There is no magic angle for mass flow when designing the cones. Mass flow is dependent on the smoothness and steepness of the hopper wall and the properties of the bulk material involved. If you were to just design all hoppers for a 70 degree slant, you may achieve mass flow, but you will also run a great risk of promoting funnel flow. With funnel flow, the first materials in are the first materials out. In a mass-flow silo, every particle is in motion during the discharge. If this is not happening, then you have run into a funnel flow. Mass-flow cone designs have advantages because they guarantee complete discharge of the silo contents.
These metal cones are curved on specialized equipment, typically plate rolls or press brakes. The specifications for these cones include the major diameter, the minor diameter, the cone height, the degree of slant, and, of course, the material. These cones can be tack welded or fully welded together.
The cone in the picture below has a major inside diameter is 52.2″ (the top opening of the cone) and a minor diameter is 14.14″ (the smaller opening at the bottom of the cone). The vertical height is 54 inches, and the slant or pitch is 55 degrees. The cone is made out of 3/8 inch thick carbon steel tack welded. This cone is for a silo that holds lime.