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Thread: What's the minimum diameter for an aluminum pour hole?

  1. #1

    What's the minimum diameter for an aluminum pour hole?

    Pretty simple - if I've got a mold what's the minimum diameter hole I need to pour the aluminum in.

    Secondary question - what's the minimum sized vent hole and is one always needed? So what size items require a vent hole or is it always recommended?

  2. #2
    Ive seen some tapered sprues that the bottoms were 5/16 (8mm), so I would say that is bare minimum imo. Vents, I think it would depend on the ability of your sand to vent, but sometimes, you dont even need to vent, but is recommended if you have a few vent holes. The metal rushes in pretty quickly, and it needs somewhere to push the air that's in the mold out, and any steam that's created from the sand out, so you want some vents in higher areas in the mold so the air doesnt get trapped. The sand is porous, but the metal rushing in that quickly, it can only dissapate so much air at once, and figure in the steam also.

    I dont really vent my molds usually unless it's a wider part, but I usually have two 1" sprues on each side of the part, so the metal can flow in and the air gets pushed out the other sprue, it also helps to see where the metal's level is so I know when to stop pouring rather than spilling it all over the top of the mold.

  3. #3
    I've been doing some small jewelry casting with aluminum. I use delft clay, and a small rectangular flask. I rarely use venting, and have had terrific results thus far. My models are pretty simple which is probably a big factor. Here are some pics for clarity.P1000536.jpgmarc's iphone 2016 200.jpg

  4. #4
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    The smallest sprues I've seen used were on the Olfoundryman YouTube channel. I can't comment on their specific diameter, but I was surprised at how small they were.
    FLAME ON...!!!!

  5. #5
    Senior Member Tobho Mott's Avatar
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    Agreed, same here and I had the same reaction. Sandrammer on youtube also calls for skinny tapered sprues similar to what we see in Olfoundryman's videos, except I think his are square instead of round.

    According to Sandrammer, it's more about the ratio of the area of the sprue's base to the cross sectional areas of the runners and gates than it is about the actual size of the base of the sprue. With the right ratios, the flow of metal from the pour should keep the sprue from freezing shut before the mold fills. But it's possible there is a lower limit I guess... IIRC the ratio is 1:4:4; I think this comes directly from the US Navy Manual if anyone wants to check. IIRC HT1 has posted about this ratio before as well.

    Sandrammer also scoffs at those who are using big wide sprues that gate directly into part and double as shrink risers, claiming that the resulting castings will surely be full of dross and sand, therefore useless. But of course we have all seen members here casting that way on a regular basis with excellent results. Whatever works, I guess... But I tend to take the advice of those guys, they seem to know what they're talking about and to have plenty of training and experience.

    When I was preparing to cast my aluminum bronze axe, the advice I got here for casting that reputedly tricky type of alloy was to design the mold so as to reduce turbulence (thanks again donkey). Since Olfoundryman's explanation of his skinny tapered sprues and offset egg-shaped pouring basins was that they reduce turbulence of the metal as it enters the mold, copying his molding techniques seemed like a no-brainer - I had no visible dross or sand inclusions in the axe I made, so that seemed to work really great for me. I didn't use the 1:4:4 ratio though, I just winged it when I cut in the gating. Next time I'll try it, and also try extending the runner past the gates to trap any loose sand or dross...

    I checked, none of my aluminum bronze axe pix show the base of my sprue clearly, but it was about a 1cm diameter circle, just a hair bigger than the one I remember seeing in olfoundryman's videos. Funny, while I was checking those pix on my phone (photobucket is now blocked from this office PC, argh), I got a notification that a new Sandrammer video has just gone up.

    Jeff
    Tobho had learned to work Valyrian steel at the forges of Qohor as a boy. Only a man who knew the spells could take old weapons and forge them anew.

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  6. #6
    Quote Originally Posted by Tobho Mott View Post
    I got a notification that a new Sandrammer video has just gone up.

    Jeff
    It's 40minutes of a baseball game. :-/
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  7. #7
    I usually use about a 7\8 spray, but for most things a 1\2 works fine
    Dave
    BrandonFoundry.com

    In the words of the Toolman: If you didn't make it yourself, it's not really yours.

  8. #8
    Senior Member HT1's Avatar
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    Quote Originally Posted by Tobho Mott View Post
    Agreed, same here and I had the same reaction. Sandrammer on youtube also calls for skinny tapered sprues similar to what we see in Olfoundryman's videos, except I think his are square instead of round.

    According to Sandrammer, it's more about the ratio of the area of the sprue's base to the cross sectional areas of the runners and gates than it is about the actual size of the base of the sprue. With the right ratios, the flow of metal from the pour should keep the sprue from freezing shut before the mold fills. But it's possible there is a lower limit I guess... IIRC the ratio is 1:4:4; I think this comes directly from the US Navy Manual if anyone wants to check. IIRC HT1 has posted about this ratio before as well.

    Sandrammer also scoffs at those who are using big wide sprues that gate directly into part and double as shrink risers, claiming that the resulting castings will surely be full of dross and sand, therefore useless. But of course we have all seen members here casting that way on a regular basis with excellent results. Whatever works, I guess... But I tend to take the advice of those guys, they seem to know what they're talking about and to have plenty of training and experience.

    When I was preparing to cast my aluminum bronze axe, the advice I got here for casting that reputedly tricky type of alloy was to design the mold so as to reduce turbulence (thanks again donkey). Since Olfoundryman's explanation of his skinny tapered sprues and offset egg-shaped pouring basins was that they reduce turbulence of the metal as it enters the mold, copying his molding techniques seemed like a no-brainer - I had no visible dross or sand inclusions in the axe I made, so that seemed to work really great for me. I didn't use the 1:4:4 ratio though, I just winged it when I cut in the gating. Next time I'll try it, and also try extending the runner past the gates to trap any loose sand or dross...

    I checked, none of my aluminum bronze axe pix show the base of my sprue clearly, but it was about a 1cm diameter circle, just a hair bigger than the one I remember seeing in olfoundryman's videos. Funny, while I was checking those pix on my phone (photobucket is now blocked from this office PC, argh), I got a notification that a new Sandrammer video has just gone up.

    Jeff
    yes!!!!
    Small

    P.S. Your question lacks so many variables .. Sand Cast. 3/8 is small, lost wax in a heated shell, 3/16 is pretty common.. really big. if you are using a heated metal mold centrifugally or vacuum cast 1/16 is BIG... and of course alloy is a hugh variable if you are trying to cast 6061 you better break out that 3/4 sprue... if you are using 364 1/2 ins far more appropriate... and square sprues are always preferable as they prevent the metal swirling which causes erosion, of course they must increase in size over their round counterparrts because of increase surface area.

    V/r HT1

    P.P.S I did a long post on venting, but it's late and I'm into my Bottle, so you will have to search for it :-(

  9. #9
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    Sean,
    It never hurts to do a little homework. The more knowledge, the better the finished product.
    I refer back to this manual quite often.

    http://archive.hnsa.org/doc/foundry/part2.htm#pg95
    Bones

  10. #10
    Senior Member HT1's Avatar
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    Quote Originally Posted by FishbonzWVa View Post
    Sean,
    It never hurts to do a little homework. The more knowledge, the better the finished product.
    I refer back to this manual quite often.

    http://archive.hnsa.org/doc/foundry/part2.htm#pg95
    we all love the Navy Foundry Manual, great info, but it was last revised in 1958, from a 1944 edition, so some of the info especially gating design is REALLY dated. I have the Navy's last Foundry "Manuel Patternmaker/molder NAVEDTRA 12207 which simple references the older rate training manuals, which reference the Navy foundry manual... there are some references to AFS system of gating which is now considered dated by some... So your library is really an issue when studying, the Navy over and over calls for the round sprue for two reasons, streamlining the metal (least resistance) and cross sectional cooling efficiency... Well if you increase the size of a square sprue, you counter the cross sectional cooling, and of course fast moving metal is likely turbulent which we never want... so you guys decide

    V/r HT1

    P.S. for the hobbiest, I really thing square sprues are overkill on most projects, but if you are pouring a 200 lb billet and bottom gating it as you should... use a square sprue

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