This is an advanced mold-making tutorial. If you need to make a multiple-part mold, or you need to make multiples of the same mold, you may want to consider making what is known as a case mold. That is, a mold-of-a-mold-part. This tutorial outlines how to use CAD and 3D printing to help in that mold-making process.
I use plaster molds to slip-cast ceramics, so the case mold in this tutorial is used to cast plaster, and the plaster mold will be used in turn to cast porcelain. The object I wish to make is radially symmetrical, so I need to make the exact same mold part multiple times to create a full mold. Once all four mold parts are complete (in your case the number may be more or less) the mold can be assembled and used to cast the final object. It can be incredibly difficult to understand this process without looking at pictures of the thing in action, so let's get started!
What you'll need:
- Computer and 3D modeling software (e.g. MoI3d)
- 3D printer and material, or printing service (e.g. Shapeways)
- Material you'd like your *mold* to be made from (e.g. plaster)
- Material you'd like your *final object* to be made from (e.g. porcelain)
- All accessory materials for mold making and casting (e.g. mold release, mold straps)
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Step 1: Model a Copy of the Final Object You Wish to Make
Using a CAD program (I use MoI3d) make a model of the object you wish to re-create with a mold. I specifically recommend using this process for radially symmetrical objects that will require mold parts that are exact copies of each other.
Even though you will be printing a case mold and *not* the final object, I recommend starting from the final object and working backward because it is easy to confuse positive and negative parts in this process, and having a concrete reminder of the end goal helps when designing a (negative) mold piece.
My final object is a ceramic bowl, so that's the first thing I modeled on the computer.
Step 2: Make an Outline of the Mold Part
Don't delete your final object until your mold part is completely outlined. It will help you remember what is original, and what is the mold. I started out dividing the mold into thirds, but realized later that the plaster couldn't come out (always be thinking about undercuts when modeling for a rigid material like plaster!!) and re-modeled it in quarters.
I started by duplicating the surface of my piece, and then designing the mold part around that. The 'container' you see in the picture is the basis for the case mold. It is the area that will eventually be filled with plaster.
The positive/negative half-spheres you see are the mold keys. In the second image (with yellow artifacts), you can see those align so that each piece of the mold "locks" into its neighbors during the casting process. Make absolutely sure you align the male and female parts properly, or your mold pieces will not fit together!
Step 3: Make Your Case Mold Printable
This is the most difficult step, both in terms of modeling and remembering which parts are positive and which are negative. At this point, you need to delete your original/final object, and model your case *outward* so that the walls have thickness and can be printed. This means building the walls into the area where your original object went. Be not afraid! The outside of the case mold doesn't matter. What matters is the void where the mold material will be poured. Just concentrate on building out from the void, and you should be OK.
As you can see from these pictures, I had to divide my case mold in two parts, because otherwise I would never be able to get the plaster out.* Remember, no undercuts! There is a simple trick to figure out whether you have undercuts in your mold parts. Look only at the part of your original object that you want to mold. Shut one eye, and you should be able to see every nook, cranny, and curve of the area you want to mold. If any area is obscured from view, then it's going to be a problem. In short, you should easily be able to see all areas you plan to mold at once, with one eye.
*If you're smarter than me, you may have noticed that the key that I made directly underneath my bowl is an undercut. I had to remove/fill it in later when I realized I couldn't get my plaster part out. I also ended up needing to apply a thick coat of physical mold release in order to get the case to come off the plaster, even without any undercuts, because the nylon print had such a rough surface. Always expect a trial-and-error period when experimenting with new materials and procedures!
Step 4: Make Lots of Mold Parts!
Once your 3D printed case mold is in your hands, you can use it to make as many mold parts as you need. In my case, four plaster parts. That means I poured plaster into my case mold, waited for it to harden, took the plaster out, and repeated three more times to create a full circle.
If you're curious about mixing and pouring plaster, look here.
Step 5: Test Out Your Mold
Now that the actual mold for your final object is finished, it's time to start using it!
For instructions on how to slip-cast in a plaster mold like I did, see this tutorial. The series of pictures here gives you an abbreviated idea of the process, and allows you to see the final object come out of the mold, looking very much like the model I originally created on my computer!
I used my mold to make a bunch of little planters, which I then had to clean up a bit once they came out of the mold, then fire them. You can see the results in the final picture :)