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Thread: Silicone Rubber Molds & Pattern Letters

  1. #1
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    Silicone Rubber Molds & Pattern Letters

    Well Guys this is what i'm going for. I have recently recieved a set of Pattern Letters from Freeman Supply. Unfortuantely. I don't have Enough Letters to spell out "Nacogdoches" & "Anthony" Simultaneously for a pattern i'm working on. These are Freemans Flat-Face Gothic Font 1/2". Now i know that i can use Silicone to make a mold for the letters but what i don't know is what is a good Silicone Rubber? I have never dealt with Silicone.

    The second deal is what to use to make the new letters out of? I have heard of Thinned Down Bondo but I don't know what to use to thin it down.

    Any Suggestions would be greatly appreciated.
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  2. #2
    As your letters are probably not intricately detailed with undercuts you would be best going for the hardest type you can get as this will be harder wearing and les prone to distortion. As for casting, some sort of resin, again as hard as you can get.

  3. #3
    Why not cast a few sets in ally.

    Donk
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  4. #4
    Senior Member metalshop's Avatar
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    Here is a smooth on video that has a link to them in the description:


    Link to them:
    http://www.smooth-on.com/media.php
    Last edited by metalshop; 08-14-2012 at 06:52 PM.
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  5. #5
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    Donk. These Things are tiny. I don't know if i can make a mold that would work for an Aluminum Cast. They are Approx 1/2" x 1/2" x <1/8" Thick.
    Clothes make the man. Naked people have little or no influence on society.
    -Mark Twain
    Failure? What failure? This was a sucessful learning experience.
    -Can't Remember who said that.

  6. #6
    Can you mold them on a backing then machine the backing off.
    1/2" isnt that small, what sand are you useing.

    Donk
    Needs must while the devil rides.

  7. #7
    Go for an RTV silicone mold and then you can pour POP to make the letters. That's what I do to copy from pattern letters to master patterns for matchplates with no trouble and not much cost.
    I found that trying to find what I need and then make it work with what I have, is more trouble than designing what I want and doing it.
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  8. #8
    Bondo works really well for replicating small pieces, and it cures fast so cycle time is kept to a minimum. Pretty durable too in the long run

    There was a post a while back about using silicone caulk + cornstarch + mineral spirits to make a castable silicone, I made some but it shrank a bit. Might be worth looking into for your application though

  9. #9
    Administrator Site Admin Anon's Avatar
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    For something that already has draft, like pattern letters, I'd be inclined to cast a POP negative and do the positives in some kind of casting material (Durham's Water Putty is what I use for fillets; it's a POP-ish wood filler that can make small castings like this). Bondo positives would also work, or some kind of plastic resin.

    The reason I say that is, RTV silicones are very expensive, and if you don't need the flexibility to release undercuts, you might as well use cheap POP instead.

    If you have a good fine sand (comparable to a commercial Petrobond; I think they usually run an average of about 110-120 mesh) you might get away with sandcasting them, but I'd be afraid of a loss of detail if you're then going to reuse them for patterns. I'd cast in a potmetal or something else with very good flowability (pick your favorite high-Si aluminum alloy if you don't have any ZA-12 laying around) and if I could get away with it, run a runner on either side and gate into the back in 2 spots with some strategic venting in the back center. That way you have minimal grinding to finish them for pattern letters, instead of having to grind the entire back off. If that didn't do it, I might run a third narrow runner directly across the back of the letters, still better than the entire back being covered.
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  10. #10
    Senior Member Bob S's Avatar
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    I posted these pictures on another thread the other day when ask about something similar. I first just molded up the drag side of the flask on a smooth aluminum plate so I had a nice flat surface to press the letters into. But I first laid these small wooden strip down on top of the sand but not pressed in. I then pressed in the letters next to these strips so the gating entered the backside of the letters making for easier clean up, then rammed up the cope, cut the sprue, closed and poured. I only needed a couple of extra letters but not wanting to waste the space or time I did more then needed since I expected some to not turn out as you can see.

    Some had flashing in and around the letters and some weren't gated to get enough metal but the letters were placed far enough from the strips so the bandsaw could cut then off without hitting the letters. Just some more of my fast and dirty casting work I do at times but I now have more then enough letters to complete longer names and/or places now. These are 3/4" high letters and cast in aluminum, originals were zinc.





    I tried this method but it didn't work out to well. I first cast a plaque with letters the size I wanted and then milled the back off but as you got down to the letter's back they would often break so I had to stop with about a 1/32 of backing still attached to the letter and belt sand the back off the letters and hope I would find them on the floor but I got the letters I needed. Bob.



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