Notes on a 3-d Texture Printer

I saw my first MakerBot last week, at Noisebridge, and I was impressed. The resolution could use some fiddling with but the results are durable and attractive. A few design revisions and they’ve got this one nailed.

It’s a great platform and I want to see it taken to the next level. They’re working on a 3-d scanner, which is key; I’ve been musing about a way to hack the MakerBot platform to make for even more awesome.

So here’s the idea: a texture printer. Takes an ABS 3-d model and puts a texture right onto the surface. This is doable, like could be done in 2010 doable.

ABS dissolves into acetone quite readily, and acetone evaporates fast. A five-axis machine should be enough freedom, given the pretty firm limitations on overhangs with the MakerBot.

The ink’s no problem, and there are plastics that resist acetone. The printer would need a reasonably sealed enclosure with a fan and a filter disk, so as to not breathe sweet, sweet acetone all day. One of the great things about this is that the acetone on the surface of the model would smooth out a lot of the minor irregularities left behind by the printing process, and the application of a texture would go a long way towards mitigating the rest.

ABS ink powders are used in the latest tattoo inks, so there’s a wide range of vibrant colors. This is a non-trivial project, clearly, but the results…

We would need software that can put the head tangent to the surface of a model. The ink heads on the market today deliver the number of drops you want, when you want them to, we pretty much just have to point them at the right places on the model.

There are plenty of questions that need answering but the basic picture is fairly clear. We’d use a MakerBot style X and Y axis, with one axis about twice as long so the piece can go under the head as well as along the side. The print head would have to be on a Z axis that moved out of the way so that the work piece could pass underneath it, and be accessed from the side. The W axis would be a turntable, and the V axis a servo to control the angle of the print head between XY parallel and Z parallel. That would pretty much put a texture on anything you can print.

There are other control geometries but my feeling is that this would allow maximum leverage of existing code and parts, which is a Good Thing.

The acetone would have to be evaporated fast enough to not drip. The simple way is to run the cabinet around 55 degrees C; the acetone will cool on the way out of the print head and evaporate from the stored heat of the model. This will require either robust electronics or a separate enclosure or both; acetone loves nothing more than eating plastic.

A side effect of this is that no parts in the enclosure can be printed or made from ABS, nor etched from polycarbonate. Polypropylene is resistant as are all metals; there’s a way to make this work while keeping to the ‘build it with your own robot’ approach.

It’s also possible that some better solvent can be found that will still chemically bond the ink with the surface. There are plenty of resins that withstand acetone however and dissolving plastic is kinda the point here.

A wood enclosure isn’t going to work for anything but proving. There’s no practical way to seal it and it absorbs acetone anyway, potentially leading to the growth of mold or bacteria, and no one wants their TextureBot smelling like cheese.  2mm polypropylene panels with Teflon cuffs would slot into MakerBeam with an acceptable seal; the cuffs could simply be PTFE plumber’s tape.

The print head should be easy, if there’s one small enough. Haven’t had a lot of luck looking into that yet, and it’s important. They make really tiny print heads for portable photo printers and one of those might use solvent based inks.

The big software part is bringing the print head tangent to the model. Mapping textures to models, I needn’t add, is a well solved problem, so picking your pixel once you have a clear shot is comparatively easy.

The body, if I’m putting this all together right, is a longer MakerBot, with the turntable functionality needed for the 3-d scanner, plus a servo lever to control the inkjet head angle.

This would be made of awesome and should totally happen. Anyone with thoughts on how, please get in touch.

Published in: on October 6, 2009 at 7:42 pm  Comments (1)  

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  1. Hardware: don’t use electric motors with their associated mass in the structure and/or intricate cable runs over pulleys; use pneumatic McKibben muscles with short “sinews” leading over no more than one pulley each, opening the valves (which should be back on the fixed base) by making them reed valves that open electromagnetically like reed relays, and send position/load data back to the controller through analogue to digital converters so feedback can control what the system achieves. Let the software do anything more sophisticated and keep unnecessary mass off the structure.

    Software: it is necessary to solve for joint angles etc. in terms of the position and orientation in the real world which are supplied as parameters, which is often tricky; as the reverse problem is easy and you always follow a continuous physical path, it is practical to solve it approximately with Newton’s Algorithm while “walking” successive small steps along that path, i.e. using each step’s solution as the seed for the next step as the steps are small enough to keep the seeds in the zone of convergence; then, throw in tests for approaches to locking up (think quaternions and gimbal lock) and apply dynamic programming to avoid them in choosing the paths (Dijkstra’s Algorithm for shortest paths may help here). Oh, and unless there is a good reason why not (like not being able to learn it), program in Forth on a bare bones target operating system, maybe using a remote PC as an “umbilical” host with editors, file handling, etc.

    Three Forths Make a Hole describes solving a related problem which may be of interest.

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