Several DIY casting questions... Lost Wax & Aluminum?

[Sporqster]

First a disclaimer: I'm really new to this, but very excited about giving it a shot. I'm also very limited on fundage, and whatever I spend on backyard industrial supplies is coming directly out of my beer making funds...

I'm currently machining a wax part about the size and shape of a distributer cap for your car (Actually going to replace the distributer cap, as it is a housing for a DIY distributerless ignition system), with a wall thickness of .1875" I would like to cast it in aluminum using some sort of investment method (I suppose) (since dummy here didn't bother to design in any draft on the part!), partially for the education of it, since there are probably some urethanes or epoxys out there that would take the heat and abuse this part needs to be able to take, but aluminum would have more bling factor.

First off, I've got a little benchtop CNC, and the detail and complexity of this part is taking FOREVER to complete even in wax. From what I've read hardly anyone casts something exactly right the first time, so I need to be able to make some copies (besides just machining another one) of my wax master. Silicone mold maybe? There are no undercuts on this part, but there is also no draft at all, and its about 2" deep. Any advice or suggestions for making accurate wax copies?

Next, onto the real mold-

Where should I get investment ceramic materials, and what specifically should I be using for an application like this? And then after the wax is coated with investment, should I bury this whole thing in sand too? Is backyard sand OK here (I live on the coast, the backyard is mostly sand anyway), or will I want to find something special?

With these thin walls I was thinking that I would need to somehow forcibly move the molten aluminum to completely fill the cavity with minimum voids... which then got me to thinking about building my own junkyard centripetal casting machine from a busted air compresser motor, a wheel rim, and a 50-gallon drum (to shield me from the flying molten metal)

So then I start to thinking, how fast should this thing spin? I assume I would want to minimze the sprue length (bringing the casting closer to the center of the rim) but maximize the G's (crank up the RPM, maybe with a pully) but keep the whole contraption balanced enough to not fall appart! Short question: how many G's should my casting see?

[SPEEDRE]

Hey Sport. First, if you are using wax for the pattern then you won't need the centrifical casting method. The idea is to build up a coating of ceramic material then melt out the wax leaving the entire cavity as the mold, then simply pour in the moulten metal. As for the ceramic slurry, thats what it's called, do a search for backyard foundry and seek a supplier there. Good luck, Sport..

[ViperTX]

Well you'll either need a vacum or centrifical caster to fill the cavity....pouring would never work for this part. Generally when you pour you're doing a sand casting or it could also be a "lost foam" casting.

The slury is known as investment for "lost wax" casting....there are different types depending on what sort of metal is being cast.

[Mcgyver]

wow. you're going so fast my head is starting to hurt

First off, you need to burn out the wax in the cavity BEFORE you pour the metal. The right equipment for this is aptly named a burn out oven. If you google or look around here, you will see stuff on lost foam casting - this you leave in and pour into the mold, but DONT DO THIS WITH WAX!. The burn out process also dries out and warms up the investment so it’s ready to pour. As Viper says you need a centrifuge or vacuum caster ($$$$). The centrifuge is the poor cousin to the vacuum. The investment is porous so under vacuum the metal gets completely drawn into the cavity (with the investment at the right temp!)

If you want to make more than one, you need to make a rubber mold and for that you need stuff like a vulcanizer. Then you split the rubber mold (in itself a skill), remove the master and cast as many wax patterns as you need.

Look to the jewelry supply industry – any one of their catalogues will be packed with lost wax casting supplies and tools.

I agree with Viper... a lot goes into lost wax casting (both $$$ and special equipment) and you might consider alternatives like sand or lost foam, or why not just turn and mill the part (after all, you are going through that process to create the master).

[Sporqster]

Originally Posted by Mcgyver First off, you need to burn out the wax in the cavity BEFORE you pour the metal. The right equipment for this is aptly named a burn out oven. If you google or look around here, you will see stuff on lost foam casting - this you leave in and pour into the mold, but DONT DO THIS WITH WAX!. The burn out process also dries out and warms up the investment so it’s ready to pour. As Viper says you need a centrifuge or vacuum caster ($$$$). The centrifuge is the poor cousin to the vacuum. The investment is porous so under vacuum the metal gets completely drawn into the cavity (with the investment at the right temp!)

Okay, so maybe I don't need to cast this centrifugally- I was looking at some crank case castings Lyle (www.landstromsfoundry.com) did for a motorcycle in plain old sand casting, and look pretty good. My wall thickness = ~.18 after I've sanded machine marks off. When I turned the workpiece upsidedown to machine the back side, it was about .02" off, I'm not sure if this was from a bad zero location or if the workpiece shifted a little :frown: but it looks nice after a little love from some 1200 grit sand paper.

Originally Posted by Mcgyver If you want to make more than one, you need to make a rubber mold and for that you need stuff like a vulcanizer. Then you split the rubber mold (in itself a skill), remove the master and cast as many wax patterns as you need.

They make this look so easy on the Freeman supply videos... but I'm starting to think I just need to be careful with my one wax master and make a plaster mold from it.

Originally Posted by Mcgyver Look to the jewelry supply industry – any one of their catalogues will be packed with lost wax casting supplies and tools. I agree with Viper... a lot goes into lost wax casting (both $$$ and special equipment) and you might consider alternatives like sand or lost foam, or why not just turn and mill the part (after all, you are going through that process to create the master).

yea- in the long run you probably have a point there... but I did cut my wax master at over 10"/min with a wood router bit (Prolly shouldn't try that in Al on a Sherline)! I don't even own a real set of high speed milling bits (yet)... I also didn't want to pony up the cash for 2 2"X6"X6" blocks of Aluminum billet when I've got an office full of raw abandoned 6061-t6 extrusions begging to be reincarnated as something useful. (can this stuff be backyard cast well without modification to the alloy, and if no, what should I do to it?) There's probably 100 pounds+ of it just trashing up my office at work alone!

Perhaps I should make a mold very roughly the shape of my finished part in sand, pour AL in, and then machine out the finished part, since I'm thinking I could never 1- melt enough aluminum in the back yard to fill up a complete 2"x6"x6" block of Al and 2- if I did, ever expect it to cool in a roughly crack-free rectangular solid-like shape, simply due to its massive volume....

HOWEVER Currently plan A (still evolving) is to attempt to make a removable 2-part mold from plaster of paris off my 1 wax master cook it to cure the plaster, and pour AL into that. My intended production run is for 1 good part.

One other thing.... I've been reclaiming machinable wax, curing powdercoated stuff, and was planning on melting wax out of an investment castings in a regular old oven in the garage, a refugee from a kitchen remodel. Why not?

[ViperTX]

It won't get hot enough to burn out all the wax...plus the investment must be hot enough so the metal doesn't chill when it touches the investment...you'll end up with a partial, porous casting.

[Sporqster]

Originally Posted by ViperTX It won't get hot enough to burn out all the wax...plus the investment must be hot enough so the metal doesn't chill when it touches the investment...you'll end up with a partial, porous casting.

Should I just make a second crucible and cook my mold while I'm melting the Aluminum?

I'm also struggling with the advantages / disadvantages of sand over plaster. it seems that a much smoother, more detailed surface can be achieved with plaster, but is seems like every example of a part I see like what I'm trying to do has been done with a wood master in a sand casting.

with these walls, would a sand casting work, or would the aluminum freeze too quickly.

I don't have a digital camera with me today or I'd send you a pic of the master, but at least here's a pic of the design in Catia:

Note, I left out drilling the screw holes in the master as 1- I thought this would be a pain to flow AL around, and 2 they might not line up after the AL part shrinks, so I thought it best to cast it without the mounting holes and drill them in the casting later.

[Mcgyver]

You are right, the most efficient way to make an irregular shape is casting. from solid it is a dodge to be used when other things like aesthetics don’t matter and IF its less time and $$$ than all the hassle of setting up for casting.

I like your attitude, after all the fun is in the making else we’d all just go a buy commercial stuff, but remember lost wax casting is a pretty tried and true process – you are going to have a tough time inexpensively circumventing its major components.

A lot of guys pour AL in their backyards with good results using a wood pattern and sand, or lost foam. I find lost foam a very cool process that side steps a lot of the tough parts of pattern making etc, but if each master is very difficult to make, you might be better of learning the wood pattern/sand cast approach

PS - since creating the wax pattern is the high value work, why not check out what it would cost to have someone with the equipment do the casting? you will get the best finish with lost wax, but the part will still require machining of mating surfaces.

[wawatobi]

If you're going to use a rubber mold to make duplicates of the part, you will have to
use a specialty mold rubber. The most common mold rubber used in lost wax casting has
a fairly large shrinkage. Even the expensive silicone mold compounds shrink more than 1%. The two part mold compounds which do not require vulcanising would be the best. Last time I bought a 5 pound kit it cost me several hundred dollars landed in Canada. I think I would try milling the part in aluminum if your machine is capable.

[Geof]

To quote Mcgyver: "Look to the jewelry supply industry – any one of their catalogues will be packed with lost wax casting supplies and tools."

He buried this advice halfway down his page, maybe you skimmed over it. Find a local hobby store that caters to people who do silversmithing and other metal jewellery as a hobby. This is widely done in that field and you do not need a fancy centrifuge. I have seen a system that worked on a spring; the mold is mounted at the end of an 18" arm mounted on a vertical axis, 'down' for the mold is on a radial line, there is a small receptacle that the molten metal is poured into (I think the term for this is a tundish) with a port in the outer side connecting to the mould, you cock the arm through two or three turns, pour in the metal, close the lid and pull the trigger. The only tricky bit is shaping the tundish so that when the arm accelerates the molten metal gets funnelled 'down' into the mold instead of spraying everywhere.

[ViperTX]

How large is the pattern, how thick are the walls. Centrifical or Vacumn casting will give you a denser casting then a sand casting. A sand casting will also have more surface texture.

[nikolatesla20]

You should just mill this sucker out of foam and do lost foam, it is sooooo easy compared to the other types of casting. Lost wax is nice but is really only needed for castings that require small accurate detail. Lost foam still gives you plenty of detail, and very good casting tolerances.

check out http://www.buildyouridea.com and look in the CNC section about "Homebrew HBLB bearings". It is excellent information that I've used, and I've been extremely successful with lost foam. It works great and I think it's the easiest way to cast for the hobbyist!

Here's a picture showing the CAD design image, then the foam part (cut by a foam CNC hotwire cutter in the background) and then the AL cast part after a small bit of filing to clean it (It only needed this because I didn't bother prepping my sand or my foam first caused I was in a hurry). The AL part looks exactly like the foam part; if you hold the foam part over it or the other way around it looks exact!

-niko