Living Hinge
A living hinge does not work because polypropylene is bendy. It works because the web is cold drawn on its first flex, which orients the molecules across the hinge and multiplies its tensile strength — well-oriented webs have virtually unlimited fold life. That is why the hinge must be flexed immediately after moulding, while still warm, and why an identical shape machined from stock has none of the endurance.
Gate the part so the melt flows across the hinge, never along it — flow direction sets the molecular orientation and a hinge filled the wrong way tears on the first fold. Keep the web the thinnest thing in the flow path. Avoid any filler: glass fibre in particular destroys fold life because the fibres cannot orient with the matrix. Polyethylene works but is softer; polypropylene homopolymer is the standard choice.
Design guidance from manufacturer manuals, expressed as ratios to the nominal wall. Moulding outcomes depend on resin, tooling, gate location and process, so treat these as the starting point a moulder will refine. A helper, not a substitute for engineering judgment.