Friday, December 24, 2010
A proper conductive polymer mix?
In which your narrator begins to test a new conductive polymer mix.
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One of the long term grails of the reprap project is to be able to print circuitry. Currently, Rhys Jones, a PhD candidate working for Adrian Bowyer at the University of Bath, is devoting a lot of his time to addressing this issue.
Heretofore, we've looked at very low temperature eutectic alloys like Rose's and Wood's metal and confected methods of extruding it into preprinted channels. More recently, Rhys has demonstrated that ordinary solder can be successfully printed onto ABS.
We've looked at various concoctions of carbon, silver and other conductive materials with polymers at arm's length. Always, though, the conductivity of these mixes has been too low for them to be useful in printed circuits. Rhys, however, has looked at a two step process which may get us by that.
The main problem with printing PCBs with a reprap machine is that PCBs are not that hard or expensive to have made using conventional, proved materials. This makes coming up with a method of printing them a bit tricky because it not only has to work, but it also has to yield a product that is not all that much difficult to use than conventional boards.
Recently, I ran across a high-tech company, Integral Technologies, located in Bellingham, Washington. They've come up with a different approach to creating a conductive polymer mix which uses conductive fibers mixed with conventional polymers. Probably the most exciting is their mix of very fine stainless steel fibres mixed with a blend of polycarbonate and ABS.
The volume resistivity for this material on the data sheet runs to 0.06 Ohm-meter. This is far too high for a useful conductive plastic. I rather shrugged the material off until I got a look at a few non-technical demonstrations of the material. This one shows two small bars of the material used to conduct 110 v current running an electric drill.
Interestingly, Bill quoted me a price range of between $9-45/lb depending on the composition. More interestingly, the PC/ABS mixes which resemble what we are already used to using in Repraps are at the cheap end of the price spectrum.
As well, they demonstrate measured resistance across an extruded plate of their material using an analog multimeter.
There was obviously a mismatch between what their data sheets said and what the non-tech demos indicated. I explored this question with Bill Robinson at Integral. It boils down to the "skinning effect" mentioned in the first clip. Simply explained, it means that the metal fiber doesn't tend to show up at the surface of an extrusion. This means that the material is partially insulated at the surface of an extruded surface.
That left the issue of how, in the second demo clip, ordinary multimeter probes registered effectively zero resistance when measuring resistance across a extruded plate of their plastic.
Bill sent me some demo plates of the material a few days ago. I have a digital multimeter rather than the analog Radio Shack multimeter.
Multimeters are not particularly good at accurately measuring low resistances. Anything reading below about 5 ohms for most multimeters is not something you want to bet your life on.
I tried duplicating the measurements in the clip and kept running into skinning effects. In places I could get readings less than 1 ohm, but others I would be in the mega ohm range. I made a cut across one end of one of the samples. You could barely see the fiber ends. Integral is using VERY fine fibres.
I then drilled 1.5 mm holes through the plaques and put the probes through those. I was able the get more consistent and lower resistance measurements. Mechanical contact between the probes and the fibre ends is how good conductivity is achieved.
The best I was able to get consistently was 0.6-0.8 ohms over about 100 mm of the material.
Bill at Integral suggested that if we used a 0.5 - 1.0 mm extrusion nozzle we out to be able to avoid jamming of our extrusion nozzle with the stainless steel fibres. Making strip extrusion as we can with a Reprap printer should tend to align the fibres much better than a simple injection moulding would do. The problem as I see it will be making a conductive connection between two traces and making a connection between a trace and a component.
The question now is whether what we know about this material now justifies having a sample turned into fibre for further testing.