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The Design Space is the most important space and complies with the imported/created solid CAD geometry. It includes any space where material is allowed to be or has to be, to provide maximal freedom for the algorithm. If the Design Space is limited, so is the optimisation algorithm and the optimisation potential is reduced.
Interfaces are all functional faces of a model so all faces where boundary conditions are applied to. With the Interface Tool these faces are selected to create both Non-Design Spaces and Machining Allowances.
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The Non-Design Space contains the necessary geometries of the part. Any part for which you need a clearly defined geometry should be a Non-Design Space. Non-Design Space have to be own solids/geometries Spaces can be created as a part of an interface or as an own solid/geometry which are covered from the Design Space.
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Retained Volumes are assembly parts which are part of the calculation but not part of the final result. Retained Volumes should always be connected via a Non-Design Space an Interface to the Design Space. The connection is a glued contact. These can experience boundary conditions and a different material as the Design Space can be assigned.
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Intersections are necessary after each optimisation to get the defined surfaces of the Non-Design Spaces. The intersection is done automatically for every iteration and can be seen in the Post Processing.