Author Topic: Investment casting using PLA 3D print as positive mold  (Read 5393 times)

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Offline Let Slip The Hogs Of War

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Investment casting using PLA 3D print as positive mold
« on: December 02, 2012, 12:36:39 pm »
For the time being I have given up on lost foam casting of PLA (pouring molten aluminum directly on plastic and hoping it burns off). After many frustrating attempts, I want to have a cast part without reinventing the wheel. So here we go.

The process is:
  • Design or download solid model
  • Print model in PLA
  • Create plaster mold of the printed part
  • Burn out PLA from plaster mold leaving a void where the part was
  • Pour aluminum in plaster mold

It seems that one of the many many many problems we had with the lost foam style casting is our lack of a real sprue and gating system. Burning plastic releases a ton of CO2 which has to violently bubble out through the molten aluminum. This turbulence causes all sorts of problems. This next first attempt will fill mold with a sprue and runner system designed to minimize turbulence and fill the cast from the bottom.

The part is a 40mm cube because stupid Alex LOVES calibration cubes.

Aluminum is poured into the upper left basin, flows smoothly over into the sprue and through the choke. The metal slows down again in the well. The charge than moves up the runner taper, into the gates which I hope fills the mold in a very controlled manner.
« Last Edit: December 02, 2012, 12:51:15 pm by Let Slip The Hogs Of War »

Offline Let Slip The Hogs Of War

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Re: Investment casting using PLA 3D print as positive mold
« Reply #1 on: December 02, 2012, 01:04:29 pm »
Last night's attempt at making the plaster mold was a dismal failure. I mixed 1:1 by volume of Plaster of Paris and fine silica sand. Then the mix was sprinkled into a bowl of water. I continued to add powder until I saw islands of powder that were not easily absorbed which (according to the internet) is when your mix is correct. Well it never seemed to do that. I just kept adding and adding. Once I got fed up and figured there was plenty in the mix, I looked at the side of the bowl to see pretty much all of the silica sand had fallen out of solution. I mixed and mixed by hand but just sat on the bottom. I attempted to degas this pitful mix by duct taping plastic sheet over the bowl and hitting it with the shop vac. That part actually seemed to work ok. I vacuumed it for ~5 minutes and then removed the plastic to get ready to pour the mix into the mold. It was already hard as a rock. FUCK. The whole mess was over in less than 10 minutes which is allegedly well within the working time of plaster. The only things I accomplished were making a giant mess in the kitchen and ruining a perfectly good Pyrex bowl.

Next steps and lessons learned:
100lb bag of sand is unwieldy.
Don't use kitchen tools that you enjoy.
Try a 1:1:1 by weight sand-plaster-water mix.
« Last Edit: December 02, 2012, 01:09:22 pm by Let Slip The Hogs Of War »

Offline Eli L.

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Re: Investment casting using PLA 3D print as positive mold
« Reply #2 on: December 02, 2012, 03:05:40 pm »
I say: forget the sand, that's what's messing it up. Also, you should be spending seconds, not minutes, degassing the wet plaster. The more you stir, the faster it sets up--so stir minimally.

When I was casting the figurines, one of the problems I had was the plaster setting up too fast. I solved that by adding only enough plaster to the water, that the "islands" were just below the surface of the water (not sticking up like peaks).

It also helps if you start with a volume of that's roughly as high as it is wide. That way, the plaster stacks up in the water better. And a flexible, disposable container is advised (eg-paint mixing pail).

BTW, I used to use the hardware store plaster, but it was too crude. Now I use casting plaster (actually, Hydrocal) from Marjon ceramic supply.

If you want some help with this on a Saturday afternoon, let me know at least a day or two before, and I'll gather some stuff to bring to XC.

Offline Eli L.

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Re: Investment casting using PLA 3D print as positive mold
« Reply #3 on: December 02, 2012, 03:10:20 pm »
PS - You'll need some way to hold the positive piece level with the surface of the liquid plaster, so that it doesn't sink to the bottom or fall sideways.

Offline scuba840@yahoo.com

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Re: Investment casting using PLA 3D print as positive mold
« Reply #4 on: December 02, 2012, 03:30:17 pm »
Make up some really watery plaster. Dip in plaster about the consistency of thin pancake batter and througly coat with silica.  Dip sprinkle dry, over and over. I think the pros use something other than plaster for this...perhaps some sort of special polymer. 

Offline KRRRL

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Re: Investment casting using PLA 3D print as positive mold
« Reply #5 on: December 03, 2012, 09:58:37 am »
For what it's worth, stale beer may slow down the setting of plaster (as does cold water).  Bull urine does the same job; at least that was what the guy at the sculpture store told me once, and they sold the synthetic equivalent.   I do not know if the additives will make the plaster softer.

I know nothing, but cannot help commenting anyhow.  It seems like you might want to add the sand after you sprinkled the plaster in the water.  And I wonder if you couldn't paint a thin layer of plaster on the object first, to prevent bubbles, and then sink the whole object in plaster.  They do this when making rubber molds.

Again, I know nothing.


Offline Let Slip The Hogs Of War

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Re: Investment casting using PLA 3D print as positive mold
« Reply #6 on: December 03, 2012, 02:09:56 pm »
I say: forget the sand, that's what's messing it up. Also, you should be spending seconds, not minutes, degassing the wet plaster. The more you stir, the faster it sets up--so stir minimally.

When I was casting the figurines, one of the problems I had was the plaster setting up too fast. I solved that by adding only enough plaster to the water, that the "islands" were just below the surface of the water (not sticking up like peaks).

It also helps if you start with a volume of that's roughly as high as it is wide. That way, the plaster stacks up in the water better. And a flexible, disposable container is advised (eg-paint mixing pail).

BTW, I used to use the hardware store plaster, but it was too crude. Now I use casting plaster (actually, Hydrocal) from Marjon ceramic supply.

If you want some help with this on a Saturday afternoon, let me know at least a day or two before, and I'll gather some stuff to bring to XC.

Ok, so I mixed the shit out of it which as it turns out is a big no-no. Like you, today I switched to Hyrdrocal and Silica Flour. It's possible my silica sand was not fine enough and the hardware store plaster has other bullshit in it. I found some awesome papers describing why the molds are cut with silica. Basically to counteract the shrinking of the gypsum as it cures and and to give it high temp strength. Gyspum-only can't handle the bakeout.

For what it's worth, stale beer may slow down the setting of plaster (as does cold water).  Bull urine does the same job; at least that was what the guy at the sculpture store told me once, and they sold the synthetic equivalent.   I do not know if the additives will make the plaster softer.

I know nothing, but cannot help commenting anyhow.  It seems like you might want to add the sand after you sprinkled the plaster in the water.  And I wonder if you couldn't paint a thin layer of plaster on the object first, to prevent bubbles, and then sink the whole object in plaster.  They do this when making rubber molds.

Again, I know nothing.

I am hesitant to go on a bull urine collection exploration. I found some issues with my mixing technique and I switched to hyrdocal+silica flour which is tried and true.