Gentlemen and ladies if there are any. I am having issues with casting aluminum. The piece is very vital for my cnc machine Y drive axies. I am unsure where to put the sprue. I had it on the outside, on top. Flipped the model over and put a sprue from the top to the bottom diagonally. even tried to vent. I am just not sure what I am doing wrong. Ive tried pouring around 680c, 700c, 740c was the best pour but lacked enough metal. But not sure why i was missing a middle piece. What temp are you all pouring at? I do not have any pictures of the pours and I cut them down to redo the projects. The media files is from fusion 360 and this model is the finished product after pouring.
Hi dom. You're not exactly giving us a whole lot to go on with just a picture of the rendering and posing the question "why won't it cast". Take pictures of your failures. You're going to need to put a little more effort into it if you want help otherwise it just becomes a tail chasing exercise. The quality of advice depends on the quality of information provided. Just looking at the geometry, I'd start with the one on the left which should suffice, but if not, then the one on the right. What are the overall dimensions LxWxH? What is the average wall thickness? Approximately, how many lbs is the pour? Generally, 1375F but this can vary depending upon the nature of the part. Thicker more massive castings slightly cooler, and thin-walled high surface area parts hotter. What are you using to measure your pour temperature, and how long does it take you to after removing the crucible (assuming you have one) before you actually pour? Best, Kelly
Hey Kelly I suppose the first few times did the casting without plaster or ceramic shell. sand fell into the hole. probably didn't pour fast enough. Ill do pictures on the next go around as I had chopped these last parts up to reuse for the next pour with plaster/ koalin and sodium silicate. The wall size is 12mm leaving enough room to mill to finale size of 10mm . Im pouring around 12kgs pretty much maxing my crucible out. I am using a K type thermo coupling after I take the crucible out. I do a finale measurement and the temp is around 850c which I allow to cool out in the open air until around 700 (other than the last pour of 740) Ill be sure to add photos with this next pour if it goes south. What is the theory to placing the sprue?
This is posted in the lost foam casting sub forum, but just to confirm, this is a polystyrene foam pattern, correct? Ordinarily we wouldn't be talking about ceramic shell, kaolin, and sodium silicate in lost foam casting. It would be done with dry unbound sand. The plaster could be a satisfactory refractory coating for the lost foam method. I used it for the first few years I cast. Are you using a pouring cup? If you have a gap in the pour that opens/uncovers the sprue, this is very bad for lost foam casting and usually causes failure. The cup can provide a buffer to keep the system filled and properly fed. For your size part I'd probably have a 25mm square sprue and a pouring cup volume >20% the volume of the casting. Using a K-Type immersion thermometer is good. I wouldn't recommend superheating the aluminum to 850c. You'll have better metal quality, especially if you are using a fuel fired furnace, if you kept the maximum temp lower. However this is likely to lead to porosity and casting flaws and not a fundamental issue with the success of the pour and fully formed casting. That question doesn't have a simple answer. Positioning of the part is important. Ideally I position the pattern so that if the part was in a container that was slowly filled from the bottom up with water, there would be no trapped air and no horizontal overhanging surfaces. I typically gate into the top/highest point and prefer to feed a more massive section of the casting. I would never top feed an open cavity mold this way but in lost foam the metal advances as function of how fast the foam is evaporated and not by gravity and cross sectional area as in conventional casting. It's in dire need of being updated, but have you perused this "Sticky Thread" in this sub forum? Confessions of a Lost Foam Caster, 5 Years on. | The Home Foundry Seeing a failed casting can provide many clues. Best, Kelly
Hey Kelly Thanks for the response. Ive tried it without plaster first, very well compacted with cement vibrator. I used a 80x80mm square tubing as the pouring basin filled it with sand up to the top of the sprue. I noticed that the sand hand collapsed on the first few. Then tried plaster, it seemed to work, but i think the gasses could not escape through the top of the sprue. The sprue is roughly around 30 to 40 mm thick. perhaps bad placement of that. I will keep an eye on the temp of the alu on the next run. I am using propane furnace from vevor. Positioning of the part is important----- And this is probably why the cast is failing. Im still learning the fundamentals on all of this. The pouring makes prefect sense from bottom up with the gasses being able to come out the top of the sprue to vent. The last 4 picture was the best 90%
Hi Dom, If you assign the physical material as aluminum in Fusion, you can draw in a simple cup and sprue and use the properties to get volume and weight of the aluminium. I have to do this as my current furnace and crucible are quite small. The casting will vent through thin dried drywall compound into the surrounding compacted sand. It is somewhat counterintuitive to regular investment and sandcasting practice. I can't say I understand the mechanisms completely, the tips I have received here definitely have improved my parts.
Just to be clear, I wasn't saying gate/fill the pattern/casting from the bottom as far as sprue placement. The previous "filling with water" remark was just an intuitive aid for positioning the part favorably in the mold. The reason I tend to "top feed" lost foam patterns into a massive section is the bottom of the pattern is the last to see molten metal and thus the coldest metal by the time it gets there. This promotes directional solidification from the bottom up to the sprue where you have the hottest metal. On many of my larger castings, I have been using large contact area feed systems which tend to better distribute the metal across the pattern and minimizes the total distance the molten metal must flow from the feed system to fill the pattern. However, this can accentuate how quickly the metal is drawn from the cup once the pattern starts to fill. There appears to very thick residual plaster on your casting. You should have a very thin coating. The coating is not intended to be structural mold support. It's supposed to be a permeable barrier that controls how fast the decomposed foam liquid/gases can escape to the mold. Thicker coatings inhibit the ability to vent gas. The coating slurry should be thinned and applied just thick enough to cover the color of the polystyrene. I actually recommend drywall joint finishing compound as opposed to just Plaster of Paris. Here in the US those compounds are non-setting and have additives that tend to make them easy to apply and more permeable when dry. Nothing is more permeable than no coating, but sometimes that can allow uncontrolled advancement of the molten metal front and localized mold collapse. How thick is the wall of that steel tube? It should be as thin as possible. If it is say 4mm thick, the metal mass will rob an immense amount of heat from the initial pour and make that metal too cold to penetrate to the bottom of the pattern. This is why my cups are low mass insulating material (moldable ceramic fiber). Instead of heavy tube they can just be very thin sheet metal folded to size. Tape the corner to seal it. The sand will hold it in place during the pour. Do not allow your sprue to protrude into the pouring cup above the sand at the bottom of the cup. The additional sprue just becomes fuel that ignites and creates a lot of smoke, fire, and turbulence in the cup. Expendable cups can also be made from sodium silicate bound sand. Best, Kelly
so here is the update to the pour This 1st one today came out prefect. The next one not sure what the hell happened. Everything was the same The last one for the X axis
Dom, I am glad to see you got one part. How are you gluing the pieces together? The Physical material can be changed under Modify>Physical Material Once a material is added to the small window, it can be dragged on top of bodies. The Appearance command functions in the same way. If you like colors, you can select an anodized aluminum for material or assign the physical material with a painted color appearance.
For the casting fail in the very first picture following this quote, it's pretty obvious that the pour was interrupted causing localized collapse around the sprue and then the casting. The usual cause of this is, especially in new lost foam casters is the infamous "lost foam pause". This occurs when you fill the cup and the molten metal just seems to sit there, and then suddenly it seems like the bottom falls out as the metal front reaches the pattern and begins to evaporate foam at a greater rate. This causes the mold to rapidly take metal from the cup and before you can react by adding metal to the cup, the sprue becomes uncovered and the surrounding mold walls collapse. Then the collapsed sand blocks the additional metal and/or gets flushed into the remaining casting. A variation on this is when flame, smoke, and/or a belch of gas back through the sprue, startles or obscures your view of the cup, causing the sprue to become uncovered and then the same failure mode. You really need to be on the ready and expecting this with the crucible poised in pouring position because the pause affect can become much more pronounced with thicker castings. A larger cup and longer sprue help. I usually advise ~6" of sprue length above the pattern. The sprue height develops some initial head pressure which tends to get the gas escaping through the mold surface instead of bubbling back up through the sprue. A larger cup gives you a larger buffer time to react to the "pause". As previously mentioned, do NOT allow the sprue to protrude past the floor of the cup. The extra sprue just immediately melts, ignites, and cause turbulence in the pouring cup. People think that the flame and black smoke are just a natural part of the lost foam pour, and while it's true that most pours involve some flame and smoke, it's a sign that the gating system and pattern coating are inadequate. My best pours have no flame or smoke at all. Black smoke is indicative of the foam burning in air. The white smoke that comes out of the sand when you demold is from foam that has decomposed in the absence of air (oxygen). If you are trying the melt the exact amount of metal to fill your mold, stop doing that. Melt >20% more than you need for a complete pour. If you can't do that, get a larger crucible. The worst fail is when you do everything right but don't have enough to complete the pour and then pour short......guaranteed failure. Best, Kelly
