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  1. #1
    Super Moderator curious aardvark's Avatar
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    Really interesting way to make fdm watertight and stronger



    Basically pack in salt and heat in oven.

    Unlike annealing - because the part is almost gas tight, there is very little dimensional loss.
    Also because you are packing it in fine salt - essnetially you are creating a 'lost wax' mould making method - but keeping the wax.

    Essentially you are producing a one shot injection moulded part.

    Ingenious :-)
    Plus salt is cheap and almost completely inert.

    Very clever bit of thinking.

    I will be trying this at some point.

    Actualy watertight vases !
    Now that's something you can actually sell.

  2. #2
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    Thanks for sharing this. I tried it last night. I have some coasters I made.20200924_085808.jpgIt was just something functional I made just for my use. I was having extruder problems when I printed them, so they didn't fill in correctly and are even less water tight than a normal print. So I thought this would be a good thing to test.The first time, I took it out way too quick. The coaster was still soft. I just set it flat on the counter and it cooled OK. You can see the sides are kind of out of shape. It is also very rough.20200924_085834.jpgI ground the salt a little more - I thought that it was fine enough, but maybe not.It was late when the 2nd try came out of the oven, so I just let it sit overnight. It did much better.20200924_090016.jpgIt's still a little rough, but better and it held it's shape. I think it did shrink a little, but I didn't measure so I'm not sure.I tested the first one with some sandpaper. I just did one corner of the bottom. I don't know if you can tell from this pic, but it smoothed out really well. 20200924_090050.jpgIt does change the color but I think it looks worse in the pics than it really is. It definitely makes the print waterproof and much stronger. I won't use it for everything, but I'm going to remember this for when I need the solidity. I tried once to make some fittings for my pond plumbing and that didn't work so well. I bet it will now.I will probably try grinding the salt a little more before I try again. With a good sanding and maybe a coat of varnish or something for shine, you could make it look really nice. I will probably try this on something that I would like to have a smoother finish just to see how it looks.Thanks for the tip

  3. #3
    Engineer-in-Training
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    Cool technique. Casting sand like Delft Clay or Petrobond would probably work for this. Casting sand is extremely fine and can take very high temperatures.

  4. #4
    Staff Engineer Davo's Avatar
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    Now that's a great idea.

  5. #5
    Super Moderator curious aardvark's Avatar
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    Quote Originally Posted by jamcultur View Post
    Cool technique. Casting sand like Delft Clay or Petrobond would probably work for this. Casting sand is extremely fine and can take very high temperatures.
    Yes sand would work - up to a point.
    bear in mind that the sand will get embedded in the plastic. And getting rid of sand is really hard. You can't 'sand' it off as it's as hard as the sanding paper.
    Silicon dioxide is tough stuff.

    The really clever thing about using salt is that it is water soluble.
    So all you do to remove the embedded salt is to rinse or soak it off.
    The other clever thing about salt is that you can grind it to different 'grit' sizes yourself.
    If you only have coarse sand - you cannot grind that smaller yourself.

    If you wanted to make an abrasive block or some custom shaped 'files' then you could use sand.
    Of just pack the area round the print with tungsten carbide dust. And use salt or sand for the bulk.

    But for non-file type uses - you do need to use salt.

  6. #6
    Engineer-in-Training
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    Quote Originally Posted by curious aardvark View Post
    Yes sand would work - up to a point.
    bear in mind that the sand will get embedded in the plastic. And getting rid of sand is really hard. You can't 'sand' it off as it's as hard as the sanding paper.
    Silicon dioxide is tough stuff.

    The really clever thing about using salt is that it is water soluble.
    So all you do to remove the embedded salt is to rinse or soak it off.
    The other clever thing about salt is that you can grind it to different 'grit' sizes yourself.
    If you only have coarse sand - you cannot grind that smaller yourself.

    If you wanted to make an abrasive block or some custom shaped 'files' then you could use sand.
    Of just pack the area round the print with tungsten carbide dust. And use salt or sand for the bulk.

    But for non-file type uses - you do need to use salt.
    The sand used for sand casting is quite different from ordinary sand. The grain size is much much smaller, and the better types (like Petrobond) have an oily coating that prevent molten metal from sticking to them. The oily ones get a bit smelly when you pour molten metal into them as some of the oil burns off, but that shouldn't be a problem with PLA. I'd guess that the non-stick coating in Petrobond would also work with PLA, but there's only one way to find out.

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