Tuesday, August 01, 2006


Nozzle idea

I whipped out a 3d model of my imagined 'wire on/off' switch on the mechanical pencil tip extruder. All theoretical at this stage, but I think it shows the idea a bit more clearly than just text alone.

First time posting images, so not sure if this will come out or not.

The purple stuff was my attempt to wrap the model with an insulation, since the heat will be spread thru a larger portion of the extruder. The idea is that air pressure drives the liquid plastic thru the tip, as has been suggested in other comments.

!! awesome

gotta love the sound of a dozen monkeys poking at high-tech concepts. Creativity is king in this game.
Cool, thanks for the 3D rendering. I have to say, I was conjuring up something like that in my head, but you beat me to it! :)

What software was that done with?
I use Blender3d v2.41 (open source, and multi-platform). I tried the Art of Illusion, but having used Blender for a few years now, the controls come more naturally to me.

The blender STL export didn't seem to work. At some point, I'll try to figure out how to correct it, but I'm not too worried about it at the moment. Worst case, I export to a format that art of illusion can read.
Thanks for the warning about Blender and the STL files. Maybe I'll try AoI out, it would just be too transfer files, I've been using Cinema4D and Strata3D for 3D stuff.

Of course, I'm not in a hurry, I'd have to build a RepRap first! At the moment I seem to be focusing on learning more electronics and programming, would like to know what I'm doing, rather than just building blindly!
There is a neat type of latching solenoid with a permanent magnet that you could use to drive that valve. That way it'd only need a current pulse to turn it on, and another (opposite polarity, so H-bridge needed) to turn it off. Check out product 250-0827 at http://rswww.com for example.
LOL! This is fantastic! I just love this new blog! :-D
Do keep in mind, guys, that the air pressure scheme was a take off on the standard viscosity test unit for polymers. You only get those really low pressures, viz, 2.5 atm. for a nozzle size of 2 mm. What the pressure requirements/flow rate profile looks like for a 0.5 mm nozzle is anybody's guess.
I'm not sure I understand what you're saying.. If I understand you correctly, some amount of pressure (presumably, high pressure) was applied to some continer with a 2mm nozzle, and the output pressure yielded ~2.5 atm?
No, the pressure that had to be applied within the extruder barrel had to be 2.5 atm in order to get get a flow rate of 18 grammes/hour through a 2 mm diameter die when the polymer was heated to 160 Celsius.

Does that help? If not, just keep asking questions and I'll keep trying to make sense.
Thanks! That makes more sense.

Does the MkII extruder need to generate the same pressures to get similar flow rates?

One reason I think air pressure might be useful is that the amount of air flow needed is very low. It should be possible to pump up a small air reservoir to 100 psi, and not bother with it until it was reduced to 80-90 psi before refilling. Assuming no leaks,
a 10 liter tank should be able to extrude a kilogram or more of plastic.
Note that I wasn't thinking of anything fancy for filling up the air tank. A bicycle pump and/or a service station with free air should get you by (Even better, if you have two reserviors.)
***Does the MkII extruder need to generate the same pressures to get similar flow rates?***

That is the question. You're talking about a 0.5 mm extrusion die rather than a 2 mm one, so you can be sure it's going to require more pressure. Just how much more pressure is an engineering study, nothing to be frightened about, but one that will require some feeling around. Best to do some experimentation and see what kind of graph you get for pressure, orifice diameter and temperature, hey?

***A bicycle pump and/or a service station with free air should get you by (Even better, if you have two reserviors.)***

As I said a few days ago, air compressors with small reservoirs running on mains power suitable for running things like nail guns can be had for $20-30. Just froogle "air compressor" and sort prices for lowest to highest. Here's a likely candidate...


By the time you organise a bicycle pump, a reservoir and the connectors to make it all work you'll have spent that much and not have a very good system, too.
Actually, the 10 liter tank I had in mind was the spare tire in my car. The spare in my wifes car would be a nice extra tank. These donut tires are supposably rated for 80 psi, but there must be some margin of error built into them, right?

On a somewhat similar topic, I did have a flat on my car, and had to use a bicycle pump to get it inflated enough to drive it to the service station -- 100 pumps on the bicycle pump and it seemed good enough to avoid any long term damage while driving it the 1/2 mile to the station. It was a nice workout. I could see this kind of machine being popular with the fitness minded person : "Build Some Muscle And Make A Plastic Comb" machine.

Okay... maybe just buying a USD30 nail gun compressor would be a better solution, long term. :)
LOL! :-D
I'd just like to point out that even without an air-driven system, it's still a big improvement in ol' drippy on the gear+filament driven setup. I know software can compensate but in something this precise I'd want all variables accounted for.

How do most commercial fabricators do it? That might be a hint as to which direction to go. Of course, that's why this group is here, so we can each dabble around and try to find the best solution.
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