Slice-form Metal Casting From Digital 3D Model

This Instructable shows how to take a 3D slice-form model and produce a solid metal casting of the object. It combines digital design using 123D Make with traditional metal foundry techniques.

Slice-form modeling is a technique to produce a 3D object as a series of layered slices. In this case, I use a CO2 laser to cut the individual layers from foam to create a pattern. The pattern is then used to make a resin-bonded sand mold for metal casting.

Materials List:

AutoDesk 123D Make Software
Access to a CO2 Laser (I use TechShop)
EVA Craft Foam - 6mm Sheets
Glue or Double Stick Tape
Sand mold making materials
  - 100 mesh sand - approx. 120 lbs
  - Resin binder and catalyst
Access to a foundry to pour the metal
Angle Grinder with Cutoff Discs and Wire Brushes to finish the piece


Production Notes:

I made it at TechShop! All of the digital fabrication in this Instructable was produced on an Epilog Helix 60W CO2 Laser at TechShop in San Jose, CA.

The sand molds were produced at the Mission College sculpture lab and the casting took place at the San Jose State University foundry.

Using 123D Make, open or download a 3D object model to use for the project.

You could also use 123D Catch to generate the model from a real-world object and then load that model into 123D Make. In that case, it would be beneficial to use the Shrinkwrap function to simplify the overall model.

With the model selected, configure the software to produce the appropriate type of slices.

Construction Technique: Stacked Slices

Manufacturing Settings: Ensure that the thickness settings match the material you are using. I typically use 6mm EVA Craft Foam which can be easily purchased from craft supply stores like Michaels.

Slice Direction: It is important to select a model that has symmetry. A basic sand mold will be a 2 piece mold which means that there needs to be a clear parting line down the center of the piece. It is important that the each half of the patten not have any significant undercuts ( a part of the pattern that runs around, under, or behind another which would lock the pattern into the mold). Very minor undercuts can be manageable because the foam is flexible and can be bent in order to remove it from the rigid cured sand.

123D Make will generate a series of EPS files containing all of the required layers as 2D vector outlines

If all you want to create is a basic slice-form model, then this step is mostly optional. I still recommend opening each of the generated EPS files in either Corel Draw or Adobe Illustrator to remove some of the additional markings.

By default, the patterns will have layer numbers and alignment lines inside each of the pieces. While this can aid with assembly, it will leave marks in the final casting so I recommend removing these attributes before cutting the slices.

Tip: Use a Sharpie to label each of the slices after cutting before removing them from the bed of the laser cutter

To add additional interest to the final casting, you can add texture to each of the slices by adding additional layers to the file. I added a layer with a vector pattern resembling circuit boards and then used this to "etch" the foam sheets before cutting out the shapes.

Cut out each of the foam slices for the pattern using a CO2 Laser Cutter. I did all of my laser fabrication at TechShop on an Epilog Helix 60W laser.

I did each slice in 2 passes - I first used a low power pass to etch the pattern into the foam (this is vector cutting the foam, just not going all the way through). For the second pass, I used higher power settings to cut the outline of each slice.

Here are some laser output settings that may be a good starting point - you may need to adjust these based on the specific brand or output power of the laser that you are using:

Pattern Pass:
Job Type: Vector
Resolution: 400 dpi
Speed: 85%
Power: 35%
Frequency: 1500 Hz

Cutout Pass:
Job Type: Vector
Resolution: 400 dpi
Speed: 30%
Power: 75%
Frequency: 2500 Hz


Tip: Even if you don't have access to a laser cutter, you can still create this type of pattern by first printing the slice outlines to paper and then tracing the outline onto the foam sheets. EVA foam can be easily cut with an XActo knife

Carefully assemble the slices to form each half of the pattern - remember not to join the halves as you need to make each side of the mold.

I like to use double stick tape to adhere each slice together. I find this holds together well enough for the mold-making process and makes it easy to separate the pattern to easily remove it from the mold.

This step is the beginning of the traditional foundry techniques. I used resin-bonded sand as my mold material. Once cured, the resin bonded sand hardens which allows the pattern to be easily removed from the mold.

Firmly attach one half of the pattern to a solid base board and construct a wooden flak to surround the patten.

Be sure to allow at least 3 inches around all sides of the pattern so that the mold is sufficiently strong. A piece like this has a large cavity which means that it is going to hold a large volume of molten metal. 

Use cornstarch (regular baby powder) to cover the pattern to serve as a release making it easier to remove the pattern from the cured sand. 

Now it is time to mix the 100-mesh sand with the resin binder (follow the instructions for the particular chemicals you are using). Be sure to wear a dust mask or respirator when working with the sand to avoid breathing in the fine sand particles.

The sand must be fully rammed into the mold to ensure that it captures all of the pattern detail.

Note: It should also be possible to use this technique with a green sand or petrobond sand mold.

Once the sand for the first half of the mold has cured (I typically allow it to cure overnight), you can flip the piece over to create the second half of the mold. 

Carefully line up the second half of the pattern on top of the first half that is now surrounded by the sand. Ressamble the flask to make a frame for adding the second batch of sand.

Be certain to heavily apply cornstarch to ensure that the two halves of the mold can be separated. Remember, you still need to open it back up to remove the pattern.

Once the second half of the mold has cured, carefully separate separate the two halves of the mold to reveal the pattern encased in sand.

Remove each layer of the foam pattern slowly by working it from an edge and bending it up to pull it away from the sand. If you added texture to the slices (like the circuit pattern), then you need to be especially careful since this texture is now a raised area in the sand and it is fragile. Work slowly to avoid damaging the texture since this will affect the finished sculpture.

The final steps that need to be done before casting:

Carve vents into the mold to ensure that air can escape as the molten metal is being poured in
Coat the interior of the mold with a mold release such as powdered graphite or Duratherm
Close the two halves of the mold and glue together with Core paste
Strap the mold together with 3/4" steel strapping (like used on large packages) to ensure the mold halves hold together
Attach a pour cup to the sprue to ensure that there is an easy way to pour the molten metal into the mold

This is the fun part! Now that the mold has been made, it is time to fill it full of molten metal. In this case, I was casting with Iron and worked with the Foundry at my local college.

You can inquire with local colleges that have metal casting programs or you can contact a professional foundry to have them cast your sculpture. Common metal choices would be Bronze, Aluminum or Iron - each has its own benefits and characteristics.

Once the metal has cooled (again it is best to wait overnight) you can break open the sand mold to reveal the casting.

Even though the metal is thoroughly cool, you still need to be careful because there are bound to be razor sharp pieces of metal called flashing that are attached to your casting. 

You will need to use a combination of tools to clean up the rough casting. I usually start with a handheld stiff wire brush to remove most of the sand that is still clinging to the casting. I then use an angle grinder with a metal cutoff disc to remove the metal that filled in my vents and any major flashing that occurred.

Be sure to wear safety glasses, a face shield, N95 dust mask, and gloves when grinding and cutting the metal. Depending on the metal you cast, it may spark and it will produce a fine dust that you don't want to inhale.

Once the piece is trimmed up, I place it in a sand blaster to remove the rest of the sand that clinging to the piece. 

After this, the piece is getting closer. You can use a combination of cold chisels, die grinders, and angle grinders to clean up the rest of the imperfections in the casting.

To achieve the desired finish, I work the piece using a combination of crimped wire brushes and knotted wire brushes on an angle grinder.

This will remove any minor imperfections and will bring a nice matte-shine to the casting.

Once the piece is polished to my liking, I finish it using knife honing oil. This oil has a rust inhibitor in it which will prevent the cast iron from rusting.