Recycled PET extrusion

[xStolix]

Nice project, I am currently working on a recycle project and am looking for information on extruding PET (single use plastic bottles) such as what is the pre-extruding, extruding and post extruding process to get this to work. I am at the point of extruding the flake I have created but hesitant to put it in my machine because of the crystallization/ extruding problems I see on the internet :)

My limited research suggests the following:

-Re-crystalize after you flake it at approximately 170 F to 200 F for up to one hour.
There probably will be an issue of the flakes sticking together in the re-crystalization process, may need to use a tumbler inside the dryer. (which is an electric oven for me)

-Dry the flakes approximately 300F to 350F for 4 to 6 hours, dewpoint of the drying air should be -20 F or lower. This is to achieve a final moisture level in the range of 0.003% to 0.005% prior to melt processing in order to prevent any chance of hydrolytic degradation.

I haven't found a successful PET extrusion video or much info from this point.

Any guidance would be appreciated!

[Maaphoo]

Thanks Stoli. We are glad to hear you are working on something similar. I haven't quite got to this part of the documentation yet. Look for greater detail related to the following comments later on in the Wiki.

Your research is pretty much the same as what I have read. In actuality I have never had issues with PET bottle flake sticking during the drying process and we don't use a recrystallization stage. We work with such small batches that we just use a bench top "toaster" oven. We are currently beginning work on a drying hopper so that the flake isn't exposed to room temp wet air between drying and extrusion. The flake must be dry and it must be clean. All bottle glue and label scraps must be removed. The main issue you will face when extruding wet flake is low melt strength and viscosity due to the polymer hydrolyzing.

I don't know the specifics of your extruder, so I can't comment too much on that. Instead I'll tell you about problems we have had in the past while extruding.

Sometimes if there is plastic in the barrel of the extruder and the screw isn't turning or there is some blockage at the nozzle, the plastic melt will flow back to a cooler part of the barrel and solidify essentially gluing the screw into the barrel. Melt plugs form in much the same way where the a solid section of plastic adheres to the screw and prevents the passage of incoming flake. We have encountered both problems in the past. To remove the barrel, we just heat until everything has melted and then pull the screw while hot. Once cool, we remove the remaining plastic by drilling it out.

Here's a link to our Digital Blacksmiths site. There is some video about half way down the page. The extrusion bit starts about two minutes into the video.

Stay in touch. We'd be happy to share our experiences and learn from what you are doing as well.

[xStolix]

I just went for it and had a successful first try. I need to work on preventing it from curling which also affects the diameter inconsistency. I think a hot bath may solve it to prevent it from getting shocked so bad.
I re-crystallized the flakes for 45 minutes at about 75C, then did further drying for 3 hours at 170C. I will do weight tests on my next batch. Extruded at 260C to 280C, the extrusion is so viscous that it likes to collapse on its self, making it really hard to start pulling. Overall it extrudes nicely and the resulting filament physical properties are very similar to virgin PETG filament. I need to do more testing and tuning but so far this seems viable.
One thing I would note is I did not let my extruder cool down with the PET in the barrel, I used HDPE (bottle caps) to "purge" the barrel, I've been extruding HDPE for awhile now and have trust in it not to clog anything.
Printing with it was pretty straight forward. It sticks to itself well and has some warping. My filament is not round and varies in diameter so there will be inconsistencies in extrusion. The part is very strong and layers are bonded well. Happy about the first try :)

[Maaphoo]

Great start! Glad to see that you went straight on through to printing. PET can be challenging.

A few thoughts that might help:

-It looks like your filament has an amber tone to it. We also had this frequently, but were able to reduce it dramatically by focusing on making sure the bottles are as clean as possible before extruding. The label glue is likely the culprit.

-The white color in your print is most likely due to crystallization and or water in the filament. Make sure your print cooling fan is blowing on the print so that it cools quickly. Also you will need to dry the filament before use. This needs to be done without recrystallizing or the filament will become brittle. We heat our filament for around 4 hours at 60-80 °C. If the temp is too high for too long, the filament will crystallize and become brittle, cracking as it is goes into the printer, so test your drying setup on smaller batches first.

Thanks for the images. Is your extruder similar in design to the Precious Plastic extruder?