Real Time Strain Imaging Tool for Additive Manufacturing

Strain-induced birefringence is a well-understood phenomenon that has been applied in conventional injection molding processes for many years as a nondestructive tool for identifying defects resulting from inconsistent thermoplastic resin flow. Although initially developed as a tool for static measurements, recent advances in digital imaging have enabled its application in real-time stress imaging in conjunction with computational modeling techniques. An advantage of this technique is that it provides information about the localized internal strain in real time. Other approaches, such as DIC, or digital image correlation, rely on surface measurements through patterning on the surface by either laser speckle or ink patterning, neither of which are applicable to in-process monitoring in additive manufacturing applications. Other techniques which have been applied, including special temperature mapping of the exotherm of photochemical reactions in vat polymerization, do not provide direct feedback on the localized internal stresses of the part. The objective of this proposal is to incorporate a strain-induced birefringence metrology tool into a DLP printer to provide real-time feedback on the printing process, providing in-process data on stresses resulting from separation forces, viscosity of the vat resin, temperature gradients, and density changes resulting from crosslinking.