I presume even though you do the low temp burn out (of PLA), your still firing above that to further cure the investment. If so, how would any (near) surface wax survive that? I'm a lost foam caster but even polystyrene foam goes liquid then gas, albeit a usually pretty thin liquid boundary between the molten metal and gas phase. There was a fella on YouTube dipping his PLA prints in paraffin to smooth them. He was shell casting but claimed it also reduced cracking as the paraffin liquified and volatilized at very low temp and it definitely improved finish for him, probably at some cost of dimensional precision. Given your comments about wax, it may not sound to appealing, but there's a whole world of lost wax casters who seem to manage just fine. Might be something to consider compared to the typical tedious solvent polishing methods. Best, Kelly
Kelly, I've used wax with ceramic shell and it works great. When I tried wax with low temp POP it melted and soaked into the investment with bad results. Unless you burnout above 800 F I'd be cautious with the wax and paraffin. PVA glues sublimate without melting and decompose into acetic acid and something else....I think CO2 but can't remember. This happens at a compatible temp to PLA melting, so hopefully it will work well without making a lot of ash. I'll have results soon. Then I'll try the UV cured resin, I'm concerned the casting resin won't sand well. PVA glues can be sanded smooth after they cure. It's just slow. I'll post when I have answers. Monty
But even when you burn out at a higher temp, don't you always transition through the lower temps along the way? So what's the dif? Funny, because in lost foam, I actually want the liquid boundary layer to exist, and I just have a thin dipped, air dried,refractory coating on the pattern surrounded by dry sand. My latest effort: http://forums.thehomefoundry.org/index.php?threads/automotive-intake-manifold-–-boss-302-independent-runner.1692/page-4#post-40080 Best, Kelly
Kelly, The problem with the low temp burnout is the temp at which the wax turns to a gas is higher than the mold ever gets....until I pour the metal in, and then the wax/paraffin that soaked into the investment as a liquid vaporizes and makes big ugly gas inclusions in the casting. The POP investment is porous enough to let the liquid soak in, but the ability to vent gas is limited. If you burn out at a higher temp, this isn't a problem because the wax vaporizes and burns off. But....the high temp creates other problems for my cheap skate Home Depot sourced investment mix... My investments are way too big to afford buying the good stuff. I have bought some USG investment made specifically for casting aluminum. It isn't too expensive, but the shipping....is horrible! I want to try some lost foam casting, I actually think it's superior for some things, finned cylinder, engine blocks etc, because you don't need to pay so much attention to venting. I've made some hybrid castings with POP cavity and sodium silicate sand cores. The sand cores really help with venting, but I wanted better surface finish....so I went off down the POP investment rabbit hole. Monty
I'm surprised, because with any PoP based casting process I would expect it to required high temp firing to liberate the chemically bound H2O before casting. The cost of investment is a real issue for larger parts. DavidF, the site owner has done a fair bit of lost PLA casting and definitely has preferred material. Search his posts. He'll probably comment eventually. I don't think you will find the available resin printing media to be economical nor rewarding. I make larger automotive parts and IMO, LF is much faster and less expensive (practical) for the parts I make. One major difference is my pattern and mold costs are essentially pennies. The molding process take minutes and the media can be reused. What is comes down to is pattern construction. Though EPS can be molded, this is really only practical in a production environment. For hobbyists and low volume, the analogous LF process to 3D printing is CNC routing. Being evaporative pattern casting, they both afford nearly unlimited design freedom. Once you are set up to use one process, you tend to apply it to everything, but when you're only tool is a hammer, everything looks like a nail! Both processes have their place. I probably need to do some updates but if you're interested in LF, have a look at this thread. http://forums.thehomefoundry.org/in...ssions-of-a-lost-foam-caster-5-years-on.1650/ If you are 3D printing, you already have CAD skills. CAM and Gcode are a bit steeper learning curve than 3D slicing/printing. I'm actually circling back the other direction, developing my solid modeling skills, so I can print......smaller parts. Also, I don't think I recall mention of the preheat temp of your investment mold.....what is it? Best, Kelly
Kelly, I take my molds to 600F. This is all that's need for casting aluminum. Initial heating of the mold is at 250F. I put a thermocouple in the mold. When it gets over 220 I know the liquid water has been driven off. Then I take it up to the melting temp for PLA around 300F. I let the bulk of the PLA melt and run out. Then I take the temp up to 500-600F to drive off the chemically bound water and burn out the remaining PLA. I pour at around 300-400F mold temp, 1300F aluminum. If you are doing bronze or higher temp alloys, then the next step up I think is 1300F with the investment?? Can't remember exactly. If I were doing higher temp stuff, I'd go with higher quality commercial investment. I started this path by emulating the RPM (rubber, plaster, mold) process for aluminum and low temp alloys. I have a lot of model making and plastic part design experience. So designing in CAD with draft for cavity and cores is second nature for me. 3D printing works great with RPM to make the rubber form. Once the tooling is made, numerous cavities and cores can be pulled from the molds. RPM yields die cast quality parts if done properly. I have always had an eye toward low volume production, so this is the path I have taken. For the prototypes, I'm doing lost PLA to avoid all the work making permanent tooling. Once the design is finalized, I'll make permanent tooling so I can do small runs, or if demand is there contract with a foundry that does RPM casting. Also, the parts I'm working with can be very complex! Here is the fan case for my engine. I'm sure I could CNC the stator vanes and case wall in foam, but it would be tricky to say the least! Monty
Looks great! Bill Jurgenson is a member who ran a lost wax art foundry specializing in sand, plaster and ludo molds, for 40 years. His info says to burn out for bronze at just 700F to reduce cracking of the investment, but he told me he had been going all the way up to 1100F for decades before he discovered he could get better results using the lower burnout temperature. I haven't tried his recipe (or any other sort of lost wax casting) myself, but I posted the investment recipe and burnout instructions he gave me here a while back FWIW: http://forums.thehomefoundry.org/index.php?threads/stacked-plaster-mold.1517/#post-34440 Jeff
500-600F is considerably more modest than what I have seen most use for ultimate burn out, but they were usually commercial investments. It would be very low for those using shell. The rest is more or less in line. Still sort of surprised you can get by with that low of a terminal of temp, even for aluminum, but what works......works and although higher temps certainly can eliminate more bound water (of course there is time at temp dependence too), they also often considerably weaken mold media, especially more common PoPs. Fan case could potentially be a lost foam candidate, though I'd choose investment or shell for a turbine impeller or maybe CNC'd wrought alloys. For the fan case, it can depend upon wall thickness and size. To demonstrate the art of the possible with lost foam, I did this project. 0.062" wall thickness, ~10" diameter. Might want to read backward from last post. http://forums.thehomefoundry.org/index.php?threads/saucer-cup-spoon-–-the-lost-foam-edition.1020/ -Keep up the good work and keep the pictures coming. Glad you decided to join the forum. Best, Kelly
Provided the heat has sufficient time to fully penetrate the mold, 500-600°F is adequate for pop. From the wikipedia article on POP i.e. gypsum plaster:
This could change everything....I was going to paint it on, but I have an airbrush! For the RPM process, I can avoid all the sanding and filling. It's a real PITA!! I use popsickle sticks with sandpaper on them to sand down in all the crevices where ribs are formed. You have to make a negative, that makes a rubber positive, to make the plaster negative....that makes the metal positive! For that I can just do a quick scrape and sand to take off the big ugly stuff, and then spray on the UV cure resin. For the lost PLA, I can use the casting resin. I ordered some and a UV light, it just arrived. I'll try it on the fan case casting. Monty
After reviewing several peoples experiences with this stuff, I feel it's probably best suited for ceramic shell/high temp burn out. Probably not well suited for low temp process. I received my resin and curing light. I have a nozzle guide vane casting that will be made from inconel. I'm not set up to deal with such high temp alloys. Fortunately, there is a foundry nearby that works with stainless and these alloys. I'm going to try printing the NGV in PLA and coating it with this resin for them to cast. For my low temp castings, I'll probably stick with PVA. I've been thinking of using a dip process to cut down on the labor involved. Probably thin the glue with water and dip the part in a bucket of mix. For the big parts like my fan case, I may try air brushing PVA glue thinned with water. Probably add food coloring so I can see how thick the coating is, and where I've sprayed it! For the RPM process, being able to smooth the parts before making the rubber molds is essential, and I still think the sprayed UV resin will be a game changer for that!
I just opened a roll of polycast. The jury is still out on this one. First of all it started having moister issues within 24 hours. Ok put it in a filament dryer no big deal. They suggest 60 deg C for 12 hours for drying. I have a SunLu that only goes to 55 deg C. Should work. Well it doesn't seem to dry it completely at 55 deg C. I keep my printer well calibrated for each filament I use. Often a few hours of standard testing on new material. IE: Checking average thickness setting up slicing profiles New nozzle (not really necessary but I thought it best for this test) PID autotune for target temp, Calibrating Z-Offset, Temp/Bridging, E-steps, Flow, Retraction........ So far I have had great printing in the very beginning. I print @.o8mm Height 40mm Speed 195° hot end 63° bed for all casting files. I printed at least a half dozen 2 to 3 hour files without issues. Now I need to prime the nozzle at least 200-300mm to get close to bubble free extrusion before I start a print. Often the print is not the quality I get from other filament. It will smooth out well using a variety of methods. Brushing for localized areas or an atomizer spray bottle for full coverage. I am still working out the best practices for smoothing. But that part works out well. The burnout is great without any ash ( I do not turn the flask over as suggested). This is the main reason I purchased this filament and I am impressed. For those who care. The average flask is 2.125x4" 90 - 120 min drying. Steamed out wax sprue 10min ( time depends on many things) then into 300° F preheated oven Soak 60 min. Ramp @ 9°F/min to 700° F Soak 90 min Ramp @ 9°F/min to 1350° F Soak 120min Then 9°/min down to whatever temp I need for that casting. So is polycast worth it in my opinion. No ash = 5 stars Cost = 2-3 stars Cost to run a filament dryer 24-7 = 0-1 star Smoothing = 4 stars (without their polisher) Printability = 2 stars I have been using HatchBox Black. It prints great for me. But of coarse it has a bit of ash. I will try Silk PLA. I believe that may be a favorite of DavidF's and trust his judgement on these things. I'm sure I forgot something...... Bill Will Edit/Question: Is Silk PLA a brand or is it a type of PLA like wood or metal
It's a type.... you'll notice it's a bit strange when it comes out of the nozzle of your printer. It puffs up rather then being a thin wire. And it prints extremely well. The main reasons I use it is that it prints easily and I have less of a problem with molds cracking when using it. Seems like everything out there that I have tried filament wise works well in certain aspects, but falls short in others. In the over all scheme of things the silk filaments have worked best for me..
Talk about pulling it out at the bottom of the 9th. Quite a save! A few lessons I learned the hard way: Always add the plaster to the water. And always measure it out so you have a little extra. Always bury your investment mold in a tub of sand, BEFORE you pour, in case it cracks. I'm guessing the reason the paraffin worked for him, is the use of the kiln for burnout. I'm sure he's taking the temp a lot higher than I am. Definitely a technique to file away. Though the newer UV cure resin printers may make the need for smoothing a thing of the past. I'm also planning to buy a router and start trying out some lost foam casting.
