Mini electric furnace

This is my new-new-idea for the final project :)
The old pages are here and here but they will no longer be updated.
Those projects were going to require a huge effort but sadly the time i can spend on FabAcademy isn't enough.
I was also thinking about a scaled down hovercraft but maybe it's and even crazier and more challenging idea.

In a fablab there is everything you need to make a good sand mold for metal casting but it's very uncommon to find a furnace, here at Crunchlab i'd like to be able to melt and cast aluminum at least.
A small furnace can be also very useful for earthenware production!

I'm planning to use refractory cement for the main chamber, 2 khantal alloy heating elements and a circuit board that regulates the temperature with a PID cycle, reading temperature from a k-type thermocouple and enabling the heating coils with solid state relays.

An electric furnace from youtube.

I don't have too much experience with refractory materials so i plan to start with a bare minimum small furnace for aluminum and then, using spiral model, increase the size, reach temperature high enough for steel and find cheaper contruction materials.

Main chamber cast

I used refractory cement rated at 1250K to build the main chamber, it's enough for aluminum melting but it's very expensive and for steel i will need to find other refractory materials.
To lower the cost of the cast i added rockwool flakes (rated at 1650K), they are also useful to reduce the refractory cement tendencies to crack cycle after cycle.

My original idea was to build an external "shell" out of standard portland cement and insulate it with "kaowool" (ceramic high temperature insulation wool) but it turned out that ceramic fibers are even worse than rockwool to breathe and VERY expensive.

For the next iteration of this project i plan to build silicone molds to make "home made" refrectory bricks out of fire clay and aluminum oxide.

More defails about the main chamber here.

Heating element

Usually little funaces are usually based on gas burners or charcoal and air blowers, but thay are too dangerous for indoor use and not very precise on temperature control.
Kanthal (FeCrAl) coils are very easy to use, just calculate the power rating with the standard Ohm's law and connect them to the mains. However their alloy melts at about 1675K so for future revision i'm planning to switch to electric arc carbon rods.

Temperature control