Help - bronze problems

Discussion in 'Home Foundry & Casting Projects' started by MRA, Nov 14, 2014.

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  1. Nov 14, 2014 #1

    MRA

    MRA

    MRA

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    Here's some photos of me trying to make a plaque for a mate's dead dog. I've had success in Al, but not much experience yet - so maybe moving on to bronze was too ambitious just yet.

    Pic 1 - pattern and impression - seemed fine
    Pic 2 - my home-made waste oil engine burner going fine. Oil reservoir us up high just to right of camera (you can see the delivery hose), and air is being blown into the end of that stainless pipe with a leaf blower. Crucible is about a pint, and full of bits and bobs of scrap bronze.
    Pic 3 - oh no. Freezes in in-gate without filling mold.
    Pic 4 - can't even get the remainder out of the crucible and pour some bun-ingots before it goes lumpy.

    There was no flux or other additives involved. I have to swap from lifting tongs to pouring shank, which takes a minute or so. So what do you think, folks - do I just need to get it hotter? Or do I need a helper to speed up the time from oil-off to pour? Or do I need to change the in-gate? Or do I need to add something clever to the mix?

    cheers

    Mark, Manchester, UK

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  2. Nov 14, 2014 #2

    jasonh

    jasonh

    jasonh

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    I'm not an expert with Bronze either - but I'm having an easier time with brass than you are with bronze, so here are my observations... :)

    >air is being blown into the end of that stainless pipe with a leaf blower
    A leaf blower is too much air and it looks like you have the end of the leaf blower and the pipe separated. A hair dryer might be a better choice- and you wouldn't have to deal with all that extra air blowing around.

    >bits and bobs of scrap bronze.
    So you have a lot of dross. Did you skim the melt before the pour? It looks like there is a lot of dross on the muffin tin.

    >Freezes in in-gate without filling mold
    I'm not sure what's going on with the gating in the other side of the mold, but it looks like you are trying to gate in through the side wall. That's really narrow. A better approach may be to have the business end of the pattern in the cope and then gate it up through the drag into the reverse side. Then you'll have to machine off the gate. It'll be more cleanup, but it will be a big gate that fills the cavity from the bottom. Also- as cheap insurance add some sprue extensions to increase the hydro-static pressure.

    >I have to swap from lifting tongs to pouring shank, which takes a minute or so.
    That's too long. When you see the pros pour bronze it is glowing- if it's going lumpy in the crucible it's probably too cold, and/or too full of dross.

    In general when you are pouring castings of a plate type nature (broad expanse, thin cross section) it's going to lose a lot of heat to the sand as it flows across and you run the risk of premature freezing. So:

    * Start hot
    * Use a flux (lower the melt viscosity)
    * Be generous with the in gate area
    * Add a sprue extension to get the pressure up

    That's my 20c - hope it helps.

    Jason H.
     
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  3. Nov 14, 2014 #3

    aonemarine

    aonemarine

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    Bits and blobs of scrap bronze?? Where did this bronze come from? Its looking more to me like you may have red brass??? Or maybe created a new alloy? Sure is alot of flame jumping out of the furnace, maybe not combusting in the furnace? Ugg, I see lots of issues, hard to pin point anything specific. I would suggest ordering a good known alloy to work with for starters...
     
  4. Nov 15, 2014 #4

    MRA

    MRA

    MRA

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    Thanks folks. Yes, there was a lot of flame coming out the exhaust (I went for a spectacular shot for the photo, I think!) - usually I throttle it down a bit on air/oil to make better use of the oil. Sure was burning in there, but I only have about 1" clearance on the radius between crucible and furnace wall, which I guess might limit me a bit for temperature. I also have say 1.5" radius of furnace lining and no insulation, but paint is still on the old gas bottle furnace, so it is not getting so very hot on the outside and losing heat there.

    I need quite a bit of air to pull the pre-heated, but sh*tty-old motor oil out the end of the tube in the home-made burner. I know when it's right since too lean and the exhaust flame thins right out, too rich and it smokes/stinks like 1000 Morris Marinas. (British Leyland joke for UK readers). The non-contact between blower pipe and stainless tube is intentional, so allow cheap and nasty fine air tuning by kicking the leaf blower more/less to one side. I have something more fancy made up for my sawdust extractor, but it seems to work no better.

    I forgot to mention that I ran out of oil 4/5 of way through melt and had to pause / refill. I will have lost some temperature there too - so will re-jig fuel tank to allow refilling without pausing things, before trying again. I have a source of spoiled hydraulic oil which I may use to thin-down used motor oil next time, and see if I get more heat that way too. Trying to get a result and spend almost no money is a big part of the fun for me!

    I was worried about pouring straight into the back of the plaque, since I thought flow straight down into the face of mold at the bottom of the pouring hole would mess it up. My top half (sorry - cope? drag?) is totally plain at the moment - I guess I could leave the bottom half as the first picture, but thicken-up the ingate in the top half, and machine it off later?

    Flux - any recommendations? Having bought the part-remains of someone else's casting hobby a long time ago (before I built my furnace / burner), I seem to own a small pot of brazing flux, and a bag of something looking like small corroded blue balls marked as '15% phos copper'. I think it may have been suggested that this was for bronze...how much in the crucible, I wonder? As you can tell, this is all new to me.

    Metal- I have some 'real' (new) material, but I want to get a feel for things first!

    cheers
    Mark

    edit to add:

    >Also- as cheap insurance add some sprue extensions to increase the hydro-static pressure.

    You mean say a bean-tin full of sand with a hole up the middle, on top of in-hole and out-hole?
     
    Last edited: Nov 15, 2014
  5. Nov 15, 2014 #5

    abby

    abby

    abby

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    I would say that the metal simply wasn't hot enough , as a rule of thumb the pouring temperature should be 100°C above the melting point , and even higher for long or thin moulds.
    I use a flux made by mixing equal amounts of borax , soda ash and salt together with wood charcoal dust.
    Put a table spoon full in the bottom of the crucible before starting the melt and sprinkle more over the scrap as you fill the crucible.
    When at pouring temperature the flux will be visibly bubbling on top of the molten metal.
    If you have phosphorus copper de-oxidising tubes skim of the flux and plunge the tube to the bottom of the crucible.
    You can often buy phos copper on ebay , if it is in lumps break it down to sand like size .
    Get a piece of copper pipe about 35-45 mm long ,15mm central heating pipe is ideal , flatten one end in the bench vice and fill the tube with phos copper leaving enough space so that the other end can be squeezed up , you have a de-oxidising tube ! saw a short distance down a piece of 1/4" steel rod about 2 feet long and open the cut to make a fork into which the tube can be firmly wedged , this is your plunging tool.
    All tools used with molten metal should be given a coat of fireclay wash , this will prevent the iron being dissolved into the molten bronze , and the tools will last longer too !
    NB. make sure everything that comes into contact with your molten metal is bone dry !!!!
    The unburned fuel in your furnace is not good for metal quality , copper alloys absorb gasses especially hydrogen this can cause blow holes and sluggish metal.
    Getting consistent results in the foundry requires knowledge and experience , keep at it and you will get there eventually.
    Dan.
     
  6. Nov 15, 2014 #6

    MRA

    MRA

    MRA

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    Great - thanks. Via Ebay I now have some Borax ordered; soda ash appears to be washing (not caustic?) soda?

    I've found some web advice suggesting pouring 5 mins after last lump melts - I do have an ancient disappearing-filament pyrometer, and it might be fun to see if it works and try to make (shock) _actual measurements_ ! :)

    I've also found web advice to use a deep runner and wide, wide gate - so I'll give this a go next time as opposed to just coming in at one spot on the rim.

    I've added more pics of my furnace and burner, in case anyone is interested.

    cheers
    Mark

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  7. Nov 16, 2014 #7

    jasonh

    jasonh

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    You have it in a nut shell. I can't vouch for the baked beans. I've only ever used mixed fruit.
     

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