Meaning
Pure shear deformation occurs in applications like film casting where the edges of the sheet are constrained by the take up rolls. Engineers analyze planar extension to understand how resins behave during the orientation phase of film production. This mode of stretching is intermediate between uniaxial and biaxial extension.
Flow Kinematics
Constraints on the width of the specimen cause the thickness to decrease as the length increases. In planar extension the material undergoes a specific type of molecular alignment that differs from simple pulling. This behavior is critical for maintaining the dimensional stability of wide plastic webs.
Rheological Characterization
Measuring the resistance to this flow provides data on the planar extensional viscosity. Knowledge of planar extension helps in predicting the onset of instabilities like draw resonance or edge beads. It is particularly relevant for the processing of high molecular weight polyolefins.
Process Simulation
Accurate modeling of the extrusion and coating processes requires data from planar tests. Incorporating planar extension parameters allows for the optimization of die designs and cooling configurations. This data ensures that the final film has the required thickness and mechanical properties across its entire width.
Researchers use specialized rheometers with rectangular nozzles to create the necessary flow field. The results are vital for the successful production of multilayer barrier films.
