Good day Asgardians,

Is there a way to DIY a machine that would transform a plastic bottle into plastic pellets?

Any advise?

Have a nice day,

how about to cut them into small pieces. then heat and mix them ?

Maybe we can heat it and then it melts, and if the temperature is high enough and under the plastics materials' decompose temperature and it must be divided into many small parts, it will form pelltes automatically in space because the surface tension of the melted material is equal in every surface point. 

At last cool down the temperature, you get pellets you want. 

There is actually a really good plastic recycling project called precious plastics. I believe if we are working in a gravity based environment then these machines (which are all open source) would be appropriate for our needs in this case.

HELLO! I use Google translate.
I've been using a 3d printer for a year and a half. It is not necessary to melt the plastic into pallets. It can be immediately melted on the rod using a screwdriver (I do not know how to correctly translate it into English) .. The plastic waste will first be pulled into the crusher (the same granule size is not necessary, the main thing is that the plastic does not exceed the required size) and then by means of auger traction It would be dragged into the melting chamber and squeezed out the finished rod for reuse in a 3D printer. Collecting such a machine for processing plastic is not difficult and expensive, as well as a 3d printer.

ПРИВЕТ!  Я использую гугл переводчик.  
Я уже полтора года использую 3д принтер .  не обязательно пластик переплавлять в палеты . Его можно сразу переплавить на пруток используя машинку с шнеком  (точно не знаю как правильно перевести на английский)..  Пластиковые отходы будет затягивать сначала в дробилку (одинаковый размер гранул не обязателен, главное чтобы пластик не превышал необходимый размер) а потом  по средством тяги шнека его бы затягивало в плавильную камеру и выдавливало уже готовый пруток для вторичного использования в 3д принтере.  Собрать такую вот машину для переработки пластика не сложно да и не дорого,  как и 3д принтер.

well, not pellets, but currently US naval ships use a heated press to turn waste plastics into 9-14 Kg disks for storage. here's some info on it:

http://greenfleet.dodlive.mil/files/2010/04/PWPEnvironmentFactsheet_v2.pdf