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Posted by johnrpm 
October 22, 2009 09:51AM
It occurred to me that if the molten polymer coming out of the nozzle could be charged at 100volts as it cools it would produce parts that have piezoelectric characteristics, so that say a bracket made on the reprap could "with suitable electronics" measure forces etc, I would imagine the nozzle having the electrodes touching the filament and as the filament cools reasonably quick may hold its polarity.I do not have a clue if this has been tried or not so apologies if it has.
Re: piezo
October 22, 2009 10:05AM
Interresting idea. I can see that it could trap a charge making an electret, but how does that relate to the piezoelectric effect?

Re: piezo
October 22, 2009 10:56AM
The electret may be useful in printing capacitors maybe, but I was thinking that if the filament holds its polarity as it cools then bending the finished part would produce an electric charge, ???????, the way its done for polymers I understand is that the material is heated to a certain state, in our case molten polymer, a charge of about 100 volts is applied whilst it cools, this polarizes the material, silver loaded paint is dabbed onto the part for wire contacts, but with the reprap, instead of heating the whole part, the nozzle contains the charge electrodes so polarization is across the filament, different polymers would be more suitable than others.


P.S this is just a theory, I may be talking rubbish,
Re: piezo
October 24, 2009 05:08AM
nophead is talking about bulk electrets, as might be found in a copy-machine or some types of microphone

qwartz exhibits piezoelectricity, shrinking or expanding with applied electric fields. But I don't know that thermoplastics do that, and you're talking about an induced static charge, not a varying applied field, so the thing would just sit there, being slightly charged, doing naught.

Fun idea, though. You may find it profitable to review your mid-level E&M with the Feynman Lectures or Griffiths on E&M.
Re: piezo
October 24, 2009 11:28AM
Yes, there are piezoelectric polymers.

Google says, " Results 1 - 10 of about 1,670,000 for piezoelectric polymer"
Apparently Polyvinylidene fluoride (PVDF) is one such, see: [en.wikipedia.org]
From what I recall, they have a strong piezo effect, and can be used both as sensors and actuators. The wikipedia article says:

"In 1969, the strong piezoelectricity of PVDF was observed by Kawai et al. The piezoelectric coefficient of poled thin films of the material were reported to be as large as 6-7 pCN-1: 10 times larger than that observed in any other polymer."

Interesting, one trade name for this is Kynar; from that I surmise that the insulation from wire-wrap wire (any fellow fossils remember that?) might be used to experiment with, in small quantities. The processing to poll (impart piezo properties) to the polymer are non-trivial, but reprappers are a clever bunch, so it might be doable. I recently scored a bunch of old wire-wrap wire. If you'd like some to experiment with, please send me a private message (and be prepared to re-imburse for postage.)

Larry Pfeffer,

My blog about building repstrap Cerberus:
Re: piezo
October 26, 2009 04:13AM
If as you say the "non trivial problems" are solved it has big potential in making embedded sensors and actuators, imagine a bracket that can measure the forces on it, or a 3d lattice that could twist or bend according to the energizing pattern?.
I think many materials exhibit piezo effects to a different degree, whale bone being one of them.
Re: piezo
October 26, 2009 07:49AM
johnrpm Wrote:
... imagine a bracket that can
> measure the forces on it, or a 3d lattice that
> could twist or bend according to the energizing
> pattern?

The electrical properties of piezo materials (capacitive + slow leakage resistance) means that they can't really be used as sensors for steady (DC) forces, after a couple time constants, the piezo-induced charge discharges through the leakage, and the signal is gone. They can be used as DC actuators (if ones drive circuitry keeps the charge up), although the performance isn't too hot. Piezo sensors/actuators are usually employed to vibrate or sense vibration. There may be reprap applications for Non-DC force or displacement sensors. I'll have to ponder what....

Larry Pfeffer,

My blog about building repstrap Cerberus:
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