Depends whether you only require it to be dry enough to avoid cosmetic issues, or structural ones.

For the most part I think that the typical printer is not concerned with the extreme limits of performance, and so drying is less of an issue.

If you were pushing the limits of the materials I have come to the conclusion that a vacuum drying method is the most cost effective. You would use a vacuum pump and glass bowl setup like what people use for 2 part resin casting.

You first heat the filament to around 130-150F (55-65C), then while it is still warm put it under as much vacuum as you can for 20 minutes to a half hour. Voila, industrial quality drying at home.

Properly dried plastic also prints wonderfully compared to the same material when "wet". Once you dry it however, if you have a suitable means of storage you can likely leave it in there for months at a time without having to dry it again.

CA loves to disagree with me here (and his reasons are valid - most people don't have exacting structural requirements), however, I process plastic for a living so I love to rant about it. Since I enjoy it so much, here's my input on the subject if you do have structural concerns:


*Long version*

From my own experiments and experience with hydrolytically sensitive polymers (like nylon, PLA, and polycarbonate), it only takes around 15 minutes of exposure to 50% relative humidity to bring most materials to a moisture content which results in measurable degradation during processing.

It is possible to dry a material to a moisture content below that which shows cosmetic defects, but still degrades, and the reason is that the water is consumed during hydrolysis. Hydrolysis splits the polymer chains, making them shorter and weakening the material. It also causes a reduction in viscosity which I have not entirely determined the effects of on 3d printing, but in molding this causes flash.
Hydrolysis can also result in degradation of certain additives which, depending on the material, may be required to achieve excellent print quality. Some additives react with water, while others do not, and since manufactures typically don't inform the 3d printing community of what is in their filament it is hard to say.

For a semi-crystalline polymer minor degradation is not a major concern. Crystallization tends to negate it as long as you stay below the cosmetic limit of moisture content. If you are running amorphous filament, like polycarbonate, it's a real killer. PC doesn't have the crystalline structure needed to support it and relies entirely on it's very long chain length.


As an interesting side note (maybe I am the only one who thinks this is neat), the reason some plastics turn clear/translucent when molten and become opaque or diffuse when solid is because the crystalline structure interrupts the transmission of light. When melted, the crystals turn back into the disordered polymer chains and do not interfere with the light transmission nearly as badly. I wondered about that for a while before I finally made the connection and then had it verified.