Introduction: Miniature Japanese Lanterns: Decorative Cement Casting

By My Etsy store
About: I Design Stuff. I've been working with 3D printers for over a decade, using them to prototype, refine, and improve all sorts of things. Recently, I've been making a foray into the world of laser cutting.

In this Instructable, I'll be going over how to cast a small statue out of cement; specifically, something in the style of Japanese lanterns. To achieve the final result, I used what's known as a "mother mold", where there is a hard shell surrounding a soft, thin, (usually) silicone layer. This is both to cut down on material usage and to provide a rigid support for areas that need it to prevent sagging. At the scale I'm working at (the larger statue is 130mm (5 inches) tall), sagging is not much of a concern, but as I plan to eventually scale these up to 2x-4x their size, practice never hurt.

Supplies

For the cement, I used Amerimix AMX 400 S, which is a blend of portland cement, lime, and sand. I have no particular reason for choosing this specific brand or product other than it was cheap, plentiful, and available at the local hardware store. Should you choose something else, two considerations you should make are A) is the particulate size suitable for the purpose (no large rocks or pebbles to get stuck in fine features) and B) will it be easy to pour into the molds?

For the silicone sleeve, I used Smooth-On's Dragon Skin 10 NV for the "hat" portions and Ecoflex 00-20 for everything else. As I've used this brand in the past, I have confidence in their products performing as expected, so I didn't have to worry about the silicone not curing, tearing, etc.

I used Smooth-On's Universal Mold Release to lightly coat the printed parts before casting the silicone. Without it, the silicone will stick much more than it needs to, causing problems when de-molding.

I used Smooth-On's Silc Pig silicone pigments. They served no practical purpose, I just wanted to try the colours out to see what they looked like. My favorite ended up being Green.

I printed the shells for the molds, as well as the masters, on a Prusa Mini 3D printer. Ensure that the printer you use is calibrated well - over/under extrusion or gaps in the walls will cause issues; leakage is likely if there are gaps, while overextrusion may result in parts not lining up, leaving rough surfaces, or capturing silicone/cement in pockets that shouldn't exist.

I also needed a mixing cup, water, a mixing implement, PLA 3D printer filament, and various lengths of M4 bolts and nuts. These are generic enough that I won't list the specific ones I used.

Step 1: 3D Modeling

While I won't be covering the process of modeling these sculptures, there are some important notes to remember:

  • Try to avoid deep overhangs that are along the line you will be unmolding. This will cause the figure to resist being demolded (due to friction/stiction).
  • An easy way to prevent this is to add a draft angle; slope the area so it will come off easier. Note, the only flat areas in my models are on parts that are the top surface that is being cast.
  • Along the same lines, remember cement is much thicker and less viscous than water and will have a harder time flowing into sections that are thinner or farther away from the pouring hole.
  • Geometry that varies greatly in width will either need to be split into sections, or will need to have a wider overall shell.

I've split the taller model into two sections, since the base is close to the same width as the main body. Since the top flares out and has such a deep undercut, it would be much easier to cast it from that surface rather than hoping the cement would fill out that area without any air bubbles.

The shorter model was split into three sections; this is both for ease of casting and to preserve the features. It would be much more difficult to get crisp corners if it was made from one piece. Also note the lip (highlighted in blue), which helps with casting the silicone sleeve, and will be explained later.

Step 2: Explanation of Features: Tall Sculpture

Here, I've provided a cross-section of the mold, and labeled the parts (click the images for labels). I've included some features to help with making the sleeve; the Centering Plugs keep the Casting Core in the same place while the silicone is poured so all the walls are the same thickness, and the "floater" is hollow to save material. There are threaded holes to hold a spacer during casting.

Step 3: Pouring the Sleeve: Tall Sculpture

The process is reasonably straightforward if you've used silicone before. Insert the core, attach the Centering Plugs, bolt the contraption together, and pour.

Since this is a 1:1 ratio silicone, it was pretty easy to measure out the components and mix them. I used a 20mL syringe and calculated how much silicone I needed using the CAD model. I poured Part A into a mixing up, added the pigment, added Part B, then mixed until there were no streaks in the cup. Then, I mixed more to be absolutely certain the components blended into each other.

One part I had a problem with is the very center - since the sculpture has a hole in the center, the silicone fused to itself. This was expected and anticipated, but still not very fun to solve. I fixed this problem with the next iteration (short sculpture), but for now, I carefully used a hobby knife to cut apart the mold halves. The silicone was made thicker in the center to make sure it wouldn't tear or puncture.

For the hat, I first poured in the entire pre-measured amount of silicone needed (measured using the 3D model), and then inserted the floater with the spacer pre-attached. This is because it would be incredibly difficult to accurately pour silicone into a 3mm gap, and the features of the hat were simple enough that the silicone would easily flow around them.

Step 4: Casting and Demolding: Tall Sculpture

For the cement, make sure it's mixed well, with no dry cement or puddles of water left. Too much water will leave the resulting cast brittle, and too little water will make it hard to settle into the mold. When adding the cement to the mold, it's best to add it in small portions, and then tap the mold to distribute the cement evenly. The more you tap, the more it settles, and the less voids there will be in the resulting cast. Be aware that small bubbles trapped in the cement can rise to the surface and pop during the process, so wear proper PPE.

With both this and the hat for the short sculpture, I used spacers to create a conical divot. This was done so that it would not slide off and would self-center; experiment with different depths and widths to find out what's right for your project. The spacers are installed just after the pour and removed immediately after settling the cement - otherwise, they would get stuck in the resulting cast (determined empirically).

After casting the cement, be careful when demolding. Depending on your specific blend of cement or even the ratio of water to cement mixture, the cure time and final strength can be wildly different. Even factors such as ambient humidity and temperature can have an effect. These are covered exhaustively in other articles around the internet, so I won't be going into detail here; however, as little as 10% of difference in water content of my casts made the difference between the final result crumbling in my hands and being solid enough to handle. I gave all my attempts at least 8 hours to cure; the casts with higher water content were still noticeably damp after 16 hours and broke easily, while the ones with lower water content were much more solid and resistant to damage much earlier.

Step 5: Explanation of Features: Short Sculpture

To help with creating the mold for this sculpture, I added a flange to the center part to create two separate mold halves. This way, I wouldn't have to cut it apart or worry about the center tearing. Also, I got rid of the centering plugs in favor of using a square "knob" to hold the core in place, which is cut off on the final cement cast.

The lid mold has an additional ring to create the fillet on the edge; it had to be made in two parts since otherwise, it would not be possible to get the final cast out of the mold. As with the other hat, there is a spacer to create the divot.

Step 6: Pouring the Sleeve: Short Sculpture

To help with demolding these, I rinsed them with dish soap after casting. This could probably have been prevented with more mold release. The hat was particularly troublesome, since it was much deeper than the hat on the tall sculpture, and so gripped the rigid shell much more.

Step 7: Casting: Short Sculpture

Not much to comment on here, but you can see how the hat mold bolts together. I was quite pleased with how smooth the result was.

Step 8: Oops!

Here are some mistakes I made along the way, in hopes of helping someone else avoid them.

  • Follow proper procedures; I didn't - I neglected to use mold release a few times - and wasted time and materials.
  • Pick the appropriate material to cast with
  • Thicker silicone with a short pot life was not able to fill all the voids before it set
  • Some cement mixtures may not be suitable for this project (Mine worked perfectly - investigate yours well)
  • Use the right ratios when mixing your cement
  • Too little water leaves a chunky mixture which does not fill voids and does not settle
  • Too much water results in a brittle, powdery cast, which will break easily

Step 9: Final Result and Remarks for the Future

Here's the final result. There's improvements to be made, for sure, but a good start and well worth the effort.

In the future, when I scale these up, I'll be adding features to route wiring for lights through. It's possible at this scale, but could be hard to pull off without adding them post-casting.