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Extrude beams from plastic waste

Andy Noyes andyn

Extrude beams from plastic waste

07/11/2017 at 18:40

In this recent video Dave extrudes a beam using a hollow square tube 3m long as a mould.

I’m wondering how the plastic flows inside the tube. Does it (A)cool and form a solid plug which then gets pushed along the tube with more material being added at the extruder end of the beam. Or does it (B) stay molten in the centre, cooling where the beam touches the walls of the tube so that as more material is extruded it flows inside the beam to the far end and is added at that end?

If (A) then it should be possible to use a shorter tube to extrude an infinitely long beam, but Dave does not do this, (he uses a tube that is at least as long as the beam).

He first extrudes red plastic then some way through switches to white. If (A) then the red would be at the far end and the white at the near (extruder) end. If (B) then it would be the opposite way around. Due to the way the video is edited it is hard to tell which way round it is. When he removes the beam from the near end of the tube you can see a small white portion followed entirely by red (you don’t see the other end of the beam). In the next shot the beam appears to have been trimmed down as it is now approx 2/3 red and 1/3 white, the small white portion having gone.

I’m guessing that it’s case (B). The small white portion being caused by back pressure extruding a further small amount of material into the near end as everything cools down and the beam shrinks.

So which is it? Dave?

And if you were to cut through the red section would it have a white core?

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08/11/2017 at 00:07

I have a question as well. Dave mentions the upgrade for the extruder that is the screw, which builds the pressure which will be useful for extruding beams. My question is has Dave tried extruding beams with the previous auger bit? If yes how did it turn out? If no will you please try it once with the old auger bit instead of the screw and post the results here. Also can PET be used to extrude beams. Please let know. @davehakkens

09/11/2017 at 10:03

So guys, I went to V3 exhibition and asked similar questions.


wanted to know why one of the beams is red outside, but black inside. I asked the team, someone told me that plastic flows as a fountain. Like first, the outer layer is formed. Then through this corridor plastic flows further. So (B) it is!

Dave said that they haven’t tried auger bit much, just the new toy conical screw. They might try it in the future. I want to try it as soon as our extruder is finished.

I think that option (A) is going to be in case of Auger bit. But this is to be confirmed.

17/11/2017 at 14:00

So if it’s ‘B’, rather than truly ‘extruding’ the beams, it’s actually working like a big very slow injection moulder. This is probably happening because the cross section of the mould is large and the plastic is able to stay molten in the centre for a long time allowing new plastic to continually flow down the middle.

If it could be made to work in mode ‘A’ this would seem to be better as infinitely long beams could be extruded (really extruded) from a short die. The best way to achieve this might be to make beams with much thinner sections, such as I beams, C channels, or hollow tubes. This would require a more complicated die, but would be a much more economical use of plastic. Weight for weight a hollow beam is much stronger than a solid one, the ladder being a good example. (BTW I’d like to see what happens if somebody tries to climb up that ladder!)

05/12/2017 at 16:55

Dave mentioned that the outside structure of the beam could be made smoother if some resistance to flow were implemented, such as a wood block “plugging” inside the tube. Has this been tried? Would that change how the beam formed, i.e., would the plastic in the core not flow as easily and clog up the space, pushing the outside along?

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