Saturday 21 August 2021

3D Printing: humidity & keeping your PLA filament dry

It is generally agreed that you should try to keep your reels of PLA filament dry.


But I'm not sure that all suggested methods on the net make complete sense!

So here are my mad ramblings on storage and relative humidity.

It is my opinion that a large number of people on 3D printing forums do not understand Relative Humidity. I also struggle with the recommendation of putting reels of filament in simple, air-tight plastic boxes with small bags of silica gel.

relative humidity

A rough working explanation of relative humidity (RH) is that; it represents the amount of moisture vapour that can be held in air at a given ambient temperature.

I call it a rough working explanation because its easier to visualise than the proper theory you'd get if you were talking to a real physicist.

But no matter! Just imagine that the amount of moisture vapour in the air keeps increasing until it reaches a point where it cannot hold any more. That's the point where water vapour starts to form water droplets ...the dew point, where the RH reaches 100%.

So it follows that an RH of 50% means that the air contains half the amount of water vapour that it could contain. OK, except it has nothing to do with air. If you created a vacuum in a container and then introduced some water vapour, the same rules would apply.

But I'll continue to use the word air.

The next point to note is that the amount of water vapour that can be supported in the air is related to the ambient (air) temperature. This is when it gets really interesting.

I've seen recommendations on the net which suggest that you should store your PLA at an RH below 50% (...which sounds reasonable, but I've no way of knowing if this is low enough). Anyway, to illustrate the relationship between RH and temperature, I created my own micro-climate using a plastic lunchbox with a good lid that clamps down onto a rubber seal.

I put my cheap temperature/humidity/clock inside the box, and gave it a few hours to stabilise.

With a recorded reading of 51%RH at 23.8'C, I then left it in the conservatory overnight. In the morning it read 62%RH at 15.5'C.

This RH reading is actually better than the maths suggest, so I will need to repeat my tests. However, I think it does illustrate that if you put a reel of PLA into a storage box on a warm day, the box moisture will effectively increase on days when its a bit cooler.

Using an online calculator; with 51%RH at 23.8'C gives a dew point at about 13'C.

So if you are not using any special methods/containers to store your filament other than a plastic bag (...which is basically me!) then you need to carefully consider the consequences of taking out a reel of filament on a day when the RH and/or the temperature is high.

When the filament reel is returned to its plastic bag, the air temperature & humidity will (more-or-less) match the ambient values. So even with this example...

I now have a cheap temp/RH meter magnetically attached to my printer

...if the temperature drops, the RH will rise, and the filament may start to soak up some moisture.


Within each filament plastic storage bag I have a small sachet of silica gel. Surely that will soak up any moisture and keep my filament safe?

Well, maybe, maybe not. There are two things to consider.

Has the silica gel already soaked up as much moisture as it can hold?

The solution here is probably just to stick the little bags in an oven at 120'C for an hour or two to dry them out.

Be aware that colour indicator silica gel may be hazardous (i.e. gel that changes colour when it is wet).

Is there enough silica gel for the volume of air around the drum?

After a bit of searching on the net, I found a statement about keeping stuff dry in a museum which indicated that 20kg of silica gel should be used for an enclosure volume of 1 cubic metre.

When I buy a new 1kg filament reel, it comes in a vacuum sealed plastic bag which also includes a small, 50x50mm sachet of silica gel, with a net weight of about 3-4g.

Doing the sums, it looks like this is about sufficient if the only void in your plastic bag (.e. the one you put the reel back into for subsequent storage) is the centre of the plastic drum. However, its very likely that your selected storage bag will also have space around the outside. In addition, as you use the filament and it reduces on the drum, there will be an increasing amount of free-space space that potentially contains moist air!

Therefore, to be on the safe side, the silica gel bag used inside a simple plastic storage bag probably needs to be between 20-40g ...depending how cautious you are!.

So my modus operandi is:- 

  • keep all filament reels in their own plastic storage bags
  • keep 25g silica gel bags with each filament reel
  • never leave a reel on the printer when not actually printing
  • don't print anything if the relative humidity is >50%
  • don't print anything if the print room temperature is going to drop significantly (e.g. overnight or over the coming days)

So you think this is a bit limiting?

Well, if you are not a tight-wad like me, you could invest in special storage containers that come with a vacuum pump. Or maybe go the whole-hog and install an air-con in your print room, and hold the temperature at 20'C and the RH about 45%.

Happy Printing!

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