They simply use an Oldroyb-B model, simply written in a different form than usual.

Greeting.

Thank you very much Dr. Jean for your response.

Please, kindly I really hop to get opportunity to discuss with you some aspect relating modelling viscoelastic flow during extrusion in my work . I used Starr CCM software and finite element formulation.  

I will very grateful for you to give me this glorious chance  

Kind Regard