That's a pretty good reason to stick with the PVAcetate. One benefits of the PVAlcohol was it was so thin, there just wasn't much of it to get rid of.......but, it might not seal the PVA very well. I dip my foam patterns in soapy (Dawn=Propylene Glycol) then dip them in the refractory. Besides the surfactant, the sanded foam surface is a little fuzzy and retains some water which thins the refractory at the surface and helps buoyantly expel the air. On fine details like wax appliques and engraving, I'll work the area with a soft brush then dip. Even with all that, I can still get a few little boogers, but they are usually just an air pocket bead that has been fed by a tiny pin hole which is the only attachment point, and they can just be knocked off with a light tap with a punch or screw drive. You might just try dipping your pattern in a thinned version of your plaster investment (or maybe drywall joint compound) without the silica. Let it dry or at least set then invest it. When I was dipping in dry wall mud I put a little dawn in the mix an that helped it wet, level, and lay down.......it would benefit from being vacuum evacuated first......just thinking out loud. http://forums.thehomefoundry.org/index.php?threads/dip-coating-polystyrene-foam.573/ Best, Kelly
Kelly, I'm actually thinking of switching to ceramic shell for the engine parts. I originally came up with the process I'm using to cast pistons. Those investments were smaller and much more user friendly to work with! The large amount of investment required for these parts, and the size and weight of the finished block makes it a bit hard for one person to move around. This part was bad enough. Handling a delicate 30lb block of hot investment is a bit of a chore. The other problem is I have to get the burnout and casting done sequentially. I can't do burnout and then wait till later because the investment absorbs moisture from the air, and additional heating increases shrinkage. With shell, I can make all of them ahead of time, indoors without regard to the weather (soon to be a concern!). Because of the high temp; burnout will be complete. I can wash the shells out with water. I can choose my day weather wise for casting. I can just heat the shells up in the oven before casting. I can use filleting wax to fill the larger imperfections in the print. Plus, I can get all the castings done in a single day. Cleaning the shell off is a chore, but in this case, I think it's probably the best method. Other than lost foam. Unfortunately I don't have access to to a 5 axis router, and I want to move forward with this project, not learn a new process! I already know how to do shell. Monty
I just poured the investment for the second go at this. I can't believe it was the end of December last time. We have had the worst weather! This is literally the first week I've had where it quit raining long enough for me to drag out the burnout oven, pour a mold and get ready to make a pour! In the interim I have completely re-built my 3D printer. Now it doesn't leave strings and goobers all over the part. REALLY saves on the clean up and prep time. It's also MUCH faster. I have the fan case and diffuser printed. The diffuser is almost ready to invest. Hopefully I'll have castings for all the major components by July.
Nuts. Almost a success. The pour went well. I was fully expecting a good result. No bubbling, no drama. But there was this popping that concerned me. I found out later either the aluminum foil or the sand at the bottom of the mold caused some gas defects. I could have fixed all but one of them. I could with difficulty even fix the worst one. The deal breaker was a shrink defect in the main part of the gear case. No fixing that. I'll have to revise the pattern by coring out the thick section. I took some video of the pour I'll post when I finally get around to editing. I've been looking back at my previous attempts and the one successful part I made for rev 1. I'm going to revise my gating back to the bottom, core out the thick area, and make sure all the flow is upwards. The real problem defect looks like a bubble that got caught in the down-flow from the main riser. I'm going to incorporate the casting filter into the gate system. Check out this gating system! I guess I'm a slacker....this guy took the belt, suspenders, duct tape, and bailing wire approach....I don't have a crucible that big!!
Are you suggesting I fabricated the whole thing!!?? I have video evidence! I just haven't had time to edit and upload. Here are some pictures of the damage: Casting looked fine at first glance: But then I found this gas defect. The first indication things were not 100%. You might also be able to see some of the cracks due to shrink in this thicker section. There's also these, which could have been fixed by welding: You can also see the remains of the sprue after I broke off the riser/pour cup at the filter. After looking at this a bunch of different ways, I have decided to go back to this arrangement: It worked in the past. The central gate feeds metal into the big riser and thickest part of the casting. I got good directional solidification with this. The central riser cools last and feeds the rest of the casting. It also eliminates the hard core in the center where the most shrink happens. I think that caused the hot tears in my last effort. I've been eyeing CNC routers.....I'm going to transition to lost foam for these bigger castings. The investments are just getting too big. I'm putting almost an entire bag of plaster into each one of these! I also want to be freed from the tyranny of venting, gas defects, and hot tears due to the core material being too hard. For production, this part would be ideal as a lost foam casting. It already has draft on all the surfaces. The main body could be molded as two halves and the central portion in one piece. They could then be assembled into the form.
I think you may have mentioned a 5-axis machine. I can certainly understand the want but maybe not the need. The rotational 4th axis may make sense for some impeller/fan patterns but you can do a lot with a 3-axis machine and two-sided machining. The big thing (at least for me) in CNC patterns is availability of stock >2" thick and Z-axis travel of the CNC. If you're willing to settle for Expanded Polystyrene (EPS), stock availability isn't too much of a problem. The Extruded Polystyrene (XPS) machines and finishes so much better it's hard to settle. The problem with laminating stock is having to machine through the glue joint. Most every glue will foul and load your cutter which will quickly destroy the piece. So the alternative is to break the part into 2" thick sections that can be assembled after machining. Suboptimal but certainly doable as a stack for your mid frame. As for Z-axis travel, most hobby cnc routers only offer 2". My machine is 35" x 35" x 3.75" advertised work space. It's really not just Z-travel, it's also height under spindle but 4" of stroke doesn't necessarily mean you'll have 4" of cutting height available. I eventually plan to raise the gantry and increase z-travel, so I can actually machine at least 4" stock. One thing you will find, it is remarkably less labor to LF cast then investment cast, especially larger parts. You'll need to make a vibrating system for your flask but once you do, fully molding that part will take somewhere between 5-15 minutes. Demolding will take whatever time needed to dump the flask....maybe a minute. If you have a bucket of water and compressed air, the refractory coating can be removed in about 5 minutes of easy work. Once I have a fully prepped foam pattern in hand, it's usually about an hour from cold start until I have a casting. Compare that to mixing, molding, baking and then demolding your investment. And you can reuse your dirt cheap LF sand so mold cost is essentially nil. Since that part fits in a 5 gal bucket it would fit in my small flask which is a steel 5gal bucket. You'd probably want yours to be 1.5-2 buckets deep. Here's my vibe rig at the bottom of this thread. http://forums.thehomefoundry.org/index.php?threads/my-new-lost-foam-casting-rig.516/ Best, Kelly
I'm sold on the lost foam, but I'll probably finish this casting up with investment. I've already got the parts modeled, and have the materials on hand. Plus I'll be re-learning everything for a new process. I'll do this whole project in investment, but I'm going to hard shell the fan case. I think I'll have hot tears with solid investment. I've already printed the form and have access to a shelling facility at work. The diffuser should cast OK using investment, and I've already got it printed, sealed and prepped. I have a 5 axis mill, but I am not about to clog that up with foam. 3 axis router is all I'm after. It is amazing how quickly Z height disappears!! By the time you clear a work piece with a tool....you essentially need double whatever Z dimension the part is. I have some parts lying around I think I'll use to build a router for foam. Since the cutting forces are so low, I should be able to get away with a little less rigidity. I'm thinking the Z needs to be at least 12 for engine parts like I want to make. I have air cooled cylinders, a gearbox, and engine castings I want to make eventually. There is an XPS billet manufacturer about 30 min away from me. I have some 4 in from other projects laying around. They have seconds, you can haul off for free. I haven't been there in a while, but I might need to re-acquaint myself... Monty
Kelly, Have you tried using tooth picks and CA (super glue) to tack the foam together? You only have glue in small blobs here and there. It's hard, but the cutter just rips it out in a small area that is easily patched. May not work that well, but I used to do this for model airplane parts. Monty
That's absolutely correct. The rigidity is challenged more by inertial loading while changing directions during rapid cutting than cutting forces. It's one area where hobby machines constructed with light aluminum extrusions for rails actually work for you because it keeps the moving mass low. We have similar needs. I'm envious. That would be a great resource. Roofing and mechanical contractors can be a source as well. I haven't tried that specifically, but in general have tried to selectively apply to avoid the tool path. Really not viable for vast majority of my parts because the walls are thin and cross sections complex enough they would have to be applied with masking. Best way I've found to cope with machining through joints is just not do it and the effort is better spent sourcing thicker stock. Lot's of boat dock flotation billet sources but that stuff is typically 8x too dense. If I had 4" XPS sheet stock and could get a 4" cutting length bit above it, that would cover a lot of ground for me.......probably 90%+ of need and I could manage the rest. Best, Kelly
After reading that long thread on gating and riser design. Looking at the Campbell stuff, I came up with this. The gates seem impossibly small, but I know it will fill from the top even if they freeze off. (I don't think they will). The pour cup goes directly to a spin trap. After that fills up the metal flows to end of all the gates, and then slowly up into the casting through the four gates at the bottom. Then when the metal gets to the top, it should mostly feed through the large central gate (less resistance) into the main riser. So the main riser gets the last hottest metal poured. If I had some hot topping, it use it, but I don't know where to get any.
This is still an empty cavity investment mold....correct? My understanding is the top of the spin trap needs to be higher than where the gates enter the mold cavity. Are the four legs part of the casting or feed system? Best, Kelly
It is an open investment mold. The "legs" are a sort of necessary evil. They allow the form to be anchored in the investment. They allow the plastic to escape during burnout, and they allow me to blow out the cavity before the pour. They can be thought of as part of the feed system, or as a part of the casting depending on how you look at it. My way of thinking is by the time the metal gets there, the area is so much larger than the main runner, the flow is constrained by the runner, not the legs. The previous casting worked with this system, even without the spin trap or feeding from the bottom. It just fed through the main riser gate and ran down to the bottom. I'm hoping this solves the slight porosity I had in the central thick area. If this one doesn't work....I'm going to change to lost foam.
That model looks a bit different than the actual casting earlier in the thread. Some of the interior features seem to be missing but perhaps they were added feed system features on that casting. Also, I thought nozzle/venturi interior was open webbed on the casting but doesn't look to be so when viewed in your second picture. It's relevant in respect to filling and packing to avoid overhangs and achieving the most stable LF mold. To your question, the function of the part is axial flow and accordingly, the features align well axially and to me lends itself to be filled axially top to bottom. I'd need to know the approximate dimensions (largest diameter, overall length, typical wall thickness, weight (15lbs?)) but assuming the typical wall is about .25", I'd be inclined to make an annular gate (I call them ring gates) that varied 1/2" ID to OD, maybe .75 to 1" tall, that tapered/chamfered down to conform and contact that outer annular section of the part. Then just make a central sprue that divides and contacts the ring gate at 3:00, 6:00, 9:00, & 12:00 O'clock. The part would be positioned and poured vertically. Make sense? I chose the (outer) smaller diameter end to gate into because I thought that was probably the more massive of the two ends and naturally flowed through the majority of the part volume, and because it looked most conducive to packing features. There would be a slight tendency to try to feed both annual sections on that end but probably not needed and as described would be very easy to degate. Might be nice to see the part sectioned through the axis. If the center is solid except for the small central hole it may pose some packing challenges. One thing to remember, in lost foam most if not all bets are off as far as the typical gating rules of naturally pressurized or AFS systems because the velocity of the metal front is dictated solely by the evaporation rate of foam and in my experience, this is always a small fraction of the maximum desired velocities associated with open cavity casting. I'd never try to top gate such a part in an open cavity casting. If you can achieve a relatively uniform and tranquil top to bottom fill, you will get a very nice side benefit of a natural bottom to top directional solidification, which should be quite helpful to that part......and the chances you'll ever see a hot tear are about nil. I'd try to get the top of the pouring cup about 8-12" above the top of the pattern as molded. An equally interesting discussion is how to cut that pattern. Because of my limitations I'd make it in axial sections on my CNC Router, assemble it, and make any of the off axis bosses and features as separate pieces that also would be hand placed/attached. Doesn't look too bad to me though. Remember, the part is never going to look better than the pattern. Best, Kelly
Kelly, This part is the "engine block" of the whole thing. It holds the fan case to the diffuser and contains the main gearbox. The outer casing attaches to the largest diameter. The internal pressure is 4 bar, structural loads are non-trivial. All of the fluid lines for fuel and oil intersect this part. There is a lot going on with this component. Air flow, fluids, and structurally, it is the most complicated part of the whole thing. Because of all these competing requirements some compromises have inevitably occurred here and there. Raw casting wt is 9.73 lbs. Largest diameter is 9.1 in. Length is 8.63 in. The raw casting has been scaled 1.3% to account for shrink. I have removed many of the features that were required for investment casting. I cored out the center to help eliminate the thick section to help with porosity and shrinkage. I have included a few pics of the finished machined component so you can see why the casting looks like this. Raw Casting Section: Finished Machined Component: Finished Machined Section: I agree with sectioning axially, and that is what I was planning to do. For production parts, I would make two halves, and the center case as molds for EPS. The center section as a single mold. Then glue all the pieces into the final form. If I understand your gating suggestions, it would look something like this: Monty
Kelly, RE the reduced cross section of the ring gate where it meets the part. Is this just for ease of de-gating the casting? Or is there some other reason/s? Monty
That helps. I'd gate it just as described. I'd use low density whit EPS for the dating system to encourage thee feed system would fill rapidly and then you'd have a generous supply of molten metal available in the ring gate to feed the entire part from the thin annular contact section. Align the clocked arms to the sprue with the corresponding webs on the casting to help feed the inner features. I think it would cast nicely. Yup, that's it. It's for sure a better packing arrangement than with the other end up because sand will flow through and around the inner shape well, whereas it would not do as well if that flat section on the other side were facing down. That should pack rather nicely and the hole through the center makes it even better. It's primarily for ease of degating in fact I might extend the 1/4" contact point (what is the width of that annular section?) for a small straight section but depends on how you would degate it. I'd mount it on my knee mill and walk around that gate with a slitting saw. It also helps the metal transition and promotes good mold stability where you have metal flow as opposed to just a square section annulus which would create a less desirable overhang. The idea is the ring gate is more massive and insures ample supply of hot metal to feed the part. Everything works pretty nice because hot metal flows through the top portion of the casting, losing heat all the way to the bottom, which freezes first and then back up to the top through the feed system. For a 10lb part, I'd think a 1" - 1.25"max square sprue would be sufficient Looking at the cross section above, to machine it with a 3-axis machine, looks to me like the center section may need to be a separate piece machined from top and bottom sectioned in the middle of the four webs and I might be tempted to machine the rest of the outer section in two halves sectioned as you've shown, if you have the z-axis reach, and then assemble. I think the bosses and all external features could be included in the machining program. I'd try to find thicker XPS stock as opposed to laminating 2" plank..........maybe even higher density EPS. Looks fun......but you still have another bite at the apple with your block investment. So what will this turbine be used for? Best, Kelly