When generating a mesh, you are provided with the option to Remesh already meshed parts of the model
If this option is selected, and you click Mesh Now, the entire model will be remeshed. If this option is not selected, only those sections of the model that have not been meshed already will then be meshed.
Typically, it is best to leave the remesh option deselected when meshing a model. This is because it is good practice to construct the mesh in a modular fashion.
For example, when you are importing and meshing a model, runner system or cooling system, the procedures described below can be followed in order to create a suitable mesh for analysis:
Create or import a part model, mesh the part model, and then diagnose and correct the part mesh.
Create or import a runner system model, mesh only the runner system, keeping the remesh option deselected, so you do not disturb the corrected part mesh, and then diagnose and correct the runner mesh.
Create or import a cooling system model, mesh only the cooling system, keeping the remesh option deselected, so that you do not disturb the corrected part or runner mesh, and then diagnose and correct the cooling system mesh.