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.
Here’s some images from the 3D model I made of the design concept. What I so far understand is that the distance between the turns of the spiral logarithmically build up towards the end of the barrel, while the diameter decreases. I now need to know what the optimal ratios for these design requirements are.Toggle replies
I’ve thought about this too. Using a wood auger bit seems a bit basic to me, it’s not designed for this and it’s unlikely to be a precision fit inside the barrel unless you happen across a piece of tube of the ideal bore. Ok, it works acceptably, but I thought I could do better. Below is a picture of a compression screw I just made, it’s not pretty and it took me several days to make on a hobby milling machine, but I’m fairly pleased with it (haven’t been able to test properly yet as I’m still building the injection machine).
I wouldn’t make another screw this way. At the time I considered building a machine to automatically grind these, which is the way I would go if I were going to make more than one. If there’s really interest from people wanting this type of screw I could look into this.Toggle replies
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?Toggle replies
It’s a manual milling machine that I converted to 4-axis CNC. The screw rotates as the cutter moves along it’s length, but as the screw is long and thin it’s very flexible and I had to take numerous shallow cuts.
The screw is designed to have a compression ratio of 2.5:1. The lead is constant and the depth of the thread varies along it’s length. I could have also varied the lead and kept the depth constant, or some combination of the two, I don’t know what real difference it would make, if any.
Another reason I wanted to make my own screw is that drill bits are right-handed, which means to feed material forward you have to rotate them anti-clockwise. I made my screw left-handed so that it can turn clockwise, which is just better from an engineering viewpoint. If you look at the pictures you posted above you’ll see that most of these screws are also left-handed.Toggle replies
I have done screw design professionally for about 7 years. I’m happy to help. There are a lot of variations in the market, but you’re on the right path to get a basic extrusion screw that will work well in this open source extruder. Here are the basic design parameters I suggest we target for this screw:
Compression ratio : 2.5 or 3:1
Zoning: 1/3 length feed, 1/3 length transition, 1/3 length metering
Pitch: square (lead equals diameter)
Channel length:flight width = 4:1
Hand: right hand. This is industry standard, and the motors should be easy to re-wire to turn in reverse if your gearing setup spins in the wrong direction.
I’d be happy to design a model with these parameters if someone would be willing to specify the shank design, length, diameter, and show where the feed port falls on the screw.Toggle replies
Here’s what I can extract from the original design files. Diameter of the barrel is 26 mm on the inside. The current drill being used has a length of 585 mm in total. There’s a distance of 365 mm between the end of the barrel and the feed of the plastic material. The drill is locked in a pipe in the back with a diameter of 13mm. Is there anything else you need to know?
Do you use a special software for generating these kinds of screws?Toggle replies
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.Toggle replies
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.
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.Toggle replies
> “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.Toggle replies
@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.Toggle replies
@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?Toggle replies
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?
@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’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?
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?Toggle replies
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