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.
Nice progress 🙂
If you use aluminium or brass, there might be a risk you chip of material from the screw and contaminate the plastic?
@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.
Unfortunately we only have solid aluminum bar with a length of about 15 cm here. But might be good enough to try and make proof of concept.
allright, keep us posted! Yeh Fablabs/ 3axis CNC would be great. But tough the find.. I’ll ask around more a bit as well. Can’t wait to test it out!
China will most likely be a cheap option (if not the cheapest) for custom stuff like this. Though it would be good to have the knowledge/skills to do this ourselves. I contacted somebody from the fablab here, so if I know more I’ll post it here.
I think it’s also an interesting challenge to see if it would be possible to get good results on a 3 axis-cnc, since that is more common. I’ll check tomorrow if we have some material to try this at our workshop.
hi @siemenc! We got some budget to make/test out your extrusion screw. Had a quick look around on Alibaba to manufacture it and it seems doable for €400 inc shipping without Barrel. Just your screw. But we are also open for other places like a Fablab. Are you interested in helping out to find a suited place that can make it? It takes some effort to dive into that world, its interesting though 😉
Alright, here it is.
Very good point. I just mindlessly copied that part without checking how the drill is actually attached to the motor. I do think it would be easier, in case this turns out to be a succes, for others to be able to replace the screw without having to adjust other parts of the machine. Or do you mean just having 3 flat faces instead of 6?
I’ve sent you the files as IGES and STEP. Let me know if you can open them. I made the tip 15 degrees and added 0.15 millimeter tolerance between the pipe and the extrusion screw. Let me know what you think.
Is there a way to post the file on this forum or is there another place files for precious plastic are being kept?
@siemenc That’s it, nice work! I would add a radius where the flight meets the root. The flight doesn’t have to come out sharp from the shank, but since the plastic doesn’t really get into that area it’s not critical how you start the flight; whatever’s easiest to machine.
The tip design is fine, but typically it’s flatter than that (like 15 degrees from vertical) with a couple of generous radii at the very tip and the root (10 or 15mm radius).
You’re going to have to be sure that there is some clearance between the barrel wall and the OD of the screw. 0.1-0.2mm is OK. More is fine, but it just reduces the efficiency of the screw.
Can you send me an Autodesk Inventor compatible file? [username] at gmail.
I had a bit of time left yesterday evening and I’ve had a go myself but I have not idea if this is the way it should be. For instance I didn’t figure out the logarithmic increments, as I wasn’t sure if this is included in the parameters you gave. This you seemed easier then I expected it to be so I’m sure I’m missing something.
What do you think?
@andyn Thanks for the explanation!
@dustintweir Cool, looking forward! Just out of curiosity, which software do you use for making the 3D model? Do you manually model this or do you have access to some kind of software package that generates the 3D model according to parameters you enter?
@siemenc: Great drawing, this is perfect. I should have a 3D model done early next week thanks to this information.
@xaviervj: Calculating throughput is very difficult to do with no prior information on similar screws and resins. Typically it’s estimated based on testing iterations of specific materials with specific designs. With that said, you can (very) roughly calculate the absolute maximum throughput by taking the free volume in one pitch in the metering (narrow downstream) section and multiplying by the plastic density and rpm. You will have less throughput than this due to compression, upstream leakage over the flights, etc. (Pi*((Dbarrel-Dscrewroot)/2)^2)*(Pitch/Flightwidthratio)*density*rpm [g/min]
@andyn is right about the flight width ratio.
> “Dustin, could you just explain what this 2channel length: fly width = 4:1″ is? I think I kind of understood the rest of those parameters.”
He means the ratio of the width of the ‘thread’ to the ‘gaps’ between them. ie. if the pitch is 25mm you want to grind the channel 20mm wide leaving the flights 5mm wide for a 4:1 ratio.
No, as of now I haven’t started on the 3D-model yet, since it seemed like @dustintweir would do this. But if he’s perhaps too busy I will have a go at it, one of the coming days.
Dustin, could you just explain what this 2channel length: fly width = 4:1″ is? I think I kind of understood the rest of those parameters.
There’s some really great discussion going on here! My main question is regarding the size of the finished piece. This is, of course determined by the die, but, if working backwards from a design, how does this determine the size, ratio or OD of the screw? In other words, is there a formula for determining whether a certain screw (I’ve go my eye on one on ebay) will be suitable for the amount of material that I want to put out? My first instinct was that the barrel and screw would have to be of a greater diameter than the outer-most boundaries of my extrusion die, but a little bit of reading and youtube watching suggests that this may not be necessarily the case. Any help would be appreciated.
For an extrusion screw it doesn’t really matter, as Dustin says you can just reverse the motor.
My screw is for an injection machine, which means it has a valve on the end which screws into a hole in the end of the screw. If I were to use a right hand screw then the valve and tapped hole in the end of the screw would need to have a left hand thread to stop it undoing as the screw rotates. By using a left handed screw I can use standard threads for the valve.
FYI the 26mm bore 4mm wall tube seems hard to find here in the UK (and I expect impossible to find in the US). Not sure about Europe. Would it be better to go for a smaller dia. more readily available size? 19,20,22&24mm bore cold drawn seamless tube is easy to get here, as is 3/4″ and 1″ pipe (21.7 & 27.3mm bore). A smaller screw means higher pressure for the same torque, which means better performance from a smaller motor, and the relationship is proportional to the square of the screw diameter (all other parameters being the same). ie. a 26mm dia screw needs a motor 4x bigger than a 13mm screw. This will be more of a factor for a compression screw than a drill bit, as the screw itself is supplying much of the heat to melt the plastic.
Nice @andyn! Can you explain a bit more on what your process was for creating this? Did you do this on a manual milling machine? Did you use an indexer or something like that? Did you use specific dimensions and ratios for the design or something that you feel like it would do the job?
Hello, I am considering making these screws by collaborating with a local makerspace who have a metal-capable CNC machine
My plan is to upgrade the CNC machine with a 4th axis
Can any of you offer any advice?
Some claim aluminium is not strong enough but, as far as I know, nobody actually ever tested. I guess it would be possible to cast it but the sandcasting is not super precise as far as I know so I’m uncertain how well the extruding will work. I think you’ll come across problems along the way but it would be very valuable information to share here in case you try it out. So in case you do, please let us know how it goes! 🙂
The easiest way to change the model would probably be to just scale it in 1 direction (this is possible in your slicer). But you’ll end up with thinner threads, so that might not work. Ideally you’d have to model this from scratch with the correct parameters. I don’t know how to get those correct parameters. The model for this screw was based on the expertise of @dustintweir.
Ok, great thread with a lot of info. I want to built myself a large 3d printer which uses granulate or flakes of recycled material. But the auger here is too large for this use. I need a smaller one because the whole extruder needs to be not too heavy/light.
I did not find ready made augers but i had an idea. Maybe it is possible to model the auger with software. Print it in pla and cast it with the lost material method with aluminium. Do you think that would be feasible? Or is alluminium too weak?
Keep checking the bazar, there will be a packaged screw and proper fitting barrel there very soon. 1 / 2nd week of April.
Hi guys, have a question:
I have found a pair of screw and tube. But the diameter of screw is 3mm smaller then the inner diameter of the tube, which means screw will be a little loose.
IS THAT OK? or should i find another tube?
You must be logged in to reply to this topic.