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Melting clear plastic (PET) into sheet Answered

Aloha, I am working on recycled PET plastic for art projects. I have read everything that I can find on melting clear PET and reforming it. My issue is it keeping it clear or at least translucent for the art projects. I cut clear water bottles into small pieces and place on a cookie sheet and place in 500F degree oven. In about 3 mins it is liquid, still clear, and in the shape of the round pizza pan it is in. Almost immediately after I pull it out it begins to cool and turn white and opaque. So, my question is, is there something I can do to keep it clear? Is there possibly a cooling process to do this? Maybe slowly drop the temperature? Any help or tips will be greatly appreciated. Aloha, Tim

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la xerra

6 weeks ago

what size of sheet/s you looking for?
i have been experimenting with flattening old bottles etc , too. pet is tricky, because, as yu both say: it shrinks. hdpe, #2, is much better to work with = molds nicely to heat... but i never saw it as transparent as pet.
i have very limited means to screw around with up here in the mountains, but i ´baked´ cut-up plastic bottles in the woodburner pressed inbetween ceramic tiles.
also: i´ll start playing around with a flat-iron over at a friends house with grid-power... try that, cut up/open big bottles and put some fabric or paper over them and start iron them like clothes.. start at low temp-settings and ramp up if needed, or if you cannnot adjust heat just put more layers of fabric/paper/etc inbetween

ps: please post some pix of your plastic-pizzas... would love to see that. gracias & suerte.

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Downunder35m

6 weeks ago

I guess it comes down to understanding the properties of the material you work with.

There is a huge difference between a melting point and the temperature where a plastic turns from solid to being formable.
You would have noticed the shrinkage when heating up - the cut pieces get smaller.
Same happens if you blow hot air through a PET bottle - it turns back to almost the same size and shape as the blank it was extruded from.
While in this semi sold, semi hard state the plastic foramable and plyable.
This is when extrusion forming happens.
Go over this temperature and into the liqud phase and the plastic struggles to turn into any liquid form.
Even extreme soft pieces stick together but don't really turn into a drop, like for example wax would.

Having said, oxygen is your enemy here, same for high temperatures.
Unless you can provide some sort of "meat grinder extruder" you are left with the oxygen part that you could control.
There are multiple way, from simple and dirty to complex and high tech.
I'll try to give you some ideas:
If you can tolerate some contaminent then melting your plastic submerged in oil could be a way out.
I had great success with LDPE and HDPE that I melted in an old pressure cooker.
The downside is that PET requires temperatures above what most oild can handle before starting to smoke or a fire.
So finding the right frying oil is the first thing on the list.
Whatever you use to melt the PET in also needs to be secured in the pot.
For example a small tin can or similar.
You need to be able to press and "stir" the melting plastic to drive the oil and remaining air bubbles out.
Once it is a more or less solid peice of molten plastic it is time to roll it out.
PET won't stick to much of anything so you should have it quite easy here, getting enough friction might be a bit harder, so make sure to wear suitable gloves.
Rolling it out removes air and trapped oil and also binds the pieces together, forming one single piece.
Folding it, pressing it flat and repeating this will allow you to melt it again in the bath but this time in a more sturdy form already.
I usually required 3 to 4 of these melting and rolling rounds to get a satisfactory result.

You can also try to melt the PET into a liquid form by providing an inert atmosphere.
These old jars your granmother used to make jam are perfect here.
Using high temperature silicone to make a new seal you can use the provided clamp to keep it all airtight.
Please make the seal first and let it fully cure as otherwise you won't be able to open the jar without breaking it.
With a glass drill you can make hole in the lid and close said hole with a high temp silicone cone shaped plug to push in.
Helium would be my first attempt unless Argon is avialable if you have a welder anyways.
Be aware that Helium is lighter than air, so you need to fill the jar upside down to drive all air out.
If the PET fully melts in such an enviroment and using higher temperatures than for just getting it soft enough you will need to let it cool down to at least 180°C before allowing any air inside, otherwise the PET will again oxydise and discolor.

Last but not least to keep in mind: Keep it clean!
All PET must be properly washed before you can try to re-use it.
With the problem of modified surfaces to prevent food contamination it might be a good idea to try sand on a few bottles.
Using quite course, clean and fully dry sand, shake the bottles long and hard or make a rotating stand.
You want the inside look like it was actually sanded with sandpaper, rough!
Do the same on the outside, here sandpaper can work really well ;)
After this give it a really good clean and try your melting in comparison to PET that was not sanded or roughened up.
I can not be 100% sure but it should save you some troubles getting to the melting point without overheating as the PET should melt together far easier.

Safety last....
If you work with hot or even boiling hot oil you need proper protection!!!
This starts by doing this outside and far away from flamable things on a single cooker, preferably of course electric and not gas powered.
For yourself you shall use a long apron made from a material that neither soaks liquid up nor fail under these high temperatures.
So the standard kitchen apron is out of the question.
A face shield is a must have, same for suitable temperature resistant gloves.
You can check restaurant suppliers for aprons and gloves used on grills and fryers for example.
And no matter what you do, never have any water around!

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alohatimashbyDownunder35m

Reply 6 weeks ago

Thanks so much for your lengthy reply with recommendations, I really appreciate it! Some of the information I have found in my research is that a person is accomplishing what I want to do with a simple toaster oven. So, since I posted this my quandary yesterday, I've attempted two additional tries while adjusting melting heat down some 50 degrees and the cooling dwell time. It has made a difference in the color density. It is approaching translucency. I will post some photos soon. Thanks again, Tim