Hey guys
I'm doing my masters and looking at a method of extrusion along 3-dimensional paths. What it involves is extruding a plastic through a nozzle (of say 5 mm dia.) along 3-dimensional paths without requiring any support material. So it's kind of like a big version of a 3D pen, with possible inclusion of reinforcement fibers at a later stage. At this stage I need to decide on a suitable material and design an extruder.

The material needs to set as it comes out of the nozzle so that it doesn't collapse. The extrusion process should also be able to be stopped momentarily to allow movement of the nozzle to another location without extruding and without the plastic/resin setting in the nozzle. Finally, the material must stick well (really well) to itself, so that extruded segments can be joined with other extruded segments.

The setting condition rules out a thermoplastic, since a 5 mm diameter extrusion is not going to cool quickly enough right outside a hot nozzle. That leaves a two-part thermoset and a photo-polymer as possibilities for a material. I may have missed some other possibilities . . .
The extrusion stoppage condition may make it difficult to use a 2-part thermoset with a static mixer, since the stuff would tend to set in the nozzle the moment extrusion stops if the 'setting point' is normally just outside the nozzle. However, this may not be an unsolvable issue, in which case a 2-part thermoset whose components have a low enough viscosity to push through a nozzle and a high enough viscosity so that the mixed parts don't run everywhere before setting could be a solution. 
A photo-polymer such as is used in SLA 3D printing could work, if the resin is light-cured just outside the nozzle. The resins used in SLS printing are diluted using monomers and typically have a fairly low viscosity which may not work too well for extrusion though. 

So I would really appreciate any advice on what materials might work for this application.

Cheers