The Shop > Metal Stuff
Petrobond moulding sand
mattinker:
--- Quote from: vtsteam on March 11, 2022, 08:20:54 PM ---There are two types of bentonite as clay used by foundry men and also potters here, Southern and Western. They have somewhat different properties. Bentonite is also used in cattle feed and is available from farm supply stores. Also it is used for a pond sealant.
I think Bentonite is wasted on most casting metals other then iron. By that I mean it isn't necessary to produce high quality castings and any good ceramics grade fireclay, like Hawthorn will work perfectly well for that.
--- End quote ---
Steve,
When I researched making Petrobond, the site I referred to talked about Bentonite and Bentone, Bentonite is apparently readily available the world over, Bentone, which is used for the oil based sand is much harder to find, at least here in Europe, apparently, it's used in cosmetics. I still haven't found a source for it!
Cheers, Matthew
vtsteam:
Haven't heard of Bentone before, Matthew. I've never used real Petrobond -- too expensive for my tiny hobby budget, both in original cost and for shipping, since it is heavy. Also, the idea of handling and breathing burnt fumes of some secret commercial oil formulation wasn't as appealing as just working with plain water and steam.
But I am curious about it. I've watched at least one video about someone "making petrobond". It turned out that he was using sanded tile grout mixed with motor oil -- 25 lbs of the grout and a quart of the oil. The video didn't inspire me to go out and try it, and it didn't seem convincing that the sand he'd come up with was the equivalent to Petrobond. I guess partly because he seemed so odd in his casting practice anyway. Also, modern sanded grout has some odd organic additives. But again it was of interest. And, who knows, maybe his results were equivalent, and the health effects not pressing. The camera work wasn't good enough to really read the casting finish quality.
My understanding about Petrobond is that casting finish for aluminum is greatly improved by it -- that's its main advantage. Also that it isn't applicable for iron, but I may be wrong in that.
I think little on the internet has been devoted to improving finish for aluminum casting using conventional water/clay based greensand. But actually there is a fair amount you can do. I think using a facing sand is important, as is moisture control, and understanding local ramming pressure to yield a high quality result.
I would like to try Petrobond to see for myself if there is an advantage in finish, compared to what I'm used to and to what degree it's improved. But as I said, it's not in my tiny budget for shop work.
mattinker:
Steve,
my understanding was that as there was less expansion with the oil than with steam, finer sand could be used as it didn't need to be as open to the expansion. This is where the Bentone comes in as it sticks the sand together using oil to "wet" it.
Here's a link to a K bond sad recepie.
https://www.homemodelenginemachinist.com/threads/foundry-sand.30505/
Cheers Matthew
vtsteam:
Matt, here's a retail supplier of a variety of bentone (there seem to be several mfr's and types listed on the net).
https://www.earthpigments.com/bentone-hd/#techdata
vtsteam:
That recipe is interesting Matt. One thing I notice is that it calls for 100 mesh sand for the K-bond. That's not finer than conventional aluminum greensand, and what I've found, personally is that even finer sand can be used in a greensand aluminum mold if it is used as a facing sand, while backing it with more porous sand. My regular greensand is 50 mesh, I believe, but I've used up to 120 as facing.
I'd really like to do side by side castings in Petrobond (or K-bond) and faced greensand, because the only way I can really tell what the advantage is (and whether it is justified in my own use) is to actually try it myself. We all do things a little differently, so I can't judge by the results others publish about it.
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