Lost Foam Pros/Cons

Discussion in 'Lost foam casting' started by Al2O3, Oct 16, 2017.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I thought I’d throw this one out there in response to a question that was posed about the merits of lost foam casting. I’d be interested in other member’s views on the subject. For the reasons that follow, I think it’s probably the easiest and most economical point of entry to the casting hobby for most backyard gamers.

    Pro’s


    • No large heavy tooling to make or store……For me the router templates which are usually just profiles are small, light, and store in very little space. In many cases you can build a library of templates applicable to multiple projects.
    • Mold packing is very quick and easy compared to conventional sand casting ….no ramming or heavy lifting and a flask is any container rigid enough to hold the sand. This usually means you don’t have to handle heavy molds because you can scoop it in in manageable amounts. A vibrator is beneficial but wrapping/tapping works to a degree. Now a days, I just dump the spent mold sand on my drive way to let it cool, off-gas, and then shovel it back into my buckets through my sifter. So other than this there is no binder management or sand conditioning to be concerned about…..i.e. no muller is required and other than sifting, no sand conditioning is required.
    • If you use a pattern coating such as drywall compound you can use about anything for sand as long as it’s dry and of sufficient refractory.
    • Foam castings do not require draft which can be helpful for designing in tooling features from which to locate and grip for post machining of castings.
    • Parts that would otherwise have complex coring or parting lines in conventional sand casting are easily handled by gluing pattern features together.
    • Pattern Detail: With liquid coatings, casting detail appears to rival lost wax and shell. Foam works very easily but I’ve had to introduce wax (although the foam pattern itself cannot rival the detail of wax) for the finer details such as fillets, embossments, and Lettering/Part marking.


    Cons



    • Odor: I store my LF sand in 5-gal buckets and with plastic snap lids so there is no storage odor. The odor while casting outdoors is very short lived. I would not under any circumstances recommend casting LF indoors….even small parts.
    • Brush coating drywall compound can be slow but it’s very inexpensive and readily air-dries. I need to work on a dippable process, but then you have that to store. Frankly, I can’t see why it just can’t be an 5-gal bucket full of thinned mud that you agitate with a drill-powered paddle before use.
    • Machining patterns can be messy. The foam dust travels. I spent this Saturday making improvements to my arm router which included a dust collection system. I also made hot-wire cutter for roughing out machine patterns to reduce the amount of foam that gets machined away. Together the two should make a big difference in speed of fabricating patterns, reducing dust, and clean up. My woodworking machinery earned its space in my shop many years ago so it’s there for the using without additional space.
    • Pattern strength: Depends upon the shape. Foam is strong in compression but in general is fragile. Thin shapes can distort during molding and it’s easy to dent or damage a pattern during handling.
    • Pattern Detail: This is a relative thing and why I list it as both a Pro and a Con. I don’t think the level of detail that can be reproduced in wax or harder materials is possible…..at least with my techniques. That’s why I introduced wax for fillets and embossments so I have sort of a hybrid process.
    • Failure means you not only get to re-make the mold but the pattern too


    Areas for (my) further development:



    • Dip coating: Might be as simple as lower viscosity mud in a 5-gal bucket….but may also require elevated temp or longer drying cycles.
    • Vacuum Assisted Lost Foam. In 90%+ of the cases I think gravity fed pours are adequate. However, I think vacuum assisted LF has real promise to further control and accelerate metal propagation speed which might increase the envelope for LF castings as far as thinner wall thickness, (larger) parts, and (lower) pouring temps.
    • Metal Quality: I honestly don’t know if this is a pro or con yet. My preliminary results suggest it’s on par with conventional sand casting.
    What sayeth all of you?

    Best,
    Kelly


     
    Last edited: Oct 16, 2017
    Wachuko likes this.
  2. Robert

    Robert Silver

    I am specifically interested in metal quality. Has anyone noticed porosity, machinability issues or other defects from lost foam that does not occur with sand?
    Robert
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The couple of my LF castings I've sectioned and examined under magnification looked as good as any sand casting with which I did the same (years ago) thus my comment above about being "on par". All castings are porous....it's just a matter of degree. Even with close furnace atmosphere control H2 will be present. Degassing certainly helps. Some of my water neck castings "sweat" hot coolant under pressure.......but I don't de-gas and I should, but I still think the issue would persist at least statistically. FWIW, I think conventional sand castings would do the same thing for the same reason and because of the thin walls. Vacuum impregnating castings is pretty much the norm for such in industry.

    On machining, I think the selection of alloy and heat treating (if heat treatable and not intended for as cast machining) is more important in this regard than sand or LF process. As soon as the metal in the cup freezes, I de-mold and quench in water to knock the coating off the casting but there's a natural degree of solution heat treatment realized in doing so because it's still very hot. I can easily do the precipitation/age bit because my furnace is electric, but having sat around for a few weeks there's also a natural degree of that as well.

    T5 is in reach at home but T6 is more difficult. I think you really must have the need for that extra little bit of strength to warrant T6. T5 machines and polishes well but you're never going to get to the level of the best wrought aluminum alloys. My castings thus far have machined satisfactorily and hold threads as well as any aluminum castings I've used. Neither LF nor conventional sand casting is the best process for critically stressed parts and that's why the design guides recommend higher safety factors be applied for parts manufactured with these processes due to alloys, porosity, and flaw population.

    I'm accumulating a quite few parts to be machined and I'll post up results in the near future.

    Best,
    Kelly
     
  4. PatJ

    PatJ Silver

    I must say I am pretty surprised at the success you are having with the foam.
    Your methods sort of take it from the very crude backyard "YT" sort of thing to a very viable process.

    Your statement above is also true, and points out he benefit of a pattern with green sand; ie: you can tweek a wood pattern and make a new mold pretty quickly.

    I think part of it is in what you are set up to do.
    I am set up for green sand type molds, and I would have to rethink the whole process in order to go to foam.
    If someone was set up for foam, then it would make a lot more sense to use that process.

    One solution to the heavy sand/mold/flask situation is to use bound sand with a custom-fit flask, and make the mold very thin; no more than 1" thick in any one spot.
    I generally start with a square wood flask, and add spacers inside to reduce the amount of sand used. The spacers can be taped in place if just a few molds need to be made, which greatly speeds the custom flask process.

    And I plane 3/4" thick wood to standard heights, such as 1", 1.5", 2", etc., so that I can easily add filler pieces into a flask.
    My flasks (generally the cope) are often only 3/4" thick, and relatively lightweight, and the bound sand allows for this.

    But seeing your foam work really demonstrates the potential of LF-type castings, and proves that there is a whole lot that can be done in that arena, and far more can be done than I think most people realize.
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    So true. As the saying goes...when your only tool is a hammer everything looks like a nail. I think there is merit to just about every approach be it rigid or loosely bound sand, LF, shell, investment, etc. I plan to use them all some day.

    Be it foam, wax, plastics, wood, plaster.....every pattern and tooling medium has its own characteristics to master.

    The only reason I started down the LF path was I had just finished a protracted furnace build and wanted to cast something and I had no molding materials of any kind. Every time I tried something with LF I thought would surely fail, it either succeeded or came so close I could see it could it could succeed. The more I learned using LF the more interesting it became.....and I'm still learning, but for the reasons stated in my opening post, it covers a lot of ground for my casting needs.

    Best,
    Kelly
     
  6. DavidF

    DavidF Administrator Staff Member Banner Member

    I want to try pressure die casting next :D
    Cant be that difficult. The casting part that is. Making molds and learning to polish them however will certainly be a journey.
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I think I'd just source any high volume production from die and permanent mold. If I had a need for higher production qtys in sand, I might do small parts in lots of 10-100, big (>25lbs aluminum castings) parts, no. ....but never say never.

    If I wanted a very high integrity casting I think I'd build a vacuum oven would probably need to be induction for high temp alloys and go ceramic shell....but just for onesies and low volume stuff. I'd probably enjoy building the oven more than using it....but totally impractical.

    Best,
    Kelly
     
  8. PatJ

    PatJ Silver

    One thing I forgot to mention about bound sand molds is that both they and the cores (which are made from the same SS bound sand) can be made in pieces and cemented together.
    I have glued duplicate molds together to cast multiple parts from a single pattern.

    I am not sure people are aware that you can cement SS mold and core pieces together, but it really expands what you can do significantly, and greatly simplifies the quantity of patterns that need to be made, ie: you only need one of each pattern.
    I will post an example and a link.
     
  9. DavidF

    DavidF Administrator Staff Member Banner Member

    I dont do enough of a single item to be able to outsource it and make a profit.
    Im not sure if any antique car part ever would / could.
    Main reasons im looking into it is for dimensional accuracy, and that some areas on certain emblems are inaccessible to polish. But those areas could be polished in the mold easily. Zinc picks up on the molds finish very well, and this would be a good enough finish in the low areas for plating, where investment cast parts still need to be polished prior to plating.
    Guess it sounds pretty crazy to be considering pressure die casting at home, but were all a little crazy on here....lol
     
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Watching posts for several years AA and then when I started my original tubular water neck thread over at AA, though there were a number of others that contributed, FishbonzWV, Sandcrab, Cactusdreams, Tobho Mott, and FrostOak all seemed to have lost foam experience and at least traditional sand cast as well. Heck, I even got DavidF to pollute a batch of sand! Was hoping to hear from you all on the pros/cons of LF. But it doesn't seem to be very main stream or taken very seriously in the Back Yard Metal casting community.

    Seems like most folks were using LF for ornamentals as opposed to my primary interest being machine parts. In many cases that seems because foam ornaments were available and economical though some were fabricating their own patterns as well. You have to be able to work foam as an expendable pattern medium and the industrial method of expanding beads with a foaming agent and steam in a die seems a bit of a stretch for home gamer whereas machining XPS with woodworking machinery or maybe even CNC is not.

    Best,
    Kelly
     
  11. DavidF

    DavidF Administrator Staff Member Banner Member

    Kelly, i assure you that it is taken seriously, If there were any doubts as to what can be done with LF you have certainly put them to rest. However i believe that the part to be cast needs to be mated to a process. It would be impossible for me to get the surface finished i need for the type of castings i do with lost foam. Could I have used it for the water necks I made? Yes but, I made 48 of those castings so far and at that point it certainly worked out better having reusable patterns. For me, I just have not had the opportunity to make use of the process yet. But I certainly will once the right job comes along for it.
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Agreed, horses for courses. That's part of what I was trying to get at with the post, applicability of the process. I would not use it for jewelry either and that's probably a close cousin to your emblems. The other bit is just that there doesn't seem to be many people doing LF, at least here and AA. A little more on YT but most of that doesn't seem to be a very serious attempt at making quality parts.

    If I knew I was making 50-100 of my thermostat housings that would probably be core and sand too, but ya know, having made the router templates (which also didn't take long), it doesn't take me long at all to make those small foamies with the jigs and if it was only that many per year and I was only going to cast 10-20 at time, it might not be so clear cut. For now it is because I'm not set up for traditional sand casting.

    The tubular water necks I've made are somewhat a different matter because that would be a much more complicated pattern and core box for even 50+ pcs. That is one thing about lost foam....it sort of offers the design freedoms of lost wax investment or shell and maybe even a little more so because my method of making a foam pattern seems less laborious then making a master then a rubber or plaster mold, then wax patterns, then shelling, and casting them. LF would be a lot less laborious without detailing and coating the foam patterns but also yield a lower quality result. The coating labor could be greatly reduced by dip versus brush and I intend to work on that. I try to machine in all fillets where possible but at complex intersections of assembled pieces I'm stuck with wiping in wax fillets and embossment applique. So I see LF can sort of be a tweener between LW shell and sand casting as far as economy and ease of use.

    I think the potential metal quality and finish is greater with LW and shell. I think LF is less total labor than LW and way less without detailing and coating the pattern.
    Coated LF seems to be on par with Petrobond as far as finish potential but LF seems limited by pattern finish whereas Petrobond more so by the mold.
    In volume, traditional sand casting and hard tools is the clear economic winner, but unless the part is small, few have the home resources to process that much sand, molds, and metal.

    I spent a day making the templates & a set of foam patterns for the carb plenum. Now I can reproduce multiples of those foam patterns in an hour......the pattern detailing is more involved. When I shared that piece with my old foundry friends, their first impression from ten feet was they were looking at investment casting. Then when they look closer and see it isn't quite as refined, they ask how the expendable pattern was made. And then when I tell them it's LF and I have 8-12 hours of total invested time, and the next ones will come much easier......they like it.

    Best,
    Kelly
     
  13. Tobho Mott

    Tobho Mott Gold Banner Member

    I have been playing around with lost foam for a few years, though less so in the more recent past. Kelly, you have covered pretty much all the pros and cons I can think of. I'll chip in my 2¢ here, but you covered it pretty well from where I sit.

    Pros...

    Easiest way to get into casting. All you need other than a way to melt and pour metal is a bucket of sand and some styrofoam. You can use free EPS packaging material for sprues and pieces where the beaded texture won't matter or if you're a really gentle touch with the sandpaper, or maybe buy a sheet of 2" XPS blue/pink foam insulation board that could last for a few years of weekend fun. It's easy to machine without needing much special equipment - you can manually mill out patterns for plaques and such with just a small drill press with a cutter bit in it, or even with a Dremel tool with a cheap plastic router attachment screwed onto it, or even just carve it by hand with sharp blades and files and sandpaper, if you are careful. Glue guns can be had at the dollar store and work great for assembling patterns. A paper towel or TP tube can be used as a sprue to get the metal right into the part similar to your metallic tape cones (but less conical). Hot wire cutters are easy to build on the cheap if you don't have a bandsaw to cut out patterns with. It can be so simple, or you can put more into it if you want to, just like with any other casting method - better sand, better foam carving tools, experimenting with coatings, sand vibration tools like your bucket or I've even seen where some people have tried getting some interesting sand fluidizing air hose setups to work, etc. So there's lots of room to have fun growing into it.

    Cons...

    Remaking foam patterns because you're too stubborn to figure out how to copy one of several cheap and easy to build pyrometer threads and poured too cold again really SUCKS! But that is not really lost foam's fault, is it? I would have had more to say here about how reliable results and thin walled parts are impossible for the home gamer if this were several months ago, but meanwhile you've gone and proved that simply isn't so. You're definitely not wrong about how messy and smelly it can be either!

    Possible areas needing more research...

    About the crusty sand lumps that come out of the lost foam bucket. Were you saying or implying in one of your lost foam posts somewhere that you don't get those when you pour hot enough, or am I misremembering? That is very interesting, something to watch out for. I remember on AA, metalbynevin (a guy with a good number of successful lost foam pours under his belt) had a theory that polystyrene vapour residue builds up in the sand, and that once it reaches a certain concentration, it can cause or aggravate sand float defects which sometimes occur if you pour the soup can Kush tool too full when the pattern isn't buried under a heavy enough load of sand to counteract the hydraulic pressure. If true, I can see how it would be easy to misdiagnose as such. I don't know if there is any validity to that, it was only briefly discussed, but if so, maybe it is possible the sand may need to be replaced (or burned clean?) every so often. If that is a real thing, perhaps it is only of concern if you've been pouring cool enough to see a lot of those tell-tale sand lumps?

    Jeff
     
  14. DavidF

    DavidF Administrator Staff Member Banner Member

    Anyone have any thoughts on how to hold a foam pattern that would need to be machined on 2 sides (top/bottom) on a cnc mill?? Something tells me that clamping it in a vise would not go over well...But what about double sided tape??
     
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    My 1/2 tubes are machined on both sides. I machined a female plug they just press into with friction fit, but I use small amounts of packing tape so they don't get sucked out of the fixture. My carb plenum is also machined on both sides and its just a friction fit on an internal MDF plug. I think the next time I may just try PoP and vacuum. If you machine the one side of the foam pattern and cast it at the parting line in PoP, then either fire or just use acetone or lacquer thinner to remove the foam pattern and you should have a real nice fitting machining fixture. You can use a lost foam plug immersed in the plaster that is cooked out to create a vacuum plenum in the plaster or simply set the plaster fixture on a wooden plenum and drill small wholes through the PoP for the vacuum. In some of my fixtures the foam fits so snug that I have to drill a couple finger holes in the back side so I can remove the foam patterns without destroying them.

    I used the above method to make machine fixtures for tough to grip castings but I powder coated the casting and then used urethane resin instead of PoP. The powder coat increased the surface dimensions enough to easily accommodate casting variation yet accurately position it. I used toggle clamps instead of vacuum for clamping. -Worked slick.

    Best,
    Kelly
     
    Last edited: Oct 17, 2017
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes, and it was actually FishbonzWVa that put me onto that. In my best casts there was nothing left but some burnt mud and dry sand.

    I have somewhere between 50-60 pours in 100lbs of sand....two 5 gal buckets. I added the second 50lbs about halfway through. The small parts like the water neck I can de-mold by grabbing the cup with a pair of pliers and pull it out of the flask. The bigger parts like the carb plenum are packed so tight the entire flask with a 100lbs of sand comes off the ground and wont release the part. So as I mentioned in the post above I just started dumping and spreading the sand on my driveway. This also helps the sand cool between pours as the next foam pattern can get soft and distort if you use hot sand. I was hesitant to dump it on my driveway at first but come on, if I'm burning XPS and drywall in it.....the driveway isn't going to hurt it, plus, I sift it anyway so leaves and what not are removed. When I dump and spread the mold it releases white smoke whereas the smoke at the cup when I pour is always black and I reason that is the difference between free oxygen burn versus whatever the oxygen starved byproduct of vaporized/decomposed foam may be.

    I've never waited more than 10 minutes to demold. If the massive cup has frozen, so has the casting. I get some but very little clumping when I sift it. Some of the sand appears to be a little moist. I'm not sure if that's byproduct from the breakdown of the foam or wax residue. I've tried and tried and can't make that wax I use burn. It seems to just instantly go to vapor and if so, may be condensing in the sand. Other than sifting I've never done anything to recondition the sand. The commercial process says sand is fully reclaimable and I'm sure heat would do it but it's so cheap, I'd throw it away and replace it before trying to cook 100 lbs of it. It's a fair insulator and would take forever and a lot of fuel unless fluidized somehow in the flame. As long as it's dry enough to flow under vibe it seems ok.

    Best,
    Kelly
     
    Last edited: Oct 17, 2017
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  17. DavidF

    DavidF Administrator Staff Member Banner Member

    Now that sounds do able, certainly worth a try....
     
  18. Ronald

    Ronald Lead

    I have been in Lost Foam for decades, and wrote an article for Live Steam Magazine: November 1984 page 10.

    Here are some photos of my work:

    DSCN4141.jpg
    Lost Foam047_3.jpg
    DSCN8116_1_2.JPG
    DSCN8114.jpg DSCN9753.jpg
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Hey Ronald that's great and thanks for the post. I think I remember seeing one of your posts at AA. Did you cast a rather large water bed frame in LF? ...And were the LF patterns cast in sodium silicate bound sand or green sand? Am I recalling correctly?

    Best,
    Kelly
     
    Last edited: Oct 17, 2017
  20. Tobho Mott

    Tobho Mott Gold Banner Member

    Those pix look familiar to me too, those castings look great!

    Jeff
     

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