Outdoor use Nylon SLS (strong & flexible) and water absorbtion

Hello,
I have been searching the internet for this for a while now, but couldn't find the information I needed. So I decided I would do an experiment myself.

First, what is the problem? I want to print a part for use on my motorcycle, which will be exposed to the weather. My preferred material would be ABS, but ABS prints apparently are quite expensive (my part turned out to be over 120 euo's on i.materialize). So I tought of SLS as an alternative (30 euro's), which would certainly fit the bill in terms of accuracy and strength for my purpose.
It is water permeable, but my part does not need to be water tight. However, Nylon (polyamide) also absorbs water. For this reason, 3D printing websites often state that Nylon (strong&flexible on shapeways) is not suited for outdoor use. But nowhere can I find why this is a problem. What exactly happens to my part when it absorbs water? (even the shapeways team couldn't tell me, as they didn't do any tests for this). For this particular part it is only important that it retains its strength and doesn't change in size dramatically (a bit is acceptable).

So I decided to do a simple test myself, on an sls part I had lying around which wasn't going to be used anymore. It's a pair of glasses, with actual glass fitted into it, a couple of hinges and snap fits. I wanted to submerge it in water for a day. If the glass still fits, and the hinges and snap fits still work properly afterwards, that would mean the part did not increase in size significantly due to absorbing water. (at least not significant enough for my purpose). The snap fits and legs are rather slim and flexible, so that would be a good "measurement" for stiffness and strength as well. (not a very scientific test, but good enough for my purpose).

So after a bit more than 24 hours of being submerged, I checked on the part and it turns out fine! In fact not much at all seems to have happened. Flexing the legs is no easier than before, and the snap fits still snap as intended, so stiffness and strength did not change noticeably. The fit for the glasses in the frame is still as tight as it was before, no tighter or looser. The hinges still work perfectly as well. So size doesn't seem to have increased significantly either.
So the water did not seem to have a drastic effect on the material. I did not measure it before hand, so it actually might have increased in size a bit due to the water absorbtion, but I can't really tell for sure; but the effects are so marginal that all parts still fit together like nothing has happened.
Since the part survived the first 24 hours, I decided to leave the part submerged a bit longer to see what happens after another day or 2, but I suppose that is beyond what the part will be exposed to outdoors anyways (not being submerged). To be continued...

All in all, I think I'll just print my motorcycle part in SLS and test it in real life. The worst that could happen is that it expands to much, and I cannot remove it without breaking it (which will not cause any life threatening situations ) and with effects this marginal, it definitely should keep working properly over time.

I not aware if anyone has ever quantified the amount of water absorption in SLS over time. That might be an interesting experiment if you use the proper techniques and equipment to measure the amount of absorbed water (or other liquids).

My guess is that the only change you would notice with water is for weight and not any kind of significant volumetric changes of the moistened plastic. An exception might be if a wet SLS part is frozen, in which case water trapped in the SLS matrix might expand as ice and crack the plastic or cause it to shed material at the surface, depending on how deep the water penetrates. Of course if you change temperatures significantly up or down the properties of even dry printed materials will change, possibly becoming softer or more brittle depending on the temperature change. Also depending on how or where you attach the plastic to parts of a motorcycle differences in expansion coefficients, for example between metal and plastic, could put stresses on the printed plastic part and cause it to crack at temperature extremes.

You could also consider sealing the part with a clear waterproof lacquer or polyurethane product to minimize moisture absorption if it's a big worry. It would also help to keep dirt from smudging the plastic surface. Also keep in mind that something like the white nylon material tends to yellow over time and is probably negatively impacted by UV solar radiation. In this case using an exterior grade plastic paint primer/sealer product would serve to protect the plastic from the UV. I've used a Krylon white plastic primer product that was just as white as the white strong and flexible material. It's been mounted on the outside of the house on for many months now so I should probably check to see how the color and primer are holding up.

Good one! I did not think of frost yet. My motorcycle will be stalled indoors during winter time, so it probably won't be a big problem, but its good to keep in mind. I think I might put that pair of glasses in the freezer and see what happens then

Other than that, the part won't be located near the engine, so temperature changes wont be very drastic. Here outside temperatures vary from 10 to 30 degrees during the season my motorcycle is outside, so I guess that are not the most shocking temperature differences either, but it might have an effect. We'll see

Regarding UV, I'm aware of that too, but I'll probably print it in black so that won't be a problem.

You might also consider the metallic plastic which I believe withstands higher temperatures (check the material spec page) even if just heated by the sun. It is more expensive, and possibly more brittle depending on the application, and probably more porous than the white or dyed nylon plastic options. The black option is also dyed white plastic, so if it is apt to be scratched you could reveal the white core below the outside dyed layer. The metallic plastic is the same color throughout the object and the grayness comes from the aluminum powder embedded in the matrix so any scratching might be less noticeable, and the embedded aluminum might also provide greater UV resistance. .

It isn't really in a place prone to scratching, so that won't really be an issue. I also know the alumide material, but I'm not quite convinced by how it looks. It is on the outside of my bike so looks are a small factor too. So I'd still prefer the black nylon

I think it's more like a fabric dye, so the black will probably also be sensitive to UV light and fade over time. Maybe there's a clear sealant with UV protection out there. It's possible someone in the forum has discussed long term exposure to the real world for various materials. You might want to do some searches before plunking down a lot of money. I think that's about all I can think of.

Ah ok, didn't know that. I guess I can always repaint it myself the color really fades badly over time. But thanks for the heads up!

Ok, so after a few hours in the freezer, nothing much changed either. The hinges were sligthly frozen together, but after a few movements they were operating perfectly normal again. So no shocking results at minus 18 celcius either!
So I just ordered my motorcycle part in black SLS. Dont think a lot can go wrong based on this experiment. At worst it will look a bit shabby after some time.

I'd suggest that you read thru this thread https://www.shapeways.com/forum/index.php?t=msg&goto=103 614
and in particular, check out the video it links to: https://www.youtube.com/watch?v=TDoR1qylB0w&list=UUhlHz5 WxKTjAk2CFh7i0dwQ

ONE cycle of immersing in water and freezing is not a sufficient test.. put that thing to some real torture <grin>...

Take that 3d printed item (the glasses) and put them half-submerged in a tray of water.
Put it in your freezer and wait till the water freezes good and solid.
Then transfer the tray to your oven, and heat them to about 110 or 120 degrees for two to three hours.
Repeat this 20-40 times., then you'll have a more valid result. <grin>


You've already indicated that the part is decorative, so I won't repeat the warning about potential loss of life, limb, or mother-in-laws from the back seat. <grin>

Haha well, I hardly think it would endure such a degree of torture in practice Especially as it wont be outside in winter much, nor will I go driving in freezing temperatures, so such radical temperature changes (-18 freezer to 120 oven) will definitely not occur...

But the best way to test is in practice. So I'll report back at the end of the motorcycle season how it endured real life weather conditions

Well, to make it a bit more relevant, I did another 2 cycles of 24 hours of soaking in water, and then freezing it. My conclusions from before didn't change The glasses still are in the same condition as before. Everything still fits together and works fine. So I'm getting confident enough that the part I ordered for my motorcycle will last at least 1 season.

Would be good though if shapeways (or any other service of the kind for that matter) would do some more scientific testing on the subject, and come with some actual measurable results that anyone can account for in their designs

Reproducibility will probably be a problem, especially when print orientation may vary and some fraction of raw material is recycled (after acting as support material in a preceding run)

Ehm, not really sure if I understand your comment within this context?

The parts have different properties based in how they were printed, and the state of the powder. So maybe a design that looks strong, will break quickly with the next print, or vice versa.