Sunday, June 10, 2007
Suntanning blotches... in an Amsterdam summer day??
I have received my UV fluorescent, a Cleo Compact of 15W. It cost me 6€ and is very easy to get in any illumination ware store. The tube allows for a fairly compact mount, as it is only 30cm long (about a foot). They didn't have the mount there for it so i bought the parts and made it myself out of this simple diagram i got from the Wikipedia.
I used a reflectant non conductive mylar plastic strip from a candy bar (Twix, to name it), to use as a reflector. The end result looks like this (left).
The bad news is that the spectrum of this lamp is adjusted to mimic the natural solar spectrum of an Amsterdam July day at noon! I'm not kidding!! I was hoping for an ozone-depleted, antarctic, January at noon spectrum! :)
The result is that after my first test-run i had not enough UV-power at the desired wavelengths to get a quick polymerization. It took 2-3 hours to get the resin to set and probably another hour or two to get it to harden... far to long! I will be ordering a black light tube (Philips TLK 40W/10R or F40BL 40W) which have an emission peak at 365nm, exactly what i need for Mechlers Ketone. They deliver 7.4 and 9W of UV radiation each, which is a lot better than the Cleo tube and the price is very similar, although the tubes are twice as big. If the results keep being to poor, I will be aiming for the 100W bulb range, which will certainly do the job, but at a higher price. I will also buy a germicidal UV fluo (philips TUV25T8), which works at the wavelength of Benzophenone.
One last thing slowing down the setting is an effect called Oxygen scavenging. It results through the effect of air Oxygen on the surface, which destroys the radicals formed by the UV radiation inside the resin mix and that are responsible for its polymerization. This causes the surface of the resin deposit to remain liquid while the interior continues curing. There are two ways to solve this problem:
1) Augmenting the photoinitiator of the mix (from 2-3 weight % to 6-10 w%)
this solution is simple but on the expensive side.
2) Adding 2-3 w% of an Amine such as N-Methyldiethanolamine, wich costs around 25€ per kilogram and is a cheap additive and which will react with the solved oxygen in the resin, preventing the radicals scavenging.
One thing i thought of too is to let some resin blotches lie in the Sun (Barcelona Sun, not Amsterdam!). And it did work! I got some slow polymerization on, and i suppose that in 12h exposure the resin would set as well.
One last important effect and that will have great implications for the deposition process. The resin remains tacky and sticky right until the end of the curing process. By timing the UV-exposure and the Amine content correctly, we could achieve seamless interlayer adhesion, almost as if the object had been cast in one piece!
Also, you might be able to flood the chamber with a heavy, non-oxidizer to prevent the scavenging. I'm speaking from ignorance here, but is the stuff water soluable? If not, you might be able to just make it in a tub, covered in water, to help block the air. the other way would be to keep a stream of CO2, or something, floating across it, but then you'd have to consider ventilation for the user.
Although, I don't know if your UV will penetrate water well, either.
I realize the stuff is organic, but so is soap, so I don't know if it's water soluable or not.
I believe this can be achieved with an UV fluorescent tube of < 40W, selected for the right wavelegth. This should keep your bill down. In case of using a 100W lightbulb, i figure that the curing time will be reduced drstically (maybe in the seconds range).
On the issue of oxygen scavengig. The price of N-Methyldiethanolamine is quite low (25$ for 1kg, having to use only 2-6 weight %, this means it would only add a maximum of 1,5$ per kilogram (2 pounds).
I guess other cheaper Amines may be considered too, but at this price it is allready cheaper than the photoinitiator and makes the resin-mix cheaper.
Plus this is a far simpler solution than pouring a nitrogen athmosphere or making a water bath, although these solutions may have other helpful applications and should not be discarded.
As you see, the price of UV-set resins will always be higher than thermoplastics, at least as long as we don't aquire large amounts of the stuff and mix it ourselves.
These guys sell 1 and 5 gallons polyester resin around 4$ a pound. Add 2-3$ per pound of photoinitiator and the 0.75$ per pound of Amine and we are talking of a confirmed price of 7.75$ per pound.
So, continued, or future exposure wouldn't "soften" the material...only make it harder, if anything. This I assume. This makes this a potential single-stage material. One that can handle itself as an extruder. Something I don't see thermoplastics doing.
Will the stuff yellow, and eventually turn brittle, since it is designed to react to light?
Will it do so more rapidly than thermoplastics?
Granted, even if, it's easier to paint out the sunshine than it is the heat.
As long as there are free monomers available, continuing exposure will harden the mix. This also means that stopping exposure before the end will leave the mix viscous or sticky.
The resulting polymers achieved through this method will always be duroplasts, so heat won't melt them reversibly. They do have an upper temperature limit where they will start to decompose, as any organic compound. This upper limit can be manipulated through addition of fillers (sand or metal powders) as they help with their good heat absorption coefficient and heat transport (to avoid hot-spots)
Plastics don't like long UV exposure, in general. They will suffer from long exposure to the sun and energetic radiation.
UV setting mixes suffer similarly from these effects, but there are many preservers (quite similar to suntanning protectors) you can add to make these resins weather resistant. If it becomes necessary to develop a resin like this it will be a very easy task.
But as you say, if you don't want to add protectors, you can always paint the pieces that will recieve heavy sun exposure.
I hope i made myself a little more clear, my writing sometimes lacks clarity i'm afraid :)
UV hardening can be stopped and restarted as many times as you want and will not affect the hardness or properties of the finally hardened polymer.
Just thinking of some of the SLA notes on the reprap forums from others.
There are spectra of the photoinitiators I use at the section i documented in the wiki:
I have more spectra of other photoinitiators that you may be interested in, I'll upload it on the wiki.
It would make a lot of people happy if you could devise a way to reprap it!
It would be a very usefull addition, not only for precision UV curing, but for cutting, etching, sintering, whatnot!
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