Tuesday, July 03, 2007
Round two: UVC versus photoinitiated resins
The Philips TUV-36W wich delivers a good 12 Watts of UVC power at around 253nm, which is Benzophenone's excitation peak and also Mechelers Ketone's secondary excitation peak.
This tube costs a hefty 50 euros, because it's glass walls are made of pure SiO2, to let through UVC radiation, when normal glass would block it.
I have also purchased a Black light tube of 40Wats (PHLF40BL) delivering 13Watts at the UVB frequency of Mechlers Ketone's main peak, at around 360nm.
Results are: UVC is a LOT more effective than UVB!
I divided the hardening time by 3 using Benzophenone and the UVC lamp. This configuration has proven to have a much higher efficiency.
Benzophenone is also cheaper than Mechelers Ketone and safer to handle, which compensates for the higher price of the lamp.
Another advantage of Benzophenone is that it dissolves a lot better in the resins I have tried. Actually, I have not reached saturation levels yet with 6 weight % BP, where as in polyester at 3w% for Mechelers I'm already close to the limit. Equally, Mechelers seems to have very little solubility in acrylic resins, which is a very negative point.
Things to be careful about when using UVC: Don't be around when that thing is on. UVC is a really energetic radiation and it will eventually burn your skin and retina. It's one of the radiations coming from arc-welding (although less intense). It also leaves a distinctive smell of Ozone in the room, like in a printer or photocopy room.
Ventilate the room after or during the operation as Ozone is not to healthy. To filter UVC, you can use regular glass panes, but not plexyglass.
To attack the problem of oxygen scavenging I was seeing with previous UV curing (in which air-oxygen solved in the resin impedes the catalyzer to do its work) I decided to try an acrylic varnish that you get from art-shops. This varnish dries on contact with air, so that I expected the combination of this varnish with my photoinitiators to improve setting times.
I was correct! Another 4 fold improvement in setting times. I goth this one to set and harden in a little over 10 minutes, with half the photoinitiator content I'm using for my regular resin.
The resulting hardened film (around 0,5mm thick) was not very tough and had yellowed a bit, as this was a surface varnish, not really thought for thick sections, but i guess that a very attractive research path would be to find out what commercially existing air-drying acrylic or alkydic varnishes could be used for our Reprap needs...
Anyway, this week I'm receiving the Diethanolamine I ordered 10 days ago(prevents oxygen scavenging) as well as Titanium oxide which will be a very useful additive to my resin mix as it should accelerate the setting as well as increase the depth of curing in the resin films.
I'm also getting copper powder which I will use with the acrylic varnish to create conducting polymer films...
It will be an interesting weekend, a lot to do!!
Personally, I'd rather work with a "dry" machine, because I think it'd be less prone to being involved in messes, particularly accidental spills. That said, if I had a choice between .5mm feature size with thermoplastic, or <.50 with photoinitiated plastic, I'd go with the tank.
Also, if the thing can make its own parts, without concern for it weakening itself in ideal use, I'd go with it. (I'm thinking about how it'd be nice to put those thermal-head units in an oven, except they are partially assembled of thermoplastic.)
Hmm. You say that UVC is the same as from an arc-welder. What about a carbon rod arc lamp then? I understand old film projectors used to use them. I don't know about energy efficiency, though. With that factored in, it might still be worth paying 68 dollars for a tube...provided it lasted long enough to pay the difference in the energy bill.
Another potential advantage of a carbon rod arc lamp is it might be feasible to make one from found materials.
I have bought some syringes with needles that have < 0.5 mm diameter, so that small features could be achievable with a deposition head too. But this remains to be tested, as viscous materials have a tendency tou swell back after pressure release when you add fillers to them.
As for resistance, acrylic, polyester and epoxy based resins would provide a growing palette of strength and toughness, not only in the vat method. The deposition method can be tuned in a way that interlayer adhesion is almost as good as an equivalent cast mold.
As for the arc welder light source, I'm not exactly aware of the spectrum it delivers. i know it contains a wild mix of radiation, as it is a mix of thermal radiation and electron discharge radiation. so you would get anything from radio, IR through visible and UV.
I would guess it's to much of a loose gun. And the electric bill would be a crazy one too! :)
Mu UVC lamp spends 40W, delivering a very impressive 12W at the wavelength i need. It is rated at 9000 work-hours. I think it's pretty good. Maybe someday i can find a cheap photoinitiator that reacts fast at UVB spectra, so that we can work with a 5 euro lamp, but for now, this is the most efficient solution i found. Also, in the US the UVC lamps are cheaper than here in Europe. I think I have seen them at around 49$ in some website.
The reason I think vats have a possible edge over extrusion is that tendency others have noticed of extruded material rebounding somewhat, either after shutting off the flow, (hence the reason they were trying to reverse the flow for awhile,) or after it's laid down, (which has caused someone to add a RNG to where the thing starts, for better results.) With this in mind, vats may be messy, and still provide more feature accuracy.
Might still want a laser, or at least a well focused spot. Possibly a projection lens, and maybe a mask, to achieve a tight point of light.
Hmm. I wonder if a mechanical shutter, or LCD one, would allow for faster tool use? I don't know how long it takes to ignite that florescent tube, or how many operating hours would be lost turning it off and on repeatedly. If the bulb could be left on, and shuttered some way, it might extend the life of the bulb, allow for faster production, (you wouldn't have to account for any warm-up time between layers/gaps), and possibly even be more energy efficient. I remember hearing something about more energy being used to ignite those things than to keep those things on.
Of course, in this case "those things" is all florescent bulbs, not just that UVC tube.
The rebound effect may also happen with resins as soon as we will start increasing the viscosity, There's a series of tests i will do with syringes as soon as I have an efficient mix to play with.
Wouldn't the simple solution be to place a shutter at the mouth of the nozzle?
For the mask idea, I has stumbled upon that idea somewhere before. The ideal solution would be to have a UV transparent screen (SiO2 glass or Plexiglas or Polystyrene...) and an LCD matrix that would block the UV when aligned properly. The question I have here is: are LCD crystals UV blocking? And even more important: do they resist heavy UV radiation? This would be important to find out. If we can make this idea happen, UV-set serigraphy would become a very feasible thing!
As for shutter/switcher, I have heard both things: it costs more energy to switch a tube on than to keep it operating but I also read somewhere that switching it off and on improves the life time of a tube significantly, so...
Taking into account that printing a layer can last quite some time, what would be better?
A) print and UV-cure simultaneously or
B) print phase, then cure phase, discontinuously ?
The old energy efficiency suggestions I once heard was, turn off an incandescent light if you're not going to return within five minutes, turn off a fluorescent lamp if you're not returning within fifteen minutes.
Personally, I believe turning it off and back on, over a short cycle, would shorten the lifespan compared to just blocking it while you don't need it. I don't have numbers for this.
Something else to think about.
After the tube finally dies, it might be feasible to use it as a laser tube.
Turning the germicidal fluorescent to a laser would be an amazing tool! It has a wide range of emission peaks that would prove immensely useful for UV setting applications!
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