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PET General / Common bottles melting help!

This topic contains 7 replies, has 6 voices, and was last updated by  Chris 1 year ago.

Edward edwardmartinhill

PET General / Common bottles melting help!

19/06/2019 at 20:48

Hi guys, I have a injection and v2 compression machine. I have an abundance of PET normal plastic bottles that I have tried in both machines but I cannot get the temp/time correct.

The attached pictures are an attempt to make a plastic sheet 30cmx30cmx1cm. It took untiil 280deg to get any form of melting occur after 20 minutes in the oven.When I got it out and cooled it down under running water and removed the mould it looked like brittle glass and is completely unusable.

Can someone point out what I am doing wrong. Should I be using a higher temp for shorter or a lower temp for longer…? I have had some success in the past for an injection mould but have never been able to replicate it  with this form of plastic. HDPE is working a char, in this mould at 250 for 14 minutes.

I hope I am just missing something simple here or is this plastic just really that hard to work with?

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20/06/2019 at 08:08

hello @edwardmartinhill,

PET is very sensitive to 2 types of degradation :

– Hydrodegradation is caused by presence of humidity or water (H20) while trying to melt. The H2O breaks the PET bonds to form other chemical bonds.

-thermodegradation is caused by being at too high T° or too long at high T° (while already being melted)

I don’t think you have encountered any of these degradations though…
Another point to keep in mind is that PET is highly thermoresistant, meaning if the thickness of the PET you are trying to melt is too big (too many layers of schredded plastic?) it will form a sort of shell and won’t let the center part melt properly.


Another important point i observed was that the material which is in contact with the molten PET while cooling will have total control on the surface’s texture and colour output.


this document  : http://www.inrs.fr/publications/bdd/plastiques/polymere.html?refINRS=PLASTIQUES_polymere_15
says  : 90°C-120°C for extrusion-moulding
230°-270°C for extrusion-blowing

i tried to find a chart for you to visualise easier the thermodegradation but can’t get a hold of it :

Here is a very interesting doc on PET recycling, in french though, but if you can google-translate it, should be perfect for you !



20/06/2019 at 14:00

Hi edwardmartinhill ,
Before I answer your question i want to say you have put in a lot of work. I know its really frustrating to put in so much effort and not get the results you hoped for. PET bottles are every where and people are really tempted to recycle them.

I will lay down some facts about PET any may be that helps. I dont want to disourage you or any one who is trying to find a solution to PET scrap. I want to inform and help people so they can focus their energy and resources better.

FACT # 1 :
PET is an engineering Polymer so it a very demanding to process it. It is not a forgiving Polymer like PE or PP. A few degree changes in temperature and time would kill PET, Litrally. PET starts to melt at 240C , commercially PET is processed at 280C.

FACT # 2 :
PET a hygroscopic material, that means it absorbs moisture from ambiet air. The moisture content can go up 3,000ppm (ppm = Parts per Million). To process PET, it SHOULD be dried to have moisture content below 50ppm. For comparison a PET bottle laying around would gain upto 2,000ppm easily. In humid conditions moisture levels can go upto 3,000ppm.

FACT # 3 :
 Heating PET with moisture will definitely degrade it. see the graph attached.

FACT # 4 :
PET is classified/Sold in Intrensic Viscosity (a.k.a IV) . Higher the number better the properties (Tough , elastic) but difficult to process. Lower the number (And you have guessed it correct,) worse gets the properties (Brittle and Fragile) and flows like water.

As imuh explained PET degradation he is totally on point but i would disagree that you are actually degrading PET. If you observe the graph, you would see with increasing moisture content (i.e 3,000ppm) you would loose all the mechanical properties and have brittle plastic within 30sec.

Sorry to break it to yoi but its really hard to work with PET also in the industry where they spent million’S on high tech equipment (been there).

Keep it up.

21/06/2019 at 17:47

If PET absorbs moisture, can it be a good desiccant?

25/06/2019 at 11:33

Hey guys,

I’m in a similar situation than @edwardmartinhill. I am working with PET biscuit wrappers and obtain pretty brittle pieces after melting 9min at 260°C.

@tajdark If I understood good, the brrittleness of my plastics is due to incorrect temperature (“killing the PET”) or the humidity (which is clearly a problem, it’s the monsoon here in Nepal).

If the humidity’s the cause, is it possible to reduce the water content of my plastics (by drying it 15 minutes at 120°C by example)? I currently wash it in a clothes bag before letting it dry in the sun.

25/06/2019 at 12:17

Answering my question, after little research :

PET-drying equipment exists and is capital.

@microtransactions tried few things for extruding PET filament:

Regarding my situation, it might just add a degree of complexity to a process that I have to present as simple and easy to replicate.

30/06/2019 at 16:02

Hello Guys,
Sorry for the late reply.
hawsan the short reply would be no. PET cant be used as as effective dessoicant.
The long answer would be as following. For an effective dessicant it needs to hold (Absorb /Adsorb) a lot of moisture (Plus other factors too but lets keep it simple). The most commonly industrialy used dessicants for drying polymers are Silica Gel, Zeolite and Molucular Sieves. They can hold moisture upto 35% of their dry weight. Comparing them to PET , it can hold only 0.3% (=3000 ppm) under very high humidy.
Also to use PET as s dessicant you would neet it to be dry in the first place.

marcvdv the brittleness is due to breakdown of the polymer chains. The chains break due to presence of moisture during melting. If you melt PET once dried then you would have good part. Unfortunately PET loves water so it is not easy to get it back from it. Industrially, PET is dried for 6 hours at 180°C.

Something like this wouls suffice

09/09/2019 at 18:47

I gave up on PET.  It was too temperamental and never resulted in anything I could use.  I stick with HPDE, its very easy to work with, melts consistently, and it’s easy to mill/work with after cooling.

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