Rapid Quantitative Assessment of Residual Stress States in PLA Components Enabled by the Combination of Photoelasticity and the Hole Drilling Method

Poly(lactic acid) (PLA) is one of the most prominent biopolymers and is considered a viable alternative to petroleum-based polymers. While it exhibits comparable properties to conventional polymers like PET, in certain applications, particularly those involving elevated temperatures, PLA has performance limitations. In addition, the properties of PLA are dependent on the processing parameters in injection molding. Non-optimal process parameters can lead to defects or undesirable effects that cannot be detected immediately after injection molding. This includes orientation and residual stresses, which significantly influence the material and failure properties. The present study investigates the influence of injection molding machine settings on the residual stress state in PLA components. Test specimens were produced using two different mold tools: an ejector pin and a full-surface ejector, while varying key machine settings. Residual stress was assessed using a polariscope and the hole drilling method. The polariscope identified distinct isochromatic fringe patterns, particularly near the sprue, indicating regions of elevated residual stress. The hole drilling method confirmed the presence of high residual stress at the specimen edges, extending to a depth of 600 μm, with a peak stress value of 47 MPa. Results revealed that the ejector pin mold induced both tensile and compressive stress states, whereas the full-surface ejector mold predominantly caused high compressive stresses at the edges. These findings highlight the importance of optimizing injection molding parameters to minimize residual stress and improve the mechanical performance of PLA components.