Hello. I want to design and print working human arm model. I have a questions about materials. Strong & Flexible plastic looks good but what thickness must details have to avoid flexibility? How thick must be outer sphere of ball joint to make it strong?
That depends on the scale of the joint you are designing. What size joints are you planning to design?
For that scale I'd go with about a 1 mm for wall thickness. That should be sufficiently strong for your application in that material.