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A screw fixation can be modelled by using a Nondesign space (red) around the screw as a shell. The Nondesign space is constrained using a clamped fixation.
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The screw (blue) will be used as a fixation and is clamped. Additionally, there should be a Nondesign space around the screw (red), which is included in the design space but will not be used as a fixation in the optimization optimisation model. This way the Nondesign space does not experience an artificial stiffening, which represents the reality much more accurately.
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However, keep in mind that the screw becomes a part of the Design space, so you need the original Design space without the screw to create a good intersection with the optimization optimisation results. Further information regarding the intersection here.