Lost Foam Casting: a Crankcase in Zinc Alloy

[vtsteam]

Since I got my Popular Mechanics boiler steaming I really wanted to make a new small steam engine to go with it. I especially like twins, and have a couple Westinghouse style versions. I was just thinking of a V-twin this time, instead of an inline,  and with OKTomT doing a really nice full size IC V twin motor bicycle engine, I wondered if I could use a similar crankcase on a steam design. So I started in on what I hope will be a lost foam casting.

I say hope because this one really may not work out well. One reason is, it's very thin walled and it's going to be tough to get full penetration of the metal. The other is, I'm going to try lost foam with zinc diecast metal instead of aluminum, which I've used before. The lower pour temperature of this metal (by nearly a third )may not burn out the foam. So all in all, my hopes aren't great for this working out. Still, you never know until you try, and it's definitely something I wanted to satisfy my curiosity about. If it doesn't work, I can always try a hotter metal.

Here is the foam crankcase as of yesterday:

[vtsteam]

I added bearing bosses and sprues today, and then plastered the patterns. Here's how it looks tonight:

[Manxmodder]

Hi Steve, this looks like it will be an interesting experiment.
Just to add something I have been thinking for a while,if the foam is a polystyrene type then the majority of it could be dissolved out of the mold by introducing cellulose thinner to the foam core.

Then apply some heat to drive out the solvent residue. How do reckon that would work?....OZ.

[vtsteam]

It's been done that way Oz. But it would need a thicker more supportive investment, and maybe heat, etc. to drive off the moisture, solvent and residues. It gets to be more like lost wax casting -- quite intensive

This way is, I believe quicker and simpler (well only if it works!). I hope to cast tomorrow, and we'll know pretty quickly!

I've cast this way before with aluminum and a larger thicker walled casting:

[Manxmodder]

Very impressive Steve,that has replicated the detail very well.I'm taking notes on this one...OZ.

[Fergus OMore]

Somewhat rudely- possibility of creating phosgene.

[vtsteam]

Doesn't seem rude at all Fergus -- an attempt to be helpful, I'd call it,  but can you give the specific reaction that will in this case do that?

ps. I mean from straight polystyrene, not if a thinner (like acetone) was used.

The only reference I remember several years ago to dangerous phosgene-like byproducts inhaled was in conjunction with an accident using an unspecified synthetic brake cleaner solvent and gas welding. I've actually referenced that link for others in the past.

I use an activated charcoal mask when doing lost foam casting, btw. I also work outside and hold my breath for the few seconds the tiny amount of foam smokes away. I stand back from the mold as soon as poured and let it cool by itself for a half hour before breaking out.  All these are I think common sense precautions.

[vtsteam]

Oz, I can't take credit for this specific way of doing it -- Dave Kush worked out the details for himself many years ago where I first read about it on his website, www.buildyouridea.com.

I was incidentally casting in the above photos "my own idea" of his original linear bearing design to try to do it with less material usage.

[Pete W.]

Hi there, Steve,

I look forward to seeing pictures of the castings.

Do I remember you mentioning a numerically controlled hot wire foam cutter in some earlier post? 

[vtsteam]

Thanks Oz, Pete. 

Pete I do have a cnc hot wire foam cutter I built several years ago for cutting model plane wings.

The casting was a disappointment. It looks as if the polystyrene melted but didn't burn off, so there are plenty of flaws in the casting. The metal didn't have enough heat to do the job, as I suspected it might not. I'll post some pics later.

I have another foam pattern ready to go and will probably try aluminum tomorrow.

[vtsteam]

Here are some pictures from today's casting:

The shake out. I was worried I hadn't got complete fill of the molds, but was happy to see in the shakeout that both molds filled -- the brown discoloration of the plaster was a good sign. The split pipe pouring basin leaked so I didn't have as high head pressure as I'd hoped. There was no flare, and only what looked like two small bubbles during the pour.

[vtsteam]

There was a small leak of metal inside the crankcase. Unfortunately this area caused shrink problems on the other side of the casting:

[vtsteam]

This is what it looked like after partial cleaning. You can see where metal flows did not fuse completely. This was because the metal was too cool and also had not flashed off the foam polystyrene. The tan-bluish area on the coverplate is investment and polystyrene foam residue mixed -- it inhibited metal fusing in some areas. It's also tough to remove. The relatively thin casting also makes it hard for the metal to retain heat while traveling and melting foam out.

I'm not sure if I can pour hotter than I did this time with zinc diecast material -- normal pouring temp is only 750F -- aluminum can pour fairly easily around 1100F or more and is a material which has a lot of retained heat. It always flash burned  the foam when I used it for lost foam casting in the past.

[vtsteam]

I made up a new pattern to try again today. While waiting for the investment to dry, I played around with the bad casting just to see what the final case should look like.

Some face milling and filing made it look quite a bit better -- it's thick enough that I probably could get rid of almost all the surface flaws if I kept going, but this was really a casting exercise, and I want to get a really good casting before I settle down and build an engine.

I'm tempted now to try diecast zinc one more time, tomorrow, instead of aluminum, just to confirm that it won't flash the poly. Maybe I should have used two sprue/risers for faster fill, more heat, and some air -- so I've done that for the next set of patterns.

[vtsteam]

Well I went with a second try with zinc alloy lost foam today, instead of aluminum. I just had to see if maybe pouring hotter and a couple of sprues instead of one to get the metal to fill faster, and inverting the pattern to get the good face down would work. I also added a lot of little vents with a needle, hoping to let out any gasses. I sealed the split line in the hinged pouring cup so it wouldn't leak, and maintain head.

I poured at 452C which was pretty hot, I figure, for zinc alloy. And the bubbling off of the polystyrene looked much better -- more like it does with aluminum, though it still didn't flare. The pouring cup stayed full, and so added head pressure and fed the casting as it cooled. Break out looked better than it had yesterday:

[vtsteam]

Unfortunately there were still flow lines and unvaporized polystyrene, though less and the casting flaws were shallower than yesterday's pour:

[vtsteam]

The surface finish was better than yesterday with fewer flaws, but nothing like the pattern fidelity I'd got with aluminum pours in the past:

[vtsteam]

Because the flaws didn't seem very deep, I put the casting on the mill and cleaned up a few surfaces with my carbide end mill. It looked good, so I will probably make an engine from it. I'll turn the front face and bearing boss on the lathe, but i just used the mill to see how deep the flaws were -- about .030" total out of nearly .250" so it's still a pretty heavy case.

For casting zinc alloy with lost foam, I'd say the patient survived, but the operation wasn't a success. I had hoped surface fidelity would be similar to the aluminum castings I'd done in the past. But the failure at high casting temperatures (for zinc) to vaporize all the foam meant flaws and poor surface finish. So I don't see it as a viable method for me in the future. Nevertheless, It looks like I'll get an engine out of it -- probably the only one I'll ever do this way.

I may yet try an aluminum version, just to check that the pattern and molding method weren't the problem for this particular casting:

[mattinker]

Hi Steve,

ZA 12 machines beautifully. Looking good, even though it was semi successful. I think you'd get a beautiful finish in ZA12  if you dissolved the polystyrene out with a solvent. Originally, lost polystyrene didn't use any dry wall "mud" just the polystyrene replaced by the molten metal in dry sand, an amazing process, incredible that is works!! I think the mud in dry sand would work really well.

Regards, Matthew

[vtsteam]

My feeling is that the mold would be too fragile if dissolved out, using a single coat of mud. I think Plaster of Paris would be  better choice since it can be built up yet still solidify through chemical means instead of drying, as mud requires. However then we get into the problem of more H20. The mud is so thin that water is negligible. Anyway, plenty more to explore. I'm allready leannng toward PoP rather than mud w/ zinc alloy.

[vtsteam]

One other problem, as I see it with the solvent-foam idea is that it doesn't actually get rid of the polystyrene. And that is the problem here.

Unless you had a way of removing the dissolved polystyrene it will simply move with the acetone to the bottom of the mold, where the acetone will evaporate (hopefully to avoid explosion when pouring) and leave the polystyrene as a hardened sludge, And possibly impregnate both into the investment.

Actually I bet it would be a long wait until all of the acetone disappeared by evaporation inside a mold. Like maybe weeks.

My guess is that acetone won't be an advantage to the problem I'm seeing which is polystyrene entrapment. Not failure to fill the mold. I could be wrong. You never know until you try.

Lost wax methods -- with a kilned mold make sense to me -- that truly does get rid of the pattern material.

And high temp metals evaporate/incinerate foam creating a gas which escapes easily through the porosity of a very thin investment and loose sand, so the lost foam method also works.

Zinc seems marginal to poor for lost foam -- as a guess, in a zinc pour, polystyrene liquefies but doesn't gasify sufficiently in a small casting like mine. And remains trapped.

My guess is aluminum creates the minimum pour temp for good results on this scale.

[mattinker]

It could be worth a try, true lost polystyrene depends on the molten metal replacing the foam with molten metal with nothing but dry sand to hold it in place! The thing about dissolving out the polystyrene is that one would have to be able to pour off the "sludge" before filling with the dry sand.

Plaster for a mold that size could be baked in a largish foundry!

Regards, Matthew

[vtsteam]

I'm starting to think in terms of feral die casting.

feral, not ferrous. Ahwooooo!

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[mattinker]

I'm starting to think in terms of feral die casting.

What's feral die casting? Google revealed nothing!

[PekkaNF]

Maybe VT is going to have animals to raise the dies? Feral dies? Or maybe just good sense of humor. Normally dies are made of steel for zinc, maybe he is thinking of using non ferrous metal? Would aluminium have problem with molten zinc?

I'm following this thread it is really interesting. I have been reading some experiments of vax coated styrofoam used as lost wax methods, but can't remember how it ended. Typically less steps you have less room for errors there is.

My brother used styroxfoam as a plug to make smooth engine intake. He laminated glass/epoxy on top of it and then dissolved foam off. Can't remember what he used but options were gasoline or MEK, had something to do in getting it all out.

Pekka