Shrinkage...or lack there of.

Discussion in 'General foundry chat' started by Gippeto, Jan 16, 2020.

  1. Gippeto

    Gippeto Silver

    I've been making patterns and playing with some petrobond over the last little while. Still very much finding my feet where this process is concerned, but making some headway and tooling up as I go. No tapered sprues or 10 rules stuff being practiced yet...in time.

    Cast up a spindle mount this afternoon, and the result has me scratching my head over a couple things...so I thought I might see what others had for thoughts on it.

    The metal seemed to all come out of the one riser, leaving the other full...not what I was expecting.

    The other thing is the apparent lack of shrinkage...I was thinking I should see ~6% depending on alloy. Melting pistons if it matters.

    Is this normal??

    Thanks,
    Al
     

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  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    For aluminum more like 1.3%. I use .013in/in.

    Best,
    Kelly
     
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  3. FishbonzWV

    FishbonzWV Silver Banner Member

    Last edited: Jan 16, 2020
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  4. Lack of shrink in aluminium can be a sign of hydrogen porosity in your castings. It doesn't make the castings any less useful so long as you don't need pressure tight castings.
     
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  5. Jason

    Jason Gold

    I wonder how much for the naked lady plumb bob???
    s-l1600.jpg
     
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  6. Gippeto

    Gippeto Silver

    Thanks guys. :) The last couple have shown very minor signs of porosity once machined, but much less than the lost foam I was doing before. Have been using the "Mikey burner" and focusing on leaning it out. Will shorten the x axis pattern to 1.5% and see how it comes out.

    Al
     

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  7. Petee716

    Petee716 Gold Banner Member

    Shrinkage occurs at a couple stages during the pour. During solidification and then during cooling from solid to room temperature. Solidification shrinkage is what causes piping in the riser and shrinkage defects and hot tearing among other things. Defects in the casting can be minimized or avoided by proper molding techniques, especially directional solidification and the use of risers, etc. Contraction is the other type of shrinkage and other than maybe varying your alloy, it is what it is. That's what the patternmakers rulers are for. https://maritime.org/doc/foundry/index.htm#pg1
    As for why one riser cooled differently than the other in your OP, there's no telling what the filling of your mold looked like, but the two risers certainly performed different functions and were subject to different dynamics so it's no surprise that they cooled differently. The gate to your full riser probably froze off before the casting did, and the casting got what it needed from the other one. Who knows, maybe there's a void in that riser. Why there's not a shrinkage defect in the more remote casting I have no idea. Either it didn't need one or the stars aligned?!

    Pete
     
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  8. DavidF

    DavidF Administrator Staff Member Banner Member

    "Pistons" some are hypereutectic castings and will exhibit very low shrinkage rates...
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Al, it's a minor point, but shrink rules are just approximations. All the Journeymen Pattern makers I learned from in the day used 5/32"/ft rule for sand cast aluminum which would be 1.56%.

    The molten aluminum shrinks as it cools too but we don't care about that because the liquid just conforms to the mold cavity. Different things happen as the metal starts to solidify but I wouldn't account for any solidification shrink in pattern making. If there was solidification shrink, it would likely be localized and I'd consider it a defect rather than something to design for and adapt the casting and/or feed system to promote directional solidification and minimize if not eliminate it.

    For alloys with a fairly narrow solidification zone, pattern shrinkage is really just what occurs at the point it becomes solid (or very near solid) and pretty much follows linear coefficient of thermal expansion which for aluminum is 11.7 to 13.3 x 10^-6 in/in/Degree F (approximately 12 microstrain/F). If you take the middle of that range and the difference between the melt point of A356 and room temp, that gives 1.3%.

    It's splitting hairs because for .2% difference we're talking a .024"/ft for something your likely laying out with a pencil and you are probably changing the mold dimensions more than that when you rap the pattern.

    Funny, I thought I'd search the subject just to see what popped up and the first two things were Wikipedia which listed solidification shrinkage for aluminum as 6.6% (I presume that is where you got your figure), and a home machinist forum thread that said to use 3/16"/ft pattern shrink for aluminum but the fellow that posted that also declared it to be 1.5%, when it is actually 1.8%.

    Besides the behavior of the alloy, mold strength can also affect modestly affect shrinkage. In the end it's all an approximation but if you are making large parts with CNC pattern precision, it probably warrants a little more precision than a shrink rule.......1.3% works for me and my processes but I think the generally accepted range for Al castings is 1.3-1.5%......just not 6% ;)

    Best,
    Kelly
     
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  10. HT1

    HT1 Gold Banner Member

    your contraction issue has been well Nuked already, I have nothing valuable to add,
    as to your risers:
    you have way way too much riser , I suspect your 2 Risers Have 4 times the metal as your casting so You would expect most of the Piping to be localized to the risers themselves . the Riser closest to the sprue ( the Hot riser) did most of the feeding because the Metal in it was hotter and stayed Liquid longest. the cold riser appears to taper in at the Top, unless it is a closed riser, it should if anything taper out ... No idea what you did . In the future if you do something like this and are concerned about open risers feeding, once the casting is full you can move to a riser and add hot metal there, slowly this will keep the riser feed ing much longer and will often make a problem casting come out, there are also exothermic riser sleeves which heat the risers, and Hot topping which heats the top of the riser to keep it feeding and piping longer, note that both of those products can contaminate your scrap with Magnesium if that is an issue


    V/r HT1
     
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  11. Melterskelter

    Melterskelter Gold Banner Member

    FWIW
    Logic would seem to suggest that those risers should have contributed equally to feed your casting. But, I have regularly seen grossly unequal feeding from two seemingly symmetric risers when I cast iron. At first this was surprising, but now it is rare to see both feed the same. I guess when the casting starts to cool and develops some negative pressure there is also some somewhat solid skin covering each riser and that skin has some strength. That strength resists allowing feeding until there is enough negative pressure on the surface of the risers to cause one to fail. Once it gives way, molten metal flows easily out of it and into the casting and largely equalizing what was a negative pressure situation. The other riser that is still intact has little motivation for feeding, so it doesn't. Casting cools and and fully solidifies with one "easier" riser having done all the work and the other smugly standing by. Only occasionally do I see symmetric feeds from similar riser pairs.

    The above is my surmise and is not based on any actual knowledge or training. Makes sense to me though...

    Denis

    Added: It turns out your risers, though symmetric in size ,are not at all symmetric with respect to heat. The one that fed is near the sprue, a source of heat. That riser was hotter and therefore weaker when the casting called for feeding. Its surface failed just as it should have. That is very predictable. The symmetric risers I was talking about are more or less equally spaced from my sprue and still the show very asymmetric feeding.
     
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  12. Gippeto

    Gippeto Silver

    All making sense, thanks guys. Next pattern will likely provide some learning via failure lol. Thinking maybe just use large risers on either end of the heavier sections and feed the two on one end equally this time.
     

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  13. Gippeto

    Gippeto Silver

    Had another play session today, casting the x axis for a small cnc.

    Figured I'd try a tapered sprue, so cranked one out to swdweeb "spec" (~5/16 to 5/8" over 6") (1.5degree/side). It was surprisingly easy to keep the sprue full, but it just KEPT ON taking metal lol. Took quite a bit longer to pour than I'm used to.

    Fed the castings from the risers because that seems to be working for me, tried feeding both equally in hopes they would feed the casting equally...didn't happen.

    Broke the one riser off trying to smack some of the petro bond off...the petrobond started on fire and I wanted the part OUT lol. At first glance, I'm happy...part looks like it'll be perfectly usable. Once again, it looks like one riser did the bulk of the work.

    After things were cool enough to handle, I cut the remaining riser off and did see a cavity..almost made it, but it seems the gates did freeze. Will see how deep they are, should still be usable. If I have to try it again, I'll likely make a wider flask and add risers mid way down on the sides.

    Regarding shrinkage...pattern was cut down to 415mm, casting came out @ 408mm once at room temp. Close enough. :)

    Al
     

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  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    1.7%. What did you measure with? Did you sample several dimensions for comparison? That's close to Bonz suggestion of 3/16"/ft. I don't see that much but am casting with A356. Wrought alloys and very high Al alloys will shrink more.

    Best,
    Kelly
     
  15. Gippeto

    Gippeto Silver

    Apparently a crooked fishermans tape. :confused: Checked the width, and re-checked the length while I was there. Also took some shots where the risers were attached...serviceable I think(hope), but not perfect. 412mm to 409mm on the length and 108mm to 107mm on the width.
     

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  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    .7% and .9% respectively. That would be very low shrinkage. I suspect you probably rapped and slightly displaced the pattern while molding and removing the pattern from the mold and this slightly enlarged the mold cavity making the shrinkage appear to be a little less. A fraction of a mm would get you into the 1.3%-1.5% pattern shrinkage range on the shorter dimension. It all matters when you're trying to make dimensionally accurate castings.

    Since my lost foam castings are dip coated and tightly vibe molded in place, I'm seeing pretty much per pattern shrink.

    Unless you are trying to make net shape castings, leave a little more material allowance for machining and call it a day Al :)

    Best,
    Kelly
     
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