R&D Saturday: Air Powered Plastruder
Ever since I got the Frostruder working on air, I haven’t been able to stop thinking about the possibilities of air-powered plastic extrusion. Today, I was able to put together a test rig and get some preliminary impressions. The verdict is that yes, air power seems to be a viable path forward, but there are definitely some hurdles to overcome.
My test rig was really ghetto, mostly because I didn’t have the right pipe fittings to make things easy. I managed to cobble together a monstrosity that did indeed hold pressure and work. The basic setup was: quick connect -> compression fitting -> PTFE tube -> compression fitting -> nozzle. I used some stainless steel nozzles that I got from McMaster. I used these because they have 1/8″ NPT connections on them. I have no idea what the nozzle profile looks like, and I had to modify them to get better extrusion. More on that later.
I used a standard extruder setup: nichrome wire + a thermistor + kapton tape. Since the hot end now had lots more metal in it (a bunch of pipe fittings connected to the nozzle) it took quite a while to heat up (2-3 minutes) but it did indeed get up to 220C. The entire assembly got really hot to the touch, so of course i ended up with lots of tiny burns. The PTFE tubing held up great in the compression fitting. We’ll need to do a long-term test on that to see just how well it actually holds up at temperature though.
As for the nozzle, I had to modify it to work. It comes with a wicked looking 2-inch blunt needle tip. I cut it down to ~2mm with a dremel. I tried to extrude with this and it just barely went. I got the dremel out and cut it flush. It extruded better, but still not so good. I started hacking into the nozzle body and cut away a good chunk of it to shorten the nozzle orifice length. That made a big difference the shorter I made it. If this proves to be a viable path, we’ll get custom nozzles machined with a short orifice distance for the best extrusion speed.
The extrusion itself was relatively slow. I didn’t measure it, but it looked like 10-15mm/sec @ 120PSI w/ 0.8mm diameter. Unfortunately this is much slower than the current MK4 which extrudes at ~26mm @ 0.5mm diameter. I’m going to try and tap a pipe plug to M6 and see how it compares when using the exact same nozzle as on the standard designs. That will be some excellent data. It definitely seems like we’re going to need to use much higher pressures if we’re going to extrude plastic with pressurized air/gas.
I started researching various options for higher pressure sources and came across a very interesting possibility: compressed CO2 cylinders. These are used all over the place, from paintball to welding to beer pouring systems. CO2 itself is really cheap and ubiquitous. C02 tanks also put out a steady 800PSI at room temperature! That is definitely going to be more than enough for our uses, and we’ll need to put a regulator on it to get the proper temperature out of the tank. The nice thing about CO2 is that it would be silent as well, compared to an air compressor which is typically very loud.
Another interesting challenge is delivering the feedstock. The air pressure does not just automatically push the plastic out the nozzle. Rather, it tunnels through the plastic and when the air pressure reaches the nozzle, it blows the plastic out and then jets air out the nozzle tip. If you push on the plastic to fill that gap, it will continue extruding and will extrude nicely for a couple minutes until you add more plastic to the mix. I was using filament in my setup and was pressurizing the entire PTFE tube that had the filament in it. This also meant that I had to de-pressurize and push the filament down in order to plug/fill the tunnel that formed. With air powered extrusion, we’d need to find a way to either feed filament into a pressurized system (some sort of o-ring?) or find a way to load pellets in at regular intervals. Perhaps a hopper with a servo-operated valve that would let the chunks of plastic enter the melt chamber?
A few of the hurdles / things to be researched:
* what pressures give us useful extrusion speeds (150, 200, 250?)
* can we find solenoid valves that handle those pressures and aren’t unobtainium?
* how much CO2 does a print need / how often would you need to fill a 1LB tank? (volume usage of a print = volume extruded + pressurized chamber size * extruder starts
* method for refilling plastic for printing (filament or pellets)
* can we use an o-ring to seal off the PTFE tube and thus be able to feed in filament continuously?
* can we create a hopper system that can sporadically fill the melt chamber with plastic pellets?
* can we do this safely?
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11 Comments so far
anonymous
Maybe you could use this as a recycling rig. Grind up old prints then put the resulting dust into the nozzle and the air pressure would push it out.
geo01005
So I think that if you had the feedstock come in through an O-ring and exit through the extruder tip, then the air pressure will provide no force to cause it to exit trough the extruder. The force on the feedstock to exit the extruder will be the same as the force pushing the feedstock out through the O-ring. So you would have to pressurize a container will the feedstock on a spool.
davidbuzz
Perhaps you’ve already thought about it, but if the desire is to use air instead of electricity, then you dont need to strow out the entire pinch-mechanism too. replace the stepper housing with a compressed air motor housing ( probably printable except for the axle? ), and still connect it to the pinch-wheel mechanism. the bonus is lower pressure requirements, no direct electricity ( not counting the compressor), and all the benefits of the familiar pinch-wheel.
someguy
The filament does not have a nice round profile especially if it’s been through a pinch wheel. Getting an O-ring or a series of O-rings to seal will be difficult even at low pressures. Not to mention finding an O-ring material that won’t get chewed up by the filament sliding through it. I would try the hopper idea, but instead of using some complicated servo release mechanism just let the pellets feed into the heated area of the extruder by gravity. The hopper and extruder would have to be a continuous chamber. As the air pressure pushes the molten plastic out, raw pellets will move down the hopper into the heated tip of the hopper/extruder and be melted. The biggest trick will be getting the hopper shape right. You’ll want it big enough to hold enough plastic and you’ll want it shaped so the pellets will move freely into the heated portion without jamming. Maybe make one giant reservoir of molten plastic?
SOI_Sentinel
You may need to step it instead. An oring sealed piston and a smalle chamber farther down. This would lower the flow rate but raise the pressure. But then we’re getting to the point where we’d be building our own pressure pump. Oops!
Zach Hoeken
Thanks for the feedback! This is a great example of why doing open source hardware rocks. Some good suggestions here.
A couple things:
anonymous: yup, if we got a pellet style extruder working like this, you could definitely just dump your old prints into it and recycle them directly into new objects. that has been a dream for a long time and hopefully we’re getting closer to figuring it out.
geo: sort of. air pressure pushes equally on all things in the system. so if we have a vessel with an o-ring on one side, and a heated nozzle at the other side, it would be pushing on both of them at the same time. i’m not sure if this would result in the filament being pushed out of the tube or not. if it does, then the whole idea of using filament goes out the door (as you’d need the same force to feed filament in as you’d need just to extrude without pressure. something to investigate for sure. if the force requirements to push more filament in past the seal are lower, then the drive system doesn’t need to be as beefy.
someguy: you’ve hit on the crux of the problem of using filament based feedstock in a pressure based system. i’m pretty confident that we could find an oring to seal onto virgin filament, but getting one to seal onto the filament after a pinch wheel is going to be difficult for sure. what i’m hoping is that if the force required to simply feed more filament into the pressure chamber is lower than the force we currently require to actually extrude the filament, then we can simplify the pinch wheel a bit and make it so it does not mar the filament. Ideally it would act as a clutch… feeding in more filament when needed, but if it feeds too much it slips on the filament and none gets fed in… perhaps using smooth pinch wheels rather than notched ones. all the pinch wheel would need to do is actually put more plastic into the melt chamber. the air pressure would do the heavy lifting of actually extruding it. of course this is all just speculation.
soi: yup, a final design would have the smallest pressure chamber possible in order to use the least amount of air, especially if we have to use something like CO2 which would need to be refilled.
i have a feeling that if we get this working, it will probably be a ‘pro’ setup since there is a decent amount of gear that would be needed in addition to a regular makerbot (co2 tanks, solenoids, etc.) still… air pressure makes things really nice, so it might be worth it.
Wade
Hey, just for comparison sake, the Mk4 Plastruder uses an 8 Kg*cm motor and a 10 mm diameter pulley – that’s about 16 kg of force on the filament, neglecting friction. Say we loose half of that to friction, that leaves us 8 kg to push the filament into the melt chamber. Divide that by 7 mm^2, the area of the filament, multiply by 9.81 m/s^2 to convert to N, we get 11 MPa, or 1626 psi. That’s roughly the maximum pressure the Mk4 Plastruder can develop, based on my back of the envelope scribblings. So CO2 canisters might be about right.
Urke
I think you could use a sand blaster setup but use ground plastic instead of sand. Then you would not have to think up a bunch of complicated mechanism. Plus you can easily substitute a compressor for a CO2 tank. Just a thought.
Matajuro
Perhaps some one can give an answer to this question. With a gravity fed hopper of recycled print parts. Air pockets would form. During the print process, when the plastic is exiting the plastruder, it would hit one of these pockets. What happens to the quality of the print? I would think there would be gaps on the print. Where the print head has moved material should have been pushed out, when there was none. The end result, being prints filled with small gaps and what not. Is my assumption correct or is that not a potential problem? As for a solution I’m not entirely sure how to fix that. Would it deal with getting the recycled bit small enough before entering the hopper? A de-airation process once the plastic has melted?
Jeff
Use the pressure to push on a piston, this seperates the high pressure gas from the plastic. I am thinking of a long thin cylinder ~ 1/2″ x 12″ filled with ground plastic, enough for a complete print. The ground plastic side of the piston is vented to atmospheric pressure, through the center of the piston, tube, o-ring seal, so that air bubbles can be released as they form. The piston/ground plastic should be kept cool (insulated from the extruder heat) So that the piston is always pushing on solid (not melted) plastic.
rbisping
If you want to feed pelets into a pressure vessel, you use a air lock system. Basicaly a multivane chamber rotating from outside pressure to inside pressure and delivering a load of feed material with each revolution. this is both trivial and difficult at the same time. the basic design is simple but you have to have good seals and mating surfaces for it to function well. Most air operated glue delivery systems use a double acting pump and a air cylender coupled to a magnetic switching valve. The feed stock is melted in a heated vat that connects to the pump from the bottom. the guns them selves are air operated pin valves with spring returns, default position is closed so air pressure must be applied to get them to open. I can take some pictures if you would like to see specificaly what im talking about.