Measuring the resistance of a short length of conducting

filament extruded from a standard hot end. The 1.75mm filament

used as input was made by rolling and can also be seen.

With the increasing number of multi-colour RepRaps out there (Ahem! Including ours at RepRapPro Ltd...), it will also be possible to use them to print in mixtures of materials with different engineering properties.We have been experimenting with putting conducting particles in printing plastic, inspired by this paper:They used carbon as a filler, which works nicely, but gives rather high resistances. So we tried silver instead (expensive...) to get the resistance down. This didn't work very well because of the shape of the silver particles.The silver particles are flakes, and the carbon particles are fluffy and dendritic. We hypothesised that the shape of the carbon allows lots of particles to touch each other (or at least to conduct between them synapse-like by quantum tunneling), but gives the high resistance. So a mixture of the carbon to get the geometry of the particles right and silver to introduce short circuits between the carbon projections might work even better.There was also a problem with the silver when it came to contact resistance at the surface. It may be that the spiky carbon tends to stick out of the surface, whereas the flatter silver ends up with a plastic film over it.It seemed like mixing different conducting fillers may be the way to go. So we have tried experiments with a mixture of both silver and carbon, which seem to work better in combination than either on their own.Full details can be downloaded from the RepRapPro Github repository here Finally, another possibility is mixing a magnetic powder into the filament, for which we don't care whether the particles touch or not, of course...