A Neat Little Casting Technique

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  • He Who Shall Not Be Named
    Moderator
    • Aug 2008
    • 12253

    A Neat Little Casting Technique

    I needed twelve little tear-drop shaped items to fasten to the forward end of the hull, just a few scale feet below the waterline. These would represent the anti-corrosion zinc anodes along the all-steel portions of the Type-212's hull. The after portion of the submarines hull is presumably GRP -- the pressure hull stepping radically down in diameter aft of the sail, making room in the annular space between after pressure hull and GRP outer hull for the many cryogenic pressure bottles associated with the submarines Air Independent Propulsion (AIP) machinery.

    You can see these external zincs in the two pictures of actual Type-212 boats below. There are six of these zincs on each side of the hull. My problem was how to fabricate these little critters quickly, with assured uniformity of form between them, and from a material that was quick curing and also easy to work with. The answer was to use the same Alumilite polyurethane two-part casting resin I used for the larger castings. However, the tools used to give shape to these zinc parts would be a simple, one-shot, open face clay mold.

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    The completed model show off the little zincs, CA'ed in place. Items as small as these tear-drop shaped resin zincs are best handled with tweezers and/or the tip of a sharp #11 X-Acto blade. Though I was tempted to leave these little detail items off the model, I found that their inclusion greatly enhanced the 'busy' aspects of the subject.

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    Here is a shot of the in-mass grouping of the Type-212 tear-drop shaped zincs. The thin connecting film is an artifact of the casting process. Removing the individual zincs from this fret was done by sliding the fret over a piece of #400 sandpaper till the film was worn away, leaving the individual zinc parts, ready for final sanding, priming, and installation to the Type-212's hull. The Mold was formed from oil-clay that had been rolled out flat on a moldboard and the tear-drop cavities formed with a piece of plastic square-section rod that had one end cut and sanded to the desired shape. To one side of this form I glued a depth-stop -- it's job to insure that all cavities pressed into the clay were of uniform depth.

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    Once all the cavities had been pushed into the clay, I sprayed on some Mann 200 silicon mold-release spray; mixed up a small amount of casting resin, poured it over the clay; then laid down a piece of wax-paper. Capillary action pulled the wax-paper down tight onto the surface of the clay, minimizing the thickness of the resin film that formed over what would become the base of the little zinc parts. Once cured the cast fret of zincs was pulled away from the clay, dunked in a container of lacquer thinner and scrubbed with a stiff brush to get rid of any clinging mold-release.

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    Each zinc was painted primer gray. Then, using the drawing of the Type-212, I plotted the longitudinal location of each zincs onto a length of masking tape. Using that tape as a guide I CA'ed each zinc in place on the hull

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    Last edited by Kazzer; 01-30-2011, 05:53 PM.
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  • redboat219
    Admiral
    • Dec 2008
    • 2735

    #2
    I read the same technique being used by aircraft model builders in FineScale Modelers when they fabricate some small items like antennas, RWR, sensors.

    Another technique would be instead of using clay to make your mold you use epoxy putty. You then use the same putty to fill in the mold cavities.

    About casting techniques, this morning I was using some high temperature RTV to seal around where the rubber hose comes into my car's radiator.
    I was wondering if we can use it to make a small mold for casting small metal items?
    Last edited by redboat219; 04-21-2010, 11:00 PM.
    Make it simple, make strong, make it work!

    Comment

    • Subculture
      Admiral
      • Feb 2009
      • 2119

      #3
      I do a similar thing with candle wax (which is self releasing). Have also used plaster for quick and dirty tooling.

      Comment

      • Steampunk
        Lieutenant
        • Feb 2010
        • 62

        #4
        Thanks for posting yet another interesting and helpful technique, Mr. Dave Sir!

        For what it's worth, since other posters are kicking in their favorite, similar fabrication solutions: I'm lately using Milliput to make both small molds - "quick and dirty tooling," as another poster had fondly termed the usage - and small castings. I'm finding it works very well, and is becoming an used-often, relied-on way for me to make subassemblies and the like. I'm liking it enough that I'm starting to make larger and larger parts with it -- a small angled turret for an ironclad, for example; or some armored-base-around-a-smokestack detail parts.

        Instead of using the standard grade of milliput, though, I'm mostly using their relatively new "black" formula, along with their "superfine white". Black mixed with whitle comes out to a nuetral gray: which I prefer, over most other color choices. Just seems easier and more pleasant to really see what's going on, with gray.

        Mixing it is easy enough. Just figure out how much of it you will need, total, for a given project: then kind of eyeball what looks like one quarter of that amount; and take that much of each of the resin and hardener materials, as supplied with the black; and ditto for the white; and when you mix all four of them together, you get (hopefully the right amount of) a material that's pretty easy to work with; cuts and sands and files and so on, pretty nicely once it hardens, and so forth.

        It takes a bit of practice to get the relatively thick (in viscosity) milliput to pick up every little detail of the tool's surface, at first: so I'll throw out two tips. One is that Merriman talked me into buying some of the Butcher's Bowling Alley Wax, as a mold release agent, ages ago. Using that on the surface of whatever I'm trying to copy (if, say, I make a little intermediate master part, that I'm making a small mold of) keeps things from sticking together, once the milliput hardens. What I found, though, is that the milliput may sort of leave fissures (for lack of a better way to describe it) if you add some of the milliput into the mold; then add another blob right next to it. Or it won't press into every tiny little cavity or detail of the master mold ... unless you ever so slightly dampen the milliput's surface, first. I've gotten some surprising-to-me, "holy bleep, that's awesome!" results, by using the Butcher's product, along with slightly dampening the surface of the milliput I'm about to add. (I made a quick press mold of a dime, and could read every letter with ease.) Due to insufficient practice, though, I don't feel I'm an absolute master of the process yet. I still get occasional portions of the surface of a mold or a casting that don't 100% cooperate -- but most of it is fine, and the rest is still well within the range of "just adjust the part, later". (Or, as David is doing: just make more castings than you really need, and pick-and-choose the best of the copies.)

        Hope that helps someone out there.

        Comment

        • Subculture
          Admiral
          • Feb 2009
          • 2119

          #5
          Devcon do a nice epoxy putty. Cheaper than Milliput too.



          Also worth looking at Sylmasta- http://www.sylmasta.com/acatalog/Onl...utties_23.html

          And of course Mr Caswell does some epoxy sticks on this site. Not exactly round the corner for me however! ;)

          Comment

          • Steampunk
            Lieutenant
            • Feb 2010
            • 62

            #6
            Actually, come to think of it, I found this bit to be pretty helpful, all by itself:

            "Capillary action pulled the wax-paper down tight onto the surface of the clay, minimizing the thickness of the resin film that formed over what would become the base of the little zinc parts."

            I'm just now trying out fiberglass, along with (slow-drying, cheapish hobby store brands of) epoxy resin, in a "learn more about it" sort of way (before I use it in any major projects, I mean) ... and I found that one of the "do it mostly to learn more" experiments / projects I worked on this week, worked out much better, once I used wax paper over a nearly-vertical portion of the surface. It kept the slow-drying stuff from oozing down, before it set. Nice little trick there, Mr. Merriman! (Of course, using a thixotropic agent or a faster-curing epoxy would have been better ... but I like learning about alternatives, as I go; and I like learning with cheaper grades of materials, up front; figuring I can always switch over to the better / more expensive stuff, once I've built up a bit of confidence with the cheaper stuff.)

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