Drying of materials Part 4.

In the previous installment we gave the basis of the drying process, in that we need to heat the material up, run a dry air stream over it and allow the moisture in the pellet to jump into the air stream passing over it.

So let us start as previously stated with factor “c” Dryness of the actual air going thorough our pellets (referred to as DEW POINT)

Dependant on the dryer that you are using there is either desiccant, molecular sieve, vacuum, and what we shall call other and non desiccant.

Please note the following about dryness of air versus dryers.
1- If using a hot air dryer you are only heating up the general air in the plant  and what ever moisture content (humidity) the air is that is what the moisture content of our air stream well be.
2- If we use refrigeration type dryer than we compress the air and take the moisture out of the air but only down to approximately 40 F. This still leaves approximately 6000 parts per million (PPM) moisture by weight. Typically used with ABS and acetate type materials.
3- Desiccant type dryers can get down to a -40F on Dew Point which translates to 100 to125 PPM of moisture content.

Now let’s talk about our DEW POINT. A simple explanation is the temperature at which dew will form is called Dew Point.  From our standpoint the lower the dew point temperature the better as at 40 F we are at 6000 ppm moisture and at -40F we are at 100 to 125ppm moisture.

What this creates is a vapor pressure differential between our pellet and the air stream. (Basically the air is dryer than the pellets in moisture content and moisture seeks an equilibrium)The greater the difference between the moisture in the pellet  and moisture in the air stream, (dryer the air) the easier the movement of moisture from our pellet to air stream, given proper temperature and air flow.  From this we are now drying the resin. If the vapor pressure goes the other way we will absorb moisture into our pellets and have issues in processing and physicals of our parts. This is sometimes why using the material directly from the sealed bag works better than using the dryer. I have done this many time to show that the dryer is not adequate for our process.

Note that some material such as Nylon may state that you can dry over a range of temperatures, but at the lower temperature it will take longer. Also that with some materials, those of a very hygroscopic nature, they may be very specific about the Dew Point the material needs to dry. An example may be PET or Rigid Urethane which have to see a minimum on certain grades of at least -30 dew point.

Thanks for the time, to be continued.

TA
Steven L Silvey
Silveys Plastic Consulting
silveysplastics@aol.com
360-882-3183