Meaning
A flow defect occurs at the boundary between different polymer layers in a coextrusion die when the layers have mismatching rheological properties. During sheet or film extrusion, interfacial wave instability causes a loss of optical clarity and a non-uniform distribution of layer thicknesses in the finished product.
Hydrodynamic Origin
The problem arises when the shear stress at the interface between the coextruded layers exceeds a critical threshold. In the development of interfacial wave instability, small perturbations at the layer boundary grow as the melt flows through the die channel. This behavior is most severe when there is a large difference in the viscosities of the adjacent polymer resins.
Defect Prevention
Processors adjust extruder temperatures and output rates to balance the viscosity of the coextruding melts. Eliminating interfacial wave instability requires selecting resins with compatible melt flow indices or adjusting the layer thickness ratio to relocate the interface to a region of lower shear stress. These adjustments help maintain a smooth boundary and preserve the mechanical integrity of the coextruded structure.
Process Monitoring
In-line optical sensors can detect the onset of these waves before they become visible to the naked eye. Identifying interfacial wave instability early allows operators to make fine adjustments to the die temperature and prevent the production of scrap material.
