Saturday, September 09, 2006

 

Just some hot air...

Here is an idea I'm working on for very small RepRapping. To give a sense of scale, the green cube is 1 mm cubed, and the little blue smudge is 0.1 mm cubed. That would be 100 micrometers (previously know as microns) wide, I think. Getting kinda small...

I'd been thinking of shrinking the MkII extrusion head and it seems to me that smaller nozzles would be more and more prone to clogging. The CAPA has to be manipulated in a home workshop setting where dust contamination could occure easily. Also from what I can understand high viscosity extrusion at smaller diameters demands high pressure. Wouldn't a doubling of extrusion rate at a given diameter imply a quadrupling of pressure? Combine both and we need too switch to alternatives from the screw drive filament feed.

The rendering shows a very fine filament of 0.1 mm in diameter, (thin vertical white line) formed beforehand and being fed through the nozzle (brass pencil thing) by two pinch rollers. Since 0.1 mm diameter filement is so small, there may be no need to use anything more than a hot air source (the strange silver cone thing) to melt it at the last possible moment when it comes into contact with the workpiece. Maybe the trick is to leave a good gap between the nozzle tip and the work surface. I'm thinking at this scale the filament should be acting like a stiff bristle, so there may be no need to guide it right down to the surface. This would give room for the heat source to melt it just at the point of contact, no sooner. (If hot air seems too low tech, how about a DVD-R laser?)

This approach may be more linear when it comes to increasing deposition speed. As long as the software and hardware matches the head's speed of movement with the feed rate of the filament the concept could work. The hot air flow would have to vary along with it, the faster filament feed, faster air flow, and it could all stop on a dime.

Hot air also happens to be used to melt solder, and this also suggests it might be possible to feed a filament of solder instead of CAPA. (Thanks to Rieyuki for that idea) Same head, just a different filament and suddenly you're reprapping electronics.

Finally, if this idea could be pulled off at a given size, it seems it could be shrunken down again without really having to change the fundamental concept. Once it's working, how small could it get?

Comments:
I don't think pinch rollers would be practical for feeding such thin fibres - I have a feeling the fibre would simply buckle and snap far too easily in the gap between the rollers and the nozzle. I'd try and use the regular, thick fibres and simply make the extrusion nozzle smaller, melting the fibre just before the constriction.
 
Good concept! It bears more thinking about.
 
While the direct .1mm hot-air extrusion may not work (who knows, it just might), it's a good reminder on thread size vs nozzle size.

I don't see why reprap can't accept 2mm, 1mm, or even .5mm threading. At these diameters, it should make it quite easy to spool the thread onto an old cable spool for extended fabrication potential.
 
Nice idea. I think the problems with this kind of system are going to be:

1. Amount of material it can deposit.

- If you can print 1 cm per second, you'll need almost 3 days solid to print a solid cube 1 cm on each side.

2. Preventing the hot air from deforming the material already on the surface.

- It's going to be a tough balancing act to ensure that you melt the thread, but not melt material already part of the object. Consider that you'll need to make many passes over the same region, and the level of detail is going to be very fine.

3. Getting the .1mm thread. You're going to have to extrude this at some point; or I suppose you could try other techniques (roll it, cut it, etc.) They are all going to be a bit difficult at these scales.

My thoughts on getting higher accuracy would be to have an additive stage, then 'shape' it with a subtractive or molding stage. This could occur on one shape, at each extrusion level if necessary (I.E, set down a layer of thread, then subtract tiny portions of it to increase accuracy. Then move to the next layer, set down a layer of thread, and remove portions of it to increase accuracy. Ad infinitum.)

An analogy seems to be appropriate: if you are familiar with pottery, then you have probably heard of coil pots. If you try to make one of these without shaping the clay, they end up full of holes, and more often than not, asymetrical and bumpy. But, shape them sponges, tools, and your hands, cut out parts that aren't supposed to be there, etc., and you can end up with a pot that looks as if it was thrown on a wheel.
 
But, shape them sponges, tools, and your hands, cut out parts that aren't supposed to be there, etc., and you can end up with a pot that looks as if it was thrown on a wheel.

...and took ten times longer to make than if you'd just gone ahead and thrown them on a wheel in the first place.

BTW, my coil ceramics always blew up in the damned kiln anyway.
 
Lol Forrest. I myself found it easier and faster to create coil pots than to throw them on a wheel. My wheel pieces tended to be lopsided and droopy.

The blowing up part certainly will happen if you don't get all the air bubbles out. One also has to be patient and let the piece dry completely before firing (or fire in stages to drive off any remaining moisture.)

Actually, that brings up another feature of RepRap -- how effective would it be at extruding something like 1mm threads of wet clay? Ceramic items and lost wax molds without the wax would be interesting possibilities. Support material would be a problem I suppose.
 
Actually, that brings up another feature of RepRap -- how effective would it be at extruding something like 1mm threads of wet clay?

Adrian's peristalic extruder should do the job for that.

Ceramic items and lost wax molds without the wax would be interesting possibilities.

Yup.

Support material would be a problem I suppose.

What would be wrong with using the same sort of polyfilla that we use for supporting CAPA?
 
My wheel pieces tended to be lopsided and droopy.

Sounds like you were trying to make pieces with the palms of your hands that were too big for them. Depending on the size, you use your forearms as well as your hands. I knew one girl in the studio art programme at my university who made huge vases and runs. Dressed in skimpy shorts if that and used her torso and legs as well on pieces she made quite a large wheel. That was years before somebody hit on the idea of mud wrestling, mind. :-D

Are those halcyon university days before things had to be so damned important to attend to at all. :-p
 
runs = urns
 
What would be wrong with using the same sort of polyfilla that we use for supporting CAPA?

Isn't it good because it is water soluable and can be washed away after extruding plastic?

Wouldn't that be a problem if you tried to wash it away from a very moisture hungry and easy to liquify clay?
 
Wouldn't that be a problem if you tried to wash it away from a very moisture hungry and easy to liquify clay?

Fire 'em together. The polyfilla is basically gypsum and cellulose. The cellulose will burn away and the gypsum will dessicate and can be brushed off the fired ceramic piece.
 
Fine nozzles are hard to drive - they do need a lot of pressure. I tried to make an 0.25mm one and not a lot came out of it.

But if you can crack that problem, then slow build times should not be a problem - just have two heads: a fat one and a thin one. Just use the latter for fine detail.

It's just like roughing and finishing in machining.

There is one potential problem with this approach, which is that the two might have a different layer thickness; but you could probably squash the fat layer flatter just by running its head lower.
 
I can't reply much now, will get back to this a little later. Several good points have been brought up!

Just wanted to reply to Adrian, about the two nozzle idea. I've been thinking maybe the big nozzle could have a deposition thickness of twice the thin one for example. Could even be 3 or 4 times thicker but that might not be so handy.

In any case, you could always make any detail the quick fat one can't do with the slow thin one, as long as they match up. If you need an in between thickness, you could overlay one thick layer with the small nozzle?

BTW, anyone have a clue as to the mathematical relationship between nozzle size, extrusion rate and pressure?
 
Okay some ideas about comments. Regarding the pinch rollers, at this small scale, the CAPA filament would act like a stiff rod, not a floppy thread. There is no major friction point anywhere past the pinch rollers, and it melts before hitting the workpiece. I've kept this render straight forward, but I've been planning a new version with several extra details, including a better match between the rollers and nozzle.

About the heat melting deposited CAPA, air heating the filament would expose the workpiece to proportionally -more- heat than what Vik has seen with his Zaphod, as much of the heat flow would spill onto the workpiece. At the same time, due to the scale, these parts would have much more surface area per volume, making it very easy to cool with air flow.

The balancing act issue might have more to do with the size and accuracy of the pinhole source of heated air.

A CD-R laser could keep a much tighter focus on the filament and might not require cooling air.

About making 0,1 mm thread. I wonder if there is already something available on the market? Fishing line for example is a strong monofilament, but it's not made of the right type of plastic... But maybe filament making is not so hard, it's a mature technology we might be able to copy.

BTW Beagle, I think you might be on to something with the subtractive step between extrusion layers, Forrest mentioned this also. But then all the cheats being used might not work, and classic CNC machine design might have to take over.

Now as for clay pots and young scantily clad women mud wrestling with giant urns... will that's a whole different topic!
 
But maybe filament making is not so hard, it's a mature technology we might be able to copy.

Yup, as long as you have about US$70K for a good used filament extrusion system.

http://www.waynemachine.com/mono.html
 
LOL! I remember that link from a while back. But then what are you saying, we will be rolling CAPA for the next twenty years? Groan...

Now if it's not possible to take inspiration from high volume, high reliability, flexible industrial machines and try to build cheaper versions in the home workshop, I guess I'd better go tell all the CNC people that their machines don't work. For that matter doesn't this mean the RepRap project won't work either? ;)

Now I'd better run before getting splattered with wet clay! :)
 
One thing to remember about pinch wheels is that CAPA is very slippery. My Mk I extruder used multiple pinch wheels, each knurled, in a stack. But the CAPA slipped, so I moved on to Vik's neat screw-drive idea for the Mk II.
 
BTW Beagle, I think you might be on to something with the subtractive step between extrusion layers, Forrest mentioned this also. But then all the cheats being used might not work, and classic CNC machine design might have to take over.

I don't think one would have all the subtractive issues. After all, we are still using additive manufacturing. In fact, one of the additive technologies is subtractive at the lowest level: pieces of paper are cut to the correct outline, and then glued together to form the overall 3d shape. (These models have properties very similar to wood.)

Some other ideas that would simplify the non-additive stages:

- use alternative means to remove the plastic. Why grind or cut it off when you can melt it off?

- Don't subtract at all.. just move it using a hot non-wetting tool to coerce the plastic that is almost where you want it to exactly where you want it.
 
Adrian, I agree that CAPA is slippery, but you were using the pinch rollers to pressurize your melt chamber, no? What if the pinch roller were just feeding the CAPA through an oversized hole, say 0.15 mm for a 0.1 mm filament?

Beagle, I hadn't thought of smooshing the CAPA around like that! Interesting idea. The mechanical accuracy of the hardware would still have to be higher though, because there would be a bit more force on the structure, and you would have to be accurate enough to make the CAPA more accurate than we are talking about with pure basic extrusion, no?
 
Right. You'll need more accuracy in the table and threads, but I don't think you need a lot more stiffness? It's hard for me to imagine that you'd need as much as, for example, a milling machine where your enemies are lateral forces, friction, and vibration. Here, you simply have the force a viscuous liquid exerts as it tries to hold it's shape. Vibration is not as big a deal (No spinning parts, except the drive motors), friction is less a factor (no surfaces racing past one another at breakneck speed), and lateral force can be optimized against the amount of time it takes to print the part.

Anyway, it just seems that a hybrid approach will be more technically easier than trying to miniturize the extruder and thread engine. I've thought that an interesting offshot from the initial reprap would be one where you cut and melt sheets of .1mm thick plastic sheeting together -- similar to the paper cutting additive machines. If you're going to use a laser anyway, might as well do a layer at a time rather than sticking to 1 dimensional extrusion.
 
Humm, interesting ideas... About the stiffness, I agree, you only need a structure that can remain very ridgid under a light load. A little bit like a delicate balsa wood model airplane. Lots of crossbracing, and no mechanical slack. Oh no, now I'm getting into analogies! ;) All of this is well mapped out by the CNC people, we just need a lighter version of such, not a show stopper.

Do you have any "non-wetting" material that might work with CAPA I've noticed silicon is interesting, cured epoxy or hot glue pops right off, but that's not what we need.

Ice is non-wetting to CAPA, not sure how practical that one is! :-)
 
Is PTFE nonwetting? Issue there is that it is a good heat insulator -- maybe a teflon coated metalic heat conductor. I'm not sure what else might be viable. It certainly would take some knowledge and experimentation to find the correct materials to use.
 
I suspect molten CAPA will stick like mad to PTFE. It will pop right off once the CAPA has solidified, but that's just like the silicon, not much use.

Maybe an oil or water coated tip could be used to manipulate the molten CAPA after extrusion...?

CAPA might have a usable semi-solid state where it is no longer molten enough to stick to things, but can be smooshed about. Kinda like Vik did with his Zaphod support brackets.

All this seems awkward... humm...

Instead, what if molten CAPA was extruded against smooth chunks of prepositioned PTFE, to form a few critical smooth faces and holes for example?
 
I suspect molten CAPA will stick like mad to PTFE. It will pop right off once the CAPA has solidified, but that's just like the silicon, not much use.

I'm not sure it will. Anyone want to try this and see? Should be an easy thing to test if they have some PTFE and CAPA available?
 
If melted CAPA doesn't stick/wet PTFE, that would be very good for small nozzles and friction/pressure issues...
 
Post a Comment

Links to this post:

Create a Link



<< Home

This page is powered by Blogger. Isn't yours?

Subscribe to
Posts [Atom]