I have been 3D printing for a couple years now with a Form2 and in general an very satisfied. When they came out with Durable as a Polypropylene like material I was really looking forward to it as most of my customers are molding PP so I hoped to give them parts as close to injection molded as possible as quickly as possible for testing and evaluation. The results however were not good. The parts I printed were not really very PP like being much too soft and pliable in comparison to actual PP. I use a Formwash and home made heater.UV cure box, but I noticed after a few weeks post cure, the parts became more PP like just sitting on my desk so I suspect my homemade cure box is not properly fully curing the Durable resin. Let me know if you are using the FormCure box with Durable and if the end results are actually PP like or still too soft and pliable as mine are.
As you can see from the Form Cure time & temperature calculator, Durable does need a good long cure. And it does make a big difference in the material properties. If Durable is undercured it can feel pretty soft. I would guess it probably just needs more time in your box.
My durable parts cure to be pretty strong. I’ve had issues with warping on thin walled parts with Durable and the standard Form Cure settings. I realized it was likely the heat so I cure for 30 minutes with no heat, just light, then cure again with the heat on as recommended. Ideally they would update the Form Cure to allow variable settings for heat and light but for now this has helped.