Help please for failed brass casting

Discussion in 'Sand Casting' started by myfordboy, Feb 5, 2023.

  1. myfordboy

    myfordboy Silver

    Can anyone advise what the issue is here. I have made two castings and both have this same defect.
    Looks OK on the outside but when the gate is cut off there is a hole in the casting and gate. On the second attempt I increased the size of the ingate. Part is about 3.5" diameter.
    1675619801225.jpg
     
  2. Have you sectioned the feeder to see if there's any shrinkage?, how was the ring cast, vertically or horizontally?. Brass shrinks a bit so you'd need to be able to feed the material from the runner. The brass bar I've cast using a vertical green sand mould and pouring straight into the mold has a shrink hole about half the length of the bar. A lot of my brass bar gets bored anyway so it's not a serious problem, just grip the rotten end in the lathe chuck. You'll likely find you could make the casting a bit longer, cast it vertical and machine off the top with the shrink defect.

    brass bar 2.jpg brass bar 1.jpg
     
    Last edited: Feb 5, 2023
  3. HT1

    HT1 Gold Banner Member

    im not entirely sure what you are doing are you pouring directly down your "gate" and then the gate acts as a riser??? if that is the case you have two issues:
    the well at the bottom is a little too deep so you are feeding it(the well) not the casting
    the gate attachment to the casting needs to have the same cross sectional area as the "gate"
    taper the "riser" from the top down to the connection to the casting , the top of your Riser is freezing before the casting completely fills leaving the void

    V/r HT1

    P.S. I hate to be that guy, but rings are a real PITA to gate, there just is not a great way to do them, a redesign to thin the casting would probably be easier in many cases
     
    Bldr J likes this.
  4. Melterskelter

    Melterskelter Gold Banner Member

    That is a shrink defect—- the gate froze, the the ring froze to the gate. There was no liquid metal to make up for shrinkage. So a vacuum defect formed. Solution is a riser to keep the gate thawed and fed until the casting freezes to the gate. I have seen similar defects iron casting. And what I recommended was the solution.

    Denis
     
  5. myfordboy

    myfordboy Silver

    Thank you all for your suggestions, I will have another go taking these into account.
     
  6. Melterskelter

    Melterskelter Gold Banner Member

    A type if riser I have been using is a cylinder with a depressed top and a has a sort of baby foot shape extendeing toward the gate. That keeps the gate hot and has been very effective at eliminating gate shrink defect.

    Here is an example fresh off the printer. Before use it would be lightly sanded and a coat of epoxy applied to smooth it. THe odd thin flat base is a part that is inset into a match plate.
    Riser.JPG

    [​IMG]
    Denis
     
    Last edited: Feb 6, 2023
  7. myfordboy

    myfordboy Silver

    Here's a pic of my 3D printed pattern. The part is the decrotive furrule on the legs of a grand piano.

    1675693923310.jpg
     
    Tops likes this.
  8. Melterskelter

    Melterskelter Gold Banner Member

    So this takes a core, I see.

    My approach to make just four, FWIW, would be to trim off the gate protrusion mostly, add a riser maybe 1.5 dia, 1.5 high positioning it within an inch of the casting and then hand carving the “babies foot” enlargement of the gate. I am pretty sure that would work. It should “draw” the last area to freeze into the runner/foot and away from the part.

    Denis
     
  9. Foundry Rat

    Foundry Rat Silver

    I think a quick lesson here is needed. When we heat metal it expands. When liquid,the metal has expanded a lot!! as the metal cools it pulls in on itself, it shrinks. So to make the part maintain the desired dimension, we give the part a reservoir of extra metal, and add head pressure, to force the liquid metal into the casting cavity.
    simple put, your casting is pulling metal from the downgate, which doesn't have a large enough reservoir to support the casting.
    the down gate should be to the side, a runner to a riser, feed the casting from the riser, the riser must have height to give down pressure, this forces the feeding of liquid metal.

    Looking at your part, it looks like you are making the ring on end, that is to say you are casting the ring standing up and coring the hole. Can be done but is not best practice, as you are going to need
    a much taller cope to add pressure to the liquid metal. Lay the part horizontal and that will automatically add more pressure to the liquid part.
    The riser melterskelter has shown is good but I would add a basin under the riser, this will keep the center of the riser liquid longer and will aid feeding. Without knowing the dimensions
    of the part I cannot give you the sizes you will need.

    Lay the part down, downgate to the side, runner to the riser, feed the part from the riser. Remember head pressure, that is critical. The higher the better.
    A point you need to take into account, Brass flows really well and a runner does not need to be large....if it is too large it will pull metal from your riser, thus the metal will feed in the wrong direction.
    You will have a pretty runner and a crappy part. Imagine solidification. What is going to go solid first? You want the riser to go last...good luck!
     
  10. Tops

    Tops Silver Banner Member

    Ben, the part is 3.5" diameter, first post right above the picture.

    I couldn't visualize it until I saw the 3D print and was told it was cored.

    Grand piano parts, what will those crazy kids think of next?

    :)
     
  11. Foundry Rat

    Foundry Rat Silver

    I was looking at the first image of the ring and that was what my last comment was direct towards.

    The leg, I have made plenty like them. Go with a 2 inch diameter riser, with a 1 inch basin below the riser, the height at least 4 inches above parting line. the ingate to feed, feed 1 inch long to a
    3/4 inch square ingate. (actually the ingate doesn't have to be precisely square)
    Do not pour down the riser but make a downgate, use a 3/8 th inch spru, the pouring cup should be about 3 inches diameter 1 1/2 -2 inches deep. A small basin at the base of the 3/8th spru, that is to calm the first bit of metal, then a 6 inch long runner to the riser, make the runner 1/2 in square, (HxW). Pour about 2,000F. When pouring keep your cup full so as to have little turbulence through the gating.
    A steady pour from start to finish. A pour that starts then stops then starts again is called a bobble, doesn't always hurt the part but is to be avoided.
    This is all rough but should give you a good part, also brass needs a really hard mold to get a slick finish.,
    The core, sometimes PVC pipe can be used to cobble together a cheap corebox, Short run? sodium silicate, CO2. not too heavy with the sodium silicate as it'll gas up and you'll get a blow, a bubble in the part, especially with brass.

    Thank you for letting me add me 2 cents worth.
    and I hope you are having fun with this : )

    Ben
     
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  12. HT1

    HT1 Gold Banner Member

    with a picture of the pattern your technique is right where I would be, but I think I would be a little stingier with the measurements, that is a lot of remelt for a small part
    and I would make the ingate an oval, corners are always to be avoided they solidify quickest

    V/r HT1
     
  13. Foundry Rat

    Foundry Rat Silver

    You are right, an oval ingate is good, just trying to give a general size to the area. There is a lot more to it, like giving a radius at edge of the ingate to stop erosion and such as that.
    The amount of remelt really can't be avoided because it is a small single impression. And truth is I do tend to go a little over as I don't like to do the parts over because I missed it by just a little.
    I do it to make money and it's cheaper to get it right off.
    Another suggestion for loose work. A good molder will "open and close" the mold. And that is, after the mold is made, cores set et al, last blown out, the mold is closed up, then opened and blown out again. Not unusual to have a crush with loose parts. So this finds the crush and gets rid of the newly free sand.
    Thanks for listening to me, rare I get to share this stuff.
    Ben
     
    Chazza likes this.
  14. myfordboy

    myfordboy Silver

    I cut the part in half and all is good apart from that defect.
    Foundry Rat, any chance of a sketch to clarify your advice?

    1675797216935.jpg
     
  15. Melterskelter

    Melterskelter Gold Banner Member

    EFA5462D-1005-45D2-9761-08AF9BC923A4.jpeg 0953E281-E3BD-4969-BDBC-3951B45317FE.jpeg 6B9741AF-36B4-4573-A862-F727D2D75EC8.jpeg
    upload_2023-2-7_11-44-39.png
    Though you did not ask, here is the riser/gate that solved that exact shrink defect
     
    Last edited: Feb 7, 2023
  16. myfordboy

    myfordboy Silver

    Metlerskelter, so a very short gate with a wide entry. You are pouring into a different part of the mould?
     
    Last edited: Feb 7, 2023
  17. Melterskelter

    Melterskelter Gold Banner Member

    Yes, short and thickening toward the riser. That helps conduct heat from the riser to the gate and makes that last area of freezing occur in the riser not the gate/casting. Doing it that way I see plenty of piping and the dimple on top promotes collapse so that the easiest place for the casting to draw metal is from the riser, not the gate and not the cast part itself.
    It makes the freeze sequence be part, gate, riser not gate, part, riser. The latter result in a starved ( my made up terminology) casting and shrink defect.

    Denis
     
  18. Melterskelter

    Melterskelter Gold Banner Member

    Here is one I just printed that uses the same principle but needs a shaped gate to match the junction to the casting
    cavity.
    upload_2023-2-7_13-10-17.png

    upload_2023-2-7_13-13-15.png

    This general style of riser/gate mayu not be entirely conventional, but it has solved my vacuum defect problems. Now that I have said that out loud, I'll probably get into trouble! Kinda like remarking about how reliavble your car has been...

    Denis
     
  19. Foundry Rat

    Foundry Rat Silver

    It looks like the ingate feeding the casting from the riser is not big enough, that or your riser is not tall enough.
    Take a close look at the last image melterskelter has shown. The riser has a section coming off it, looks a bit like a foot (less the toes) ...that aids feeding a lot.
    you can help that further by making the ingate more round or oval, open the ingate some, add height to the mold. Are you pulling the riser thru? That is, open the riser to the top of the mold.
    In risers the height is everything, the taller it is the more pressure.
    Also, in troubleshooting casting defects, a look at the gating, risers helps.

    Ben
     
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  20. myfordboy

    myfordboy Silver

    I do usualy use much larger feeders and risers. The metal was melted in my small elecric furnace and what you see is the capacity of it.
    I thought I might get away with it. I'll will have to fire up the oil fired furnace for the next attmpt, plenty of capacity in that.
     

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