Investment Castings problem -- surface defects caused by ...?
Folks,
I sent some very nice waxes off to be cast in sterling, but a few came back with surface defects. My casting guy says it's because the cast pieces were "too heavy." I think it was a problem with the investing technique. What is your diagnosis? I posted pictures at www.eliasbing.com/surface_defects.html. Thanks, Paul |
Investment Castings problem -- surface defects caused by ...?
On Mon, 07 Apr 2008 19:17:11 -0700, in rec.crafts.jewelry Paul Wilson
wrote: Folks, I sent some very nice waxes off to be cast in sterling, but a few came back with surface defects. My casting guy says it's because the cast pieces were "too heavy." I think it was a problem with the investing technique. What is your diagnosis? I posted pictures at www.eliasbing.com/surface_defects.html. Thanks, Paul I'm not sure, but I think I see what appear to be two things going on. One seems like surface displacements, fissures, or the like. That might be too wet an investment, incorrect water temp or mixing time, or some other factor giving rise to water marks or small cracks, etc. But I'm not sure I'm seeing that. Those marks could be just more of my second observation... The main thing it looks like, unless these were CAD milled or grown waxes and that texture really was on the wax, is that the texture looks sort of like dentritic crystal structures. Now, the metal always has that structure anyway, to a degree. Crystaline, at any rate. But usually it doesn't manifest itself at the surface as a texture unless the metal is cooled too slowly, allowing that crystal growth, and it can suggest metal starvation too, from too small a sprue structure. If these things are heavy medallions fed with just one edge sprue, there's no way that would have been enough metal flow to properly fill the mold quickly, and that could give you that. If that's the situation, then your caster's statement that the items were too heavy is correct, but incomplete. Complete would be "too heavy for the size sprues used". And if the item is that heavy (and three and a half ounces is heavy), then it's also very possible the mold was too hot, again allowing too slow a cooling / solidification time. Something that heavy is almost an ingot in bulk, and the mold would have needed to be much cooler than what would be appropriate for, say, the typical ring. Depending on casting method, a ring might get cast with the flask at anywhere from 700 to 1000F. A three ounce heavy medallion, though, I'd guess would be better off cast around 400 to 600F, maybe considerably less depending on the thickness. There's another clue in the photos too. Note that the dendrite structure is most visible on the head, where the medallion is thicker. The edges, with the lettering, have a better surface. Some of those areas seem just fine. That supports the thought that the center, being heavier, solidified last, and being metal starved (because the rim silidified first, and now the sealed off center section is solidifying, plus it's solidifying too slowly, means that as it cools, metal shrinkage drains still molten metal from the surface towards the inside, leaving the dendrite structure visible. Again, it's a combination of mold temperature, weight of the item, and insufficient metal feed (spruing) Things that will help: if you can find a way to cast this via centrifuge, rather than vacuum assist pour, the higher force on the metal will help a lot, plus allow you to use an even cooler mold. But I'm guessing this is too large for most people's centrifugal casting machines? Barring that, a cooler mold, and much heftier and short sprues, with a bigger button, so there's an increased chance the molten button will still be able to feed metal to the core of the thing as the core solidifies. If you can set up a temperature gradient, so the bottom of the mold is significantly cooler than the middle, and the middle is significantly cooler than the top edge or wherever sprues enter, and the sprues and button are hottest yet, then you'll have a better chance of getting progressive solidification, which is what you need to avoid the shrinkage and cooling problems that I think you've encountered here. Hope that helps. Peter |
Investment Castings problem -- surface defects caused by ...?
To:
Subject: Investment Castings problem -- surface defects caused by ...? From: Paul Wilson Date: Wed, 9 Apr 2008 16:47:37 -0700 (PDT) On Apr 7, 9:43*pm, "Peter W.. Rowe," wrote: On Mon, 07 Apr 2008 19:17:11 -0700, in rec.crafts.jewelry Paul Wilson wrote: Folks, I sent some very nice waxes off to be cast in sterling, but a few came back with surface defects. * My casting guy says it's because the cast pieces were "too heavy." * I think it was a problem with the investing technique. What is your diagnosis? *I posted pictures at www.eliasbing.com/surface_defects.html. Thanks, Paul I'm not sure, but I think I see what appear to be two things going on. *One seems like surface displacements, fissures, or the like. *That might be too wet an investment, incorrect water temp or mixing time, or some other factor giving rise to water marks or small cracks, etc. *But I'm not sure I'm seeing that. Those marks could be just more of my second observation... The main thing it looks like, unless these were CAD milled or grown waxes and that texture really was on the wax, is that the texture looks sort of like dentritic crystal structures. *Now, the metal always has that structure anyway, to a degree. *Crystaline, at any rate. *But usually it doesn't manifest itself at the surface as a texture unless the metal is cooled too slowly, allowing that crystal growth, and it can suggest metal starvation too, from too small a sprue structure. *If these things are heavy medallions fed with just one edge sprue, there's no way that would have been enough metal flow to properly fill the mold quickly, and that could give you that. *If that's the situation, then your caster's statement that the items were too heavy is correct, but incomplete. Complete would be "too heavy for the size sprues used". *And if the item is that heavy (and three and a half ounces is heavy), then it's also very possible the mold was too hot, again allowing too slow a cooling / solidification time. Something that heavy is almost an ingot in bulk, and the mold would have needed to be much cooler than what would be appropriate for, say, the typical ring. Depending on casting method, a ring might get cast with the flask at anywhere from 700 to 1000F. *A three ounce heavy medallion, though, I'd guess would be better off cast around 400 to 600F, maybe considerably less depending on the thickness. There's another clue in the photos too. *Note that the dendrite structure is most visible on the head, where the medallion is thicker. *The edges, with the lettering, have a better surface. *Some of those areas seem just fine. *That supports the thought that the center, being heavier, solidified last, and being metal starved (because the rim silidified first, and now the sealed off center section is solidifying, plus it's solidifying too slowly, means that as it cools, metal shrinkage drains still molten metal from the surface towards the inside, leaving the dendrite structure visible. * *Again, it's a combination of mold temperature, weight of the item, and insufficient metal feed (spruing) Things that will help: *if you can find a way to cast this *via centrifuge, rather than vacuum assist pour, the higher force on the metal will help a lot, plus allow you to use an even cooler mold. *But I'm guessing this is too large for most people's centrifugal casting machines? Barring that, a cooler mold, and much heftier and short sprues, with a bigger button, so there's an increased chance the molten button will still be able to feed metal to the core of the thing as the core solidifies. If you can set up a temperature gradient, so the bottom of the mold is significantly cooler than the middle, and the middle is significantly cooler than the top edge or wherever sprues enter, and the sprues and button are hottest yet, then you'll have a better chance of getting progressive solidification, which is what you need to avoid the shrinkage and cooling problems that I think you've encountered here. * Hope that helps. Peter Thanks for your insightful comments. I too was drawn to the dendritic structure on the metal surface. I thought it held a clue, but what? It turns out only 3 of the 9 medallions (all different) had bad surface issues, so I think we're close. We do, in fact, use a centrifugal process (an old Jelenko Thermotrol). Most of my waxes are too thin to cast, so this project presents other issues it seems! |
Investment Castings problem -- surface defects caused by ...?
On Wed, 09 Apr 2008 18:26:01 -0700, in rec.crafts.jewelry Paul Wilson
wrote: Thanks for your insightful comments. I too was drawn to the dendritic structure on the metal surface. I thought it held a clue, but what? Another way to think about it is this: The metal will always crystalize as if solidifies, but the only way you can see the physical shape of that crystal structure in relief on the casting surface, is if the remaining liquid, as the crystals form, is drained or pulled back away from the crystals by and the surface by the shrinkage of the cooling metal. What you need to be happening is for the surface layer of metal to be the first to solidify when it hits the surface, and solidification to then proceed from the surface inwards, as well as from the bottom up towards the sprue. If the metal that first hits the mold surface freezes up before the insides do, then it will conform to the mold surface without additional texture, and subsequent shrinkage would then be either pulling more metal from the sprue feed, or once that's blocked, would simply be creating voids / porosity in the interior of the mass of metal. If you're almost where it needs to be, the surface can freeze, but shrinkage pulls that formed surface back or out of shape before the solidified layer gets thick enough to support itself. This would lead to those few areas where it seemed small sections of otherwise properly conforming surface bits were displaced, cracked, etc, or to situations where an entire cast surface may end up being bowed in, even when the mold is not. As I said in the first post, the solution is whatever will give you better progressive solidification from the end back towards the sprue, and from the surface towards the interior, rather than the whole mass solidifying all at once. Key to that is a cooler mold temperature, so the surface layer chills more quickly. One other thing you might try along with a lower burnout kiln temp (and thus mold temp) at casting time, is before putting the mold in the machine, let it sit a minute or so, sprue hole down against an insulating surface, and the back end up in the air, and thus cooling in the air. You could accentuate the resulting difference in temperature between the two ends even more by playing a torch flame directly on the sprue hole area for a bit, warming up the investment in that area, before casting. Again, the idea is to promote the metal at the end of the mold chilling faster than the metal at the sprue and button area. This idea comes actually more from a useful trick when casting larger flasks with a bunch of rings sprued to a central thicker tree trunk style sprue. The flast is pulled from the oven a hundred degrees or so higher than the actual recommended casting temp, but allowed to simply sit for a few minutes, sprue hole down, before casting. The steel flask and outer layer of the mold starts to chill, while the interior core of the mold, where the sprue is, doesn't so much. So the rings extending from that sprue are now in a temperature gradient, which substantially improves progressive solidification, helping with both filling and porosity problems. Your large coin is a little harder to do that with, since the symmetry of the thing doesn't so nicely match the way a flask cools, but you get the idea, I'm sure. It turns out only 3 of the 9 medallions (all different) had bad surface issues, so I think we're close. We do, in fact, use a centrifugal process (an old Jelenko Thermotrol). Nice old machines. Most of my waxes are too thin to cast, so this project presents other issues it seems! It's difficult to get something too thin to cast that would still have been servicable in metal. Especially if using a centrifuge. But sometimes it takes playing with extensive sprue feeds, and surprisingly high flask temps. I've done things which were literally filligree delicate, but casting with a flask temp left right at the 1350 high point from burnout. Way too hot for most things, but the items I'm thinking of, in 18K yellow gold, needed that, and came out fine when cast at those temps. Many of those shapes were not much more than a third of a millimeter in thickness, with even more delicate surface features (edge textures, etc) Think about it this way. If you used an investment material that could withstand the temperatures at which your metal actually melts, and you cast your metal into a mold that started out above the metal's melting point, the metal would have all the time it wanted to fill the mold completely, and so long as your details weren't so thin that surface tension itself kept the metal out (and the force of the centrifuge could overcome that), everything would fill. Then you'd have to simply cool the mold. Shrinkage might present problems, but with really thin sections, since shrinkage is a percentage of the thickness, chances are you'd not notice it much unless you mixed very thin sections with substantially thicker ones... cheers Peter |
Investment Castings problem -- surface defects caused by ...?
Paul Wilson wrote in
: www.eliasbing.com/surface_defects.html Why aren't they being die struck, if I may ask? -- Saint Séimí mac Liam Carriagemaker to the court of Queen Maeve Prophet of The Great Tagger Canonized December '99 |
Investment Castings problem -- surface defects caused by ...?
Séimí mac Liam schrieb:
Paul Wilson wrote in : www.eliasbing.com/surface_defects.html Why aren't they being die struck, if I may ask? Very often this won´t help. More important is the thickness of the casting sprue and the mold/metal temperatures. Here you find a ppt presentation. At slide no 7, there is a picture of a casted silver coin underneath a casted part of a watchband-clip. The surface is completely even nearly like the original coin. http://butschal.de/download/Karlsruh...Bu tschal.ppt Mit freundlichem Gruß, Heinrich Butschal -- Schmuck Gutachter und Schmuckverkauf http://www.butschal.de Schmuck nach Maß anfertigen http://www.meister-atelier.de Firmengeschenke und Ehrennadeln http://www.goldschmiede-meister.com Schmuckmanufaktur http://www.schmuckfabrik.de Schmuck gut verkaufen und günstig kaufen http://www.schmuck-boerse.com |
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