So, you have three phases, not two. You should define the solid material "solid plastic" and enable melting/solidification model to get solidification.What is you time step size?

Why three? The first phase is air (initially the domain is filled with the air) and the second phase is the plastic. Plastic's viscosity is temperature-dependent. As it cools down it should loose heat and get solid. Do I need to enable solidification and melting?Time steps: 1e-10n