Anyone mold silicone or urethane?

Discussion in 'Fiberglass and Composite Boat Building' started by aaronhl, Oct 13, 2013.

  1. aaronhl
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    aaronhl Senior Member

    Looking to mold a small square gasket about 1/8" thick. I have been researching online for the last several hours to find some products that may work on smooth-on.com.

    I was wondering if I would be able to use a urethane rubber mold over a plug and then pour liquid urethane rubber to create the gasket? I need a mold setup that will be able to accept liquid rubber that when hardened could be flexible, not tear, and resist warm temperatures and gasoline. What do you think?
     
  2. CDK
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    CDK retired engineer

    There was a gasket making kit on the market, using 2C polyurethane. The large can contained a black syrup, the small one was a clear liquid. And there was a small plastic bottle of oil and a brush.
    The instruction leaflet said you should use plaster of Paris to make a mold, coat with a thin film of oil and pour the thoroughly mixed resin.

    I bought it long before the need arose to make a gasket, so when I needed it a year or more later, the black syrup already had changed state, probably because of too high storage temperature...
     
  3. PAR
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    PAR Yacht Designer/Builder

    A latex mold over your plug would be the most common way to go. It can be poured or brushed onto the plug and when dry, you apply whatever your gasket material will be.

    What is your "liquid rubber" product? As I would think this the determining factor. I've molded polyurethane, polysulfide and a number of other materials. Release materials can be tricky with some of these.

    Seems a lot of bother for a small square gasket. Why not just make a wooden female, wax it up with several coats of automotive wax, then pour in your gasket material? I've made window gaskets like this several times.
     
  4. aaronhl
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    aaronhl Senior Member

    Thanks for the help guys. I am really looking for ANY ideas how to mold a small 1/8" thick 5" x 6" frame gaset. The gasket needs to be tough silicone or soft rubber. Not really sure what products to use? Specifically for the gasket to pour in the mold.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Again, make a wooden female mold and you can use 3M-5200 (or your goo of choice) for a tough, polyurethane gasket. I've done this a number of times, it works. Last time I did it for some windshield gaskets on an Owens, previously I made opening port gaskets, for some old, odd shaped bronze pieces.
     
  6. aaronhl
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    aaronhl Senior Member

    Great information, I am going to look into that
     
  7. PAR
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    PAR Yacht Designer/Builder

    If you want an 1/8" thick gasket, make a wooden (wax, foam, cardboard, or whatever) perimeter that defines the outside edge. Do the same for the inside edge, of course, using 1/8" or ever so slightly thicker stock, to account for some goo shrinkage. You can incorporate dowels for fastener holes too. Glue these pieces down to a smooth substrate, like sanded plywood and a very small fillet of glue along the contact points will help with release. Seal it with polyurethane paint, epoxy or something fairly tough and let it dry for a few days, so it's good and hard. Once it's ready, coat with PVA mold release, automotive wax or whatever works on the goo you'll use. Lastly, apply the goo of choice, maybe using a plastic applicator to smooth off the top or just use another piece of waxed plywood, that'll cover the whole shooting match and place a brick on it, until it's good and cured. Most polyurethanes will need a few days to be hard enough to remove from a mold like this without tearing when you try to peel it out. If using polyurethane, a high humidity area will speed up the cure significantly.
     
  8. aaronhl
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    aaronhl Senior Member

    Wow great info...I was thinking of using an arcylic floor, how do you think that would work? But wood would probably be easier

    If I use black 3m 5200, what mold release would work best? wax and pva?
     
  9. dougfrolich
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    dougfrolich Senior Member

  10. SukiSolo
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    SukiSolo Senior Member

    If you are after a flat gasket, why not visit your local upholsterer or a good rubber and foam supplier. I suspect a decent quality Nitrile rubber could be cut by knife and I think that material may be enough for what you want. Other materials such as EPDM are available and some Silicone sheet too.
    If you want to mould it and it is not a flat shape you need to shake it or vacuum it to get the air bubbles out after you have filled the mould. This should be done even if you have vacuumed the material prior to pouring.
     
  11. aaronhl
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    aaronhl Senior Member

    Yes it's a flat gasket, measurements are down to the 32nd, so the .140 diameter holes need to be accurate, shown below. 1/8" thick nitrile 60 dura works, maybe something not as stiff would work better.

    If I knew how to cut it accurately I would do it. Right now I have a shop that cuts them on a machine but they are very expensive. Any idea how to cut the holes?

    [​IMG]
     
  12. PAR
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    PAR Yacht Designer/Builder

    Sweet God that's so simple you can use an Exacto knife. A mold for that, including the fastener holes, would take less than an hour to make, with 1/8" plywood, some TiteBond III and some Turtle Wax. EDPM isn't the best thing for certain solvents and fuels - what's this gasket for?

    If looking to mass produce this gasket, I'd have a steel mold machined and pour or pump in liquid gasket material, such as Nipol 1312 (TSE).
     
  13. aaronhl
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    aaronhl Senior Member

    This forum is great because everyone is so helpful in my threads, everyone has a different way of doing things and its always got me thinking!!

    The gasket is for a watertight lid on the square box in the boat on the right side

    [​IMG]

    [​IMG]
     
  14. SukiSolo
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    SukiSolo Senior Member

    A mould for that gasket, is like PAR says a very simple thing. But you can also knife it and use a leather or hole punch to get the 10 through holes. That's good enough to let you assess materials, and maybe thickness.

    BTW I only mentioned EPDM as a material because for certain applications it is the best choice. Not for this one here. Mostly you can select the correct material by chemical resistance and mechanical properties charts. However nothing beats a bit of real world application and use!.

    Oh and my car rides on hydraulics (no springs) so the seals etc have to work with 2600 psi from the hydraulic pump....and she's 22 years old.
     

  15. PAR
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    PAR Yacht Designer/Builder

    If you just need these occasionally, I'd knife them from whatever material you find suitable. If you want to mass produce them, a simple machined mold (two piece) wouldn't be a difficult thing to make up and you can injection mold or cast them, again with the material of choice.
     
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