View Full Version : Rubber compound needed

10-27-2008, 12:14 PM
Our shop has made a simple mold that seperates in two halfs for a part we need. My problem is that I cannot find a rubber compund that will dry in the the airtight mold. The only thing that we have found that works is repro-rubber inspection putty.
Does anyone have a good idea for this, we are looking for a rubber compund that dries in a airtight mold and is oil resistant, with a stiffness a little softer than a tire.
Any help would be appreciated.

10-27-2008, 06:02 PM
If it can stand a little heat maybe Vinamold Yellow will do the job (200C Melting point)

10-27-2008, 09:18 PM
Will a urethane casting resin work?

10-29-2008, 10:19 AM
Try a two part isocyanate/polyol polyurethane foam mix. We have just bought a machine to make parts with this stuff, having been outsourcing it. It basically comes out as a sort of synthetic rubber. You have to heat the mould to about 50 degrees, and then keep it there with active cooling, to get a good finish. Some of the chemicals are a bit nasty, but we are looking into that at the moment.

I'll be interested to hear anything anyone else knows about this type of thing.

10-29-2008, 11:55 AM
A putty like the Repro rubber works nice because the mold seperates in two halfs. A thick liquid would also work nice.
Does anyone have any ideas?

10-29-2008, 12:15 PM
Perhaps using latex in liquid form then heating it to vulcanize it inside the mold would work?

10-29-2008, 12:17 PM
....Does anyone have a good idea for this, we are looking for a rubber compund that dries in a airtight mold and is oil resistant, with a stiffness a little softer than a tire.
Any help would be appreciated.

Hypalon, Google it for more information.

It is a synthetic rubber that is cured by heating to just over 100 degrees C.

You will need to find someplace that does rubber vulcanizing and see if they will sell you some. It has to be kept refrigerated to stop it curing slowly at room temperature.

I have done a lot of 'garage shop' vulcanizing of parts using Hypalon in aluminum molds that I heated in an electric frypan. It is easy to work with because in the uncured state it is like putty so you can measure out a block the same volume as the mold cavity and then just clamp it in the mold. You need some bleeder holes to let the trapped air and excess material to squeeze out.

Much, much easier to work with than RTV Silicone or polyurethane mixes, no measuring and mixing, no vacuum degassing, no goopy stuff leaking out of the mold. Once you have the weight needed to just very slightly overfill the mold and the time/temperature needed for complete cure things pop out very reproducibly. In addition you can scale up from prototype to initial production just by making some multi-cavity molds.

10-29-2008, 02:28 PM
I cannot find any for sale. It sounds like what I have been looking for, any ideas on where I could buy some?

10-29-2008, 04:25 PM
I cannot point you in any specific direction. I got it from a local rubber vulcanizing company; this is why I suggested you try and find a company near you that uses it.

I think it might be something you need to by in huge quantity when you are getting it from the original manufacturer.

There are other synthetic rubbers that are also oil resistant such as neoprene; I just know that it is very easy to do prototype molding with Hypalon because I have done it.

10-30-2008, 04:35 AM
You could try "Potting Compound"- I used this to form a rubber vibration absorbing layer around large infra-red windows for attack helicopters.

I'll try and find out what the name of it was, it came in two parts, I added black powder dye and injected it into a gasket mould. As Geof rightly says though, it was messy, what with de- gassing and all, but there was no heat curing and the resulting layer (about 5mm thick and 15" length around the perimiter was nice and solid but springy.

One thing I would advise- to get the stuff to stick to the surface I wanted I used a red primer (again, can't remember it's name just now), but more importantly in your case- to get the stuff NOT to stick to the mould face I sprayed the surfaces I didn't want it to stick to with a silicon dry lubricant and lightly polished with a lint free clean cloth. I cleaning the surface with acetone and IPA each time prior to use.

This resulted in an homogenous, solid and springy vibration buffer for the windows which were made of monocrystalline germanium and so quite costly (helicopter vibration would have chipped and shattered them).

11-02-2008, 04:02 PM
These places carry a huge variety of Urethanes.


(arrgh...I just noticed yer in the UK,)
These guys probably have what ya need


I've used lots of it, and you can get it from super hard to fleshy squishy.
For rubber with the qualities of a tire, I'd use a Shore 50 or above.
Be sure to use a non-pouros mold, (plastic, -SEALED- wood/plaster, metal, glass, or of course silicone, or other urethanes.)
RELEASE AGENT CRITICAL! stuff sticks like....well....ya know..

I like Smooth-On Universal Release, but Carnuba wax works too...
and heck, both together work great too....

Mixing throughly is critical. 'specially for the rubbers.


11-05-2008, 04:10 AM
The stuff I used was RTV silicone two part polymer. I can't recall the actual part number but a google search of RTV Potting Compound should get you close to what you need.