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Rheological weldability of thermoplastics considers the materials flow characteristics in determining the weldability of the given material; Rheology is the study of material flow as well as how a material deforms under an applied force. Rheological properties are typically applied to Newtonian fluids but can also be applied to soft solids such as thermoplastics at elevated temperatures experienced in the welding process. The material properties associated with the rheological behavior include viscosity, elasticity, plasticity, viscoelasticity, and the materials activation energy as a function of temperature.

To understand the rheological properties of a material it is also important to recognize the stress strain relationship for that material at varying temperatures. This understanding is attained through experimental measurement of the resultant deformation as a function of the applied force. A materials rheological behavior is also influenced by a combination of the materials microstructure, its composition and the temperature and pressure acting on the material at a given time. The rheological and viscoelastic properties of a polymer melt are sensitive to the materials molecular structure; including molecular weight distribution and effects of branching. As a result, rheology can be used to develop relationships between differing material combinations.

Also, melt rheology has shown to be an accurate method in determining the polymers molecular structure. This is beneficial in determining weld compatibility between materials; as materials with drastically different flow characteristics will be more difficult to join compared to those with more closely matched viscosity and melting temperature properties. This information can also be used to help determine weld parameters for the given welding process to be used. Knowing the viscoelastic behavior allows for adjustments to temperature and pressure during the weld process in order to improve the weld quality.

Welding of polymers is dependent on intimate contact resulting in molecular diffusion and chain entanglement across the weld joint. This action requires the polymer to be in a molten state where the melt viscosity and flow behavior have a drastic influence on the amount of diffusion and entanglement. Therefore, the rheological weldability is best between materials with matching or very similar melting temperatures and melt viscosity. More so welding of semi-crystalline to compatible semi-crystalline material and amorphous to compatible amorphous have exhibited the best results. Ultimately production welding should be prefaced with series of testing to verify compatibility between both base materials as well as the process employed.

Similar to welding metals, the solidified polymer weld experiences residual stresses inherent to the joining process. With polymers, these residual stresses are in part due to the squeeze flow rate leading to a specific molecular alignment direction, ultimately influencing the weld strength and overall quality. Having a thorough understanding of the rheological properties of the materials being joined can aid in determining the resultant residual stresses; and in turn provide insight to processing methods that could reduce these stresses.