Metal casting question.. any metals offer 1:1 casting size?

[hydin]

Mainly, as most men, I am worried about shrinkage.

I have a screen used piece that I want to replicate in metal, and the only way this seems feasible is through a mold and then molten casting. There is entirely wayyyyy toooo much detail to try and get this thing cgi modeled and printed at shapeways, and I think a cnc machine would have a heart attack at trying to make it out of aluminum.

So... does anyone know of a metal that offers minimal shrinkage when casting? I really would rather do this than go the resin/aluminum powder route.

Chris

 

[kursosawa]

Do you know how much shrinkage occurs? I forgot who brought it up, but someone here once suggested making the thing larger than the original piece to compensate for shrinkage before casting the piece in metal in order to get around this problem.

 

[GeeksGoneBad]

Sadly I think that anything molten will shrink when cooled and I think they are all roughly the same :

Metal Pattern Oversize Factor* Finish Allowance** Win Wall mm/(in)
Aluminum 1.08 – 1.12 0.5 to 1.0% 4.75 (0.187)
Copper Alloys 1.05 - 1.06 0.5 to 1.0% 2.3 (0.094)
Gray Cast Iron 1.10 0.4 to 1.6% 3.0 (0.125)
Nickel Alloys 1.05 0.5 to 1.0% N/A
Steel 1.05 – 1.10 0.5 to 2% 5 (0.20)
Magnesium Alloys 1.07 – 1.10 0.5 to 1.0% 4.0 (0.157)
Malleable Irons 1.06 – 1.19 0.6 to 1.6% 3.0 (0.125)

 

[hydin]

Can't really make the piece bigger, since it's going to be using a mold off a screen used piece.

It's got some insane detail in there, so I can't really get it printed by shapeways without spending a fortune, or really imagine a cnc machine doing all the little detail work.

I think I might have to just suck it up and going the old fashioned resin/aluminum powder route. Crap.

Appreciate the help
Chris

 

[exoray]

Sadly I think that anything molten will shrink when cooled and I think they are all roughly the same :

Not everything, water first shrinks then expands upon the transition to a solid when cooled

Back to the OP's question, is the shrinkage that much of a concern? So what if it's slightly smaller or is the size crucial for some other aspect?

There are some novel but impractical for the hobbyist ways to cast metal without losing size... I read about a 'lost water' method where the metal part is traditionally molded and then castings are made from a custom water gelatin mixture that is tailored to expand the same ratio as the metal shrinks, pull an investment mold off of that expanded water cast and in theory there is no lose in size when cast in metal...

 

[hydin]

I don't mind much shrinkage, but the size is pretty important to me. I want to make it as close as possible to it's "real world" equivalent.

Not trying to be all mysterious, I just seriously doubt anyone has ever heard of this thing outside a small community of people.

Chris

 

[exoray]

I don't mind much shrinkage, but the size is pretty important to me. I want to make it as close as possible to it's "real world" equivalent.

Not trying to be all mysterious, I just seriously doubt anyone has ever heard of this thing outside a small community of people.

Chris

Well a 1% shrinkage is certainly measurable but hardly noticeable in most cases...

Example

If the item is 6 inches long, 1% is approximately 1/16 inch, unless side by side or end to end I doubt anyone would notice the difference between a 5 15/16" copy vs the original 6" version...

 

[Contec]

Not trying to be all mysterious, I just seriously doubt anyone has ever heard of this thing outside a small community of people.

Chris

you might be surprised...

 

[brimst0ne13]

Knowledge of what this mystery prop is would shed light on this dubious situation good sir.

 

[Eaglewood]

It's got some insane detail in there, or really imagine a cnc machine doing all the little detail work.


Chris

I work with CNC machines and cutting tools that can engrave a logo on a human hair-- so the detail work is possible

PM me for details.

 

[hydin]

The mystery prop is a replica of the Bob White UFO fragment . If you know what that is, well, I am surprised :lol

Don't have any pictures handy of mine, but a quick google search nets :

I own the piece used in the discovery channel episode that was talking about this. It's the hero with some light up LEDs in it to simulate it "glowing from heat" when it landed.

I really want to get this done in metal to have more of a "realistic" feel, but as you can see from the somewhat bad picture, it's got tons of details/undercuts on it. It's making it hard to work out a way to recast it.

Chris

 

[DeadRinger]

What method will you be using to cast this? Mold max 60, sandcasting? lost wax? I guess it depends on what type of metal. I'm interested because I'm trying to figure out the most efficient way to mold/pour metal for prop building.

 

[DrewSmith007]

I would be more worried about the sprue that you'll have to cut off from the cast piece than the small shrinkage of the metal.

 

[hydin]

Hoping the people I get to do it will be able to handle the casting part without causing too much variation on the design. Thinking that it might be best to cast it with the "bulb" portion at the top, so the undercuts are more of "overlaps", and cutting off the sprue means just having to sand down the bulb a little at the tip.

Thinking about checking some local foundries and seeing if they can swing it. I would LOVE to get one made out of silicon but I don't even know of a place that would have the ability to cast it in that, or if it was even possible. It'd go great with the big chunk I have as another "ufo relic"

Chris

 

[hydin]

I know it sounds kinda crazy when you go "Oh, pfft, 10% is something like .25" or so", but it's just that feeling that YOU KNOW it's off.

It might be perfect, it might be terrific, it might be every detail accurate, but if it's just tad bit smaller... it's just annoying. I think the only other person who would agree with me on that is asavage though, and that's only from watching the youtube clip of him explaining the maltese falcon bronze casting problem :lol

Eh, there's worse things in life.

Chris

 

[DrewSmith007]

I completely agree with you. I can't stand it when I make something and have to settle for close enough.

 

[Mike J.]

Quoth the Wiki:

"Bismuth expands 3.32% on solidification; therefore, it was long an important component of low-melting typesetting alloys, where it compensated for the contraction of the other alloying components."


-MJ

 

[Greg in MO]

Hello Hydin - this thread is pretty old, but if you are still looking for a solution...

For your item a flexible silicone mold could be made, and then a metal cast of low melting temperature Tin / Bismuth can be made in it. Multiple casts actually. Here is a link to the material - I recommend the 281-338 (check out the shrinkage data also - virtually zero)
Bismuth Alloys, Cerro Alloys

Due to the complex detail on your item, I suggest the silicone be poured in one piece around the item and then be slit with a razor knife just enough to allow the master to be removed.

Color: There are various ways of putting a patina on this metal. or it can be painted. This metal has no lead so can be safely cast and handled.

 

[Organic Mechanic]

Lead-Free Pewter has a ridiculously low shrink rate. .03125" per Foot.

 

[exoray]

Bismuth Alloys, Cerro Alloys

Some of the extremely low melt stuff is fun... I have been playing with a few pounds of Wood's metal (Low 158) that I picked up last year... Although not good for handling due to the lead and cadmium, it's fun stuff to play with... I melt it in double boiler using plain water and you can cast it in just about anything... I have been using it to make temporary metal jigs for holding parts while milling, it's nice in that it doesn't bond to much, so I can build up cardboard or clay walls and literally cast right on top of the part I want to hold down and get a prefect custom fitting jig no matter what the shape...