MakerBot Industries

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?

We have been crowdsourcing printed pulleys for the last few batches and it’s been great. With batch 10 we’re switching up the design for the pulleys a bit and so we’re putting a stop request on pulleys. If you’ve been printing out pulleys, we’ll buy them or give you credit in the MakerBot store, but we need you to send them in right now so we can use them.

We love that we can pay people to use machines to make parts for other machines and we’ve got plans to reinstate this program with different parts. If you’ve got a machine that is waiting to be put to work, we recommend you start printing out Mendel parts.