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.
You must be logged in to reply to this topic.