The Ruttmeister
Contributor
...like properties.
PLA+ is still Polylactic Acid, but a different blend and with a bunch of additives (like calcium carbonate of all things).
According to the actual polymer experts I have worked with, Nylon is going to have the best long term resistance to sun and salt water. Followed by things like polypropylene or HDPE. But PP is a nightmare to print with and HDPE is at least as bad. Yes, nylon absorbs a bunch of water, so its dimensional stability isn't great. But that's only an issue if you need dimensional stability (for things like screw-together parts or something). Mostly I'd use PETG. The strength and durability of ABS is greatly overstated, its real advantages are that it can be chemically welded and polished, which PETG, PLA and Nylon cannot. I have some CF-PETG I should probably try in the water, but I don't have anything I need to get wet.
But most of the polymers out there will last well enough, the plastics world has figured out how to put enough stabilizers in everything that you need fairly novel requirements before you'll have issues. PMMA, PLA and PC are going to have the worst UV stability IIRC, which is probably the main concern.
For maximum strength:
High contact area between layers.
So wide extrusions. In normal use you want to be around an extrusion width to layer height ratio of 2:1. Ratios of 3 or even 4:1 will greatly increase strength (eg, 0.5mm layers with 1.5mm extrusion width).
Because of the limits of the reology, its much easier to do this with larger nozzles and thicker layers. Which is a trade-off, but if strength is critical, ugly low-resolution prints are the way to go.
And I suspect that carbon fiber filled polymers are getting much higher bond strength, as their rough surface finish increases surface area for the bonding, but I haven't tested that.
Inter-layer fusion temperatures.
This.
The boundary between layers will be the average of the temperature of the two layers. So if you let the part sit too long that temperature will drop below that needed to fuse the polymer.
You can compensate in a bunch of ways, lasers, silly plasma nonsense... but for those of us not trying to 3D print satellite components.
Higher extrusion temperatures (this is where PETG has a huge edge over ABS, in that its usable extrusion temperature range goes much higher than ABS. PETG is running like water long before you'll start to carbonize it, where as ABS just ups and clogs your nozzle when you get too high). This will push that average high enough.
You can also slow down the rate of heat loss from the existing layer, either by using a heated chamber, or by keeping the parts small (this is a challenge if you are trying to print fully dense parts obviously).
Even a heat lamp pointed at the print can make a huge difference.
The biggest limit is not having the part so hot that the nozzle just pulls it around.
In my experience, PETG, with nice wide extrusions, printed nice and hot, will have close enough to the strength of monolithic polymer that it will break perpendicular to the layers. The limits are the inherent strength of the polymer, not the process.
Oh, and ALWAYS DRY YOUR FILAMENT!
Even PLA loses significant strength if its not as dry as possible when you print it.
And if you need it to be really really really strong, design it out of metal
I'm shocked at how cheap laser cut metals have become in the last year.
