The SpoolHead concept relies on several untested ideas, both specific to our implementation, and also fundamental in concept. As it stands, these are:
1. The wire can be controllably fed by clicking a mechanical pencil
2. The solenoid we've chosen will be strong enough to click the pencil
3. The wire can be firmly bonded to the plastic part by melting the PLA surface and inserting it
4. Our heater design will be able to melt the PLA surface without contacting it
5. The wire can be cut by our rotating cutter without fouling the device
6. The solenoid we've chosen can actuate the cutter
7. The wire can be bent and dragged without serious difficulty
This is enough question marks to make any design engineer nervous. It all works just fine in my head, but will that match reality?
To get some certainty, we've decided to perform another series of benchtop experiments. The biggest assumption is number 3: The wire can be firmly bonded to the plastic part by melting the PLA surface and inserting it.
If this turns out not to be the case, then our whole design needs to be re-evaluated to make use of a new material, such as superglue or UV-curing resin. That would be a considerable setback.
Bing melted some PLA with a lighter and pushed in some 0.3mm magnet wire. It went in easily with little force, which was good (a mechanical pencil can hardly supply any, and even if it could, the wire might buckle if too much force is applied). It was pushed in to a depth of 3mm. We let it cool for 20 seconds, and gave the wire a tug - it didn't budge. That was a good sign: the quality of the bond between the PLA and wire surface - in this case, enamel insulation - appeared to be quite good.
To see just how good, we started hanging weights from the wire. At 590 grams, the wire snapped - but the bond held firm. That means that the bond was stronger than the wire, and held at least 2N/mm along its length (quite likely more).
We tested again with other wires: 0.25mm steel piano wire, and bare 0.1mm wire. These results were also both similarly successful, although the piano wire was stronger than its bond (unsurprising, given how strong that wire is). That bond also held over 2N/mm, failing when the whole assembly was jerked upward. The 0.1mm copper wire was a very good test, because its stiffness is extremely low (like a sewing thread); thus, the fact that it could also be push-inserted without buckling is very encouraging. The copper-PLA bond held tightly as well.
Why are we concerned about bond strength? Our printing algorithm is to click the wire forward until it has bonded to the surface at one point, let it cool, and then rely on the strength of that bond to feed the wire further as the print head moves in a straight line.
As a side note, the 20 second cooling time was probably longer than necessary; the bond already felt stiff to a gentle tug by 10 seconds.
Conclusion: Bonding wire to a heated thermoplastic surface appears to be a promising and viable method.
More testing to come...