How can I cast a urethane rubber item without this craziness?

[Art Andrews]

I recently got an original prop and sent it off to my molder (who has molded and cast hundreds of items, so he is no newb). He used Smooth-on's Rebound 25 for this project, which is a Platinum silicone. After allowing ample time to cure he opened the mold to find that all of the rebound had fully cured except a VERY think layer that was directly touching the prop itself. Where the silicone touched the prop, the silicone had only partially cured and had developed a very pock marked surface (the surface of the prop is fairly smooth and without any texture).

To say the least, we were sorely disappointed and quite confused. We contacted Smooth-On and they said the ONLY substance that Rebound 25 wouldn't cure against was urethane rubber. We don't know what the prop is made from but it IS relatively rubbery and quite flexible. After consulting another molder, we were told that urethane rubber leaches alcohol and that is what we were seeing in the silicone, tiny bubbles from the alcohol.

It was suggested that we clear-coat the prop and try again. We tested a small area with this and while it was better, the silicone still didn't fully cure.

The final suggestion we were given was to use a tin based silicone instead of a platinum silicone.

So, before we go buy a bucket of tin silicone, we wanted to see what experiences you have had and what suggestions you might have for casting this piece up.

Here is a picture of the uncured and heavily pock-marked silicone.

 

[Beaker]

you could clear coat it and then put on three layers of Inhibit X from us...
That should work fine, but still do the small scale test...
http://www.smooth-on.com/tb/files/INHIBIT_X_INHIBIT_STOPPER_FOR_PLATINUMS.pdf

 

[porkandbeans]

Are you concerned about damaging or altering your pattern prop? Different coatings may help.
How about changing the mold material from silicone to a faster curing urethane? Kick either under pressure or vacuum?

 

[Art Andrews]

Are you concerned about damaging or altering your pattern prop? Different coatings may help.
How about changing the mold material from silicone to a faster curing urethane? Kick either under pressure or vacuum?

Definitely trying not to alter or damage the original piece if at all possible.

 

[porkandbeans]

Definitely trying not to alter or damage the original piece if at all possible.

Poopers then. Guess I won't suggest coating.
Might reverse engineering be an option? Not sure what the financial constraints are, but starting fresh with a new pattern would solve everything - maybe even improve some things along the way if that would be desired.

 

[hansicle]

You may need to pressurize the mold, that will help reduce the bubbles, and you may be able to use a heat band on the pressure tank to bake out the alcohol as it's curing, and you may need to bake out the alcohol once you've removed the piece from the mold. Also make sure the proper mold release is being use on the original. You can spray a layer on and brush it into the item with a brush, allow to set, another light coat, and brush, then a few more light coats before casting.

Hope this help

 

[robn1]

I like to paint my master parts with Tamiya spray. I've used three different Smooth-On platinum silicones and they all had trouble curing against this paint. It takes three days to fully cure, even for the six hour rubber. I cure my molds under pressure so I had no problems with bubbling, the molds are usable but three days don't cut it. I switched to Mold Max XLS II and it's been excellent, with no inhibition problems. 24 hours to cure, but it's better than 72.

If you're using Rebound for a brush on mold, you can add Cabosil to make any silicone brush on.

 

[bookface]

In my experience platinum silicone is also inhibited by anything containing sulphur, hence the existence of sulphur free clay.

Art, I have a couple of urethane rubber pieces here in my studio. Happily for you, I'm also in the middle of moulding some pieces with tin cure silicone. I will do a small scale test on a couple of my rubber pieces and let you know how it works out.

 

[Art Andrews]

Thanks for the advice everyone and thanks for the offer to test a tin cure silicone, bookface!

 

[bookface]

OK, test pieces done Art Andrews . Both pieces cured without any issue on two completely different rubber props. In case it's important, I was using Polycraft general purpose silicone, but I would imagine any tin cure silicone will react the same. Hope that helps!

 

[OldSkoolEffects]

You could always try going all old-timey and use a brush-on latex in thin layers backed with a mother mold. I would think latex would suck up anything that the urethane is pushing to the surface.

 

[robstyle]

I dont use Smooth On materials but have had to mold existing rubber parts for work projects. Epoxy Parfilm dusted into a fine surface coat on the master and a quicker kicking silicone (or add some kicker to it) always does me well. Regardless the master will have some surface residual effects but it shouldnt be anything more than a gloss type surface that should wear off on its own.

 

[Duncanator]

Bookface is right. Platinum silicones are susceptible to sulfur as well. You do have to watch for incompatibilities.

I have found platinum silicones to be much more persnickety in general. Platinum silicones are great for certain applications, but are expensive and less tolerant than tin silicones so I only use them for specific applications.

I use tin based silicones all the time, and they are my go-to general purpose silicones. Sulfur can be a problem for tin silicones too, but not as bad as platinum.
Also, platinum silicone will not cure when touching tin silicone, but tin silicone will cure when touching platinum silicone. So you have to be careful not to cross contaminate your silicones by re-using the same mixing sticks, buckets, etc...