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  1. #1
    BobL is online now Member: Blue and white apron brigade
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    Default Tapered compression screw.

    One of the chaps at the mens shed has been beavering away on his own for more than a year at setting up a DIY plastic recycling system. His goal is to produce extruded rods/filaments of various plastics from things like HDPE milk/ containers.

    He's from South America and didn't speak much English but fortunately I could understand some Spanish and his english has improved significantly over than the last year. Although he is not very technically skilled, he is a fast learner and does come up with some very good ideas that are mostly beyond his abilities so I have been called in from time to time to assist. I don't really want to get involved too deeply because it then means I end up doing most of the work so I step in and help only when he gets really stuck.

    So far I have helped him design the teeth for a plastic shredder and he has had these laser cut and I am leaving it to him to have a go to work out how to mount them on a pair of shafts. I also helped him source some motors and he cottoned on quick at how to use gum tree and has been sending me lots of links to motors "for sale" to check out. Recently he found a suitable 1HP 415V 3P one with a nice 15:1 gearbox attached to it and he brought it around yesterday for me to test and it ran it from a 240V VFD just fine. I said I would convert it for him if needed but for the moment he will try it out as is.

    His next big task is to find a tapered compression screw for use in the extruding process.
    screw1.jpg
    Has anyone ever seen anything like this or similar around?
    The screw sits inside a tube and the shredded plastic is fed in at one end and the heating process starts about the middle section and the plastic is extruded out the end.

    Thanks

  2. #2
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    https://capotexl.de/wp-content/uploa...2480434822.jpg

    Use a drill bit maybe. Google filastruder I believe that is what he is trying to build?

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    Hi Bob, I don't know anything about plastic processing but I assume that the tube is an induction heater to melt the placka.
    The screw should be easy enough to make if you had a TTA long enough, (maybe a bit of time there tho).
    I think my TTA is only 10 or 12".

    What is the finished tolerances? + or - what ? Is that 0.8 for the land and 0.02 for the screw?
    What is the material? I'm guessing that it would need to be heat treated and ground to size?

    cheers, shed

  4. #4
    BobL is online now Member: Blue and white apron brigade
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    Thanks for the info guys.

    Quote Originally Posted by Com_VC View Post
    https://capotexl.de/wp-content/uploa...2480434822.jpg

    Use a drill bit maybe. Google filastruder I believe that is what he is trying to build?
    Yes it is a filastruder but he wants to also be able to extrude stuff a bit larger in diameter than 3D filament.

    Quote Originally Posted by shedhappens View Post
    Hi Bob, I don't know anything about plastic processing but I assume that the tube is an induction heater to melt the placka.
    The screw should be easy enough to make if you had a TTA long enough, (maybe a bit of time there tho).
    I think my TTA is only 10 or 12".

    What is the finished tolerances? + or - what ? Is that 0.8 for the land and 0.02 for the screw?
    What is the material? I'm guessing that it would need to be heat treated and ground to size?
    Thanks Shed. I don't think the finished tolerances need to be anything special and neither does the steel after all its only processing plastic.
    I was thinking a long SDS concrete drill bit like this but he seems to think it has to be tapered.

    Screen Shot 2018-08-10 at 5.33.38 pm.png

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    I was thinking that the reason for the taper is because the material changes volume when it is heated (liquefied) and it also likely needs to move out the nozzle fairly quickly before resolidifying, without the taper you would have (maybe?) the wrong flow rates at each end of the screw? if you used a straight non tapered screw.

    cheers, shed

  6. #6
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by shedhappens View Post
    I was thinking that the reason for the taper is because the material changes volume when it is heated (liquefied) and it also likely needs to move out the nozzle fairly quickly before resolidifying, without the taper you would have (maybe?) the wrong flow rates at each end of the screw? if you used a straight non tapered screw.
    Yeah its a volume change thing but probably less to do with a phase change and more likely to do with the pellets or shredded plastic par melting melting and then taking up less volume. I was thinking the back pressure of the die might be enough and that is probably OK for filament but may not be for for rod formation.

    I'm going to try and get him to just see if he can extrude filament first.

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    Could the taper be on the outside diameter of the screw rather than the root diameter. How about grinding a taper on one of those drill bits and putting into a tapered cylinder?

    Steve

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    Those sorts of screws are used because as has been mentioned, the 'volume' of the plastic changes as it melts. The screws also really push the plastic together.
    Looking at the drawing, the screw is over 500mm long - that would take quite a long TTA. Possibly could be better done with a CNC lathe. The other option would be helical milling it. Either way, not a quick/ cheap thing to do.
    An option may be to talk to places who run injection moulding machines. The screws do wear out so an old one may be available that is good enough to prove the principle though.

    From memory I think you can get different screws (different shear and compression ratios) for different sorts of plastics. What he is trying to do is simple in principle but like a lot of things, getting the process optimised is harder than you might think.

    Michael

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    I can think of some pretty crusty ways to mill one on a manual machine but all of them require a driven universal dividing head and ideally a universal table.

    You could also make the screw in lengths and join them.

    Also you could not bother and use some kind of piston?

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    Also I notice that only a short section of that screw is actually tapered, I'm pretty sure you could do that on a conventional Tta.

    Also note im not volunteering because that thing is 20:1 length to diameter.

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    True. The length also makes it difficult for most people's machines as you would need at least 20" travel on a lathe, so probably 25" to 30" between centres? For me the nastiest bit is the tolerance on the OD - 0.05mm is 2 thou. Not impossible on a lathe, but if there is wear in the middle of the bed...

    While that tapered section is only 157mm long, all the TTA's I've seen are only good for around the length of a MT2 (now why would I know that? ), so you would have to do that section in two parts. Again, not insurmountable but as the surface finish requirement is 0.8 micron, the transition between one taper and the next will need to be spot on.

    There will be a fair bit of torque on this screw, so making in parts is probably not practical as joining becomes a problem (and remember the axis needs to be straight to 0.02mm so that likely knocks out joining methods like welding)

    Michael

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    Quote Originally Posted by Michael G View Post
    While that tapered section is only 157mm long, all the TTA's I've seen are only good for around the length of a MT2 (now why would I know that? ), so you would have to do that section in two parts. Again, not insurmountable but as the surface finish requirement is 0.8 micron, the transition between one taper and the next will need to be spot on.
    My TTA on the Monarch CY will do at least 400mm, probably close to 500mm. It masses more than a Hercus lathe and is really *built*. Main reason I've kept the lathe TBH.

    Not volunteering either. Too many projects already. Also don't have a travelling steady though it probably wouldn't take long to knock one together.

    I can think of a way it's easily (!) milled on any turret type mill though. I think I posted a page or 2 out of the book somewhere, sometime. Can't find either post or book ATM though.

    PDW

  13. #13
    BobL is online now Member: Blue and white apron brigade
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    FWIW he's down sized his shoe setup and gone for some sort of a wood auger. When I'm next over at the mens shed I might take some photos of how far he has got.

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