Why polish screw before doing injection molding prodction？

Screw is very dirty and many dark spots on it especially after making flammability V0 material for parts,when produce white or transparent products,reject rate is very high,many black spots on the visible surface of parts.
So have to polish screw before doing mass injection molding production.
Save time and energy,also we check all parts one by one as no defective to custom,be sure shipping all on time.