Hello all. Been here a while, lurking, reading, but this is my first time posting. Kelly called me out for pics in another thread, so figured I'd start a new thread and not threadjack. I've done a bit of green sand casting over the past few years, but this is my first time trying lost foam, and am at failure #2 so far. The first failure was 100% my fault as I didn't have enough sand to completely cover the pattern, and made a hasty decision to make a secondary containment tube out of duct work to just surround the sprue above the part. I thought I left enough sand over the top of the part, but the venting gasses bubbled up and blew out that portion while pouring and overflowed into the bucket, ruining the pour. Lesson learned, I bought more sand.... 2nd failure is as shown below. I made another pattern, and I'd molded this one up with the sprue vertical to fix an overhang problem for the first one where I laid the pattern down on an angle, and sand didn't fill an under cut. Here's the pattern Here's it after a coat of thinned drywall mud Here's how I had it sitting in the bucket of sand. I vibrated it in 1-2" lifts. Ya, I know I need better pouring basins.....I'll get there..... I don't have a foundry furnace built yet, so I built a heavy duty steel crucible for my forced air propane ribbon burner forge. A dedicated foundry furnace is top ten on my very long project list... Shakeout seemed promising.... Nice detail on the wax filleting I did. Pretty happy with that. But I got some pretty nasty cold shuts. When pouring this one, the basin fill up right away, then slowly drained a bit as I kept up filling it, then it bubbled up and the bottom fell out and I pretty much just dumped the rest of the crucible in as fast as I could. After thinking about it all day, I think the vertical sprue was the biggest cause here, and some metal got in initially, but the venting choked off, not allowing anything else to fill, until it bubbled out and that's when the basin emptied causing that cold shut. I have a better idea on how to mold this for the next one that will prevent any overhang problems, but tilt my sprue to allow better filling and venting I'm thinking something on the left would allow complete sand fill/packing, but tilting the sprue would allow the metal to flow down the bottom while allowing the gases to pass overtop on the way out. The one on the right is an alternative idea with all the vents/sprue drill out to allow gases to escape immediately. Sorry for the long post, hope that gives enough detail. I welcome any criticism, or tips to make the 3rd time the charm. Thanks, Dan.
Hi Dan, maybe something like this? It is based on guidance I received for my last lost foam parts. https://forums.thehomefoundry.org/i...size-lost-foam-project.2921/page-2#post-54143 PS Seems like I am seeing more about people using forges as furnaces lately. There was a recent 'Mr. Pete' YouTube post in the Iron Pot Factory thread where he melts aluminum in a similar fashion.
Now we're talkin'! Getting some sand depth above the pattern is important to avoid "sand float" and maintain mold stability. This is caused by the static pressure created by the liquid metal depth of the sprue. Aluminum and dry silica sand are about the same density, so keeping the sand stable is further aided by good packing, so when casting aluminum, you don't need a vacuum membrane, or weights on top of the mold surface which can become necessary for denser metals like bronze and iron. This is really no different than floating the cope in conventional sand casting and also why I always bury the pouring cup, because even a couple inches of metal in the cup can cause a "run-out"/(sprue leakage) at the mold surface. You don't say what you are using for vibration. That's not a particularly difficult undercut/overhang to get sufficiently packed. I'd say as long as your sand was very dry, vibration and sand depth adequate, you should have been able to get away with that positioning and gating. This is the preferred position/method. I might suggest moving the sprue contact location to the very top of pattern. Best way to describe positioning is to imagine the flask being slowly filled with water from the bottom up in a way that no trapped air pockets and preferably no horizontal surfaces occur. -Sometimes easier said than done. One thing from conventional sand casting that really doesn't apply in lost foam are the notion of vents. A vent tube like you have drawn will not be a vent until the foam is consumed by molten metal and that won't happen until the mold completely fills and by then, the potential damage to the pour you are trying to prevent has already occurred from belching back through the sprue. In lost foam, the entire surface of the coated pattern becomes the vent. Your pulley looks pretty thick and chunky. If it needs to be so, so be it, but very thick items like squatty cylinders and spheres (the worst) are actually the most difficult to cast with lost foam than thin uniform wall parts. This part of casting design is shared between most if not all methods of casting. In lost foam, the problem it creates is a large volume of foam vapor that needs to be expelled through a relatively small surface area of the pattern. This makes parts with higher surface area/volume ratio (thin walled parts) easier to vent to the mold surface. If you can stand it, pull some stock out of your design. When liquefied and vaporized foam is created more rapidly than it can be expelled through the coated pattern surface, it will buoyantly bubble up through the sprue. Some people just accept it and use a larger sprue, but it creates a lot of undesirable things. When that belch reaches the cup and comes in contact with air, it will ignite, rapidly expand, wreak havoc and create turbulence in the cup and black smoke obscuring your view. This can/will create defects in the casting after/if it settles down and fills. This is also why I advise against having the sprue protrude above the surface in the cup because the exposed portion immediate melts, floats, and ignites and then you have to pour through it. The rapid creation of vapor and belch can also accentuate the that pause that causes it to gulp from the cup but this also happens naturally as the metal front spreads across a larger percentage of the pattern. The belch can also create localized mold collapse if the void doesn't fill fast enough The defects in your last pour may be cold shuts or also could be "fold defects" caused by oxide layers in the metal fronts caused by cup turbulence. The oxide layers can prevent the streams from joining/fusing. You should come up with a way to measure your metal temp before pouring. I wouldn't want to cast without my contact pyrometer. You know you have a good feed system when you experience no flame or smoke. I was amazed by the improvement realized when I started using square offset pouring cups and square sprues. Round cups and sprues tend to aspirate air due to Coriolis affect. -The finer points. Best, Kelly
This explanation (filling bottom up, no trapped air pockets or horizontal surfaces) really helps, thanks.
This is the orientation I had in my first pour. Seems like it probably would have been fine if I'd had a couple more inches of sand on top of the pattern....I will go back to that on the next one. Using the forge as a foundry furnace was more a path of least resistance than an ideal solution at this point. I built it a couple years ago, and also ordered enough blanket and refractory to build a foundry too, just have not go around to it yet. Building that quick steel crucible was going to be a lot quicker and easier than building the foundry to cast this part, so I went that route. I made it so that I can also use it as an ingot mold with draft on all sides for bulk scrapping at a later point. I have a few more projects to finish catching up on before I get around to building my foundry furnace so that will get me by for bigger pours until then. For smaller volume stuff, I have one of those import electric benchtop ones, that works pretty decent.
Good info, thanks. I only had around 2" above the pattern at the point of blowout, which I "thought" might have been enough, but I learned a lesson the hard way. To be honest it was a hasty decision made because I only started molding once I have the forge lit and crucible in. That how I've always done it with my benchtop foundry, and it usually gives me about 45 minutes to get my molding down before it's ready to pour. This one liquefied that crucible in about 15 minutes and caught me off guard which caused me to make some hasty decisions. I initially was going to walk the 50' to my blacksmith shed and grab my old green sand to fill the rest of the bucket, but that duct was sitting there, and I thought it would work to reduce the amount of sand needed to cover the top of the sprue. I also had the idea of just tossing a bunch of aluminum soft jaws in there to take up some volume as you can see in the pic, but realized that wouldn't be enough so abandoned that....Probably another reason for the underfilled overhang, as I was in a hurry to vibrate and obviously didn't do it enough. I tried a couple different things from a DA sander, to a jigsaw, but the most effective was the thera gun run around the outside of the bucket. All the sand was baked dry in the oven prior to using it, but I guess I didn't vibrate it enough to pack up under that undercut. I like your various vibrating buckets and dollies, and would like to make something similar eventually. I was really just hoping to get this one part cast, and the project finished before falling down the big slippery slope making all the related things for casting at a later date. I always try to think like that water when molding something, but had some blinders on in the 2nd molding of this. So you think the original way I had it, shown above would have probably been fine had I had more/better vibration to fill the undercut, and more sand above the part? Yes, understood. That's why in the 2nd pic, I was thinking about drilling out the majority of the foam leaving clear air space and just a shell to hold the sand back, so it can "vent" right away. It was just a thought, but reading more of your post I realize it's most likely not necessary and most likely a waste of time. The middle gets bored out for a taperlock, so I could possibly removed some foam there to pre-rough that out and save some time on the lathe. The rest of it is a pretty chunky monkey that really doesn't need to be that thick everywhere, but I have a habit of overbuilding stuff sometimes..... That info is gold right there. Thank you. Some finer details you pointed out, but can have some drastic effects on the pour, and mask themselves as other causes I'm sure. I need to make a better pouring cup than a can, but again, I was just hoping to get through this one part before getting too deep into all the extras needed. There's machining allowance on every surface almost, so I was just hoping for a full pour and if it was ugly looking was going to machine it all around. If acceptable, just the bore and od/face of the rim. I like your moldable cups, but that is a bit too much money for me to throw at this for now. I have an idea that might be cheaper, and perhaps suffice for now. I have a ktype thermocouple around here somewhere I bought years ago. I should dig it out and make a pyrometer. All my previous casting experience has come from one of those electric benchtop units, and with the PID control, I have more control over temperature. Sure, it's not the actual temp of the metal in the crucible, but it's not far off and a pretty accurate way of repeatably controlling the variable. Casting with the forge, I was just eyballing it by viscosity compared to what I was seeing with the electric one. As with most things, controlling variables leads to better results almost always. You mention lack of smoke or flame. Is that just for foam, or would that apply to the wax for fillets as well? I had a decent amount of wax on this pattern for the fillets, and was concerned about it causing issues burning out. I assumed most of the flame coming from the Sprue was due to the wax, but am not 100% sure. I appreciate your comments that confirm I was kinda on the right path the first time regarding part orientation. The most important thing with failures is to understand WHY something failed, so that you can make the right changes moving forward. The 2nd molding I wanted to orient the part vertical to eliminate the overhang that didn't fill, but in doing so created more failure points I didn't fully understand or realize until now. The next one I will go back to my layed down orientation, and come up with a better vibration system. I have a couple ideas on how to do that without making a science project out of it. At this point I would have been better off to make another batch of green sand, and cast it with a split pattern, but oh well. Lessons learned the hard way always seem to sink in better for me it seems.......I'm sure paying my tuition on this one.....
Nothing wrong with it and a lot of people get a long ways down the road with such and steel crucibles. Aluminum is easy to melt, a little more difficult to maintain high metal quality. If your using scrap and material properties aren't that critical, it's very practical. If you want good material properties and machinability a lot of things matter. Some people get by just thumping the side of the bucket but for good packing that really doesn't cut it. As a test, you should get at least 10% compaction with granular silica sand. I routinely get 15%. Meaning, fill the bucket with sand by just scooping it in. If there is 12" of sand, you should be able to get at least 1.2" of settling with your compaction method. 1.8" even better. I think a (molten metal) contact pyrometer is an invaluable tool. Without it you're just guessing and that's not a good place to be for success and repeatability. You do need to protect it from the molten metal with a sheath. I use carbon gouging rods. There are threads here. You can also buy commercial probes. Could be in part due to wax but also could just be as I mentioned in previous post. I don't always get smokeless pours but I do strive for them. I use a wax formulated for lost foam casting that melts at the same temp as the foam. I do use it sparingly. It's much denser than foam and adds to the mass that must be burned out. That's it, along with develop your processes and stick with/repeat the ones that work. Best, Kelly
