Problems with PetG

[Ama-fessional Molder]

Vacuum drying is almost the ideal process in terms of manufacturing, because it is a rapid process with minimal exposure to heat. To my knowledge the only negative is cost of the units, but they are amazingly fast and often used on high precision processes.

Desiccant gels or beads are actually extremely effective, the problem is you need airflow through the material for it to be effective in a reasonable amount of time. The way a proper polymer dryer works is that cool air is forced through a bed of desiccant, thus removing nearly all moisture from the air. Following this, the air is heated to a set process temperature and forced through the bottom of a hopper full of resin pellets.

The air is cooled off somewhat before returning to the desiccant bed because warm air wants to hold more moisture and transfer less to the desiccant.

You are correct in that simply sticking desiccant gel or beads in the warm chamber is less than ideal. They certainly will absorb moisture, but you are limited by the fixed temperature point.

All that being said, the other primary issue is this:

Overdrying is impossible.*

You cannot dry a resin too much that it causes degradation. I see the word thrown around a lot in 3dprinting and even in my own industry. "Oh, it's just overdried." No, the proper term is oxidation. The material has been oxidized, and in most cases this destroys the properties of that material. This is what happens in the cases of people drying their spools for days or weeks at a time only to have them snap like dry twigs. It is not an issue of being "overdried".

They leave their storage container at an elevated temperature for far too long and the materials actually begin to degrade from interaction with the oxygen in the environment. In theory, one could store them at any temperature below Tg in pure nitrogen and never have an issue, but that's pretty impractical, and not necessary.

The correct solution is limited drying time. For an example, on particular batch of ABS might call for a 170f drying temperature and dry time of 4 hours. After 4 hours at 170f, using a desiccant bed forced air drying system to maintain a very low dewpoint in the air (the "standard" is -40 degrees but some materials work fine with higher), the material should be down below the moisture content required for processing.

Of course in 3d printing, I don't know the exact requirements for drying, but the polymer processing requirements should be identical to injection molding. Thus far the closest thing to proper drying I have ever seen is the filadryer - but even that is ineffective for proper drying and does not get the moisture content low enough for reliable processing.

Drying your material first, then mounting the spool and printing will no doubt give acceptable results, but the issue lies in the following hours during the print. Many materials can absorb moisture from the environment at a rate that would give you wet material within 30 minutes. A specific grade polycarbonate that I use only takes 15 minutes at 50% RH to exceed the minimum moisture content. That's a typical well controlled home environment.

I have some ideas in mind for a solution to the improper drying and storage I am seeing in this field, I just need some time and motivation to develop the drying unit. Once I do however, I will freely share it and my results with everyone. I just don't want to see some manufacturer steal what might turn out to be an effective design and start making money off of all of us. I guess I should look into open source copyrights? I digress.

And again, this is all coming from someone who doesn even own (yet, it's on the way) a 3D printer. I just know how polymers behave during processing conditions. The idea I have right now could very well turn out to be useless or totally impractical.

*Some materials actually do require a specific moisture content during processing, however, you can always put moisture back into a material and run it through the drying process again. These are usually rare cases of highly engineered, custom resins that I believe would be extremely unlikely for you to use in 3d printing.