Drying Method

How to dry flakes out of a washing line

 

Drying, together with cutting, is one of the difficult step of any plastic washing line.

Drying is easy when talking about tick flakes of any elastic material.

For example drying HDPE flakes from plastic drums is the easiest job in this world because thickness is in the range of two or three millimeters, the material is elastic so you can put it into any spin dryer and you get them dry.

The story becomes a little different when talking about thin flakes out of a film line, and for thin we mean 20 microns (of millimeter) or less, or brittle materials like Polystirene, PC, etc.

Let's see one problem at a time.

This film first.

If your centrifugal dryer leaves one milligram of water per square centimeter (or change the units but the general concept remains) and your flakes weighs one gram, moisture content will be 0,1%  and everybody is happy.

Because the centrifuge performs the same way, doesn't matter the thickness of your flakes, the same one milligram of water per square centimeters means 10% if the flake weighs 0,1 gram.

And a thin film flake can weigh way less that this.  Got the point ?

We'll talk a little later on this page about what can be done to dry flakes to almost zero but we would like to talk about centrifugal dryers first and other mechanical drying machinery after this.

Most of everybody uses hot air for final drying, and because hot air cost a lot of energy, one matter is to start from a moisture content of 2% and a completely other one is a10% starting point.

So, before sizing the hot air drying system, you better care about the % of moisture out of your dryer, whatever it is.

We do love centrifuges for quite many reasons:

First is the fact we can get a very low moisture content, even with very thin materials, and very proud of this.

Second because any centrifuge, while drying, it washes as well because it is the combination of friction with water presence.

Third because even the most sophisticated centrifuge is nothing but a rotor spinning into a screen basket that means it is an easy to maintain machine and cannot give lots of problems.

Because the purpose of a centrifuge dryer is to dry, the fact many of these machines are equipped also with the "static centrifuge" make us die laughing.

If you still have water coming out from the perforated cyclone (the real name of the static centrifuge) it simply means the centrifuge didn't make its job, that was supposed to be removing water from flakes.     Isn't it ?

We don't like also to talk about the "squeezing press" because of simple reason: the concept is (almost) right but to make one that works the right way cost so much money nobody is prepared to pay.

So, as far as today, the very best way we know to get plastic flakes dry, is to go with a well performing centrifuge. Period.

Centrifuges can be very different and the one that makes thin film dry will make 30% fines running Polystirene so diameter, shape of the paddles, size of screen holes and may be something else should change accordingly.

After getting flakes mechanically dry, for some plastics, we will need further drying to get the best possible quality.

We'll talk about Crystalline Polymers later in this chapter because they need something very specific.

For all other polymers, what we do suggest, is to get a good extruder with a venting, better with two, and feed it straight with the flakes you got after the centrifuge.

This what we do in all cases but one; dealing with film, doesn't matter the thickness, washing and drying is done with flakes of about 30-40 mm. in size and any force feeding (we know of) accepts only way smaller flakes so further cutting is needed.

Because any granulator will make friction by cutting, meaning developing heat, and then, by blowing, flakes lose some more humidity and by the time they get to the extruder, will be dry enough for pelletizing.

For all other situation, no hot air is needed because a good centrifuge MUST deliver a flake with less than 1% mixture and all extruders will be able to easily remove it by venting.

Crystalline Polymers, as we said, need a chapter by its own.

PET and PC in fact don't like the mixture of water (moisture) and heat together plus the fact they are hygroscopic by themselves.          (PA performs the same way even not being crystalline)

So if you dry and leave them in a storage place they will get moisture from the air and the energy you have been using to dry them is just thrown away.

Drying should occur just before extrusion and moisture content is measured in PPM and not a %

Both material need a dryer where they stay for sometimes (few hours) with or without vacuum and this because they should release the moisture that's inside the flakes, or pellets.

If these polymers are not completely dry, during extrusion IV (Intrinsic Viscosity) will decrease a lot and polymer loses its characteristics.

Mechanical drying at the end of the line should deliver a flake with a moisture content of 0,5-0,6% and after this it will be a dryer unit job to decrease it to nothing  (a good dry flakes should have 20 PPM before extrusion)