Author Topic: Lost Foam Casting: a Crankcase in Zinc Alloy  (Read 21554 times)

Offline vtsteam

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Lost Foam Casting: a Crankcase in Zinc Alloy
« on: August 07, 2014, 08:13:30 PM »
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:

« Last Edit: January 13, 2018, 05:55:57 PM by vtsteam »
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #1 on: August 07, 2014, 08:20:05 PM »
I added bearing bosses and sprues today, and then plastered the patterns. Here's how it looks tonight:

« Last Edit: January 13, 2018, 05:56:50 PM by vtsteam »
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Offline Manxmodder

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Re: Lost foam casting a motor
« Reply #2 on: August 07, 2014, 11:19:50 PM »
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.
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #3 on: August 07, 2014, 11:43:39 PM »
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:







« Last Edit: January 13, 2018, 05:58:17 PM by vtsteam »
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Offline Manxmodder

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Re: Lost foam casting a motor
« Reply #4 on: August 08, 2014, 12:48:20 AM »
Very impressive Steve,that has replicated the detail very well.I'm taking notes on this one...OZ.
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Offline Fergus OMore

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Re: Lost foam casting a motor
« Reply #5 on: August 08, 2014, 03:08:40 AM »

Somewhat rudely- possibility of creating phosgene.


Offline vtsteam

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Re: Lost foam casting a motor
« Reply #6 on: August 08, 2014, 08:36:02 AM »
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.
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #7 on: August 08, 2014, 09:04:30 AM »
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.
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Offline Pete W.

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Re: Lost foam casting a motor
« Reply #8 on: August 08, 2014, 09:25:26 AM »
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? 
Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest design change-note!

Offline vtsteam

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Re: Lost foam casting a motor
« Reply #9 on: August 08, 2014, 04:52:42 PM »
Thanks Oz, Pete.  :beer:

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.
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #10 on: August 08, 2014, 08:01:47 PM »
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.

« Last Edit: January 13, 2018, 05:58:56 PM by vtsteam »
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #11 on: August 08, 2014, 08:03:59 PM »
There was a small leak of metal inside the crankcase. Unfortunately this area caused shrink problems on the other side of the casting:



« Last Edit: January 13, 2018, 05:59:27 PM by vtsteam »
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #12 on: August 08, 2014, 08:12:50 PM »
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.

« Last Edit: January 13, 2018, 06:00:02 PM by vtsteam »
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Re: Lost foam casting a motor
« Reply #13 on: August 08, 2014, 08:19:51 PM »
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.



« Last Edit: January 13, 2018, 06:00:38 PM by vtsteam »
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #14 on: August 09, 2014, 06:04:39 PM »
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:

« Last Edit: January 13, 2018, 06:01:19 PM by vtsteam »
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #15 on: August 09, 2014, 06:09:40 PM »
Unfortunately there were still flow lines and unvaporized polystyrene, though less and the casting flaws were shallower than yesterday's pour:

« Last Edit: January 13, 2018, 06:01:54 PM by vtsteam »
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #16 on: August 09, 2014, 06:13:05 PM »
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:

« Last Edit: January 13, 2018, 06:02:34 PM by vtsteam »
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Re: Lost foam casting a motor
« Reply #17 on: August 09, 2014, 06:25:23 PM »
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:

« Last Edit: January 13, 2018, 06:03:19 PM by vtsteam »
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Offline mattinker

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Re: Lost foam casting a motor
« Reply #18 on: August 09, 2014, 06:43:17 PM »
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

Offline vtsteam

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Re: Lost foam casting a motor
« Reply #19 on: August 09, 2014, 09:58:21 PM »
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.
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Offline vtsteam

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Re: Lost foam casting a motor
« Reply #20 on: August 09, 2014, 10:32:49 PM »
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.
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Offline mattinker

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Re: Lost foam casting a motor
« Reply #21 on: August 09, 2014, 10:43:26 PM »
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

Offline vtsteam

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Re: Lost foam casting a motor
« Reply #22 on: August 09, 2014, 10:52:17 PM »
I'm starting to think in terms of feral die casting.

feral, not ferrous. Ahwooooo!

werewolves of london
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Offline mattinker

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Re: Lost foam casting a motor
« Reply #23 on: August 09, 2014, 11:47:55 PM »
I'm starting to think in terms of feral die casting.

What's feral die casting? Google revealed nothing!

Offline PekkaNF

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Re: Lost foam casting a motor
« Reply #24 on: August 10, 2014, 01:50:54 AM »
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

Offline mattinker

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Re: Lost foam casting a motor
« Reply #25 on: August 10, 2014, 02:22:10 AM »
Al would be great for casting Zn and  Zn alloys.

Regards, Matthew

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Re: Lost foam casting a motor
« Reply #26 on: August 10, 2014, 07:14:31 AM »
Feral as in not being part of the social conventional but also not apart from it in a way that uses what that society discards to support something creative.

Yes I was thinking of aluminum as one possibility for a material. Though I am sure that industry has probably already tried it somewhere early along the line of die casting technology, and moved on. Still, they didn't leave notes. So, maybe I'll just go ahead and repeat their early mistakes....

I have about ten experiments I'd like to perform today. Too bad I'm not ten people!  :bang:

My big problem is I can already smell winter coming, and I shouldn't be doing any of this. I should be figuring out how to make an accessible heated shop in winter, or all this will stop for 8 months as it did last year. Time is slipping by.
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Offline Joules

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Re: Lost foam casting a motor
« Reply #27 on: August 10, 2014, 09:11:27 AM »
Figure a way to burn snow  :thumbup:
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline vtsteam

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Re: Lost foam casting a motor
« Reply #28 on: August 10, 2014, 05:26:10 PM »
Had another go at zinc today. Altered the pattern, and it went well. This time I've got good fidelity -- the texture of the foam is plainly visible. Even the little blob of hot glue attaching the sprue got cast as zinc.



« Last Edit: January 13, 2018, 06:04:44 PM by vtsteam »
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Offline mattinker

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Re: Lost foam casting a motor
« Reply #29 on: August 10, 2014, 05:36:01 PM »
That looks satisfying!

Regards, Matthew

Offline vtsteam

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Re: Lost foam casting a motor
« Reply #30 on: August 10, 2014, 05:52:55 PM »
It is Matt. Here it is off the sprues:

« Last Edit: January 13, 2018, 06:05:21 PM by vtsteam »
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Offline Manxmodder

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Re: Lost foam casting a motor
« Reply #31 on: August 10, 2014, 05:57:45 PM »
Al would be great for casting Zn and  Zn alloys.

Regards, Matthew

Matt,I can confirm that aluminium works well for zinc casting molds.

About 20 years back I made some latch components for a customers double glazed windows to replace the original plastic items which were prone to fatigue failure.

The mold I made was a several piece jigsaw affair milled from aluminium,screwed together and preheated before each pour.

The detail and tolerance of the latch components was really good with the milling marks from inside the mold being clearly reproduced on each of the components.

Steve, that last cast you've done looks really good,looks like you've mastered that technique. :clap: :clap:

I'm looking forward to seeing more of this project.....OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline vtsteam

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Re: Lost foam casting a motor
« Reply #32 on: August 10, 2014, 07:35:09 PM »
Thanks Oz!  :beer:

In case it helps others, these are my notes re. zinc and lost foam on this scale casting:

Pour temp 450C

A sprue is needed every 35 mm or so on perimeter of a 6mm wall, 2 spue minmum before loss of detail.

Max depth of 6mm casting wall is about 25 -35 mm before too much heat is lost.

Horizontal 6mm plate doesn't work. Keep mold cavities vertical.

Deep pouring basin/riser needed -- there is a great deal of shrinkage and this kind of thin casting needs good head pressure to maintain detail.

Use plaster of Paris in preference to drywall (calcium carbonate). It speeds up the process (pattern can be cast in 3-4 hours including cure and open air drying instead of 24-36  hours open air drying. )

And, critical difference noted today -- young plaster of Paris absorbs molten polystyrene, which drywall does not, thus removing it from the casting. That enabled good fidelity in this margnal low temp lost foam process. While zinc pour temps can't flash the foam into gas as aluminum can, poP makes up for this by absorbing the liquid polystyrene away from the melt. Calcium carbonate traps the polystyrene inside the mold.






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Offline mattinker

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #33 on: August 10, 2014, 09:12:30 PM »
Are you using pure Zinc or a ZA12 alloy?

Regards, Matthew

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #34 on: August 10, 2014, 09:21:43 PM »
Zamak 2
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Offline Eugene

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #35 on: August 11, 2014, 04:17:18 AM »
Steve,

May I ask a question?

How do you make such beautiful moulds, their accuracy and sharpness looks excellent. I'm on the beginner slopes with hot wire cutting and my efforts look distinctly agricultural when compared to yours. Could it be a CNC foam cutter?

Eug


Offline vtsteam

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #36 on: August 11, 2014, 07:09:22 AM »
Eugene, the pattern is the most important part of any mold. People often get caught up in the technology of sand, or furnace refractories, burners, metal composition, gating and risering, fuels, etc. in casting. Yet knock a couple pieces of wood together for a pattern.

Casting starts with the pattern and its finish.

I spend a lot of careful time with very fine sandpaper, even on the foam cutouts to get them as good as I can before using them. More time than they took to cut out  by machine. I use little strips of sandpaper under a strong light. Oudtoors on the porch in summer is perfect!

I do the same with wooden patterns, and use filler and sanding sealer coats, each sanded down, and several coats of spray colored lacquer to finish. Most of my practice for this kind of thing comes from building model airplanes, which I've been doing as a hobby since I was 14.

Oh one other thing about cnc hot wire foam cutting. You need to go VERY slowly for a machine process. 1mm/sec to get good quality cuts. and you need thin wire and low temps to match. I use .011" stainless steel fishing leader as a hot wire.

If you don't have a good pattern, all the time and money in the world invested into the latest metal casting forum technology will just get a person a lump of scabby metal. Sand break outs happen because the pattern is rough. Inclusions ditto.

Conversely, I've found that the crudest casting equipment will turn out good faithful castings with good finish if the pattern is good and the technque is sensible. My iron foundry is made from ordinary firebrick, and clay and sand. The fuel I use to cast in aluminum is store bought charcoal briquets or home made charcoal. For the last few zinc castings I just used wood for a fuel. Ordinary plaster of Paris with no additives, no bake out, air dried for a few hours and ordinary scraps of foam were used for a pattern.

I like simplicity, and I am impatient, and I hate to waste things. I use what is available and try to do a good job with it. I study what happens and I gradually think through why it didn't go as I wanted. Eventually the problems get worked out by trial and error until I understand my own equipment and materials.

Anyway, it is great that you ask, and the answer is a lot simpler then I've ended with here. Just take a little sandpaper and do a very careful job, with a light touch. A few times through it, and you'll get good at it. It's nothing more than that.  :beer:
« Last Edit: January 13, 2018, 05:48:18 PM by vtsteam »
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Offline vtsteam

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #37 on: August 11, 2014, 07:52:18 AM »
Eugene, re-reading your question, I see you probably don't have  a cnc foam cutter, and that isn't a problem for getting good patterns at all -- you must be using a hand bow, or vertical fixed wire. I did that for many years. The main thing again here is to have good templates --smooth ones, just like the need for smooth patterns in casting.

The way I made foam cutting templates was to cut them out of sheet aluminum with tin snips (after pasting a paper printed pattern on top) and then filing to the line with a fine file. The fine file is essential.

For small patterns (rather than wings) a vertical wire and table is best. After that comes sanding the foam pattern, as it does with CNC, as well.

Low temperatures, slow smooth movement, thin wire are all helpful, but the blank is only a blank, and hand work refines a pattern to what it should be.
I love it when a Plan B comes together!
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #38 on: August 11, 2014, 08:55:05 AM »
Steve,

I don't think that we've had the pleasure of pictures of your CNC hot wire cutter. It sounds interesting, any chance that you could oblige?
Andrew Mawson
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #39 on: August 11, 2014, 09:55:33 AM »
Hi Andrew -- my wire cutter is just a plans built wooden drawer slide style wing cutter -- quite crude, and looks like the original here:

http://www.drayconstruction.com/foamstuffs/

except that I used 3/4" birch plywood instead of the plans shown MDF.

It's not a particularly good or accurate "machine" as you can imagine but it does cut wing cores, and an occasional pattern. I bet practically anybody here could design and build a better one. One big problem with the drawer slides as designed is they protrude off the ends as they slide to the far end, so you need a far bigger area than the cutting area.

In general, I don't like cnc hot wire cutting, and for all tapered wings I now use a pivot wire (manual wire attached to a fixed point, used with a single airfoil pattern). It's much faster and not prone to CNC programming errors -- which can quickly eat up a LOT of good foam. If I had to cut the same wing in production, maybe CNC would make sense. But for occasional one off, it seems I waste 3 panels getting one good one, and it takes hours of first programming, generating G-code, cutting a blank and positioning it for wire cutting specing the taper and finally cutting at 1mm/sec, meanwhile watching it in case something goes wrong.

I wish I could say that it's magic, and easy but frankly, I'd rather cut a one off part with a hot wire by hand.

I'll show you my crude hot wire jig saw instead, which, with a simple foam pattern to do, can cut far more quickly than the CNC would.  Back with pics later.....

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #40 on: August 11, 2014, 10:10:16 AM »
Thanks Steve.

I suppose this stems from your RC modelling activities, an arena I've not been involved with. I'd have thought with careful construction that wouldn't do too badly, after all you're not looking for +/- 1 thou are you  ?
Andrew Mawson
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #41 on: August 11, 2014, 10:58:04 AM »
Steve,

Thank you, that's been a great help. I've already dropped the power (and hence temp of the wire) considerably and got a much better result just working freehand; time for some sheet ally now.

Your set up isn't "quite crude", this is crude ... !



The only excuse is that I made it yesterday from leftovers from the roller shutter cupboards which were themselves made from left overs; like you I hate waste. The only difference between it and a thousand others might be in the power supply which is from an old PC.



I just made a contact block to take the various power leads.Switching them and moving the position of the crocodile clip on the wire gives pretty fair control; something I did after reading your reply. The thing that looks like the end of a safety pin between the green and black ground wire is actually the end of a safety pin between the green and black ground wire; it just provides a load to switch the unit on.

Today I scrounged an old (Polish!) propane bottle which is just the right size for a charcoal smelter, got the brass tap out and filled it with water. It'll stay that way till Friday when I'll be back int t'hut.

Again thanks,

Eug

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #42 on: August 11, 2014, 11:31:16 AM »
Andrew, sorry, you're right it is quite accurate enough for wings since movements are relative and we tend to sand things, cover them etc. I just wanted to warn exacting machinists that the accuracy isn't what they would expect from ball slides and a rigid gantry, etc for a CNC mill. and metal work.  And for pattern work, we usually expect surface finish to be at least foam texture, and likely having a machining tolerance as well.

A hot wire has a variable kerf width also depending on the direction of cut. Because the wire is cutting by radiant heat (properly) rather than contact, and the melted foam gasses travel upwards an upward vertical cut is wider kerfed than downward, and horizontal varies with angle from horizontal, etc. Also depending on software, speed of cut tends to vary around curves, somewhat -- naturally the wire is in an area of foam longer when doing a tight radius and can actually melt out a section even though the linear speed stays the same. etc.

Some software compensates for this (somewhat) or some people play the heat control manually. Still others accept the slight variation (yours truly!).

All this tends to make for even more wasted material when you suddenly lose a section that on paper (in CAD, I mean) it looked good.

When you work manually, the knob between your shoulders does some amazing compensation at times after a little practice, seemingly without any calculation, just by the appearance of the cut. And quickly adjusts placement of a part to best utilize remaining scrap after a botched piece -- something that's difficult to re-program into a CNC machine as quickly.
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #43 on: August 11, 2014, 11:42:36 AM »
Eugene, that looks great!!! Much better than my vertical wire. Now I'm ashamed and no need to post mine.

Also, if you want variable heat control with your computer supply DC output -- assuming you aren't "attached" to your alligator clip method (we have alligators here, crocs very rare and confined to the Keys) try one of these:

http://www.ebay.com/itm/DC-12V-24V-Single-Color-Adjust-For-5050-3528-LED-Light-Strips-Dimmer-Controller-/171104731465?pt=US_Lighting_Parts_and_Accessories&hash=item27d6a30149
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #44 on: August 11, 2014, 11:58:58 AM »
A comment from someone who knows nowt about hot wire cutters:

Why are you bothering with DC power supplies - I'd have thought a suitable low voltage transformer (transformer not wall wart) powered  by one of the very cheap phase angle  'power controllers' available on ebay would have given you full control as needed and mains isolation.

http://www.ebay.co.uk/itm/High-torque-50V-220V-10A-AC-Motor-Speed-SCR-Controller-Power-2000W-UK-Seller-/281387453688?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item4183ffccf8
Andrew Mawson
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #45 on: August 11, 2014, 12:54:05 PM »
Andrew,

Two reasons ...... first I never thought of it :(  and second I had the PC power supply to hand anyway. Steve, at the bottom of my "thingies box" I've got a 220V dimmer switch which I can press into service if I need it. When I've done the immediate casting that I need, I have a cunning plan for the ATX unit, so again I'll be recycling the already recycled! I've got a DC motor with a 2 rpm reduction gear not unlike a windscreen wiper motor that the unit drives a treat ..... it might very well do for a fine feed on my wee Myford.

Eug

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #46 on: August 11, 2014, 01:40:51 PM »
Eugene you can't use an AC light dimmer on the input of a regulated computer power supply!!!  Don't do it.

It can be used on an older transformer type AC or bridge rectified DC supply, like a "dumb" unregulated automotive battery charger. Or even on a transformer alone for AC output (as long as it's output is safely under say 25 volts or so.

However, The DC LED Dimmer can be used with your supply -- it is five dollars and change postpaid --or equivalent more or less in your money. It goes on the Output of your PS, not the input.

Andrew -- hot cutting wires can be run on either DC or AC as you probably have guessed -- just a resistance wire. A computer power supply is fine as long as it has enough current capability, and you do DC power adjustment on its output (not its input)



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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #47 on: August 11, 2014, 01:53:39 PM »
Steve, I don't think Eugene, and certainly I am not, suggesting that you use one of these devices on a DC power supply.

The suggestion is to use them on the mains feeding a low voltage transformer  (this is how the current is controlled on many welders of a certain vintage)
Andrew Mawson
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #48 on: August 11, 2014, 02:12:30 PM »
No I didn't think you were, Andrew, but I thought Eugene was considering it. Apologies if I misunderstood.

I've run all three for cutting wires:

A  25V transformer and dimmer on the AC side with 24V AC output to the wire
An old style auto battery charger with a router speed controller on the AC side with 12V DC output
A computer power supply with the 12V DC side feeding a DC LED light dimmer like the one I've shown.
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #49 on: August 11, 2014, 08:38:08 PM »
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 we can see 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 the 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

« Last Edit: January 13, 2018, 06:06:17 PM by vtsteam »
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #50 on: August 12, 2014, 04:32:28 PM »
Steve, dozy beginners question ..... what gauge ally do you use? I don't have any to hand so I can buy whats best for the job, anything from .5 mm upwards.

Eug

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #51 on: August 12, 2014, 08:19:50 PM »
Eugene the best aluminum for casting, in my opinion is scrap that was a casting itself. Try going to an engine re-build shop and asking if you can buy some pistons pulled out of old engines. They will likely sell them to you for a very small price.  They make very good material for casting.
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #52 on: August 13, 2014, 04:58:23 AM »
Steve, my bad, I meant the gauge of the stock you use for the templates.

I'd already decided on old castings only as raw feed material; I think a couple of car wheels from the local scrappers is the right way to go, and / or plus pistons as you say. 

It will be a while before I'm fully kitted out for casting; my first issue as always is safety, so I'm making sure everything is strong and stable and that I'm in the right PPE. I qualified as a Safety Manager some years back, so it would be a bit infra dig (as well as painful) to become the owner of a bootful of molten ally.  :)  I plan on making a wheeled trolly that won't be in danger of tipping when the smelter lid is opened, plus having the casting box on a dry sand bed. 

Eug

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #53 on: August 13, 2014, 07:25:28 AM »
Eugene, sorry, I misunderstood re aluminum for templates. I just use hardware store aluminum utility sheets used for patching etc. I don't have my calipers here in the house right now to measure the gauge. I have also used aluminum roof flashing, though that seems somewhat thinner, and I like the utility sheet better.

Aircraft pattern tinsnips will make short work of either. Just like using a pair of scissors.

I print out patterns from my computer, cut the paper out slightly large then spray lightly with contact adhesive, and the aluminum template stock as well, I do this outdoors on the porch. Let dry briefly, then stick the paper down.

Cut out the templates just outside the line.Then clean up to the line with a fine file.

I punch or drill holes in templates to accept straight pins or finishing nails for temporary attachment to the foam while hot wiring. Sometimes I use a bit of scotch (celo) tape to hold the pattern on, too, if needed. The hot wire will cut through it.

ps, always clean your hot wire before cutting (or after, if you're an organized person!) with a quick wipe with a rag or paper towel wadded up and the wire hot. Your cuts will be much better.

Working outdoors, or proper ventilation is essential when wire cutting.

I don't think a trolley is necessary for pours of 5 pounds or under, and don't own one. Proper holding shanks and lift out tongs are essential. I like my new springing ring shank for iron or heavy pours a lot (see my oil furnace thread, I believe for a picture). It grips the crucible with, well, an iron grip.....

For aluminum pours I like a cast iron plumbers pot as a crucible. If you're finicky about iron in the melt, just coat the inside of the pot with sand and fireclay mix as a wash. I did so with my zinc alloy pours.

If used bare, it has the advantage of heating up faster and being able to be gripped with a long handled pliers arrangement by the lip, which would shatter ceramic crucibles. I added tube handles about 18" long to a pair of large long nosed offset pliers when I first started pouring twelve years ago, and got in the habit of using it. The advantage is you don't have to transfer the pot to a shank, but just pour direct. This is for small pours, under 5 lbs, again.
« Last Edit: August 13, 2014, 07:53:07 AM by vtsteam »
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #54 on: August 13, 2014, 08:44:42 AM »
I qualified as a Safety Manager some years back, so it would be a bit infra dig (as well as painful) to become the owner of a bootful of molten ally.  :)
Eug

Heavy trousers with the legs outside your boots, or alternatively wear a pair of leather spats as sold by most of the foundry suppliers
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #55 on: August 13, 2014, 09:45:18 AM »
Leather boots these days tend to have synthetic fabric tongues, which hot metal will go through like foam. Never had it happen while pouring, but did when a spatter from welding hit the right spot. The tongue and lace area are a perfect shape for trapping hot metal.

I have a pair of welder's leather boot protectors now -- purchased cheaply on the Internet -- basically just a flap of cheap cowhide with leather straps the wrap around the bottom of you boot to hold it in place over the top. Metal will bounce and roll off leather as long as there is no recess to trap it. You could probably even make some kind of guard out of old welding or work gloves. Just something to cover the tongue area of a boot.

I get more serious with leather protection for iron than I do for aluminum -- but my aluminum pours are small. Jeans for aluminum in hot weather. For iron, I have a leather shop apron that goes below the knees, and a leather welder's cape for topside protection. I wear a welder's helmet with a clear glass when pouring any metal, and of course welder's gloves
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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #56 on: August 13, 2014, 10:06:01 AM »
Thank you both.

Steve, the trolly is at least in part a matter of storage; I'm so hard pressed for space it's impossible to keep the smelter and the bits and bobs indoors. If it, the pouring table, and tool rack are all of a piece I can make a waterproof cover for the assembly and stash it behind the kennels along with sundry other crap err vital equipment.

As a kid I worked in drop forges and die casting shops. In those days foundry boots were wooden soled with steel rims and heel segs nailed on; the heavy leather uppers were a bit like a wrap round jodhpur boot and fastened with a buckle at the back of the ankle. They were surprisingly comfortable and easy to wear. Looking back I probably spent the first 15 years of my working life in boots of some sort and even at my great age my tootsies are in fine fettle; the rest is falling to bits but the old plates are A1.

Eug

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #57 on: August 16, 2014, 11:21:42 PM »
Neat steam engine crankcase.. I tinkered a bit with a triple expansion one years ago. (H2O2 steam generation by silver catalyst)

Casting:
You must really pay attention to the clothing, I was using a cutting torch and my "polypropylene" tennis shoes caught fire.
My old instructor would have been happy with the way I was flip kicking that foot trying to put the fire out. Finally the shoe came off and blazed in the corner while I hopped about. (plasma cutting in flip flops is worse)

Cold weather Snow-mobile suits, zip up style, are also highly flammable.

My green welders jacket that came from a Nuclear job, the fire retardant was "washed" out long ago by improper laundering. I think it is cotton, which smolders and burns, but does not burst into flame.

I seem to burn my beard back to the welding helmet monthly.. It is half the length of my profile pic, never burns back evenly, always one side or the other.
I Hung a 24 foot Ibeam this morning in the ceiling by myself, programmed a Arduino this afternoon for a solar project, Helped a buddy out with a electrical motor connection issue on the phone, then cut up a chicken for Hotwings. I'd say it has been a "blessed day" for myself and all those around me.

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Re: Lost Foam Casting: a Crankcase in Zinc Alloy
« Reply #58 on: January 13, 2018, 06:08:19 PM »
Photos restored after Photobucket broke the links.
I love it when a Plan B comes together!
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