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RTV, hard or soft
6 replies to this topic
Posted 17 February 2008 - 08:09 PM
Before I make my next purchase of RTV, does anyone have any opinions on the advantages and disadvantages of the hardness of the compound or have any preferences. Also, which brand do you prefer.
My first purchase was thixotropic RTV, it mixes to the consistency of a thick paste. I went for this product in light of all the threads discussing the expense of RTV and how to cut down the cost. Thixotropic RTV goes where you put it and usually ends up the shape of a cow pat. With a little practice, it makes a reasonable mold, but clamping the two halves together is a nightmare, so never again, I would sooner pay the extra.
Posted 17 February 2008 - 08:50 PM
I have little experience with it, and what little I do have wasn't good. I bought some from Contenti with the intention of making a master part for a jig.
The stuff took forever to set, which the instructions stated like 16 hours. Once set and after several days before I tried to use it, it was still too flexible for me. When clamping the two halves very lightly together, it distorted the shape of the part. The RTV I had was too soft for my liking and I was unaware that there were different hardnesses to it. I am sure that much of the failure was my doing, but a bad experience none the less.
Posted 17 February 2008 - 09:04 PM
I can see how that would be discouraging. 16 hours does seem excessive, the stuff I used was solid in less than an hour and I could throw the second half of the mold after two hours.
I can see how a softer RTV would give clamping problems, as I did experience some of that myself. This is a good enough reason to go for a firmer product.
So, an additional question: What are the normal set times that other members are experiencing?
Posted 17 February 2008 - 09:50 PM
When we first started the site I experimented with samples from quite a few manufacturers. I tried some with a high hardness of tire rubber on down to very soft durometer similar to heat set plastisols.
My preference is with a mid range shore hardness @25-30 These types work well for me no matter what I'm casting........ foam, urethane, plastisol, whatever.
A harder RTV will rip/tear more easily, but they supposedly last longer.
Much depends on what you intend on casting & the detail needed in the mold.
Higher shore hardnesses usually come with higher viscosity & mold detail can suffer without proper degassing equipment.
For cure times, I like Smooth-on products, particularly Oomoo RTV silicones.
Pot life is around 15 minutes & de-mold in about an hour & a half.
My second choice would be Quantum Silicones who make a good high-temp product.
Posted 17 February 2008 - 09:51 PM
I am going to make a "high tear strength" RTV mold soon. I will let you know how it turns out. The regular stuff was OK, for me, but I had problems with the mold "degrading"( meaning it didn't seal as well and the edges started to tear and loose some detail) after a few hundred pours.
Posted 17 February 2008 - 10:43 PM
Thanks Redg8r, good information.
Oomoo 25 seems to fit the bill, 15 min pot life, 75 min cure and shore hardness of 25.
Oomoo 30 has a cure time of 30 min, but the cure time is 6 hours and a shore hardness of 30. I don't think I have the patience for that cure time.
Hawk, good luck with your project.
Posted 18 February 2008 - 09:28 PM
I used moldmax 30 last time by smoothon and liked it quite well.