Rinsing

After you got your flakes washed and all dirt is gone, you may find out you need to remove the media you did use to remove the contamination itself.

In other words, when you use chemicals, surfactants, detergents or anything like this, you should remove it as well.

Sometimes it is more difficult to get rid of a chemical media than the dirt itself because it can be even more sticky.

One example for all is caustic soda used to remove glue from PET bottle flakes.

After the flakes has been deeply washed, and caustic did make its job to remove glue, you flake is shining and you're smiling because you finally got it completely clean.

Then, you go to extrusion and your pellets come out black or very close to black.

What did happen ?

The few PPM of caustic left on the surface of the flake did react with the Polyester chain and oxidised the whole polymer.

And you're in damn trouble.

Rinsing is an important step of the washing line, specially if you use any kind of chemicals.

Some of them require only a short step into little water, always together with friction, and everything will be gone while, for some other application where the final product needs to be absolutely pure, rinsing should be as deep as the washing itself.