We've just launched our map. Add yourself by clicking here!


Me Speaking out loud: solar extruder

  • This topic has 6 replies, 4 voices, and was last updated 2 years ago by Stan.
Bjorn bjorn

Me Speaking out loud: solar extruder

30/08/2019 at 13:27

The idea:

To make a large scale extruder, to make a timber replacement product, which is

Solar powered and uses mixed plastic.


There’s nothing new about mixed plastic “timber”, you can get park benches made this way.


But here I would like to go through the practical challenge of making this happen and what it would take to use the sun.


The Extruder has two main elements: the heat input and the pressure pushing it out. i’ll start with the heat source:


option 1:  a solar powered heat source could be made with a concentrated solar array, pointed at a sand mass which the body of the extruder would be within.


option 2: concentrated solar with a working fluid (such as motor oil) and pump it around a jacket which might give better temperature control.


option 3:  photo electric panels leading to a “standard” electrical extruder but the efficiency would be very low leading to greater cost.

however this does mean that “standard” equipment can be used



Next the motive force required to push the plastic out i.e. pressurise it.


option 1: “standard” electrical motor with photo electric panels,  geared as required


option 2: steam powered piston, batch process, large cylinder head to “gear up” the pressure.

Basically pool a reservoir of molten plastic inside a syringe attached to a bigger piston.

its simple, can generate alot of force and with proper safety valves….



option 3: steam turbine or sterling engine geared as required to replace the “standard” motor


option 4: gravity feed melting funnel i.e. get enough pressure of plastic on top that it is forced out the bottom.

not practical because plastic isn’t very dense, so would require a very tall column and assumes it doesn’t stick to the sides.

Also worth remembering that there is energy required to lift something up that high in the first place.



energy input to melt 10kg of plastic?

energy/force required extrude 10Kg of plastic? (or calcs to find that info with variables)



Steam pressure tables: https://www.engineeringtoolbox.com/saturated-steam-properties-d_101.html


6 replies
3 subscribers
0 saved
sort on most likes
30/08/2019 at 17:35

Hi Bjorn,
welcome to Precious Plastic. Yes, solar was few times mentioned in the forum and the monthly news but we couldn’t get hands on the details.

Not sure for which area this is but according to our calculations for sunny Spain you can (assuming you have at least one array of 6 panels, panel amount = voltage )
– power the extrusion motor (1-2 Kw) by connecting the panels directly to the VFD. I can find the details back if you like. It’s not a large extrusion but enough to make good 4×4 beams
– power the heatbands as well but you need an extra device for around 700 Euro.

30/08/2019 at 20:40

When you pick your plastic, the specific heat and heat of fusion will tell you the minimum energy per Kg. If your extruder is shearing the plastic, that energy goes into the plastic as well.
If you are going to use motor oil, keep an eye on the temperature limits and flash point.
I think most of the plastic lumber I’ve seen has been a mix of plastic and other materials.
I think a solar anything in Sweden is a challenge…Good luck

31/08/2019 at 10:35

Hopefully you dont get to the point of taking a plastic to its melting point where heat of fusion comes into play. Basically what heat of fusion is is at this point the plastic does not change temperature but changes state from solid to liquid. You just want to find the temperature that with a specific extrusion speed will take the plastic right up to its glass transition temp. but if it is “melted” then heat of fusion and specific heat. if glass tradition specific heat use will be fine. cp*m*delta t basic equation. cp is specific heat of plastic and m is mass of plastic.

My thoughts on this are super heating water then using it on steam engine. Steam engines are not 100% efficient on using all the heat into steam engine. so send that remaining heat across the barrel for the plastic to absorb and send it around to be heated again in the solar exchanger to be sent through the steam engine after. Now you have a 40-60% efficient machine.

31/08/2019 at 11:44

refering to connecting solar panels to the motor  with a VFD, then the heaters to another unit:

you could take the heaters off the same VFD output as AC (as long as the VFD could handle the wattage).



Yeah solar in sweden isn’t happening, we have an electrical grid for this.

The idea is for the rest of the world.



You read my mind: flowing the heat from the engine to heat the plastic.


I’m still thinking the batch process of a steam piston has some merit.

Because this going to be for places where electricity wont find and the simpler/cheaper the better.


I still love continuous process but standard methods have that covered (with a generator)  plus the volume to actually make the product.


Time for some math……….(I may be some time)

31/08/2019 at 12:09

@bjorn, as far I remember, this won’t work well or at all, at least that’s what the solar expert told me when we explored the solar idea running certain machines in our machine shop. Possibly I can ask him again about if you like 🙂

31/08/2019 at 16:34

@thegreenengineers , The glass transition temperature does not have much to do with the usual recycling process. It is the temperature below which the material becomes brittle. For example for HDPE the glass transition is at -120 C. I don’t think that even in Sweden you are passing through the glass transition temperature. http://www.matweb.com/search/datasheet_print.aspx?matguid=f9470672aa5549cb9c7b157677d02062  The melting point of HDPE is above +125 C, so yes, you typically are taking HDPE above the melting temperature. In my injection machine I typically use 200C. If you have been able to extrude or inject HDPE below its melting point, please post the process.

Viewing 6 replies - 1 through 6 (of 6 total)

You must be logged in to reply to this topic.

Support our projects on Patreon so we can keep developing 💪