Step 1: Recall what spinning the mould actually does.
In centrifugal casting the mould rotates at speed, and this rotation creates a centrifugal force that pushes everything inside outward, toward the mould wall, with the strength of the push depending on how dense the material is.
Step 2: Compare the densities of metal and impurities.
The molten metal itself is dense, so it experiences a strong outward push and is thrown firmly against the outer wall where it solidifies into the main body of the casting. Impurities such as slag, dross and non metallic inclusions are noticeably lighter than the metal, so the same spinning motion pushes them outward far less effectively, and the denser metal essentially displaces them inward instead.
Step 3: See where the lighter material ends up.
Since the heavier metal always wins the outward race, the lighter impurities are left behind at the innermost region, near the axis of rotation, meaning they collect at the centre of the casting where they can later be machined away, leaving a clean dense outer wall.
\[ \boxed{\text{collected in the centre of casting}} \]