Recommendations for RTV molding materials

[pnolan48]

While I've pretty much settled on casting materials, I've had a little bad luck with my mold materials. My first supplier seems to have left the business (everything is "temporarily out of stock" for the past six months); my experience with a second supplier with polyurethane molding materials was middling; I've recently been using "wet" ReoFlex 40 from Douglas & Sturgess, by way of Smooth-On, I think. That's been OK.

I've realized it is foolish for me to scrimp on mold-making materials. What do you folks recommend--regardless of price--as the best mold-making materials?

Oh, this is probably important: I've finished, for now, the casting of big parts such as hulls, and am now making the molds for items such as 21' rescue boats. My most important criteria is ease of use, and I probably want to stick with 1:1 mixing, unless someone makes a case to 10:1 materials. I am open to any comments.

[DKS]

After a year or so mucking about with casting high quality product in relatively high volume, I've pretty much settled on the Smooth On line. For molds I use various grades of Mold Max, which is a 10:1 product. The different grades (10, 20, 30, etc.) relate to different hardnesses, which comes in handy for molding different items. Then I use their Smooth Cast line for resin casting. Again, I use different grades, which translate into different pot lifetimes, from 10 minutes to over an hour. These distinctions are vital when designing part and molds. And I also pressurize the molds as they cure, likewise the casting resin. I don't use vacuum, and I also use no mold release on anything. I have not found a need for it; the only catch I've found is that the mold material will fuse permanently to unprimed wood, so I always paint wooden masters prior to molding. Oh, and I also use Smooth On So-Strong dyes to tint the castings. That's about all I can say; customers seem pretty happy with my products, so I must be doing something right.

[pnolan48]

Thanks, David.

I guess I've just had problems deciphering the Smooth-On website--so I end up buying their products from a California firm, Douglas & Sturgess, which answers my questions quickly and has good prices. As I've gotten older, my brain has just gotten denser. I guess I'm just going to have to move to the 10:1 products for the best selection.

I guess I should pressurize the smaller molds and castings. As I mentioned on another thread, I'm not sure I can safely pressurize molds or castings for large items like hulls and superstructures. I've found that drawing a vacuum on the mold materials not only reduces bubbles, but also removes a lot of moisture, which is a problem in SW Ohio. Yes, it can create a mess when foaming, but I've learned to use large containers. I no longer use vacuum for the materials for large castings, as I've seen no difference. I've also found that, as shelf life increases, the amount of time in the vacuum should also increase, until I'm using 30 minutes of a 6 CFM pump. If I don't use a vacuum, I'm getting soft spots on the molds, because the RTV materials have been opened too many times, even in an A/c environment.

[DKS]

I have a humidity issue as well (>60% in the summer). So far, I have not had any foaming issues. In fact, the only time I do have a problem is due to operator error. I'm also using these materials constantly, and they don't hang around long. For those materials that I do expect to have for a while, though, I give them a dose of dry blanket spray.

[peteski]

I also use SmoothOn resins, tints and rubber.  They usually have a local distributor in your local area who can be reached over the phone and can assist in figuring out what is the best material for the items you're casting.