Definition:  Over; 1) in excess of, more than, 2) too great   Dry; 1) free from moisture, or excess moisture,  

In the previous section we discussed the level of moisture as related to the term overdried. In this section we shall discuss the other issues.

All resins have some sort of processing aids in them whether this is lubricants, plastizer, stabilizers etc. In the use of a dryer we must understand that we heat the pellet creating a thermal expansion which than allows moisture to leave via these pathways when drier air flows over the top of the pellets. At the same time other additives which may be in our resin may now also have a pathway to escape.

An example is with cellulose materials such as Butyrate and Propionate which suggest using a vapor trap, and or refrigeration type dryer so as to prevent plastizer from contaminating the bed of the desiccant if that type of dryer were used.

If we dry for extended periods of time, not only are we drying, but we may be taking away the processing aids added to the material itself which in fact is really changing the composition of the resin pellet. The results can mimic that of very low moisture content such as stiffer to flow, brittleness, poor weld lines. 

Other issues with drying for extended periods of time and or at high temperature are the oxidation of the material. Put into a simple form this is a surface effect first and a molecular breakdown secondly. It occurs at a temperature and time factor for each material, the most prominent one to do this is Nylon. The material well go yellow and than physical properties well start to go down, and in terms of flow it well increase and flow easier as the molecular weight declines.

In some cases the use a vacuum dryer can prevent this as it eliminates the presence of oxygen. The chemical process is slowed greatly due to this.

So when some one says over dried we must consider all facets of the statements.

1-     is it that we have reduced the moisture content of the resin too much

2-     is it that we have blown off the processing aids that were in our material

3-     is it that we have oxidized the material

Yes it is complex but paying attention to the details of what is going on is critical. And the following are areas to be concerned about

1-     dryers are left on over the weekend at normal operating temperatures

2-     dryers are left running during shifts when no processing is occurring on that press

3-     We have dryers with the wrong hopper for that material, thus material flows slower at edges than at core. (December article)

4-     We haven't calibrated the dryers or monitored the temperatures of actual.

Solutions to the above:

1-     Put or buy dryers with programmable clocks for starting stopping the dryers.

2-     When the press is down decrease temperature material is held at, and or turn off.

3-     Pace material in a correct hopper, and or figure out if your hoppers are correct.

4-     Have your equipment on a calibration schedule.

As stated previously flow of material will tend to change, have you documented the fill time, pressure at transfer and packing pressure. Have they tended to change at the same time you noticed that the parts are more brittle and or cannot fill? A good indication of what is going on.  If in fact you have run a shear viscosity curve previously when manufacturing good parts than run another now and see if the relative viscosity has changed and document.

If you have access to a moisture analysis than document that.