Tuesday, January 6, 2015

Once more unto the 3D Breach, dear friends.

Haven't posted in a while. I built two flying robots recently, but everyone does that these days. My David Winstead V3 Tricopter kit turned up last week, and that's going to be a joy to put together.

Yes, this one...

But amongst all that, I've decided to get back into 3D printers.

Well, substitute "decided" for "saw a UV laser and complete 20kpps galvo kit on eBay and couldn't pass that up."

UV lasers, 30 years ago, were the kind of thing you needed to be in the Defence Department to get a hold of. These days, $20 on eBay. That's what quantum mechanics has done for us lately.

"Galvos" are the laser afficionado's slang term for galvanometers that are specially designed to have a mirror stuck on the end. They're conceptually no different from the "meters" that have pointy dials attached to magnets, so that currents in a nearby field cause them to twitch to and fro, as invented by Galvani. Except your average multimeter needle isn't designed to accurately hit the mark 20 thousand times per second.

Galvos are. And while moving significantly more mass than your average meter needle. Which is why they need big-ass driver boards and +/-24 volt power supplies. "Closed Loop Galvos" even have feedback systems so the galvo knows how wrong it is, and can correct. Necessary at those kinds of speeds.

Why am I buying lasers and signal-driven mirrors? To build one of these:

Which is a later-generation version of these:

Which are both hobbyist (but still multi-thousand dollar) versions of a fairly old idea called "Stereolithography". Basically, using light (from lasers) to selectively cure a special epoxy resin.

A 3-D laser printer.

Now, all of these printers have suffered a problem that took a while for me to really appreciate - the "release" part of the layering cycle. You get the impression from all the videos that the object builds downwards from the platform like a stalactite from a cave roof, but in fact each layer forms upwards from the optical window until it reaches the previous layer, which it hopefully sticks to,

Then the "build platform" is supposed to move upwards a fraction, and that's when the issue comes in. The epoxy is stuck to the window. And even if your window is "less sticky" to the epoxy than the layer above it (you hope) there's still a moment where you have to pretty much rip the newly formed layer off the optical window and prepare for the next layer.

This is the infamous "clunk" that sometimes yanks the protective silicone coating right off the bottom of the tank, especially if you've in-filled too much of the previous layer. And once that coating degrades, you have to recoat or prints start to fail badly.

I actually though the achilles heel of these machines was the cost of their "toner". But that's come down to the point where you can by a useful quantity for $50 from a place like makerjuice.com

Bit it's not. The worst thing is literally the sucking sound of the next layer being ripped off the optical window's protective surface. If you're lucky, you get ten coatings out of a $50 bottle of the stuff. And you'll go through the coatings in less than a dozen prints. So the tray coating alone is a $0.50 per-print consumable.

There is only one 3D printer which has managed to avoid this form of sucking. The "Peachy" printer - which uses a liquid float system, and builds in the opposite direction.

It's full of elegant ideas, but some terrible design decisions. (like trying to make his own galvos)

After thinking about it for a while, I've come up with an compromise that combines the key idea of Peachy (liquid resin float) with the more repeatable inverted Z-axis build tables of the B9 and the Form 1+. It's even a fix that could be retrofitted to those machine, with some work.

My key idea is to replace the solid silicone protection layer with Wax.

Why? It's second only to teflon in having a non-polar surface, which Epoxy doesn't like to stick to. (That's why carnauba wax is used in mold release agents) It should contaminate the epoxy less than saline solution in the peachy. And finally, it's incredibly cheap and available everywhere.

Possibly even use Paraffin oil, although I'd need to float a clear layer above the epoxy to provide the non-stick optical window, due to relative specific gravities.

If there's no solid surface, there's no sucking, so the Z axis drive can be weaker. A careful retract - extend would still be advised to pull in fresh epoxy to heavily infilled areas, but maybe this method can 'continuous build" thin-wall structures without doing a release cycle. (ie: the way people actually think these machines work.)

Now, there's a couple of minor things to consider when combining flammable waxes with high-power lasers, but I really don't need very much in there. Just a shallow layer. I'm planning on a much smaller build volume, too.

So, I'll let you know how it all goes. Clearly the machine has to be called "Waxer".

And any day now, I expect a breakthrough or two with the UV epoxies that are the expensive 'toner' these machines consume - probably by replacing the metallic catalysts with cheap and safe organic dyes squeezed from colourful fruits, similar to what's happening with dye-activated solar cells.

The Fused Deposition (melty plastic gun) printers never really excited me, I've got to say. But the photolithographic ones - I think - are,on the right path at last.

Writing with light. Writing solid plastic with light.

Update: November 2015

Well, it worked. For a while. Then I burned out the UV laser. (don't worry, It was a crappy laser) But in the meantime I took some videos of the setup in action. As always, most of the work was in the software.

I learned a lot in the process, which was the important thing. The basic theory is still sound, but there are a few nuances I hadn't appreciated before. Hint: in early tests, my laser lines had a 'wobble' that exactly corresponded to the blinking USB LED on the Arduino. Current consumption matters when your DAC is moving only millivolts per LSB.

1 comment:

  1. A lot of research has turned up the basic issues with a liquid float system. Nothing insurmountable, but probably requiring the talents of a good chemist.

    The epoxy resins typically have a specific gravity (weight per volume) of about 1.2 g/cm^3, where water is neatly 1.00 on that scale. So, epoxy sinks in water. Although it doesn't readily mix with water, there's an interaction at the surface which tends to draw catalyst molecules across the boundary, and leave the epoxy 'waxy'.

    The peachy makes the epoxy float by using brine - water with so much salt in it that the specific gravity goes up to 1.3 or more. That's not only going to waxify the epoxy at the bottom, it will probably contaminate it with ionic salts too. Also, if the water gets too warm, the epoxy is miscible with it.

    Paraffin wax (ignoring it's flammability for the moment) would be a brilliant substance to float epoxy on, since it's a non-polar liquid and basically as immiscible with epoxy as it is with water. Except for the fact that it's lighter, at 0.8-0.9 at best. Mineral oils, or any liquid oils, tend to be lighter than water. Duh.

    Tracking down a cheap and available UV-transparent heavy non-polar liquid turns out to be surprisingly tricky. Chemically, you want something like Flourinert - a short-chain liquid fluorocarbon (rather than hydrocarbon) with a specific gravity of 1.8! But there's a good reason we don't use flourocarbons anymore. (Ozone, it turns out, is quite useful to have in your atmosphere)

    One possibility I've picked up is Glucose. Specific gravity 1.54, available in most foodstores, surprisingly inert (for a sugar). I can't find a damn thing in the literature about the boundary interaction of glucose with epoxies or acrylides though. One of those "try it and see" things I'd love to get around to. I've still got to think it would poison the epoxy less than brine would. And there might be some interesting positive effects (when glucose oxidizes it produces H2O and CO2. 3d resins are generally not oxidizing except when the photocatalyst is active. So it might create a submicron-thin gas separation layer at the boundary where the epoxy is hardened, which might help. Who knows.

    Liquid float processes are a staple of chemical engineering. If I was going to brute-force this, I'd float the 3d resin between layers of paraffin and fluorinert like a very expensive and flammable cocktail.

    Baby oil and warm glucose might be a far cheaper solution, but the chemistry starts getting very convoluted, especially since most resin makers don't reveal all the components of their mix.