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As I started to learn about 3D printing, the gist I learned was "it's better to store the filaments in a drybox." As I rechecked these notes, they were to a good degree from an era when PLA was rather new to the market and ABS was the goTo.

Then I learned "PLA is not really hygroscopic and can be stored freely."1

Now, I know some materials are pretty hygroscopic, but not all. So, let's try to pin it down:

Which materials are hygroscopic enough to demand a drybox?

I know that it is good practice to store all filament in a somewhat dehumidified or airtight box, but there are some materials out there that become unprintable and need drying before printing if improperly stored. This question is to point out these "special storage mandatory" filaments only. If a material can't be printed without dry storage it belongs here. If it is a nice to have, it doesn't.

This is a Back It Up question: answers need to provide one of two:

  • explain personal experience, marked as such.
  • provide an authoritative source (scientific paper/videos, manufacturer papers, quote from an experienced maker).

1 - For some time (month?). I do store my PLA in a closed but unsealed IKEA container with all the desiccant bags I can find as it is clearly benefitial.

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  • $\begingroup$ Isn't this the same (or similar to) as Filament Storage? $\endgroup$ – Greenonline Sep 26 '18 at 3:30
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    $\begingroup$ @Greenonline While Filament Storage asks "Do I need a drybox" This is "which filaments absolutely need a drybox". Its scope is in intent not filaments like PLA or ABS that everyone has at home but those that one might want to know about to be hygroscopic before purchase. $\endgroup$ – Trish Sep 26 '18 at 6:15
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PVA

From experience I can tell that PVA filaments need to be stored with silica beads in a plastic bag or in a specific dry-box. PVA is soluble in water and is very hygroscopic. With moisture it gets soft and swells. My Ultimaker came with an open spool of PVA which popped when heated (steam bubbles popping) resulting in very poor quality supports and clogging of the nozzle. A newly bought spool which was properly packed did not have these issues.

Nylon

Some Nylon filament brands require to be stored dry or need to be dried before printing. I have a spool of Nylon that has taken up some moisture although carefully packed, the only time it was out is when it was being printed. This experience applies to Ultimaker Nylon; I've learned now that not all Nylon filaments are behaving the same, it is suggested to look-up the specifics from the manufacturer or from reviews prior to buying.

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  • $\begingroup$ Is this an all inclusive list? $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 Sep 24 '18 at 20:16
  • $\begingroup$ @Pᴀᴜʟsᴛᴇʀ2 Just sharing my experience, we could make it a wiki so other people can add. $\endgroup$ – 0scar Sep 25 '18 at 4:37
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All filaments should be kept in a low-humidity, dry environment. The OP suggests that PLA doesn't need to be dried. My experience says otherwise. I had miserable trouble printing with a PLA spool until I dried it in an oven at about 60 C for a couple of hours. It then printed well.

A comment was made, and then refuted, about FDM being the same as injection molding. I agree that, for this purpose, it is. Both involve heating the plastic so that it softens, and neither process will work well if the filament is outgassing steam. They both have a confined melt chamber, and steam will be pressurized with the plastic. When the pressure is released, the expanding steam creates defects in the object.

It is true that some filaments are more hygroscopic than others, but all filaments should be treated as if they are moisture sensitive and should be stored in a dry container.

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Necessary:

  • PVA otherwise it will become unprintable, because it is VERY hydrophilic and becomes soft, but it is dryable

Better to store boxed:

  • PLA otherwise it can became brittle, I have some of such PLA, it brakes from time to time if it is just loaded, but luckily it doesn't broke while printing so far.
  • Nylon is also hydrophilic

Generally you should google for the material you want to use and look for hydrophilic properties or other, e.g. gas releasing, properties and so on.

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You should ask the maker or supplier of the filament which ones really need to be stored in dryboxes and which ones are safe.

Technically, they can all benefit from being stored in dryboxes, but some can handle being left in the open more than others, so it also depends on how fast you go through your rolls of filament. If you use one roll of PLA a month, using a drybox for it is not necessary, if you use it in a year, then a drybox becomes useful.

Though, as a rule of thumb and as was pointed in previous answers, PVA absolutely needs a drybox and Nylon also benefit greatly from being stored in one.

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    $\begingroup$ It should be noted that the length of time will vary depending upon the humidity of the country/environment. A month could be too long in a country located in the tropics, for example. $\endgroup$ – Greenonline Sep 26 '18 at 11:52
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PEEK (polyetheretherketone) will print better if has less moisture, and should be used at under 1%.

PPSU (polyphenylsulfone) prints terribly unless it is extremely low humidity (under 0.1%), and is really only reliable when run from a drier to the print head without any exposure to "normal" humidity from the air in the room.

The point at which you have visible cosmetic issues comes at a different point than that at which you have visible structural issues, but even when no cosmetic issues are visible, there are still potential structural issues at a microscopic level.

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All filaments need a drybox. Before any kind of plastic is injection molded, it has to be dry. I'd advocate a vacuum sealer. If they are not free of moisture then the water will turn to steam and cause little steam explosions inside of the nozzle. This blows the molten filament out of the nozzle causing blobs, gaps and other z scaring artifacts. This is the same thing that happens in injection molding machines. Before the plastic can be placed into the hopper the plastic pellets (of any kind) must be thoroughly dry.

I have personally noticed surface condensation on my PETG filament on a rainy day; here in the tropics. A spool of nylon filament that I have would not stop crackling and poping during the printing process. Its moisture absorption was so high that just having it outside while printing was enough to cause printing problems. The resulting part that I printed was also hydroscopic; so much so that it expands if you place it in water and shrinks after is is allowed to dry out. I had tried to make a cover for a water container with the nylon and foolishly turned the container upside down on the counter just to prove that it was a good seal. I came back a few hours later to find water all over my counter and the floor. The cap expanded, let go of the container, and all the water leaked out. It then dryed and shrunk back to the original size. This caused me much confusion as to why the water leaked out, as the cap was still fitting snugly on the top of the container when I came back to it.

The following link from "CNC Kitchen" highlights these issues in more detail.

https://www.youtube.com/watch?v=FAXUjZZER5E

In short the video says that:

  • Wet filament will have stringing, bubbles and substantial oozing.
  • Moisture can break up the long chains of the polymer changing the mechanical properties of the plastic, resulting in embrittlement.
  • Air bubbles end up in the printed part and result in delamination.
  • The surface finish of wet filament will be rough and inconsistent, requiring post processing (sanding, filling etc).
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  • $\begingroup$ As the original comment has now been deleted, you may want to include your comment "All filaments. Before any kind of plastic is injection molded, it has to be dry. I'd advocate a vacuum sealer." in your answer (or maybe just the vacuum sealer part) - I'll leave it up to you, but I thought that I'd point out that the comment has been deleted. :-) $\endgroup$ – Greenonline Sep 26 '18 at 14:13
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Some brands of PLA+ and colors of filament seem to have issues if left in the open air for too long a period. These then need to be dried back out in a dry box. The filament seems to be softer and the hobb seems to strip it out more easily causing print failures. The colors I have had the most troubles with are transparent Blue, Dark Green, and silky Copper.

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