Metal Casting with 3D Printed Wax Patterns

Casting is one of, if not the, oldest techniques used to create metal parts.  By pouring liquid metal into molds, complex shapes can be made simply, and even geometries that would be impractical to machine (thin and spindly, for example) can take shape quickly.  Pouring the metal is quick, but making the mold isn't always easy.  But with a wax 3D printer and plaster investment, it comes about as close to easy as I think the process can get!  In this article I'll run you through the process, in the context of a small art piece being cast from a silver-copper alloy (Ag/Cu).

The process starts, you guessed it, with 3D printing.  In order avoid issues down the line with burnout and casting, we want a material that will melt quickly (as opposed to swell, which could crack the plaster around it).  We also need a material that will burn cleanly, so that any residue left behind doesn't become ash that will foul our casting.  Finally, we of course need a material that will print nicely and that's easy to clean, as once the part is cast in metal things will become a lot harder to fix.  

At POP, we have just the machine: 3D systems Projet 3600W.  It's loaded with a 100% wax build material -- arguably the ultimate investment casting medium -- and can produce ultra fine details and smooth surfaces with its high resolution of ~750DPI (XY) and ~1,600 DPI (Z).  

Progress feels slow with this machine as mere microns of height are added each pass, but before too long the part was finished, and looked great.  

Top immediately after printing and removal from build bed, with white support material

Another view. You can see how precise and smooth these surfaces are!

Support removal happens with the help of isopropyl alcohol (IPA), which dissolves the white support material but does not affect the blue model wax.  With time and gentle heating and agitation, even complex parts like this one with internal channels can be fully cleaned.  

Supports all gone!

Next, we need to sprue this part -- basically connect it to a pour cup so that once the whole thing is entombed in plaster, we'll be able to easily pour metal into the hole left behind by our melted wax.  Because the print material is 100% wax, this process, delicate as it may be, is pretty easy with the help of a wax pen and some wax rods. 

Melting the sprue to weld our wax piece onto

Fully joined!

Next up: time to invest.  Not talking 401(k) here, but plaster!  We used a gypsum-based plaster for this pour, but chemistries can vary slightly depending on what metal is going in and its melt temperature.  The raw plaster can be pretty dusty, so best to wear a mask for this part until it's mixed with water.  

Once the plaster has cured overnight, it's time to add heat.  The flask goes into a kiln for a standard (wax) burnout cycle.  A slow heat melts the majority out, and then a gentle ramp up to about 1,350F burns anything that remains out cleanly (remember this is just wax -- not resin).  Afterward, it slowly ramps back down to ~950F, where it holds until it's time to cast.  Keeping the flask hot helps to ensure that the metal that's poured into it remains liquid long enough to fully flow into all of the details of the mold and create an accurate casting.  Melting the metal is a quicker process, about 45min in an electric furnace (a graphite crucible helps to maintain a neutral environment and prevent oxidation).  

Once everything is hot and the workspace is ready, the process moves quickly -- it has to before things cool!  

1. Remove flask from kiln
2. Place into vacuum casting machine (this pulls air through the porous plaster flask and effectively vents the pour and sucks metal into any deep nooks and crannies of the mold)
3. Stir crucible with metal mixture to ensure that things are properly molten without excessive slag
4. Remove the crucible from the furnace
5. Pour into flask
6. Allow to solidify (place some flux onto the "button" to protect it from oxidation during cooldown)
7. Quench the flask to instantly remove the bulk of the plaster from the cast piece

Once the metal part is freed, it's all a matter of cleanup.  First a "pickling solution" (mild acid bath) to remove some surface oxidation and soften any flux that's stuck to the metal and plaster.  Removing the sprue happens with the help of a saw or dremel (or snips, depending on part size).

A spin in the magnetic pin tumbler cleans things up even more and produces a nice, burnished finish.

Because this top is a functional air-powered device, we needed to do some final post-op machining to get a hole reamed out for the sapphire (ruby) ball bearing.  That happened on our lathe, where we took the liberty of also turning the bottom face to reveal a beautiful metal finish.  No porosity like some other metal 3D printing processes!

Finally, it's time for a test!  Keeping video length manageable here.... but long spin times are definitely possible with this one as it's nice and heavy and appears to be quite well balanced!