Stability Tests – Long Term Mechanical

We’ve had a couple of queries in from designers considering the longer term stability of the Fishy Filaments recycled nylon product both in terms of use cases such as wet/dry cycles in water sports and for washability in wearables/fashion.

In the first of two posts we’ll look at some bodies that we printed a year ago and have kept in water since then. In the second we’ll look at some wash tests on dyed forms.

So first off, remember a year ago when we carried out some ‘short’ term (17 day) immersion and wash tests ? Well we kept those going over the last year. The forms shown below have been stored in water for almost exactly a year.

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The aim was to see if interlayer bonding was effective or whether water ingress and algal action might prise the layers apart with time.

The clear glass jam jar was filled with fresh water so that we could ensure that we did get some algal growth but didn’t end up with a storage degradation issue. The rationale for the choice of storage vessel was that jam is not an inert substance and that its storage technology has been honed over many decades. As you can see we got a little rusting on the lid but the water was not otherwise contaminated.

I didn’t feed the algae (apart from that little bit of rust) and its apparent volume reached a maximum after a few months and then held despite regular and vigorous shaking at least once a month. Its a bit of a stretch scientifically but this might suggest that we did have a closed system and that the algae was not metabolising the nylon at any appreciable speed. This would seem to support the idea of nylon as a persistent material with a lifespan measured in centuries rather than years, but I’ll leave the often quoted 600 year figure to others to confirm.

Mechanically there was very little to choose between the long term soaked forms and the original shorter term soak test forms that we kept as a long term control. The original control forms from a year ago and stored in uncontrolled conditions, but that were never soaked, remained noticably stiffer. This seems to support the contention that nylon reaches an equilibrium with the amount of water it can absorb and release under normal environmental conditions, and that the elevated temperatures required to dessicate the polymer prior to printing (somewhere over 70C) are absolute ‘need to knows’ for anyone considering using nylon as a 3D printing material.

TIP – you cannot leave it to a silica dessicant pack to dry pure nylon 6 or to keep it dry, you have to heat it to between 70-80C and keep it there for several hours. Once you have driven off both surface and internal moisture from the polymer you have a limited time in which to use that filament and when you can expect optimum performance. How long that period is will depend very much on local conditions but a warm, dry storage system can extend that envelope very significantly and we recommend considering solutions such as Richard Horne’s heated drybox, ESun’s eBox or if you are a bit wealthier Mass Portal’s filament drying systems. Unheated options such as Polymaker’s Polybox are useful but have more limited application. It might feel like a bit of a hassle but stable 3DP materials are a rarity in the manufacturing world. Most materials (leather, metal, ceramics) need both pre and post manufacturing processes to make them stable over the longer term.

But once printed that ‘absorb and release’ process does not seem to degrade the 3D printed form, suggesting repeated immersion in water is not going to cause rapid degradation. This should give some reassurance to the several designers looking at wet/dry use cases, but of course we advise carrying out your own tests too.

After a quick rinse and scrubbing off the algae, we looked at the colour. The jar was kept on a window sill that gets direct sunlight year round. Of course two layers of glass severely limits the amount of UVB incident on the forms, but some UVA should still have got through. PA6 (our kind of nylon) is considered to have ‘Fair’ resistance to UV degradation. Also I think it is worth pointing out that some of the more obvious UV damage seen in low-end 3DP filaments is bleaching of cheap masterbatch pigments that manufacturers add to them rather than the underlying polymer itself.

Long Term Stability Tests
Left – 1 year fresh water soak. Centre – 1 year dry storage. Right – 1 year dry storage after 10x boil washes and 17 days immersion

The underlying colour of our nylon appears to have changed ever so slightly over the course of a year long soak in fresh water, but nowhere near as much as it did after repeated boil washes with detergent, and in a different way. On closer inspection the apparent muddyness of the long term soaked forms is due to algal growth into some of the layers at surface, and that scrubbing with a toothbrush did not dislodge. It is noticable that surface roughness is the primary determinant of the location of algal growth suggesting some interlayer penetration by the algae. We should note that these forms were all printed on a minimally modified Wanhao Duplicator i3 at a relatively coarse 0.3mm layer height.

So in summary; soaking in water over long periods does not seem to impact the mechanical properties of 3D printed nylon forms unduely but surface roughness does seem to promote algal growth. UVA does not seem to cause significant damage to our Nylon 6, but UVB cannot be ruled out by these tests as a possible source of degradation.

Hopefully this will help designers when evaluating material choices.

Next post will be on colour stability through various wash cycles.

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