I’ve always wanted to try making a double-walled teacup, but the complicated and technically tricky assembly always made me pause. The concept of a double-walled cup is to insulate your hand from the interior temperature of the vessel. For mugs or glasses, it can also be used as insulation to help keep your drink warm. For a small teacup, that’s not really the concern, since you’ll probably drink the small amounts of tea or espresso before the liquid would cool down. Really, it’s just a different, interesting way of making a handle. Since you have two walls, it is possible to cut away part of the outer wall to create a decorative effect. My concept was to cut significant portions away from the exterior to make a simple, bold, linear pattern. I thought that this sort of delicate line work would give a unique effect (and one that would be incredibly difficult to achieve if you were building this by hand.) I settled on a simple chevron pattern radiating around the edge of the cup. This way as much as half of the exterior wall might be removed, leaving a network of legs to support the cup.
Whatever concept you have for your vessel, I'd encourage you to dig into the details. The shape, size, and weight of your container will all contribute to it's functionality (not to mention how it looks.) The more unconventional your design is, the more likely that it will be tricky to print or get to function properly. I certainly wouldn't discourage designers with crazy concepts, but you just might have to do a little more homework to get everything in line.
Step 1: Design Considerations (What Will Make Your Vessel Look Good and Work Well)
For those of you who are new to ceramic design, there are a few design considerations that ceramic artists obsess over. Some of these are easier to tackle with a computer-generated design, and some are more difficult. But they are definitely things that you'll want to think about as you come up with your design.
Base: The shape and proportions of your base are terrifically important in contributing to how your vessel will feel during use. Too narrow a base, and your vessel will be easy to knock over. On the other hand, if you have a thick, wide base, your vessel might feel chunky and heavy. It all depends on what effect you want to create with your design.
Lip: The counterpart to the base is the lip of your vessel. The shape of the lip is particularly important if you are designing a cup, where it will be touched by your lips. Often handmade ceramics will have a rim that is thinner than the wall of the vessel. If the lip is flared out, you will get more of a gentle pour (think teapot), if the lip has a sharp angle, you will get an abrupt cut-off when your pour from it (think white wine glass.) Pay attention to the different lip designs of your drinking glasses and mugs, and try to figure out which lip style will work best for your design.
Handle: If your design contains a handle, you'll want to know what that will be like to hold. Take note of handles on mugs and glasses that you use. (And take measurements of the ones you like.) You can even make a mock up in clay or play-dough to work out what shapes and sizes work for handles.
Weight/Balance: This might just be the trickiest thing to figure into a computer design. For ceramic artists who throw pots on a wheel, they get lots of time (and many chances) to hold the vessel in their hands before it gets fired. Usually the challenge for wheel-throwing is to make vessels that are not overly heavy and chunky. In a computer-generated design, you can make your wall thickness incredibly precise (and minimal), but you won't have the chance to get a feel for the balance and weight of your design. If your vessel is very tall, you might want to taper the wall thickness so that the bottom of the vessel has a little added weight and stability. Also take note of similarly-shaped vessels and see how the center of gravity affects how it feels in your hand.
Step 2: Design Constraints (What Might Make Your Design Unprintable)
3D printing in ceramic has significantly more constraints than printing in other materials. Unlike other printing media, once your ceramic form has printed it has just begun its journey. Afterwards it must be dried, glazed and fired. Anyone who has worked with ceramics before knows that just because something looks good going into the kiln, it doesn’t mean it will look good when it comes out of the kiln. Here are some general ceramic concerns that ceramic newbies might not be familiar with:
1: Wall thickness. With 3D printing, you’ll generally want to minimize your materials to get the most cost-effective print, and with ceramics this is important for another reason. The key to getting your product safely through the firing process is to first get the clay completely dry in one piece, and then let it get completely heated in one piece. This means that dramatic differences in wall thickness (or even really thick walls) can cause problems. A thick wall will take much longer to dry out than a thin one, and since clay shrinks as it dries, your thin pieces might just crack off while the thicker ones are not yet dry. Generally, aim for a uniform wall thickness throughout your design. The same thing goes for sharp edges-- they will dry (and possibly crack) before the rest of the piece has dried. Remember, too that the glaze will add to the thickness of the finished piece (as much as a millimeter on each side). But even though it will make your walls thicker, glaze will be molten during firing, so it will not contribute to the structural integrity of your design.
2. Structurally sound shape. Ceramic is a very strong material, but when it is fired it is heated to a point where it is nearly molten. At this point heavy, unwieldy structures might sag and collapse. If your design is mug-sized or smaller (which it probably is), you don’t have thaaat much to worry about. It is much trickier to get very large vessels to stand up, but smaller vessels will likely stay upright, even if the shape deforms (see attached image). If you have any dramatic overhangs, or very thin walls, you might want to think twice, or at the very least, support them well. For rounded vessels parabolas are the structurally ideal shape. If your vessel is more than 3 or 4 inches, I would design the walls to be slightly thicker at the base than at the top. Also, (depending on the shape of your vessel) you’ll need to plan for it to sag a little during firing.
3. Glazable shape. Lastly, your piece needs to have a shape that is possible to glaze, and an unglazed surface where it can sit in the kiln. If you were to glaze the entire surface of a ceramic piece, you’d fuse your piece to the firing shelf with glaze. So that means that you’ll want to design the bottom of your piece with a stable base that sits on a surface which will be left unglazed. If your object doesn’t have a clear base, it’s likely that it will be fired on a little metal stand. This allows your piece to be more completely glazed, but it will also create little pucker marks at the points that your piece sits on the stand. (Check the bottoms of lots of commercial plates and you’ll see what I’m talking about.) You’ll also need to have all surfaces of your design be accessible with a sprayer. For instance, a mostly enclosed area would be very difficult to pull off. It also seems like a good place to note that the added surface of the glaze can cover up subtle engraving and patterns. Remember that the glaze will be a liquid, and it can run and pool into fine patterns, so make sure any pattern or surface articulation you create is a bold enough relief to show through at least a millimeter of glaze.
Step 3: Scale the Interior Volume of Your Vessel
Scaling your design: Because 3D printing ceramics is still quite costly, you’ll likely want to minimize the cost of your print by making sure that your vessel is exactly the right size for your purpose, not larger. My design was for an espresso/tea cup. I figured that 2 fluid ounces was the smallest size I could make the cup, and still have it be useful. (At least a double espresso.) I knew that I was already spending more on the double-walled option (which would always use more material than a single-walled cup), so I wanted to pick the shape for the interior vessel that would be optimal.
Set up your drawing: Most 3D printers use millimeters in their design guidelines. I found it easiest to set up my drawings with millimeters as the base. I also like to set up a few construction lines to make aligning objects easier. I tend to draw in 2d, then create a copy of whatever I created with an easily accessible reference point. This makes it easier to see what you're doing when you're drawing (since you're working on just one piece at a time), and also if you don't like what you just did, you can quickly go back to the previous version and change it.
Draw and measure the interior volume: In AutoCAD, I drew a section cut through my desired volume, and revolved it to create a solid. Using the _massprop command, I found out what the volume of my solid was. I could then calculate the scale factor to scale my model up or down so that it equaled two ounces. (You can convert cubic millimeters to fluid ounces here.) I took the measurements of my new volume, rounded them up to the nearest millimeter. So, there is: the inside profile of my two ounce cup!
Step 4: Create Detailed 2D Drawings of Your Design
Step 5: Create Your 3D Model
For those of you who want to get into the nitty-gritty software stuff, here's a rough outline of how my modeling went:
1. Revolve the interior cup polyline along the center axis.
2. Revolve the exterior wall polyline along the center axis.
3. Draw a closed polyline of the wedge that will proscribe the width of the leg.
4. Use the _helix command to draw a spiral half the height of the cup.
5. Sweep the wedge shape along the spiral path.
6. Mirror the wedge spiral to create the bend in the chevron pattern. Merge the two spiraling wedge pieces using the _union command
7. Copy the exterior wall profile and the spiraling wedge profile and line them up. Create a new solid where the two solids overlap using the _intersect command.
8. Use the _3Dfillet command to round the exposed edges of the leg.
9. Create a polar array of the legs around the center of the cup. (I used 10 legs)
10. Explode the array and align the finished legs with the interior cup.
11. Use the _union command to consolidate all the pieces into one solid.
12. Copy the completed solid into a new file. Prepare the file for export to a CTL file according to your printer's instructions.
Step 6: Refining and Perfecting
I do love my little teacup, but there are a few things I will modify to make the next print even better. First of all: the lip. The angle of the lip sagged significantly (the finished height was diminished by almost 5mm.) This was enough to change the volume capacity of my cup to 1 3/4 oz. rather than my intended 2. So on the next go around, I'm going to change the profile of the lip, and probably add in some more support to help combat the sagging. I'll also up the clearance between the exterior and interior walls. With sagging and the added thickness of the glaze, there were several spots that the legs fused to the interior cup. Heat transfer is still fairly effectively minimized, but I think it might be a cleaner connection if I added another millimeter of clearance.