• Moisture permeability of common plastic bags

    Adam Demuri • 08/17/2024 at 00:11 • 0 comments

    Motivation

    As with most people who own 3D printers, I have a large collection of filament. Filament is moisture-sensitive, so it's best to store filament in a moisture-free environment. This can be done with purpose-built cabinets, but these are expensive and bulky. I prefer to store filament in plastic bags. This allows me to store the bagged filament ad-hoc in my existing storage.

    A gallon ziploc freezer bag fits all of my filament rolls nicely. However, I'm not sure whether it is an effective moisture barrier. I bought some thicker (4 mil) plastic zip-top bags. They seem to work fine, but they are harder to close, and do not fit all of my filament spools.

    So, I decided to do a simple experiment.

    Methodology

    The goal of this project is to determine which bags are suitable for preventing filament from absorbing moisture. 

    I put one 20g silica gel with indicator packet into five different types of bags. I left them for four months in a medium-high humidity environment (50%-60% RH at around 70°). I then compared the color of the packets, to determine the amount of moisture which ingressed into the bag.

    The types of bags that I used were:

    • Gallon ziploc freezer bag [1]
    • Clear plastic filament bag from Polymaker [1]
    • Mylarized filament bag (I think this was also from Polymaker) [1]
    • Control - the bag that dessicant packets came in [1]

    [1] I had previously used these bags, so they are representative of "normal wear" scenarios

    [2] This bag was new when the test started

    Results

    Note: this photo was taken using daylight through a window. The leftmost three packets are blue, indistinguishable (to my eye) from fresh out of the bag.

    The packets in the thick zip-top bag, the mylar bag, and the gallon ziploc all appeared to be totally blue. This means that within the precision of this experiment, no moisture ingressed into the bags.

    The packet in the clear polymaker bag was slightly purple. The zipper on this bag is rather fiddly - it is somewhat difficult to close. The moisture in this bag is likely due to me achieving a slightly imperfect seal on this bag.

    The "control" packet was the most purple, at "mildly purple". This is somewhat surprising, as I would expect the bag in which the packets ship to be effective as a moisture barrier. As with the clear polymaker bag, I would expect that this is due to an imperfect seal. However, unlike on the clear polymaker bag, the zipper on this bag is not too difficult to use. So, it's possible that this bag admitted some moisture. Additionally, this bag is three years old - it's possible that it is damaged or less effective than when new. There is no visible damage to the bag.

    Discussion

    Overall, all of the bags performed well. Opening and closing the bags will likely admit more moisture than leakage through a closed bag of any of the types tested.

    The fact that the control showed some change is a little suspect. Repeating the test with multiple samples of each bag type could provide more data. However, given apparent total success of several of the bags, I think that this is unnecessary.

    Given that the gallon ziploc freezer bag appeared to admit no moisture, it is now my choice for filament storage. These bags are inexpensive, easily available, and the easiest to work with. I find that the closure on these bags is the easiest to operate. Additionally, unlike the other bags, ziplocs are soft and not stiff, which makes them easier to work with.