Monday, March 8, 2010

Wires pt. 1: More Benchtop Experiments

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...


  1. Addendum: >2 N/mm pull strength on 0.3mm wire means the bond has a breaking strength of at least 2 MPa.

  2. This is incredibly interesting work! Congratulations!
    I have a few questions:
    1. Do you have any idea about the chemistry/physics at work that causes the metal-plastic bond?
    2. Does your printing algo cater for laying copper-wire outside of the X-Y-plane? What are your thoughts on laying wire in a 3D configuration.

    I am very impressed with your work, and follow your progress closely.

    Marius Botha
    Pretoria, South Africa

  3. Thanks!

    1. I don't really know much about the chemistry. But doing some background research, it looks like the strength of thermoplastic bonds like these depend on the temperature at which the bond is formed, and the materials involved (of course). Our temperature wasn't tightly controlled in this experiment. And it remains to be seen if the joint will work as well with ABS; however, we have no ABS so this is something someone else will probably have to try.

    2. We were originally going to try for XYZ wire printing, but deemed it too ambitious for the timeframe of our project (just one month remaining!). So XY it is. But using a similar setup, some XYZ parts could also be printed. It would require good software to keep the wires from interfering with things, a reverse-feeding wire head (ie, two mechanical pencils), and it would only be able to print one wire at a time that moves uniformly upward (like a coil, rather than a knot).

    Thanks again! I hope our next experiments turn out to be as successful as this one...