Introduction: Bronze Casting Your 3-D Print
The Xerocraft Hackerspace in Tucson has put together our first 3-D printer and several of the artists immediately wanted to make their art pieces in a more durable form than the thermo-plastic that the printer puts out. We decided to do a sand cast in a two part, cope and drag mold. This is the finished piece after cleaning and we have a great set of photos shot by Karl of the whole process. In this photo you can see the detail we captured including the contour lines of the original print.
This method only works when the original pattern has no undercuts that can be pulled away from the mold material.
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Step 1: The Materials
We started with supplies purchased from a local Lapidary Supplier.
from left to right we have 1. talcum powder as a mold release and parting agent, 2. Play-Doh for sprue forming, 3. sticks of Bronze alloy (ingots) as our casting material, 4. Borax flux (jar with the red band, powdered borax for laundry use from the grocery store works just as well.), 5. Lapidary casting sand (this is really fine sand with a special binding agent that contains no water and can be used immediately) 6. the two-part Cope and Drag mold for casting small objects, 7. a crucible for melting our Bronze ingots, and in the center on the plastic box, 8. our 3-D print.
Step 2: Preparing the Pattern
To start, we coated our 3-D print (pattern) with talcum powder (from Walgreens) to create the first mold release and allow us to separate the pattern from the sand when the time comes. This covering is really thick but we gently shook off most of the powder to leave a nice powder film on the pattern.
Step 3: Inserting the Pattern
The first box (technically the "cope") is partially filled with casting sand to give a raised center area for the pattern. Although not visible here, a piece of plywood has been set into the underside of the cope to hold the sand in and act as a pressure plate later in the process.
The pattern is gently forced into the sand in a straight down motion with the area that will be the sprue (the neck in this case) being close to the preformed sprue opening in the cope. The low areas in the cope are then filled in with sand that is gently packed to bring the entire surface level to the top of the cope.
Step 4: Cutting the Sprue
In order for the molten Bronze to flow into our mold, I cut a sprue opening in the sand from the preformed sprue funnel down to our pattern. Since we are using a one sided pattern with a flat back-side, the area of the sprue is wide enough but not quite thick enough. Due to the nature of the sand, cutting into the mold sand leaves some raw edges and a little sand debris.
Step 5: Tamping the Sprue
It is important to tamp the fresh cut sand to minimize any debris that could get loose during the pour
Step 6: Securing the Sprue
Once the sprue is finished to my satisfaction, I used a filler (Play-Doh) to pack the sprue so we wouldn't mess it up during the next steps.
Step 7: Mold Release for Separating the Cope and the Drag
Once the Cope is ready, I sprinkled it with talcum to create a mold release layer for allow the finished mold to be separated when we are ready to remove the pattern. This application is a bit spotty and a fine dusting would have been better.
Step 8: Filling the Drag
The Drag is now attached to the Cope and filled with casting sand. This will be filled almost to the top leaving enough room to insert our pressure plate (plywood) an eighth of an inch into the Drag.
Step 9: Setting the Pressure Plates
The Cope ad the Drag are held together with two registration pins inn the semi-circular "wings" of the box to keep the mold boxes aligned. I now insert the second pressure plate in order to "ram" the sand and make a perfect mold of our pattern.
Step 10: "Ramming" the Mold
With both of my plywood pressure plates started into the Cope and Drag, I proceed to force the pressure plates in using a large rubber mallet. It is important to use smooth, square strikes that use the full face of the mallet so the pressure plates compress evenly. I have use plywood sine the commercial Cope and Drag actually has a bevel to the side walls that is narrower toward the center of the mold. The plywood will crush on the leading edge as it wedges into the steel box to prevent it from backing out.
The pressure plates stick out from both sides about equally so that they compress equally and do not move my pattern away from my sprue hole.
Step 11: Opening the Mold
Once the ramming is complete, the Cope and Drag can be separated. The talcum mold release is visible to show that it did it's job of preventing to two parts of the mold from sticking to each other.
Step 12: Pattern Removal
We are now ready to remove the patern and the sprue filler very carefully using a knife point to minimize any damage to the mold. At this point you will find out if you have any "undercuts" in your pattern that may have caught any of the sand of the mold and lifted it away from the mold impression.
Step 13: Inspect the Mold Impression
After careful removal, the mold impression is examined to see that the pattern is complete. We also examined the original pattern to make sure that there was no sand attached to it that should have been left in the mold. If there has been damage, it probably can't be repaired and you will have to start over. This is where you see how good a job of mold release application you originally did.
We had one spot of Cope and Drag separation damage just to the right of the impression. Not fatal to this casting but it shows where technique is important.
Step 14: Preparing for the Pour
Test assembly of the Cope and Drag showed that the sprue hole in the sand appeared to be too small for a successful pour. If the hole is too small, the metal will chill at the sprue throat and plug the sprue ruining the cast by preventing the molten metal from filling the cavity.
I expanded the sprue hole in the Drag side knowing that this will result in a thicker piece of metal behind the neck on our piece, but this can be ground off later if necessary.
An examination of the closed mold now shows that the sprue opening appears to be adequate.
Step 15: Preparing the Melt
Now that the mold is ready, it is clamped closed and set up for our pour. We previously chose a generous amount of Bronze ingot to equal the volume of out original pattern and some extra to allow for the sprue so there would be adequate material to fill the mold.
This was placed in our crucible with a teaspoon or more of Borax flux in the bottom of the crucible. This is just the beginning of the Borax since more will be needed once the metal reaches a near-melt temperature.
We are using a closed grip type of crucible tong that will stay attached to our crucible during the melt. Since the tong is steel that will melt at a temperature 800 degrees Fahrenheit higher than the Bronze, we only have to keep the wooden handle far enough away from the furnace.
Step 16: The Melt
Our furnace is a small, propane-fired knife-makers forge made from spare parts. This forge brought our crucible full of metal up in about 15 minutes with a two-inch wide blue flame coming down directly onto our crucible.
I watched the metal by checking it's color and glow. Initially the ingots take on a dark color that gradually lightens to a pink as they begin to glow. Once they take on a light glow when pulled from the oven for a quick check, we started sprinkling more Borax on them to stop any surface oxidation.
As a final stage before the true melt, the ingots developed spots of silvery metal sheen that looked like mercury drops before the ingots slumped into a pool of molten metal.
I continued to heat the crucible until the metal was fully fluid like water. If the metal is sluggish, it will want to "chill off" too quickly during the pour.
Step 17: The Pour
Once the metal is molten, it starts to form a crust that is a mixture of liquid Borax glass and oxides. At this point you are ready to pour. Do not wait too long once the metal is at temperature since you start losing some of the lower temperature alloy metals into the slag on top. Here I am pouring the metal into the mold and using a green stick to push the slag back away from my pour.
The green stick is a small branch I just cut from a tree in the yard that has a high moisture content and will not directly burst into flame.
After the pour I have fully filled the sprue, and there is some leftover material in the crucible. The brighter material on the right is molten Bronze and the darker material in the bottom is Borax glass from the flux. I poured the metal out onto our concrete block mold stand and a layer of Borax glass will remain in the crucible for the next melt.
Step 18: The Moment of Truth
After about 15 minutes, we gently opened the mold box by separating the Cope and Drag to reveal a black formless mass where we expected to see our gleaming bronze casting. The black was the sand and binder that had been heated to a decomposition temperature and had "cooked" around the cast. When the cast was removed from the sand most of the black sand came with it. the little bit of black sand left in the mold was brushed out with a fine brush and discarded. the red sand in the box can be used over again.
The mold was too hot to handle but not excessively so. The cast was definitely too hot to handle for another 10 minutes or so.
Gentle scraping and brushing at the piece started to remove the black sand and revealed that we had a good cast.
Step 19: Final Product
After some diligent cleaning, this was the final cast. From digital 3-D scanned image through a 3-D printer and into a Bronze art piece. I want to thank Karl Whitaker for the excellent photography and the original 3-D printed pattern, Alison Aragon for the original artwork, and the crew of the Xerocraft Hackerspace for their moral support