Silicone+Rubber+-+Mouldmaking&Subcategory=51&Subdivision=178) from my local sculpting store of the product.

Never thought about this possibility.

I was pondering on buying some Silicon thinner.
Anyone ever used it? By how much would that lower the viscosity?

The lowest viscosity silicone in the world won't help you. Pour water on that wheel and you'll see that you still get bubbles. You have to "paint" it into the crevices with a brush as drunken monkey suggested. After brushing on a thin layer you would then chase out the bubbles using a toothpick.

I did the "painting" trick on the dozen wheel molds I built using standard RTV rubber, and have not had a single bubble no matter the complexity of the wheel design.

It works, try it!!

(and when casting the wheel, try brushing the mold with talcum powder, it acts as a surface tension eliminator, and avoids having bubbles in the resin)

Jano: I got troubles with bubbles but with resin, not sylicon: I found great to use a "vibrating table" , there are various way to do it (old game consolle controller, by yourself with a electric motor, I use the same air pump used in the PE etching pool, but you'll link a V12 for achieve mor powerful :D), simply with fast vibrations I see bubble go up like nut.

Also what suggest DM is not stupid at all: you'll achieve bubbles but far from parts >> great, I'll try next time.

Thanks guys!

I'll use all these ideas!

PS: The talcum powder trick for the resin works, I tried it 2 days ago and no bubbles, plus excellent demolding!