The quest for the ultimate extrusion screw
In the plans of the current extrusion machine a basic drill is being used in the barrel to push and melt the plastic to the nozzle. This seems to work, however, in the industry a different type of screw is used which compresses the plastic while it’s melting. These are way less accessible than the drill, for obvious reasons. So Dave had the idea of possibly trying to produce these ourselves. Therefore this topic will be dedicated to the research on trying to design/model/produce this.
I was quickly doing some research on this yesterday and it seems to be a world on its own with different designs of screws, different compression ratios and different “zones”. If there’s anybody that might have some experience or knowledge on this that could help out, let us know here.
I turned the basic idea for the screw design into a quick 3D model, to see if I would be able to model it. Now I need to find the right design requirements to turn that into the eventual 3D model, so we can turn this into a physical prototype.
@davehakkens So, I’ve been talking about this with Jens Dyvik, who is currently working on an open source parametric cnc-milling machine which can be produced on another cnc-machine. (https://github.com/fellesverkstedet/fabricatable-machines) And turns out he has been researching this as well before and already worked on a parametric version of the extrusion screw! See image attached.
He’s also thinking of incorporating a 4th axis in the machines for exactly this kind of stuff.
Until the 4th axis is done, I think we can do it at the Fablab outside of Oslo, they have a 4-axis machine. But we will need to wait until the machine guy is back from holidays so he can explain how the machine works. And I’m afraid by the time he’s back I’ll be out of the country for two weeks. :/ So we’d be talking about at least another month before we can try it out, unfortunately. We’re eager to try it out though! And if it works, I’d be willing to send it for testing, if material and shipping cost would be refund?
I’m also wondering whether aluminium or brass would be strong enough. Is there a lot of force applied to the screw in the extruder?
Nice progress 🙂
If you use aluminium or brass, there might be a risk you chip of material from the screw and contaminate the plastic?
Cool! I’m curious of the result.
Can’t go wrong with a company called “Five-star machinery manufacturing” 😀
Wanted to toss an idea out there:
In twin-screw extrusion there’s a method where the screw is fabricated from inter-meshing kneading blocks as shown.
Pros: May be cheaper in theory than a machined screw, kneading blocks can be adjusted post-fab, and you get the benefits of twin screw (higher mixing)
Cons: More parts, probably slower extrusion
This definitely isn’t industry standard but could maybe work for this application. I’m new here but have followed the project for a little while now. Also- I’m finishing up a polymer processing course now so I don’t have 20yrs. experience but if anyone wants notes and slides explaining this process better let me know.
Interesting. Maybe I’m missing out on something but how does the plastic get pushed forward then?
In principal, screw extruders are using shear flow and a pressure gradient to push polymer forward and a lot of the shearing and viscous dissipation imparted by the screw is what’s melting the polymer.
The single screw and twin screws are both operating on the same principles but twin screws are more difficult to model. Additionally for twin screws: the two screws can rotate in the same or opposite direction.
Honestly, it seems like compression molding might be worth looking into as well because you can get a decent variety of molded parts, it’s simple, and it’s least expensive.
Interesting, looking forward to seeing how this will turn out! (Pun not intended)
I’ve been trying to find a material supplier where I don’t need to buy a 6 meter rod (and them not being able to chop it up into smaller chunks), with little success. But tomorrow I might meet somebody that can help me out.
I finally got some material… Now just need to find the time to try and mill it out!
Has the Chinese one arrived yet?
I love Alibaba and AliExpress. I hope the prototype works well. They will be getting a bunch more orders if it’s more efficient than the current version!
I decided the screw I made above wasn’t good enough. Though it worked well with injection moulding pellets, the channel in the screw was too small to feed shredded plastic consistently, even if it was screened quite small. Making a bigger screw was a challenge and I had to build a couple of things to do it. The new screw is a lot closer to Siemen’s design, 25mm diameter, 25mm pitch, it’s even right-handed!
Haven’t tested it yet as I have to rebuild the whole machine around it. A couple of comparison pics below and here’s a video of how I made it: https://youtu.be/cf4dQ2xw4OM
That looks pretty sweet! @andyn Pretty crazy lathe skills there. How long did this take? Which material did you use?
In the mean time we got the indexer running at the Fablab in Moss. It’s been a very slow process of setting everything up. The indexer was now just standing on the machine with not positioning whatsoever so we had to spend some time fixing that (and to be honest I assume there will be some deviation on the positioning as well, as we were manually trying to put it in the correct place with the tip of a V-bit as an alignment tool on the y-axis).
We did a test run in wood which seemed to work quite well but looks horrible because of the wood chipping off from the wood grain. The tricky part with this setup is machining the conical part. One reason being the big distance between the clamping on the side, the other reason because of the geometry.
It took a lot longer than I expected, over a week. The problem was the small motor I used for the milling attachment would overheat after 10-15 minutes and then I had to leave it for 1/2 hour to cool down.
The material is bright mild steel (EN32B).
Not sure what you mean about the tapered part, is is just hard to set the machine up square and parallel?
The problem I found with milling it that way is that to do steel, you need a very rigid setup which is why I went with the lathe. Can the Fablab machine mill steel?
Yeah it is hard to set it up square and parallel. But what I actually meant was that the front and the back part of the screw can be made with a straight milling bit fairly fast. But for the taper you will get small steps with the straight milling bit. And setting up that part in the cam-software is less straightforward than we expected. I can provide images to explain better if you don’t get what I mean.
It is possible to mill steel at the fablab in the center of Oslo, but we haven’t tested it on the machine in the fablab outside of Oslo yet. We’re first going to try in aluminium so we can check whether we can make that work. After that we can potentially dig into milling steel on the indexer.
wow that looks nice @siemenc! We got our screw from china installed in our extrusion machine. It fits perfectly and works super nice. It sin’t necessarily smaller but it does have waaay more pressure! We are trying to make solid tubes with it. Filling up a hollow tube with the extrusion machine, we already managed to make a 40mm x 3M. II have to say it goes beyond our expectations. Super impressed by this screw!
No for some reason it just works. Get a metal tube and fill it up. That’s all. Super easy. Haha it has enough pressure to fill up the entire 3 meters. Haven’t really pushed it to the limits yet. Think we are currently about 40 mins. @mathijsstroober has been working on this, he knows the details.
exactly what Dave says. Temp 235-220 with extrusion at full speed (@Davehakkens what is the speed). Filling took almost an hour. Then let it cool down and it is really easy to slide out. We were really surprised, first time something goes in one way as we hoped.
Later, we put a small wooden block in the tube to build a bit of pressure to get smoother surfaces. I will show a small stool, hopefully, next week.
Aha ok gotcha! For a second I thought the plastic shoots like a canon all the way to the end of the tube and fills it up like that. :p Now I understand it just pushes the dried plastic further through the tube, right?
Does that mean that you can create plastic bars with infinite length? By making the length of the tube a bit shorter and perhaps cool it somewhere in the middle and leave the other end of the tube open so the hardened plastic comes out. Perhaps from a certain length it needs some help with feeding the plastic bar out of the tube. Or is that what you did now as well?
Ok, I see what you mean now about the taper! But I really don’t think it matters, I kept my cutting tools parallel to the axis of the screw the whole time, I could have angled them easily, but there’s no need. Yes the compression zone will appear to have steps between each turn when viewed from the side (mine are only about 0.33mm and not really visible) but the channel in the screw is a smooth transition from one end to the other.
In fact if you were to cut the taper at an angle, you would have to blend each end in with the straight parts either side which would be more complicated and give a less smooth transition between zones. I don’t know if @dustintweir still reads this forum, whats the industry standard way of doing this?
I’m glad you’ve had success with this type of screw. Did you find the plastic chips need to be smaller than with the wood auger bit? I found my first smaller screw needed really fine chips to work consistently, and it took too long to shred and filter them.
@siemenc we used a really long bar. We wanted to do what you said (using cooling to get infinite bars). However we found that the newest added material is going totally through the beam untill it gets to the end. So if I use for instance black, green, blue. Blue will be in the total beam in the core and the end of the beam, than the green, than the black. Maybe @davehakkens can show an image of that.
We guess it is because friction and the cool temperature of the tube that the new material is traveling true the core till it reaches the end.
Because of the molten core the plastic has not so much struggle to travel true the tube.
What you mentioned is done in the industry but requires a lot of cooling.
Industry extrusion beams
Btw shooting plastic sound really cool, this is a slow process.
@adyn we use still the same shreds. The screw gets only smaller were the plastic already is molten.
I would love one of those for my extruder. That is a much better than the drill bits hands down.
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