On the left is a successful end cap for T-slot aluminum extrusion. On the right is a bowl full of failed trials. Each of the failures didn't fit correctly or couldn't handle repeated use.
Each end cap has to fit snugly. You don't want it to slip out if gravity or some other small force pulls on it. But it must also be removable and reusable. So you need a very slight snap fit. But this puts stress on the plastic. And the more you stress the plastic by reusing the cap, the weaker the snap fit is.
With my first designs, only a couple of uses would render the cap useless because the plastic would permanently deform. By tweaking the design, I was eventually able to get to a couple of dozen uses when 3D printing in ABS.
After reaching the limits of ABS, I switched to a plastic called Alloy 910. It took some time to figure out the best way to print with the new material and finalize the design that worked best with it. But Alloy 910 is less brittle than ABS which means that it can be used many more times without permanently deforming and losing the springiness which is necessary for a snug fit. The success on the left can be used more than fifty times and still work well.
I will now be able to make these end caps myself instead of ordering them. In addition to being cheaper, they are much higher quality. The injection molded end caps I was using before began to deform and become less useful after only a handful of uses.
This makes the many dozens of attempts and hours of tweaks and trials worthwhile. These end caps are a very small thing. But I now believe that I might be making the best end caps on the planet.