Alongside the main RepRap build, I have been interested in the idea of printing with food - specifically chocolate. Fab@Home have had reasonable success printing with chocolate and sugar pastes, and I thought it would be interesting to have a go and see what I could build.

There has been a lot of forum discussion around a 'granule' extruder, and how this might work. I went for a slightly simpler approach - a small hopper that would contain low-temp melt, and manually refill if required.

Chocolate seemed like a good choice - reasonably cheap, low melting point, fairly liquid in melted state, tasty and fun.

Design

Construction

I spent several weeks scavenging and finding parts that might come in handy. DIY and plumbing shops provided several useful plumbing parts. One initial problem was how to build a good screw thread. I cheated and bought a pack of Auger drill bits from the local market stall. One of them was 14mm which *exactly* matches the internal diameter of UK copper water pipes.
Other useful parts:

• 14mm Auger drill
  
• Short section of 15mm copper pipe
• Tank Flange (used to attach a pipe to a tank)
• Compression end cap
• Fire Cement, Nichrome wire, and thermistor left over from my RepRap building
• Silicone Trivet (pan stand) 
• Non-stick pudding bowl
• Two old PC slot covers (handy small chunks of metal)
• 0.6mm MigMate Welding tips from Halfords (5 for 4.99)
• Tesco Value rechargeable screwdriver (3.6v), £4.98.

I cut a short (100mm-ish) section of copper pipe. The flange has a compression fitting which attaches to one end, and the pipe end to the other. I then constructed a fire-cement heating element around the pipe (details on the BitsFromBytes video ) - basically it's a nichrome wire wound round a firecement base, covered in fire cement. I also inserted a thermistor for temp monitoring.

I cut a section of Alminium from a Coke can, and wound it round the fire cement assembly to provide some structural support. A chunk of the silicone sheet , with a couple of holes, makes up the outer layer of insulation.

The pipe end has an M5 hole drilled and tapped into the end. A drilled-out welding tip then screws nicely into the end to provide a 0.6mm nozzle.

I cut a 1cm section of pipe and cut along the length, so that I could overlap it a bit so that it fits inside the heater copper pipe. The end of the nozzle is several mm into the pipe, so if the drill is  inserted, it butts straight onto the nozzle and blocks the hole. The 1cm spacer gives a gap so that the nozzle entrance is not blocked, and a couple of M5 washers provide a bearing surface around the welding tip.

The PC slot covers provide a couple of wobbly supports to keep the drill from moving up, and a hole provides access for the screwdriver blade.

Testing - v0.01 - chocolate - score (8/10)

Takes a while to warm up to 60 degrees C : about 5 minutes.
Filled with chocolate - melted quickly
WORKS!

No motor : chocolate slowly forms a drip (5-10 sec per drip)
Auger drive down: thin stream of chocolate extruded (several mm/sec) - was able to draw lines by hand on a plate
Auger drive up: chocolate drip is actually sucked back up the nozzle! - oozebane would definitely help.

Initial testing - v0.01 - water - score (1/10)

Once built, I tried it out for leaks by filling it with water - good, it is watertight - but turning the auger did not seem to produce any real extra flow or pressure. I suspect the screw pitch is also far too large to drive the water down and any pressure can push back up the thread. Molten chocolate should be more viscous, but I suspect it won't hold up well.

Problems:

• Not enough pressure to either start or stop water flow
• Tool steel rusts in water. Lovely rusty stains, although they wipe off easily. Drill needs to be sealed in some way for actual food use. 
• Motor bracket extremely wobbly - although it works.