Trying Lost Foam with Iron

Discussion in 'Lost foam casting' started by SRHacksaw, Jun 3, 2022.

  1. SRHacksaw

    SRHacksaw Silver

    I'd like to try casting lost foam using iron. I think for a first try I'll shoot for a simple tube of 1-1/2" dia and 1/4" wall.

    I don't have commercial refractory wash, though I do have both wallboard compound and plaster of Paris. Also no thermocouple, so this will be a seat-of-the pants first foam/iron casting.

    I have done limited LF casting in aluminum and zinc alloy in the past, and actually found that for me, the plaster of Paris coating had better permeability than wallboard compound -- on zinc alloy at least.

    I also wonder if a pouring cup would hold up for aa single melt if made of plaster of Paris. Again, I don't have on hand any moldable ceramic fiber for this first try. The reason I am curious if poP is usable for an iron pouring cup is because Steve Chastain once mentioned years ago that disposable versions were used commercially.

    Anyway, things to think about/try......
  2. Al2O3

    Al2O3 Administrator Staff Member

    The lightweight non-setting joint compounds are usually better. The same ingredients that make them light, spread, and finish more easily make them more permeable, coat the foam, and dry better. ESC seemed to get ok result with drywall mud in that thread I linked. He poked some adiditional needle holes for enhanced venting.

    Never heard of plaster in iron duty. Moldable ceramic fiber isnt suitable either for iron and expensive material for probably no gain. I'd be very careful about a PoP cup with iron. Hard to get rid of the chemically bound water before the pour, and during the pour would be a very bad time for it to occur. I think you'd be far better off with a sodium silicate or resin/epoxy bound sand.....and bake them before use.

  3. SRHacksaw

    SRHacksaw Silver

    Hi Kelly, I wouldn't have made a cup of plaster without first checking the reference, I was just wondering about something I'd remembered. I just looked it up again and The reference is to riser sleeves In Chastain's "Metal Casting: A sand Casting Manual For the Small Foundry Vol. 1", and yes you're right, for melts including cuprous metals, not iron. I definitely would have dried it to get rid of bound water -- Chastain recommends a minimum of 400F.

    I was also just mentioning my own experience re. plaster of Paris vs Wallboard compound in zinc. Here's a comparison photo from 2014:


    My notes back then:
    "This morning with the sun coming in low through a window it was possible to highlight the difference in surface finish and detail fidelity of the second casting and the third (last) one.

    "On the left the zinc separated from the mold walls by layers of molten polystyrene. The drywall compound was not absorptive and the plastic stayed inside the mold.

    "On the right you can see that the actual texture of the foam has been recorded and transferred to the metal. Polystyrene was pulled out of the mold by absorptive plaster of Paris allowing the metal to cool against the mold sides and record the very fine detail of the foam texture. And a blob of hot glue which was used to attach the sprue to the pattern is also preserved in perfect detail on the left."

    I'm not saying that this is always true with all brands of compound, all molders, and all metals, just what I experienced with what I had and did then. I'd like to try both again, I'm sure the variables are all different now, so not necessarily the same result.
  4. HT1

    HT1 Silver Banner Member

    disposable pouring cups are made from refractory material, a very simple fireclay with something in it to lighten the piece, probably foam beads or vermiculite...

    Be forewarned this info is now 35 years old, so about the time SC was writing

    V/r HT1
  5. SRHacksaw

    SRHacksaw Silver

    Thanks HT1, I am now thinking of just making a few small samples of materials shaped like angle-iron ingot molds, and pouring a little iron into them to see how they handle it.

    Like you say, probably just plain sand and fireclay, baked into a pouring cup, makes the most sense, and is easy. I do have some Satanite and could wash the cup with that to get fancy. It does smooth surfaces nicely, and can really take the heat.


    Today I cut out some foam pieces with a 1-1/2" hole saw, and drilled them 1" with a Forstner bit, and glued them together with good old rubber cement. Then sanded. Worked well, though the hole saw left a fairly coarse finish compared to the Forstner. Sanding with 220 was no problem, the paper didn't load and the seams sanded flush.

    I then swept the dust and mouse droppings of 8 years off of a CNC foam cutter I built out in the big shed. Hooked up an old Win 98 laptop through the printer port, and actually cut the same dimension foam out. The center was a little eccentric, so I'll try again Monday wih some adjustments. I was kind of amazed the contraption still worked!
  6. ESC

    ESC Silver

    SR, this was my first lost foam cast iron pour. It came out great so I didn't learn much from it. I've had mixed results since then and my plan "B" has been a quick and dirty split pattern.

    I couldn't find the thread of the pouring basins I made for the Indian heads, but I used linsead oil bonded sand and bake them so I have a selection that can take CI temperatures. In the above example I believe I poured directly into the foam. Beginners luck to have all three come out.

  7. SRHacksaw

    SRHacksaw Silver

    Thanks ESC, that's great!
  8. Al2O3

    Al2O3 Administrator Staff Member

    I think that's a good plan. Sounds familiar in fact. You'll learn something from it for sure. I wouldn't put much effort into the pattern. In fact, just hot wire some 1/4" foam sheet and glue (I'm guessing Elmer's rubber cement ;)) together some box section tube, and finish or detail to whatever degree you wish.

    I had to go look at the USG website for the lightweight joint compounds compositions. They list limestone as the primary constituent. That's calcium carbonate and it melts at about 1500F, way below iron temps. PoP is Calcium Sulfate. Calcium Sulfate, melts at 2600F, so based upon that, I'd say PoP is marginal but still a much better choice for an economical/practical iron coating at least as far as refractory goes.

    Out of curiosity, in your previous trials, how do you know the PoP was more permeable?

    For the pouring cup, just a tin can should suffice for a practice run but burry it to the top with sand on the exterior. Give yourself at least 6" or more of head with the sprue and if you can, make the cup volume comparable to the casting volume.

  9. SRHacksaw

    SRHacksaw Silver

    This is the basic foam shape cut out.

    Size was actually chosen as cylinder liner stock for a steam engine. I'd recently cast a 1.25" dia iron bar for piston stock, and 1.5" x 1" was chosen to accomodate inner and outer machining allowances.

  10. SRHacksaw

    SRHacksaw Silver

    I cut the raw stock into a couple of shorter lengths:

  11. SRHacksaw

    SRHacksaw Silver

    And added a solid length of light expanded bead to the tops:

  12. SRHacksaw

    SRHacksaw Silver

    I coated the foam and hung it outside to dry. I'd decided two test two coatings, wallboard compound and Satanite, which I use to line the blanket of my iron furnace.

    I chose wallboard compound because Matweb listed a (conflicting?) melting point of 1330C (2430F) which seemed close. I had already decided against plaster of Paris because of the sulphate portion. Sulfur is just bad for iron.

    I haven't made a decision yet on how I want to do the pouring basin, but I have another couple days of indoor drying to think about it -- weather won't be good for casting for a bit, rain predicted.

    Tops likes this.
  13. Tops

    Tops Silver

    Will it be hard to get the sand up into the centers if they are hanging from the closed ends?
  14. Al2O3

    Al2O3 Administrator Staff Member

  15. SRHacksaw

    SRHacksaw Silver

    Thanks Tops and Kelly. Tops, I thought I'd just prepack and tape. I've done that in the past in sandcasting when making a pillow block from a bronze bushing, poured in place with aluminum.

    I like your solution Kelly.

  16. Al2O3

    Al2O3 Administrator Staff Member

    Positioning and gating LF patterns can be a bit nebulous.

    What I came to understand was unbound sand really isn't unbound. It just doesn't have binder. It's bound by the mecahnical interference and friction of the media. That increases dramatically with good vibration and compaction. If you have an object mounted on a stick, place it in a flask and pour sand on it, you can pull it right out. Vibrate and pack it well and you will pick the entire flask up instead.

    Blind cavities and overhangs are the least desireable features in LF casting. Sometimes they can't be avoided. I never had much luck prepacking them because no matter how well I prepacked them, they were always destabilized when I packed/vibed the rest of the mold. It was better with bound sand but still not great result. In the example above the top surface will become loose where the metal impinges and create wash and mold media migration. Adding the opening will help and the coating helps prevent the initial erosion of the mold media surface.

    It's always best to create a vehicle that allows sand to flow through the cavity via gravity and vibration because if it can, it will pack and become bound. I came to think of filled cavities just like cores in conventional sand casting. It doesn't take much to create the sand flow, usually a small hole peircing the top of the cavity will do it. It helps to think of the entrance and exits to the cavity as locations core prints would suspend a core in conventional sand casting......the more area in contact between the mold media inside and outside of the pattern, the more stable the interior will be, just like hanging a solid core, and just like conventional sand casting, if the window/core print area would be insuffcieint to hang a solid core in a bound sand mold, it most likely will be in LF as well.

    I suspect all this is even more the case with iron because being ~3x as dense as aluminum, the metalostatic pressure on the mold surface will be 3x greater as well......and much easier to displace and float the mold media.

    Here's a couple links that may be of interest from years ago when I was experimenting.

  17. SRHacksaw

    SRHacksaw Silver

    Thanks Kelly, it was very interesting reading. I'm very tempted to just drill a hole through the top piece at an angle, and recoat that part. Other benefits besides the sand fill would be faster drying of the interior at the top of the cavity, and the ability to inspect the interior of upper part. Iron is even more unforgiving of trapped moisture than aluminum. Frankly, I can think of many reasons why this particular attempt will fail, and especially with iron.

    But, also, If I don't try it this way, and just jump to plan B, no matter how sensible or advisable, I will never know if it might have worked.

    Possible differences: this is a dead simple straight tube pattern by comparison with even the water neck, and a shallow one (5"). I don't have a vibrator, so hand rapping is going to be the limit of compaction. Maybe the loose sand core, also hand rapped while filling, won't settle as it would with hard flask vibration. I also noticed that pushing the pattern into bulk sand put pressure on the tape, like a diaphragm and further compacted the core. No idea if that will help prevent problems, but it's at least a positive factor.

    Bottom line, I don't mind if this first try with iron doesn't work -- in fact I expect it not to. Hard for me to imagine hot iron not simply destroying the entire mold. The coatings seem paper thin, the foam highly flammable, sand unbound. And I'm using non-industry materials and methods. So, what are the chances?

    If it doesn't blow up, I may quite possible see first-hand the same results you got with the pill and test engine blocks. That's not a problem for me, and maybe iron will add a few more fun twists to those. So be it.
  18. SRHacksaw

    SRHacksaw Silver

    Changed my mind, after all that. Impatience won the day.

    The top of the core is just drying too slowly, and with all the other reasons, I decided to notch the upper plug, cutting in square at the joint and tapering out upwards, like simple notching a tree before felling. I do have to re-coat them now, but a slightly thicker coat will just feel better, anyway, whether it is or not.
  19. SRHacksaw

    SRHacksaw Silver

    Here they are with cuts. They hadn't dried near the top inside after a day and a half, before. With the cuts and recoating, it only took a couple hours to fully dry this afternoon.

  20. Al2O3

    Al2O3 Administrator Staff Member

    Wont know until you try.

    What are you going to use for a flask? Make a mark at the height of loose sand and see how much settling/compaction you achieve. Letting the bucket drop from an inch or two can produce some compaction too.


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