Sand casting an odd shape - sprue, runner, gates and risers

Discussion in 'Sand Casting' started by Ando, May 13, 2024.

  1. Ando

    Ando Copper

    Hi All, I'm about to cast (or attempt to) part one of an art project, in aluminium.
    This piece is approx 430x250mm x 25-30mm thick, in the curved shape as drawn.
    This first or four pours will be approx 2.6L + gating etc, so I'm using the A20 crucible.

    Been reading/watching vids and gathering info on all aspects. I've cast before, though not in green sand and not in this volume, so this is a little daunting.

    I have a series of articles written by Col Croucher and now putting his advice on runner and gate ratios to work.
    The sprue is 12mm diameter, tapering up 3 degrees (I will add a pouring basin) and approx 100mm down to the casting.
    Runner is 30x15mm, so approx 4 x the sprue area
    Gates are 18x10mm, so approx 3 x the sprue area


    I can't find a specific formulae/suggestion on risers, so I'd thought of going with two, 25mm diameter risers as shown.
    Am I on the right track here? Any suggestions ?
    The pattern is mostly in the drag, with the runner. Only 20mm or so will be in the cope.
    Last edited: May 13, 2024
  2. HT1

    HT1 Gold Banner Member

    it that part a full half round (half a pipe?) or is it just a section
    a 1inch thick aluminum part will give you huge shrinkage problems that you will fight with

    as pictured its terrible, the risers should be on the same side as the runner to get hot metal into the risers so it will feed longer

    my first instinct is always to pour any plate shaped item( or billet) standing up so you can riser one end or even just extend that end if it is simple so the end acts as a riser to be cut off ,

    in your case I would try mold manipulation instead put your risers on the end farthest away from the sprue and raise that end of the mold up 15 to 30 degrees, this will allow the metal to very gently fill the mold, the angle will create directional solidification( it will help the risers work) , and yes the risers and the sprue need to be built so they are straight up and down when you pour. so in the mold they will be crooked whatever degree you decide to tilt the mold

    V/r HT1
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  3. Ando

    Ando Copper

    Hi mate thanks for the reply. The section resembles, say an 1/8th of a pipe. It's quite shallow.
    The shrinkage may not affect me too much as it's an art piece. Dimensions can vary.
    I've rammed up one piece last night and about to pour, so I'll report back on the result.
    Ended up going with one 50mm riser but have it on the other side from the runner (unfortunately)
    I've been trying to look for a similar shaped/type casting to get some tips; closest found is in the Art Casting thread with Tobho Mott.
    Can't see any risers in the pics but they may have been removed already?
  4. Tobho Mott

    Tobho Mott Administrator Staff Member Banner Member

    Nope, we didn't use any risers on the rock parts so far that I can recall. Every piece cast so far is shown in the art casting thread with all its gating still on if you want to check more closely, I believe. Going back out for another session on Thursday.

    Aspects of those molds might not be the same as I'd ideally like them to be if there was unlimited time and resources to get those castings made. Some of the parts are definitely too thick not to have risers, at least on paper. And carefully measured and ratioed gating patterns are off the table with all these one offs with different shapes and sizes and differently irregular partings in play. I've got to cut the gating all into the sand by hand and eye on those. Gating into low points where practical to encourage smooth laminar filling rather than tumbling falling metal inside the cavity. Sometimes though, the flask won't let you and you have to gate in elsewhere, where there's room inside the flask.

    Still, we haven't noticed any shrink defects. If they're there, they'll mainly be welded up inside a hollow sculpture anyway. With said timeline we're having to pick our battles.

    Not saying it's right to just embrace the shrink I'm sure there's good reasons not to, besides appearance. And definitely not trying to argue with HT1! His advice has always worked out well for me. When I use them, I try to put my risers on the hot side of the mold too. Rather than on the cold side. For what it's worth.

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  5. Ando

    Ando Copper

    17157476101057819558929451287159.jpg Thanks Jeff, I did the pour as it was already rammed up. Haha, a bit of a disaster, though like I tell my kids, you're either winning or learning.

    Biggest issue was the vent holes in the drag. I lost probably over a litre there. There were about 20, punched through with 1.6mm TIG wire.
    I've seen vids of people venting the drag on ally without a problem. I'll vent only the cope on the next pours.
    Facing sand worked a treat and what's there has the desired finish.
    Last edited: May 14, 2024
  6. Ando

    Ando Copper

    HT1, as mentioned above the vents in the drag really stuffed the show this time around. I haven't used the A20 before so on the plus side this was a good trial run for the next 4 pours.
    I'm going to go with the 15deg tilt as advised and of course riser on the runner. Sprue was between the two gates on this pour. Would I be better off with the sprue at one end and the riser at the other end? Or between the gates?
    The sprue was only short so I added the uplift. Basin was 50mm dia and worked a treat, just had a little spill with the full crucible, hence the burn mark.
    I've also reduced the patterns to approx 15mm thickness.
    Last edited: May 14, 2024
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  7. Ando

    Ando Copper

    Hi Jeff, HT1,
    So taking some guidance from your fine selves, I poured again this evening.
    Note the flask at 15deg. Vent holes in cope only.
    Same runner and gates but sprue at the end and the riser central. Placement of the pouring basin was much better too.
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  8. Ando

    Ando Copper

    End result below. The facing sand was a little on the dry side so a couple of very minor cave in, though they will be on a weld seam so all good.
    The casting is about 2.5mm shorter than the plug. 428.5/430mm
    Note the shrinkage on the sprue and riser; both were above the mould/ basin when I stopped.
    Perfect result for the project. 20240515_203908.jpg 20240515_205334.jpg 20240515_205357.jpg
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  9. Ando

    Ando Copper

    17157720138545729603437664432470.jpg Sprue, runner, gates and riser seem to have worked well.
    I'll mull the facing sand and add water.
    One down, three to go.
    Next cast will be similar to this.
    The last two will resemble a bonnet/hood shape, as pictured.
    Thankyou again for your input.
    Last edited: May 15, 2024
  10. HT1

    HT1 Gold Banner Member

    Glad you got a Good piece

    Now For the Cursing : who told you to vent in the drag????:mad:, Vents don't work if they re full of metal , vents create a path of least resistance for gas, remember your sand is supposed to be permeable and let gas pass through, vents create a path of least resistance to overcome poor permeability in sand ( or being too wet where too much gas is created), now pop offs( May have other names ) are vents but have a special purpose , they are at the last location where metal will reach or an isolated location , this is where lots of gas can be trapped, so a vent allows it to escape easier, these are normally open vents completely through cope,... NOW they are overused (people put them at the end of runners ) also they often cause a shrink defect where they attach to the casting, this is easily prevented by not venting entirely into the mold cavity , the proper venting tool is a icepick or a piece of 1/8 welding rod sharpened venting should be done while the pattern is still in the mold , i personally try to measure and vent about 1/4 of an inch from actually contacting the pattern ( this creates the path of least resistance without jeopardizing the casting surface finish) if you cannot help your self and want to vent the drag , this is the way it must be done or you are very likely to get a porcupine part

    again glad you worked it out

    V/r HT1
  11. Ando

    Ando Copper

    Thanks for the advice HT1, the vents on the cast part can be removed when I dress that interior surface.
    In saying that, I'll use your method of stopping vent holes short of the pattern for the next castings.

    I reckon I've watched a couple hundred clips and read a lot of blogs etc in the last few months. In some cases where the drag was vented, there was a piece of ply which would have contained the metal. This is my first sand cast so alot to learn and sorting the wheat from the chaff is part of that I suppose.
    Glad I came across this site though.


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  12. crazybillybob

    crazybillybob Silver Banner Member

    Cast Iron guys like to vent because the iron isn't a fluid as aluminum or bronze. But I don't know any of them that vent through the draft. maybe a few that vent at the parting line. but like a porcupine all the vents need to stand up.
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  13. Tobho Mott

    Tobho Mott Administrator Staff Member Banner Member

    @MichelHandJello are we being pranked? o_O;)

    Lol, these are amazingly similar to the boulder sculpture castings. We did another molding and casting day on that project yesterday, I'll make sure to post some pics that show gating etc. over in the art casting thread once I sort through and grab some screenshots from the video clips I got.

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  14. HT1

    HT1 Gold Banner Member

    Ok just think about it for a second the gas in the mold is HOT what does HOT GAS do? it rises, I really cannot think of a good reason for a hobbiest to ever vent the drag (On a really big(tall) MOLD im talking thousands of pounds ) it might be helpful
    the techniques I have advocated work and thats what matters

    V/r HT1
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  15. Ando

    Ando Copper

    20240518_113410.jpg 20240518_113415.jpg 20240518_214538.jpg 20240518_214555.jpg
    Poured another part to the project today. Pics below. Same technique/15deg tilt, vents in cope but only 6 this time.
    I made the facing sand more moist this time around so no cave ins. Very happy with the result.
    Once complete and welded together, with permission of the artist, I'll post the finished piece.
  16. Ando

    Ando Copper

    Cocky from the two successful pours, I rammed up the next piece....and had a drop out. The cope has a fairly large piece to fill the void in the drag and coupled with a layer some dryer facing sand it failed.
    I'll ram up again tomorrow, this time adding gaggers to keep it all together. There will be pics.
    Last edited: May 18, 2024
  17. Ando

    Ando Copper

    Here is the second attempt...finished piece/opening/ramming the flask
    The gagger worked well...cope projection stayed put and the pour was a success.
    20240519_144628.jpg 20240519_141920.jpg 20240519_141848.jpg 20240519_121134.jpg 20240519_120432.jpg
    Last edited: May 19, 2024
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  18. Ando

    Ando Copper

    So now I'd worked out how to stop the cope from dropping out, I quickly mulled the sand and rammed up the last piece which is very similar.
    The next pour would be over in a jiffy; right.

    Since the total weight of the cope/drag/sand is around 115kg, I've been opening the flask(s) vertically (standing it on it's side)
    I've got an overhead gantry but the wire rope sometimes bunches up on the drum and can drop and jerk. It's pretty rapid as well, even with a double fall; so I didn't even consider using it for this job. Didn't want to risk shocking the flask and losing the mould.

    Anyway, for some reason the last mould (cope projection) just kept dropping out.
    I checked the pattern for negatives, that flat face at the end is about 5 degrees so it parts easily. Parting powder everywhere, sand seems correct, nothing seemed to be binding; seriously frustrating.
    The first couple of times the projection broke at the gagger. Third time it broke an inch under the gagger. (just to note the bar was coated with clay and dried on each ram up)
    I rammed it up 4 times to have it fail, thankfully while breaking the flask, rather than during a pour, so no lost metal.

    On the forth drop out, I only dug out what cope sand was left on the pattern in the drag and cut out what was left of the projection in the cope.
    Ran the sand in the muller again and added some water (pretty windy and dry here today) and rammed again.
    This time I added a couple of 1/2" dowels either side of the gagger as per pics.
    In addition to that I laid plastic over the inside of the pattern to help the break. This inside surface of all these pieces will be machined, so finish out of the mould isn't a big issue.
    This time was the charm; it was quite a relief when I saw the metal top the riser.
    If I had the time, I'd rethink the flask and orientation but these pieces HAD to be done this weekend.
    Thanks again to all those who commented and gave advice.

    Weekend totals:
    Green sand produced 250kg approx
    4 successful castings, one failed casting, 52kg of bar stock melted and poured, some of which ended up on the floor.
    A handful of bar stock pieces from what was left in the crucible after each pour.
    1 x 8.5kg LPG refill so about $30
    Sore arm from ramming the same @%##ing mould over and over LOL
    These big flasks wore me down, so the next planned pour will be very small!!!

    Closeup of the 1/2" dowels added in desperation. Worked quite well.
    20240519_211130.jpg 20240519_211116.jpg 20240519_204729.jpg 20240519_141931.jpg 20240519_134256.jpg
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  19. Tobho Mott

    Tobho Mott Administrator Staff Member Banner Member

    We have been opening the big molds for the boulder sculpture in the other thread standing on their side too, to keep the sand from dropping out. Usually works but a couple molds the pattern slipped instead of staying put and we had to redo those molds. Might have helped to have the mold oriented still on its side but rotated 90 degrees. But David did add some bars (based on pictures in Ammen's sand casting handbook) to one of the flasks so that the molds could opened normally.


    I believe he has used that flask since I was last out there and got another casting poured. However I haven't asked if he had the mold on its side to part it or left it lying flat. I like the dowels, we might have to steal that.

    Last edited: May 19, 2024
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  20. Jeff is right, this is eerily similar to what we are doing.

    My most recent pour (this time without Jeff holding my hand) I did an 18 x 18 flask cope was 3.5" deep but thrust about 7.5" into the drag. Drag was 9"deep.

    I put support bars in both for manoeuvrability and used a hand crank genie lift to move the completed mould because it weighed at least 225lbs. I didn't document well, but I added long support bars to the cope part was through the ramming. Pretty good results and really stable mold. I still chose to open it on it's side because it was just too darn heavy.

    No riser, and the gate placement was far from ideal. The slope to the pattern was too steep to have a bottom feed without the metal falling turbulently. I did tip the mould for the pour so that most of the filling would be up hill.

    Attached Files:

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