(And a few minor setbacks.)
Tests of our first prototype revealed some problems. Mainly, we haven't been able to heat the plastic effectively by convection / radiation alone. We tried making contact with the PLA surface, but this didn't work either: If we let the heater cool and then try to lift it, it stuck to the PLA. If we lifted it before it cooled, the wire pulled out from the still-hot plastic. The heat didn't diffuse very far into the PLA.
The ineffectiveness of the first extruder at heating the plastic lead us to machine some new ones to replace it:
Also, our 0.3mm pencil has proven itself very unreliable at feeding the wire; clicking the pencil doesn't always result in advancing the wire. It could be a problem with the wire, or the pencil itself. Fortunately 0.5mm pencils have proven much more effective, but they are more difficult to click - our solenoid will certainly not have the power. This should be solvable with some re-engineering.
So, heat transfer first. I machined two more extruder tip designs. We've abandoned the flange for now, and are using a narrow tip instead. The tricky part is to try to get more heat going down to the PLA than moving up along the steel tube. Thinking about the heat transfer situation we're facing led to this second design, which uses a flared conical tip. Thermal conductance is proportional to cross-sectional area, so the conductance gradually increases along the length of the cone toward the bottom. To do this properly I'd like to do a pen-and-paper calculation, combined with finite element modelling in SolidWorks. But for now I'm just going on intuition. To put it in the language of circuits, the heater tip is like a current divider. I could make a much more complicated and accurate model, but this one illustrates the theory very well:
The PLA surface is assumed to be a heat sink, as is the stainless tube length above the heater. Heat will tend to flow both up and down from the nichrome wire, but we can skew things to make it prefer to flow downward by having good thermal contact to the PLA and having a high thermal resistance going up. I hope that the cone's bottleneck will act as a thermal resistor to keep heat moving downward. Ideally I'd make the bottleneck much longer, but we'll start with this and see if it works before we move to more fragile designs.
So our other new tip uses the same cone design, but the cone is made of alumium and screws onto the stainless shaft. This is done for the same reason; aluminum's thermal conductivity is about 15 times better than stainless, so now the stainless itself should have a high thermal resistance compared with the aluminum path to PLA. Aluminum has the advantage of being easy and quick to machine compared with stainless, but I'm apprehensive about making thin structures from it because it's pretty weak. Also, its high thermal conductivity downward comes at the cost of high thermal conductivity upward. I'm not sure the cone will make much difference here; without a doubt a lot of heat will flow up the aluminum. The thermal contact between the aluminum and stainless is quite poor, but the aluminum piece might be large enough to act as its own heat sink anyway:
So I tried to make it as thin as I could. We'll see. Sometimes it's quicker to just do the experiment than to over-analyze these things.
Mo used a screw jack to manually simulate the RepRap's Z-bed, and mounted the heater nozzle on a clamp. Bing did it up with nichrome and fibreglass just like the real thing, so this test would be more authentic (no more bic lighters). We started with the all-stainless nozzle. Again convection didn't seem to be enough to heat the surface, but when we made contact, the heat penetrated very deep into the PLA. And so did the wire. Penetrating a few millimetres means we can remove the tip while it's still warm, because the wire won't pull out.
(Sincere apologies to SparkFun Electronics for using their logo as a test piece).
Here's a wire bonded this way, by our tip, to a coat hook.
The wire here is 24-gauge (0.5mm) tinned copper, from McMaster-Carr. It fits beautifully in a cheap 0.5mm pencil, but the best thing is that it's not insulated like magnet wire, so it might actually be useful. The downside is that it's quite stiff, which might make it hard to print with. Our impression is that if we bond it at regular intervals and when going around bends, we shouldn't have too many problems.
The aluminum tip will be tested next.
These tips don't yet have cross-drilled holes for the rotating cutter. But with Bing's observation that the cutter seems to act as a powerful heat sink, it might be worth considering a back-up plan. I'm thinking of using the solenoid directly; solenoid bars come with holes drilled in them already.
We'd lose the mechanical advantage, but when it comes to solenoids I'm not sure that's such a bad thing. A solenoid ideally has a force proportional to the inverse square of the pull distance (although for very small pull distances, magnetic saturation makes it more linear). With a mechanical advantage of 2, we'd amplify our force by two but need to pull over twice the distance, so at the far extent the force the solenoid can provide will drop by a factor of four. It's hard to tell at this point whether it's beneficial or not, because the strongest force is really needed right at the end of the pull, when the wire gets cut, where the mechanical advantage and solenoid non-linearity work together to provide a strong force.
So it could go either way. At least it's worth keeping this alternative in mind. It is, after all, a fair bit easier to build.