The use of certain materials coming from wastes can outline different problems for a good processing.
One of the most usual problems that we can find is that we have a waste of certain quality but whose viscosity is not the appropriate one to the process in which it must be used. To modify or to adapt this viscosity we can add a percentage of another plastic material so that the resulting blend is the appropriate one. 
In the present work it has been studied the influence that viscosity exercises in the incompatibility of plastic waste during  recycling process. Three different blend systems, which represent real problems at industrial level, have been studied. 
In the first blend system, which is composed of two compatible materials with similar viscosity, some available models have been applied to predict the properties of the blend. For the prediction of the process behavior , a method has been adapted and thus allows us to know the rheologic parameters of the blend from the individual rheologic parameters  of the non-blended materials.
In the second system, the process incompatibility has been approached, with the objective to know the limit of viscosity modification for a poliolefin without complicating in a great extent its transformation, by studying the relationship between the final properties of the blend and the decrease in viscosity obtained.
Finally the most unfavorable blend system has been studied. In this system, addiotionally to great viscosity differences, the individual materials show incompatibility
An industrial case that represents this system is the PET waste, which is contaminated with PE coming from caps. 
In this case the influence of the PE in the PET injection process has been determined, furthermore, its influence on final properties of the blend has been studied.