The goal of this instructable is to make a full size 12" diameter cake mold, with a custom insert of a nyan-cat.
Nyan-cat was popularized in early april 2011, and features a kitten with a poptart stomach (or tummy, for short)
Further details on the glory that is nyancat are detailed here: http://en.wikipedia.org/wiki/Nyan_Cat
To create a cast cake mold, using the lost wax method, a wax positive of the required mold needed to be created. As a 12" diameter cake pan is too large to 3d print, and fairly complex to mill, a new approach was investigated: laminated lasercut LDPE,
Step 1: The Design & Lasercut Phase
Using CAD software, a 3D model of the mold was generated. With a full size model, a number of 1/8 inch thick slices were made and exported as drawing files to be read by an epilog laser cutter.
With the files <included> a number of 12" square plastic sheets could be lasercut. By stacking these sheets together, a full size model could be quickly re-built and used as a positive mold. For this mold, a lightweight, low cost, low melting temperature, lasercutttable plastic was chosen. LDPE fit the bill for these requirements, it even has the texture and flexibility of hot glue!
LDPE sticks together fairly well, with a hot-glue gun. Each layer was hot-glued together, while ensuring that edges and features had the lions share of the bonding. By gluing around the edges, it prevented the mold from having gaps which result in a poor final finish.
Step 2: Plaster Casting
With the lasercut positive mold assembled, insert it into your springform cake pan of choice. The cake pan used in this instructable is a heavily modified deep frying pan, to be used as a springform pan. The modifications include bandsawing the pan in half and adding steel hinges and latches to the sides for easy cake removal.
After inserting the mold into the pan, use masking tape and aluminum foil to seal the insert to the walls of the pan. This seals the insert, and prevents plaster from seeping down between the cake pan and the insert. For this experiment, a simple mix of plaster of paris [link], mixed 1:1 with fishtank sand [link] was used. Note, follow the directions on the plaster container, the consistancy of the plaster should be akin to a goopy pancake mixture.
When the mixture is ready, pour into the (now sealed) cake pan
Step 3: Drying the Cake Mold and De-LDPE-ing
The plaster will take at least 2 days to start to 'dry', after which, removing the LDPE sheets can begin!
Using an x-acto (or equivalent) knife, slowly remove each layer of LDPE. Note, using a heatgun [link] makes removing the LDPE a lot easier. Being meticulous and slow is better than rage-cutting, as the plaster is fairly delecate. Over the course of two hours, a negative mold of the nyan-cake-mold emerges!
Prepare for molten aluminum covering the entirety of the mold by wrapping it with spring steel and holding it together with steel rivets. Use some more plaster at the interface between the spring steel and the mold, and smooth it down so that the aluminum doesn't pour down the sides; similar to what was done before in order to keep the plaster for flowing down the sides of the pan.
Finally, once the LDPE has been removed from the plaster (which still contains quite a bit of water) and the sides sealed in by the spring steel walls, it's time to bake the mold!
Step 4: Aluminum Casting Prepwork
While waiting for the plaster to dry, why not make a furnace?
A furnace is a device used for heating, and for ours, the heat energy is provided by charcoal. Combustion of the charcoal takes place within the main chamber, made of concrete, into which air is forced in from the side to create a vortex within the chamber. The forced air provides oxygen, the limiting reagent, and allows the charcoal to burn much hotter than it usually would; hot enough to melt aluminum.
Pick up 40lbs of quick-set concrete mix, a large plastic storage tub, a short (~5") length of steel pipe, duct tape, a ducted fan, and a concrete tube at your local hardware store.
Begin furnace-making by mixing a small amount of concrete and creating a flat layer of concrete (1-2") at the bottom of the tub for the base of the furnace. Allow this to set (doesn't have to be completely dry) before continuing on. This forms the base of the forge.
While waiting for the concrete to set, take the concrete tube and cut a hole into the side, about 2 inches above the bottom. Cut the hole so that the steel pipe fits into the tube at a slant. This is in order to create a vortex when air is forced into the forge using the fan. Once the hole in the tube has been cut, set the tube into the tub. Cut a hole through the tub as well so that the steel pipe protrudes slightly into the tube and out of the tub. The inside of the concrete tube becomes the chamber in which combustion/heating occurs.
Optionally, add handles to the furnace by passing more steel pipe through the tub near the top. Carrying 40lbs of cement and steel can be a pain without handles.
Seal all edges around the cut holes, along with the ends of the pipe, with duct tape in order to keep the cement from leaking out.
Mix the rest of the cement and pour into the tub, outside of the concrete tube. Allow the furnace to dry for a few days. Note, this step cant be skipped, a damp cement furnace can and will spontaneously chip apart and throw projectile cement flakes as it heats up!
The fan for forced air will be attached to using flexible piping to the steel pipe sticking out of the bottom of the furnace. Secure the piping to the fan using a large gauge wire as a twist-tie. Find a reliable power source for the fan.
Find around 10lbs of scrap aluminum and cut into 1.5-2" chunks for melting. We were able to get the scrap aluminum from a waterjet shop, and used a bandsaw/hacksaw to break the parts into smaller, higher-surface-area chunks.
Afterwards, remove the plastic tub, duct tape, and concrete tube from the now-dry concrete furnace and get ready for some casting!
Step 5: Baking and Casting
Obtain lighter fluid and a large amount of charcoal, probably around 40-50 lbs. Be sure to refill the charcoal in the forge during casting as needed.
Have the furnace set up outside on a non-flammable surface, ideally a steel sheet or concrete driveway. In our case, it was the shelf of a steel shelving assembly. Note, spraying down the work area (back yard) with water helped prevent any flying ash from causing problems, as always be aware of your work environment! Have any surrounding lawn or flammable areas well-dampened using copious buckets of water. Attach the ducted fan assembly to the furnace and hook it up to a variable power source.
Pour charcoal into the center of the furnace to about 3/4 of the way full. Then light it, turn the ducted fan on a low-power setting, and place the still-damp plaster mold onto the furnace. Allow plenty of time for the plaster to dry out, watching for water evaporating from the plaster, and adjusting the heat accordingly.
Once water has stopped evaporating out of the plaster mold, keep it toasty by storing it with live coals in a fire proof container such as an old charcoal grill. Ideally, when pouring molten aluminum into a mold, the mold will be around the same temperature in order to allow the aluminum to settle into the mold slowly. Be sure to cut the power to the forge before taking the mold off, and wear protective clothing and eyewear.
Next, take a few of the smaller chunks of aluminum and put them in the crucible (in our case, a steel milk jug with a cast iron pot lid), and set it all on the forge before lighting it again, this time at full power. We recommend using smaller chunks of aluminum at first because of higher surface area, making it easier to melt down.
Wait for the aluminum chunks to begin melting down, adding charcoal to the forge as needed. When manipulating the steel jug or lid, use appropriate hot working gear and steel tongs where appropriate. When the smaller chunks begin melting down to a soupy consistency, begin adding larger ones, a few at a time. Once all the aluminum chunks are melted, pour the aluminum into the plaster mold. Be sure to wear a respirator for the process, as the fumes are toxic.
Allow the now-smoldering mold to cool for at least a few hours.
<update> now with video of casting process
Step 6: Finishing Passes
Now that the molten aluminum has set, it's time to remove the plaster.
As the plaster is fairly dry and brittle, it is easily chiseled away with a screwdriver and mallet.
Following this process, the faces of the insert can be milled down for increased smoothness.
Once the insert has been finished, it can be placed into the pan assembly, and then presented to the rather startled receiver or used immediately to bake nyan-cakes.
Many thanks to the wonderful land of MITERS, where plaster and de-ldpe'ing occurred.