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Thread: OT Foam for "lost foam" casting?
01-12-2007, 03:24 PM #1
I have need to cast a couple of small parts that cannot be sand molded, as parts of a larger Petro-Bond casting. I know that the foundry industry uses expanded polystyrene foam, because of the near instantanious and residue free void left as molten metal comes in contact. My foam patterns must be formed over irregularly shaped sodium silicate/CO2 sand, (or any other kind) cores.
In spite of the fact that modern automotive foundry practice utilizes millions of intricate lost foam patterns, patterns that because of their intricacy and volume, must be formed in molds, I can't seem to track down the exact process, nor base materials.
My searches seem to indicate that expanded Polystyrene is distributed as tiny pre-expanded beads that are somehow delicately fused in molds, but the proccess escapes me. I guess my question is, "how do you make a Styrofoam coffee cup?
Yes, I've whittled and smoothed Styrofoam patterns but accuracy and surface finish is lost and volume approaches nil.
I tried Polyurethane spray foam in a mold, it made a foam pattern with fine detail, however, it's not nearly as afraid of molten aluminum as Styrofoam and leaves nasty residue and mal-formed castings, in spite of the fact that I provided large passage ways, (and will for the Styrofoam) to vent the large volume of generated gasses.
No, in this project, large blobs may not be cast and excess machined away. Billet is not an option.
I've haunted foundry forums to no avail, so all answers much appreciated, even those that start, "Are you stoopid?" then list a thousand easily attained, spot on links, or, "Did you look in the 1918 edition of Machinerys Handbook where an entire chapter....." My ego doesn't bruise easily, my need is much greater than my pride. [img]redface.gif[/img]
Checked one more time, yup, still says, "Welcome to the Manufacturing Forum." Note; this is my very first request for help, still, no need to be kind, just informative.
Again, thank you very much in advance, Bob
01-12-2007, 03:47 PM #2
They are "pre expanded" and then steam heated and pressed together. AFAIK they are expanded by steam heating or previously CFC's?
I believe that you can use acrylic sheet to do the same thing with a preheat to burn it out. It obviously won't burn out with the metal coming in but did give castings that were good looking.
I know that the molder had to put fillets in the corners but other than that have no idea what the process was. I believe you can buy wax sheet to do the same thing.
You need to search for "EPS beads"
01-12-2007, 04:21 PM #3
Let's back up a bit.
Long before lost foam there was millions of parts cast with patterns having "loose pieces" cleverly dovetailed (or whaterever) to main body of pattern. These stayed in the sand when the main body was "pulled" and are gingerly lifted out of mold after.
What is to prevent this being done to the pattern for the petro bond casting?
01-12-2007, 04:21 PM #4
For me lost wax and investiment moulding, is the better option, excellent finish and can be very intrincate. good luck
01-12-2007, 05:07 PM #5
EPS is done with hollow styrene beads that are preconditioned with steam then injected into a closed mold with more hot steam which causes the beads to expand and fuse to each other. It is like magic. Never heard of it being done on a small scale but seems like you could. BTW, white bead board EPS is no good for modeling, try the extruded EPS ie pink or blue styrofoam, this is easy to cut and sand and gives a decent surface finish. You can heat bond it together. Sculptors use this for bronze casting.
01-12-2007, 09:31 PM #6
Thanks for the responses guys,
Chris, thank you for the leads, "EPS" didn't even occur to me.
John Oder, without a doubt, loose pieces are a favorite of mine and lord knows I've tried with this one, without success. The features in question are shaped like a horn of plenty, tapered and deeply curved. There is a large cavity inside the mold, only the small end faces it and I have not been able to break down the loose pattern enough to squeeze it through.
Outside extraction is foiled by the curve, as the big end hits the mold before the tail rotates free of the cavity.
I'd be pleased to avoid further complicating the proccess with the addition of lost foam but I can't see how.
Servidor, oh yes, I'd be very pleased to have the entire piece formed by the very accurate lost wax system, however, with the part alone at more than 22", I don't begin to have access to large enough capacity for burnout and producing the complex wax form would be very difficult.
Surplusjohn. great tips, at least you've aimed me toward better products for hand carving and hope for discovering the "steam magic."
Thank you guys anything else? Like that steam consolidation? John Oders already got me back to the drawing board, re-attempting loose piece. Ooh, loose parts surrounding a balloon.. with step locks that release on baloon collapse...gotta' go..computer stays on..
01-12-2007, 09:41 PM #7
I e-mailed you and would be more than willing to make some helpfull suggestions if you e-mail me a picture. Toad
01-13-2007, 12:11 AM #8
Join this group
then search for "decoy" and "Pentane" will find discussions and links of highlights of LF pattern making.
Pentane is one danger of LF pattern making, other than the steam or hot water used.
Also the beads have a short shelf life.
Also search for "polystyrene"
01-13-2007, 06:45 AM #9
Has no one mentioned the generation of 'phosgene' gas?
I recall the matter coming up when we were doing alloy casting with polystyrene packaging. It's over 30 years ago but someone may come up a more up to date comment.
01-13-2007, 11:33 AM #10
Trent, thank you for your email!
Ray, great leads. Yahoo doesn't like any name I've yet fed it to join, will get on though, thank you!
Hi Norm. Only you with "phosgene" gas but if I properly understand the constituent parts of EPS, it consists solely of carbon and hydrogen, with a miniscule pentane activator that is mostly dissipated by the "rest" period, where air takes it's place. That and the fact that I will work outside and live in the sticks, I hope will negate any ill effects. But thank you, off to google with phosgene, not to be confused with gargle with sodium chloride. [img]smile.gif[/img]
01-13-2007, 04:15 PM #11
I work for a firm that machines lost foam cast aluminum parts, the foam cores are goued together from many pieces to form water jackets and oil galleries.....they run "specials" now and then to try to make better castings and you can tell by reading what the original process is to make the foam and what they are trying that the size of the beads and many other things is very important.
01-13-2007, 04:43 PM #12
I am coming up with reports on Google.
Not of phosgene but severe problems. Sadly, time precludes further digging.
I can, however, report dense white fumes on dropping molten 'ally' as the stuff melted in the molds.
01-13-2007, 09:13 PM #13
Bill, any chance you can track down that "glue"? I don't want anything in there to not evaporate and cause a cold shut.
Norm, caution taken, thank you. In this instance the two parts are small, (but very important) won't consist of more than 4 or 5 cubic inches and I'll take care to stand up wind.
01-14-2007, 01:49 AM #14
So are you replacing the two loose pieces with foam? If so I would think that wouldn't be too hard to do with a hot knife. It would get pretty close to finish size and could still be shaped up a little by sanding using the pink or blue polystyrene board, it comes in thicker section like you would need and get you to the prototype stage that it seems you are looking for. That is correct isn't it that this is for just a couple of one or two off castings?
01-14-2007, 01:39 PM #15
a hot air gun will weld psa, put the two faces partly together at an angle, blow the hot air against both faces and press together quickly. it takes some practice but it works well. Also works with Ethafoam, ie pe foam which is a devil to glue.
01-14-2007, 02:10 PM #16
Trent, thank you for your professional analysis of my email to you and for this latest info. For now, I believe that I'll follow the advise of you, and others to hand shape pink or blue foam, (which thanks to you guys, I now know about) for prototyping, but will persue info on precision foam molding for the future.
John, cool! Heat gun, gonna' run out into the shop and give that a try. Like it, no foreign material.
Oh and John, every time I step on the pedal that engages that new 1/4HP GE pony motor on my RPC, that you sold me for 25 bucks!, I think of you. Great deal!
01-21-2007, 12:18 AM #17
Bob, here at Hawver Aluminum Foundry we have used " ram up" cores to produce castings that would otherwise not be possible. You part the small shape on it's side, to make a small core then glue the halves together. The core is then indexed on the pattern & the mold rammed up.The core stays in the mold and there you go. We used to cast a radiator end with an integeral filler neck cast on it this way. good luck Jim