Friday, December 24, 2010

A proper conductive polymer mix?

In which your narrator begins to test a new conductive polymer mix.

Do you want to read more?

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.

Sunday, December 19, 2010

Getting into threading...

When I bought my Sherline lathe, I went for the loaded option.  As I am finding out, it was a very good decision.  I don't think I'd have been able to use at all it without the digital readout box.  I understand how people in times pass could have used a micrometer to do measurements between cuts.  I doubt that I could have had the patience, though.

After my last attempt at doing a deep drilling and cutting down a piece of aluminum bar stock, the next thing I wanted to do was use the threading attachment.  Between Joe Martin's Sherline book, Tabletop machining and the Sherline Accessories Shop Guide I was able to puzzle out how to re-rig the lathe for threading.  My motive in getting a threading attachment was to have the possibility to cut my own lead screws and thrust collars.  The studding used in the Rapman and Mendel 3D printers, not to put too fine a point on it, suck the middle out of canned Vienna sausages.  When you buy studding it is rarely, if ever, straight.  When you try using it as a z-axis lead screw, it does not very pleasant things to your print quality.

As well, I am quite fond of the polymer pump design on the Rapman.  My primary reason for wanting to do my own threads was to be able to make the threaded drive shafts for those, too.

The biggest shock for me in this adventure happened when the instructions book told me to demount the lathe motor.  It turns out that you cut threads on the Sherline lathe with a hand-powered handle.  Using it reminds me of my grandmother's old foot pedal powered Singer sewing machine that I learned to use when I was about four.  It has a very similar, 19th century feel to it.

By and large the threading system design is quite good.  The real pain in the tail, however, is that little black lever that you see below the spindle in the above picture.  Getting that to work right had me taking the whole lathe apart about three times.  Sherline needs to rethink that part of the design.  It really sucks.

Other than that, the threading attachment was quite easy to use.  It is set up to do standard and metric 60 degree threads.  The carbide cutting tool did that quite easily.  

For a first try I did a really coarse metric thread, 2 mm.  

I figured that that deep a thread would require the most muscle.  It was a bit tiring but not at all impossible.

For lead screws, the Sherline will handle about 15 inches of threading.  If the diameter of the stock is less that about 9 mm, a longer unthreaded piece of stock can be snaked through the spindle.  Given that manufactured lead screws about cost $1/cm and thrust collars typically cost $30-35, this capability will save me some money over the mid-term.  

Monday, December 13, 2010

First lathe experiments

I finally got a few days off, so this morning I began to do things with my new lathe.

My initial motive in getting the lathe was to have the capacity to make my own extruder hot ends.  Given my total lack of experience with lathes, I decided to see if I could try sinking an 1/8 bore in a piece of aluminum round stock using a fixed drill bit in a chuck inserted into the tailstock.  An idea of what I did can be seen here.

After a considerable time, I was able to sink a 50 mm hole into the centre of the round stock.  The steady rest that you see supporting the end of the aluminum round stock nearest the drill bit reduces vibration and misalignment.

When I worked with Tommelise 1.0, I used a 5/32 inch braised copper tube (1/8 inch ID) hot end several inches long with its top only heated using nichrome wire.  I was able to get well past the whole issue of having filament melt in the PTFE thermal break with that approach.  These days, I hope to do something similar using aluminum.

Here you see the drilled round stock after I've trimmed it down to 12 mm outside diameter and trued up the end.  It takes a nice finish.