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Thread: Casting parts

  1. #1
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    Casting parts

    Had my first go at metal casting. I like the Umarex Custom Shop Colt but it has no front sight. Made a high temp rubber mould and cast one in pewter to fit into the comp slots on the Colt barrel. The casting can be smoothed out better than shown here.

    Baz

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    Thumbs up

    Nice job!!

    I've worked in a foundry for 39 years, we used to make all sorts of stuff when the gaffer wasn't around, can't do it now though as there's cameras everywhere

    My bro in law's lad had a 1377 crosman some years ago and the plastic enplug snapped so I used it to make a mould then poured one in aluminium, it turned out fine and he could carry on with his plinking!!


    John
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  3. #3
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    Castings not a bad option. One possible advantage of resin though is it can be coloured so you don't have to 're black the sights if they get rubbed or wear. Alloy has dome advantages too. Done lots of grips in resin, black or mottled brown, some odd ones too in creamy ivory colour with a vein in it. Made a few bright coloured ones too look ok till you put them on a pistol then they look a bit out of place. Everything's worth a try I suppose.

    Small parts with large moulds can sometimes take the heat away, cooling the molten metal giving a wavy pattern, in the little casting I've done I found a quicker pouring technique was better but obviously you've got to beware of the risks if it splatters or you get a blowback from trapping air. Not so problematic with resin but then some of them set almost as soon as they are mixed, even without an acellerator/catalyst.

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    Here is the sight after fettling and ready for painting satin black. I have come to the conclusion it is not worth casting these parts as the finished article for some reason is a few thou up on all surfaces and does not fit as my the original one I made from plastic metal to make the silicone mould. Does anyone know why this happens ? Had to take a lot off the height of the sight to get bb impact in the right area.

    Baz

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    Last edited by Benelli B76; 08-06-2018 at 07:29 PM.
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  5. #5
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    Baz, I think there are quite a few varriables that can interact with one another in the casting processes.
    Many materials will contract as they cure, usually dependant on the chemical type & the filler loading. Generally eoxies shrink less either as a mould material or as a part, polyurethanes shrink more as a general rule, but they can also expand due to moisture content or moisture pick up during storage, mixing & curing. There other chemistries & hybrids too. The hardeners used for polyurethanes in particular can absorb moisture on storage so we used to keep them under dry nitrogen. Silicone mold making stuff can absorb moisture too, this can make them swell. Then there's thermal expansion with heat, softening of the mold with heat might allow it to move under the pressure/weight of the casting medium. All a bit of a game, upshot is that if the cast item ends up a little larger at least it can be worked down to a good fit, I I think that's better than it being too small. Not so much that can be done if it casts on the small side.

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    Quote Originally Posted by trajectory View Post
    Baz, I think there are quite a few varriables that can interact with one another in the casting processes.
    Many materials will contract as they cure, usually dependant on the chemical type & the filler loading. Generally eoxies shrink less either as a mould material or as a part, polyurethanes shrink more as a general rule, but they can also expand due to moisture content or moisture pick up during storage, mixing & curing. There other chemistries & hybrids too. The hardeners used for polyurethanes in particular can absorb moisture on storage so we used to keep them under dry nitrogen. Silicone mold making stuff can absorb moisture too, this can make them swell. Then there's thermal expansion with heat, softening of the mold with heat might allow it to move under the pressure/weight of the casting medium. All a bit of a game, upshot is that if the cast item ends up a little larger at least it can be worked down to a good fit, I I think that's better than it being too small. Not so much that can be done if it casts on the small side.
    Very interesting, thanks. Looking at this makes me think the silicone mould is too soft and moving under the weight and heat of the molten metal. As you say it can be fettled down which is not too much of a problem if you are making a one off for yourself, but a pain if making multiple copies. Think I will look around for a harder silicone mix.

    Baz
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  7. #7
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    Quote Originally Posted by Benelli B76 View Post
    Here is the sight after fettling and ready for painting satin black. I have come to the conclusion it is not worth casting these parts as the finished article for some reason is a few thou up on all surfaces and does not fit as my the original one I made from plastic metal to make the silicone mould. Does anyone know why this happens ? Had to take a lot off the height of the sight to get bb impact in the right area.

    Baz

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    Good work tho Baz, I'd be interested to see how this works out, I'll have a look when I'm round next.

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    Quote Originally Posted by cozzamec View Post
    Good work tho Baz, I'd be interested to see how this works out, I'll have a look when I'm round next.
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    Quote Originally Posted by Benelli B76 View Post
    Here is the sight after fettling and ready for painting satin black. I have come to the conclusion it is not worth casting these parts as the finished article for some reason is a few thou up on all surfaces and does not fit as my the original one I made from plastic metal to make the silicone mould. Does anyone know why this happens ? Had to take a lot off the height of the sight to get bb impact in the right area.

    Baz

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    Nice Job,
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  10. #10
    ccdjg is offline Airgun Alchemist, Collector and Scribe
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    Nice work Baz. I am curious about the pewter that you used. What was its melting point? If it is too high I suppose it could cause some distortion of the mould even if the rubber resin is claimed to be high temperature resistant. I know you can get casting metal alloys based on bismuth that are very low melting, even in hot water, so perhaps one of these would give sharper definition and need less fettling.
    Cheers
    John

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    It looks like it was designed to be there, very well done for achieving that look!
    Its strange to see a revolver with no way of aiming it, Im assuming this would have been a very CQB style of pistol.

    Just as an aside what were the slots originally designed for?

  12. #12
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    Quote Originally Posted by Rob Anybody View Post
    It looks like it was designed to be there, very well done for achieving that look!
    Its strange to see a revolver with no way of aiming it, Im assuming this would have been a very CQB style of pistol.

    Just as an aside what were the slots originally designed for?
    Revolver is a copy of Colt used by Stallone in film Expendables, the slots are compensator slots they cut into barrel on real firearm to reduce recoil by exhausting some gas vertically.

    Baz

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    Quote Originally Posted by ccdjg View Post
    Nice work Baz. I am curious about the pewter that you used. What was its melting point? If it is too high I suppose it could cause some distortion of the mould even if the rubber resin is claimed to be high temperature resistant. I know you can get casting metal alloys based on bismuth that are very low melting, even in hot water, so perhaps one of these would give sharper definition and need less fettling.
    Cheers
    John
    John, it is supposed to melt at 330 C and the silicone is rated at 370. I have found this pewter to be quite hard and strong, very surprising.

    Baz
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