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Discussion Starter · #1 ·
So i was listening to a podcast the other day, Still Untitled, and one of the guys mentioned pva casting on it briefly and me being ignorant and curious at the same time started digging on what that is. Turns out PVA is what a 3D printer prints out. After that i looked up how they do the casting part if it, and its almost exactly like lost wax casting except you melt it out at a higher temp.

My question about it is how precise can you make a casting, detail wise? Could you print out a set of risers, put it in a mold of sand and plaster, melt it out, pour, and take it out and basically pop it right on the bike? Or would it still take some sanding to make it smooth? With a plaster/sand mix i would assume it would very detailed (from what i see thats the most common way to make a mold), seeing as how its basically a liquid that you pour over it. Could you get threads in it? Can you print threading into the part? Idk lol

What are yalls thoughts? I may do this when the time comes, buy aluminum scraps melt em down and have someone print out some risers. And i'll also make me a forge (i want one anyways) to melt it all down and such.
 

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Wannabe
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You can get decent detail, but it will take some cleaning for the simple reason that where you pour and where you vent will also fill with metal. Anything that you want looking nice will need polishing, and likely some cutting. But no matter what you're going to need to do some cleaning.

The better you do the prep and work the nicer it will turn out. Casting is also terrifying in that a single drop of water in there will cause a BIG bang and molten metal all over the place.

You might not believe it, but if you want to see some awesome investment/lost wax casting you should check out the Man at Arms channel on youtube. He's a blacksmith who recreates a lot of fantasy weapons. But a lot of the details are wax carved then cast in sand.

https://www.youtube.com/watch?v=xogheZdAO18
 

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Discussion Starter · #3 ·
I watch their videos all the time actually, thats kinda where i first started thinking about it. They do a bunch of brass pommel molds on there. I'll try to find a link to the video i watched specifically for PVA casting.

Anything i did would be powder coated or something like it. So unless its supper rough i would just clean it up well. Maybe soda blast?

But anyways i want to do a little bit of practice casting before hand just to get the process down.

Edit: Here it is

http://youtu.be/0bxOzCgUj_U
 

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lɐʇuǝɯᴉɹǝdx&#4
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3d printing on of the sort that uses PVA can't print usable theads. You would have to do finish machining any place where you wanted a mechanically flat surface, threading, etc. Still a damn useful technique for many things.
 

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Discussion Starter · #5 ·
I didnt think you could do threads but you could at least have the holes there and ready to be tapped. I want to do this for a set of rear sets as well, just seems like a great way to get a very manly one off part lol.

What about doing prep work on the printed part? Sanding and such and making it as smooth as possible for pouring the mold over it? I know, depending on the printer, it can have a rough finish that needs finishing. So why not do it before hand? Like making a plug for fiberglass
 

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lɐʇuǝɯᴉɹǝdx&#4
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Might work, although I doubt the dimensional tolerance is good enough to tap. Maybe if you drilled the pla, and scaled it up to account for shrinkage. Surface finish can be improved- prints an abs treated with acetone vapor can look like die cast. Something similar may be possibl for pls, or just sanding as you say.
 

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Discussion Starter · #7 ·
Thats a good idea, i'll have to figure how much it may shrink. I heard that you can prevent shrinkage by using risers so that as it shrinks it is replenished by the extra metal. Like i said i've never done it before so i'll need to look up more about it
 

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lɐʇuǝɯᴉɹǝdx&#4
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Risers don't prevent shrinkage of the 'cast version is 3% smaller' variety, which is simply due to the fact hot, solid metal is bigger than cooled, solid metal. What they really do is elevate the cup enough to provide pressure to force metal into the details of the mold. They do act as a resevoir as the liquid shrinks and cools, but obviously can't do anything about the fact that solid metal shrinks as it cools.

Old school pattern makers use "shrink rules" scaled for the metal they are working in. If they want the final part to be 5.50 inches, they just measure that on the ruler... but the resulting model is actually 5.65 inches, because an inch on the 'shrink rule' is actually 1.03 inches (or whatever is needed to account for shrinkage). With 3d modeling and printing, this isn't needed; you just scale the model up before printing!
 

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Discussion Starter · #9 ·
Ok that makes sense to me. So if i were to have someone print up a riser i would need to up scale it 3% all around so that when i pour it it will be the same size. Except the holes for the forks and the steering stem right? Cause it would try shrinking to a solid piece of mold and not get smaller, correct me if i'm wrong though.

Sounds like you've done this before lol, you know quite a bit about it
 

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lɐʇuǝɯᴉɹǝdx&#4
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I've done casting before, but only for art. Mostly I've read about it, a lot of that being 'history of technology' type info, more than how-to stuff. Same for 3d printing. Its also the sort of thing that comes up in conversation fairly often when you hang out with people at a hackerspace, often over adult beverages.

The 3% figure I gave is just for example purposes. It would vary by metal type, maybe even alloy, and might be much smaller than that. The holes would shrink along with everything else. If you have 3% shrinkage, a 1.00 inch hole shrinks down to .97 inch. Usually the mold just cracks in such places, or crumbles a bit. But you'd want to machine any clamping holes after casting anyhow, so would make the holes smaller than needed (to allow for machining) instead of shooting for exact dimensions.
 

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lɐʇuǝɯᴉɹǝdx&#4
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PVA (poly vinyl alchohol) is a low temp thermoplastic used in most of the less expensive fused filament fabrication type 3d printers. FFF is pretty much the only low cost 3d printer type their is, but the (slightly) more expensive ones can also do ABS (and sometimes nylon). Compared to ABS, PVA has the advantages of not producing toxic fumes (good for both home printing and burning), and burns out of molds much more cleanly than ABS would. It also tends to give better quality (but weaker) prints, mostly because it doesn't shrink as much when cooling.

PVA (poly vinyl acetate), which is what common Elmers glue is made from, has exactly the same acronym but is different stuff.
 

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Discussion Starter · #13 ·
I've never been to a hacker space before, i dont even know that there is one near me. I would like to go sometime though.

I do this all the time though, find something that interests me and then research the crap out of it. I have some clips for one of my helmets (to hold the visor) that i want to make into aluminum because i broke one once and would prefer something more solid than plastic. So that may be my test run for casting
 
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