It is long known that flow can dramatically increase the crystallization rate of molten polymers, but a molecular description of the underlying reason is still lacking.  The ability to predict and control FIC may be critical to effective welding of semi-crystalline polymers and their composites in 3D printing.  We are developing instrumentation and models to understand how the crystallization rate is changed by the 3D printing environment and the ultimate effect on part strength. We have developed the rheo-Raman microscope