nylon/CF or nylon/glass for stair bracket?

[JohnInOttawa 104]

Good morning everyone.   This morning I am working on some brackets to hold pre-fabricated aluminum stair treads.  The treads are pre-drilled to mount on brackets made from aluminum 'T' extrusions, wall thickess may be 1/8" or 3/16", but the brackets are welded to beams at an angle I can't use, so I have to make my own.

 

I was about to hire a Tig welder to make up a set of brackets, likely out of 6061 angle, but the price is rapidly spiralling.  These brackets are small enough that I can fit two sides of the triangle on my printer. 

 

So I wonder - with all of the claims about the strength of materials like NylonX, Nylforce, XStrand, etc, would it actually be safe to print these brackets?  And additional consideration is that these will be used in a wet environment, with the bottom brackets likely underwater for much of the time. (fresh water, not salt).

 

Thoughts?  Thanks!

John

[geert_2 558]

Technically, it will probably work well if you design the parts thick and massive enough, if load is distributed well (=no stress points, correct orientation of layers), and if you print them correctly (temp, fill, flow), and regularly test for degradation and replace them.

 

But personally, I would not really trust it. I wouldn't trust any plastic parts for such purposes. I have seen too many plastic things crumbling apart or breaking after a couple of years: dustbins, bottles, gardening tools, plastic toys, food boxes, garden tables and chairs, car bumpers, all sorts of composite casts,... And these were injection moulded parts, or cast parts, so they even didn't have the 3D-printing problems like layer-adhesion, stress-inducing entrapped air, uneven cooling stresses, etc.

 

The biggest risk will probably UV-light degradation, plus to a lesser degree ozone, chlorine (if in contact with tap water or swimming pools), hydrolysis, drying-out (evaporation of plasticizers), fatigue,...

 

I am not sure, but I do believe that here in Belgium weight-carrying plastic connections on ladders and stairs are even forbidden. At least, they are forbidden in our university: all connections have to be welded or rivetted.

 

Without photos or drawings it is difficult to give advice, but what about using rivets? Or nuts and self-locking bolts? Maybe with use of copper-grease to prevent corrosion?

 

[JohnInOttawa 104]

Thanks @geert_2.  Your response makes perfect sense to me.  My default is metal for the same reasons as you cite.  I just thought, maybe I am being old fashioned.  But given the consequences of a step giving way, I think maybe old fashion is best.

 

I will look at rivet options.  The nuts and bolts would probably be strong enough, but I am trying to have one flush surface and would rather not have to thicken the metal for a countersink or counterbore. 

 

Much appreciated!

John

[geert_2 558]

I just took a look at the portable ladder in my lab: it is both riveted and welded. This makes it feel rock solid. If only riveted, each connection could still pivot a bit, causing a wobbly feeling. If only welded, the welds could be superficially glued instead of really melted together. I can't weld myself, but I heard from a professional welder that welding aluminum is difficult due to the very high temperatures required to melt the oxide layer. If not high enough, the connection may slightly stick, but is not welded and will separate under load.

 

If yours will be mounted to a wall, pivoting will not be a problem. But if portable, make sure you provide multiple connections per step.

 

[geert_2 558]

Plastic folders crumbling apart after sitting in the daylight for a couple of years. And this was even behind sun-shielding green glass, which should catch most of the UV-light.

 

These folders are usually made of PE or PP, but I have seen similar problems with other plastics. Last month I had a dustbin falling apart. A couple of years ago I have seen this in "weatherproof" garden chairs...

 

That is why I would not trust plastics too much for load-bearing outdoor use, unless they are specifically tested and approved for it. 3D-printing materials are relatively new, and we don't have a lot of experience with long-term durability in this regard.