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In sheet forming, the creation of wrinkles in the flange area of the blank commonly occurs. When diaphragm forming aligned-fiber-reinforced thermoplastic composites, observations reveal that the wrinkles tend to occur at angles oriented at forty-five degrees to the axis of the fibers. Intuition suggests that this is related to shear, but this may be misleading. The results of a stress-analysis and buckling analysis of this flange wrinkling problem are discussed. The forming of the thermoplastic sheets occurs at elevated temperatures and the sheet is essentially inextensible in the fiber direction. This leads to high anisotropy ratios of in-plane stiffness properties. The out-of-plane bending of the sheet will be strongly influenced by the presence of polymeric diaphragms. To investigate the buckling problem for this forming situation, a finite-difference formulation was developed. The solution revealed that only when one considers the combined effect of the thermoplastic composite and the polymeric diaphragms does the analysis predict the observed forty-five degree buckles. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]