Making an extruder screw

[Dr Dean]

So I have jumped right into another hair brained idea. My wife wants a patio but as usual I just can't bring myself to do anything normal. So I have been experimenting with recycling plastic grain bags and I've decided that the best way is to extrude it into a mold. I have a piece of 1.75" I'd 1/2" wall pipe and I would like to make the screw to go inside of it. I think I want to use a 1" pitch on the auger flight but my little craftsman lathe only has gears down to 4 tpi. I need some ideas to get the lower pitch, obviously I will be making some parts to make it happen I'm looking for suggestions.
What I have in mind is a .25" thick flight with .75" between them. The center od will start at probably .75" for 12 inches then taper out to 1.25" over 12" then run that size for another 6". I do have a taper attachment so the taper won't be an issue.

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[Scotts]

That sounds interesting. However, why do you need a screw?
If you are only using one resin and not worried about mixing it with a screw you may be able to inject it just like a syringe, albeit a heavy duty syringe.

If you have an idea of the volume of the part you are making you could design your syringe around that. Do you have a hydraulic press to operate the syringe?

https://youtu.be/cVsRX0jnsqE?si=U3FrO5bpLUE6zyDZ here is an animation of the injection molder with the screw and all. You might be able to make your parts just by heating the tube with a torch and pressing the plastic into a mold.

Here is manual molding machine. https://youtu.be/5AMkaljguJg?si=Dga28-Z-boqyoDA8

You might know all this already and I did not know how to make the screw on your lathe.

Scott

[arizonian]

What lathe? Quick change gears? Give us a photo of the gear train. Do you have means to cut new gears?

In case you missed it, I added a gear to my gear train to make 11-1/2 TPI by fudging the gear train.

[dubby]

I have no idea on how to make the auger either, though I've got a fair bit of experience in hand molding plastics.

I'm not sure whether to ask for more information on what type of parts you're planning to make or just stay outta the way on this one.

[midmosandblasting]

How are you planning to melt .

[Dr Dean]

Well what I would like to do is make planks, say 1X6X8 foot. I had thought about a piston pump type arrangement, like a concrete pump however heating and making it started to sound like a lot for an experiment, honestly making a screw seemed easier on the surface.
For heat I was planning on a sleeve around the extruder tube with a propane burner.
I've had several thoughts on getting the pitch I want. One is to make a coarse lead screw with coarse half nuts. Another idea would be to chain drive the lead screw off of the part being turned. I'm just tossing some random numbers because I haven't checked the thread pitch on my lead screw. Say it's 10 threads per inch so my gear increase would need to be 1:10. 10 tooth driven with a 100 tooth drive.

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[Dr Dean]

BTW I started out bright and early this morning with big intentions. About 10 minutes in I decided to move a mold I had made from 16ga sheet. Well I hadn't deburred it and it got away from me. Seventeen stitches later my right wrist isn't leaking anymore.

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[toprecycler]

Ouch. Do your best to stay out of those medical places. Sick people hang around those places.

Arm looks bad, but could have been worse.

Now this project has to continue thru, just because you can’t let it beat you.


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[midmosandblasting]

Commercial is about 3 inches between highs of flight . I am guessing lower distance between highs will give better melts .We had a member that worked on injection molding equip .

[arizonian]

So, 1x6x96 = 576 cubic inches. That's a lot of heating and then cooling before the mold can start the next run. Let's say the density is 0.05lb/in3, that's almost 29 lbs.

Best guess is the lead screw has 8TPI. That lends itself well to different pitches via the gear train. You will need to cut gears that will overdrive the lead screw by a factor of 4 when set up for 4TPI. Not familiar with the Atlas/Craftsman lathe, I take it there is no quick change?

On edit, you will have to make multiple upon multiple passes, that little lathe will not cut a 3/4" wide swath.