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Wednesday, September 25, 2024

First attempts at rock casting

I've been hesitant about casting rocks and starting mountain detailing, but a couple of days ago I decided to just jump right into it.

Over the summer I made about a dozen latex rock molds - a few are shown here. The idea was to give me some variety and spares in case I ruined some, which it turns out I did.

Up there is my first attempt. I used CelluClay as the casting material. 


After around 6 hours I peeled off the mold and that is what I was left with. The casting is a bit furry and rough as I haven't cleaned it up, but it seems rocky. I should note I used a mixture of castor oil and IPA as a mold release. I think when I tried casting rocks back in the 70s I used vaseline as a mold release; however, I read somewhere recently that a mold release shouldn't be based on petroleum products as it will attack latex. I don't recall what happened to those 70s molds, but I figured better safe than sorry as I won't be able to make any more molds until next spring.

I should also note that after 6 hours the casting was still soft. I figured that was due to limited air exposure while the mold was still on. After peeling off the mold, and allowing the casting to dry with its outer surface exposed to the air, the casting was hardened by the next morning.


I also tried making a casting with Great Stuff spray foam. I squirted some in a mold, made sure the mold was evenly coated, and applied it to the mountain side.


This wasn't successful. The upshot was I was left with a gloppy mess and a ruined mold. I think this was due to little air exposure for the foam and a not thick enough coating of mold release. I may revisit this by spraying foam in a mold, letting it hardened, and then gluing the foam rock to the mountain. It's a good thing I made a few extra molds. 

Anyway, foamy blues aside, it's happy days here making rocks at all hours of the day and night. 

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4 comments:

  1. Michael LeddySeptember 25, 2024 at 9:11 AM

    Rocksteady.

    ReplyDelete
  2. DerrellOctober 30, 2024 at 12:35 AM

    When casting parts using either epoxy or Urethane in RTV rubber molds it's a good idea to use a parting solution. In the case of parts I've settled on a mix of Vaseline Petroleum Jelly dissolved in paint thinner and I use an airbrush to apply it. Let the thinner evaporate away for a few minutes and then mix and pour the casting material. Wash the castings in soap and water so paint will adhere.

    So. I wonder if this would work on a latex mold and the spray foam?

    This tip is from my friend Doug Heitkamp of Design Tech Models

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    Replies
    1. J D LoweOctober 30, 2024 at 7:24 AM

      Thanks for the tip. I'm almost done mountain work on this project, but I have another little rocky layout in mind that I may give it try on. I think spray foam has a lot of potential and I don't want to give up on it yet. FYI: For mold release on this project I used a mixture of castor oil and IPA. I heard somewhere that petroleum based lubricates could attack latex, so castor oil, being made from the bean, was a good alternative. It did work ok for papier mache castings, but it clearly wasn't suitable for foam. Back in olden days I used vaseline and it worked fine without adverse impacts to the molds - as you recommend, I should try that again.

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