The plastic recycling -can he need the addition of virgin plastic material first?

Not necessary, but if can add if need to maintain standard required by the customer

It is better to differentiate between scrap plastic and recycled plastics. The level of contamination and impurities existing in each category are the main factors controling the purity of the plastic. The other main factor is the degradation level sustained by the polymer during either processing or service life. The higher the degree of degradation the lower the performance/quality of the recycled plastics.

not necessary  , but to get more pureity , it's better to add virgin plastic to reused plastic 

yes, to be purer and strength.

For better purity . processability and strength