Friday, February 29, 2008


UV plant-derived resins for RepRap

Inspired by Fernando Muñiz over on the RepRap Builder's blog and by meeting Norman Frost of Sustainable Composites Ltd. (who kindly donated a sample of their UV-cure resin to me) I have been experimenting with thermosets. (There is plenty of time between my tweaking the parameters of my new Darwin as it makes a splodge of polycaprolactone almost resembling a cube.)

The idea is to mix the resin with a glass filler like this stuff from Tomps to make a paste, and then put that in the paste extruder with a ring of UV LEDs round its nozzle to set the stuff after it's been laid down.

Norman said that the ideal cure wavelength is 365nm.

The first thing I discovered is that cheapo vanilla 400nm LEDs won't touch the stuff. It just sits there and stays sticky for hours. So then I got some NSHU550A LEDs that emit at 370nm. The picture above shows one of these zapping a drop of resin about 5mm underneath it using a forward-bias current of 20mA. That forms a thick skin at 10 minutes, and is solid to the touch in 15.

Now. All we need is for each layer to be mechanically rigid enough to support the next, so that should be fine. And, of course, layers underneath will be further hardened as the layers above are laid down, because the UV will percolate down through the resin and the glass.

As you know, we've been working on using polylactic acid in RepRap because it's a plant-source thermoplastic that can be home-made, and when it's finished with it can be locally composted. Widespread use would take CO2 out of the atmosphere then put it back, and so would be carbon neutral.

But Sustainable Composites' thermosets are plant-derived too. So using that resin to make stuff that gets thrown in landfill rather than recycled would be even better. Thermosets are very stable underground (think amber), so widespread takeup of them in replicating RepRaps would be carbon-positive...

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That rocks, Adrian. I've seen small UV lamps (fluorescent) for use by geologists for IDing rocks. Will try to find the frequency but that might pump out more photons for faster curing.
I've got some dental resins. Want me to send some?

Vik :v)
I hear these things are more efficient when driven with a small flyback transformer, rather than just a resistor in series. What is your driving circuit like?
Homebrew thermosets, eh?

In that line of inquiry: I wonder if it would be possible to manufacture glass microballoons at home. They're made from water glass, which, so I've heard, is fairly easy to make using sand and lye...
> might pump out more photons for faster
> curing

Yes - the LEDs just about hack it, but something with a little shorter wavelength and more watts would definitely be better. A small 12v UV source would be perfect.

Vik - yes please to the dental resins; or do you want to try them through your paste extruder? (P.S. I'm about to do an enhancement to the design that should make it much easier to load and pressurise - you may want to wait for that.)

Joel - I just stuck in a series resistor and an ammeter, and turned the volts up till it said 20mA :-)

And home brew glass microballoons would be brilliant, or even just home brew glass powder. There are some people who make self-healing thermosets with microballoons filled with liquid resin - when a propagating crack breaks a balloon, the resin leaks out into the crack, sets when it contacts the catalyst that set the original resin and that's still in there, and repairs it...
The extra efficiency you can get with an inductor instead of a resistor means you waste less power rather than getting more light. It's only really worth doing for battery powered equipment like torches or very high power LEDs, or large numbers of LEDs.
Can we use this to solve the 'filler material' problem? What happens if you turn off the UV lights? Would the material stay stiff enough to support the next layer until it sets? Can we focus the UV accurately enough to selectively cure the material?

With this kind of material could we go the direction of the 3D printers that use a tank of laser-sensitive materials - progressively lowering the work platform and curing just the areas that need it?

Are there UV lasers that are cheap/powerful enough to do that? Perhaps the laser in a blu-ray player would do the job (those emit 405 nm which is right on the edge of the ultraviolet) and whilst they are quite expensive right now, by the time RepRap gets going, they'll be dirt cheap and any old discarded blu-ray player or PS-3 would contain one that could be recycled. A blu-ray WRITER would be much more powerful than a reader.

Furthermore - for small parts - you might even be able to build a RepRap using the parts from the BluRay drive itself - the head stepper motor could be used to step the laser in and out (radially) and the main drive motor could simultaneously spin the platform and drive it down into the goop using a finely threaded rod.

This is all wild speculation of course - I have no idea whether 405nm light will harden the material.
Oh - and if you really are working in the UV - you're going to have to consider eye protection while you're working.
Nichia are supposed to make a 250mw, 365nm device as well, the NCSU033A.

I can't find a price though.

...has UV LED flashlights at around $26 containing one LED or more upmarket UV flashlights for $57 with 5 UV LED's in I'm guessing you should be able to pick up the LED's for $10 or so.
The real efficiency gain I saw included both a capacitor and an inductor. They report:

> If the brightness of the LED is equal to a DC voltage of 4v and a current of 10mA, the circuit we have produced is slightly more efficient than delivering a DC voltage to the LED, even though there are some losses in the transformer and transistor.
This proves the fact that LEDs driven with a pulse, are more efficient than being driven by a DC supply.

Details here.

As to UV solid-state lasers, they'll be expensive for the foreseeable future, since they're the part that wears out the fastest. They're very inefficient compared to red lasers, which means they run much hotter; that, and their more-delicate constitution means that they bake themselves to death before the game console becomes obsolete due to software considerations.

Oh, and GaN epitaxy isn't getting much easier, either.

Your cheapest solution might be frequency-tripled Nd:YAG, diode-pumped. This is similar to the green laser pointers you can buy, except with a somewhat more-difficult and less-efficient process of wavelength conversion.

If your pump diodes are coupled to fibers, the power electronics can all be on large, well-ventilated boards, with only an optical connection to the write head.
Those reports are true when they say that pulsed LED's are more efficient than constantly driven ones, but don't forget that they assume the *human eye* is the intended recipient of the output!

A chemical reaction doesn't care about persistence of vision, just about how many photons it can soak up. So if you drop the duty cycle of the light source, your reaction rate is going to drop by a similar amount.
I thought the efficiency gain came from having the majority of current flow at the minimum device temperature. This would mean that a given diode could put out more photons per second at a given average temperature.

The reaction kinetics aren't that fast, are they? I don't believe that pulsing would slow the chemistry; the extent of curing should just depend on integrated intensity.
Sorry to double-post, but I just re-read perlrun's comment, and would like to clarify:

I don't think the average current should be chopped down to a lower duty cycle. Rather, I suggest we bunch it up into extremely short pulses, so that, over all, more light is produced from the maximum average current that the LED is rated for.
Here's a place with a load of different UV devices, prices range between 999$ for complete large devices to 11.95$ for single bulbs. Since this will be custom mounted one of the small "U" shaped bulbs would probably be best, wiring up a high-frequency supply should be a snap for some of our communities brilliant electronics engineers. Operating frequency: 351 nm. should work great at 15w.
Oooh, I've thought of an even simpler UV source, TEA lasers:
and here's a Maker page:

I figure one of these could be miniaturized and driven by a flyback coil and a 555 oscillator circuit. Figure total size of 1x4x9 inches. Capacitors would be external of this if needed. Cheap.

Nice thing about this is no need for vacuum, advanced epitaxy or toxic chemicals (ignoring the minor amount of ozone created) plus they look really easy to make.
> Can we use this to solve the 'filler
> material' problem?

I think the paste extruder that I want to use for this will solve that problem, but not using UV resins. Best bet IMHO would be to run a water-soluble paste through it for that.
Just how finely can we focus one of these things, and how thin a layer of material can be reliably laid down prior to setting?

I mention this because presumably as the resolution of the device increases, the radiation is focused into a smaller area and hence it might be possible to get away with lower power UV sources - in this case it might actually be easier to try building a high resolution device straight off rather than make increments starting with chunky, low resolution devices.

I wonder if geometrically curved mirrors might be a good way to approach optical curing - I reckon that the resolution we can get with the current generation of FDM reprap hardware would be just right to construct fairly accurate conic section forms for a mirror of the right size to go around a normal sized lightbulb - the steps could be smoothed with some kind of filler before a flexible surface could be bedded down onto it, perhaps polished aluminium foil at a pinch.

Just thinking aloud (or, rather, into the keyboard) - anyone think there's anything good in my ramblings?
the guy:

TEA lasers sound ideal. I'm glad to learn of their existence. They sound difficult to maintain, but hopefully some miniaturization would help.


generating light close to the target allows for much finer focus spots. It will also be limited by the optics inside the device; for LEDs, the optics might be decent or very poor depending on what they were manufactured for. Building one's own oscillator, as for a TEA laser, would require impeccable optics, and so would allow exquisitely fine focus (and, as a tradeoff, a complete lack of function as soon as one mirror is bumped a little...)
How about UV lighting from below?
Aoneled lighting is one of professional LED lighting manufacture in Shenzhen, China.Such as,LED light bulbs,spotlight, GU10,MR16,E27,LED fluorescent light,LED Downlight,LED Wal washer light,Strip light,Rope light and LED RGB controller...
Can i get some more info on this?

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