Question

why d orbital with 18 electrons has less sheilding effect than d orbitals with 8 electrons?​

Answer 1

Explanation:

It is down to the properties of the wavefunction which corresponds to the orbital (indeed, an orbital and a wavefunction are the same thing).

already answered your question, so I don't really have much to add. I had started a draft with an answer which I didn't manage to complete earlier but I decided to post it anyway.