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 re-meshed. 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 re-mesh option deselected when meshing a model. This is because it is good practice to construct the mesh in a modular fashion.
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 re-mesh 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 re-mesh option deselected, so that you do not disturb the corrected part or runner mesh, and then diagnose and correct the cooling system mesh.