Filament Drying
I have been doing some thinking about the subject and process of 3D print filament drying systems. I own one branded from SUNLU but there are many other that work on the same concept.
First, the term Relative Humidity (RH) needs to be clearly defined and understood. It has nothing to do with the absolute content or volume of moisture in the material ( air in most cases, but includes wood, plastic, and many other materials.)
RH a ratio of the saturated dew point / temperature. Dew point is when the substance is fully saturated at a prescribed temperature and the gaseous water vapor will begin to condense to liquid water. At this condition the RH is at or extremely near 100%.
True moisture content for air is expressed as grains (weight) of H2O per pound of dry air
20 grains / 50F = .39 or 39%RH. 20 grains / 26F = 1.0 or 100% The same AMOUNT of moister is in the air at both temperatures. But at 26F is where the air has met it’s due point and water vapor will start to condense into liquid water. It’s Raining! err... snowing!
Warm air can hold more grains of water (as vapor) than cold air.
Now for the filament.
The filament drier doesn’t probe into the plastic and take a direct RH reading. There are systems for wood and other materials that do probe to take a direct measurement.
So what we see with filament drying is a “relative” Relative Humidity. The sensor in the dryer is measuring the RH of the AIR in the dryer and not the filament itself.
Heat the air in the case from room temperature, say 75 F @50% RH and the RH should change to 12% @ 120 degrees just because the hotter air can hold more grains of moisture. So the RELATIVE humidity goes lower.
If it stays the same RH or goes higher as the air is heated, then the filament must be adding some of its contained moisture to the air.
The issue is that because we are also raising the temperature of the filament, it too can hold more moisture vapor. But the drying system is trying to maintain a balance between both materials, so some moisture vapor transfers from the filament to the air.
The warm air leaks out carrying some absorbed moisture (from the filament) and is replaced by cool air entering and being heated. Now attracting more water vapor from the filament. The net effect is a drying of the filament material.
As a side note “Freeze Drying” is a far more efficient method of removing large amounts of moisture from product, but requires a vacuum chamber and refrigerant cooling systems. A heated box is more primitive but easy to implement.
The heated warm air driers (food dehydrators) require the warm air to leak out of the container. It’s a similar science to filament driers.
Chemical desiccants adsorb moisture from the air and chemically bind it to its materials. They work best in closed and sealed containers. Large amounts of desicant are available for use in closets
If you really want to be scientific study adsorption verses absorption, But all we want to do is dry our filament… <grin>