Investigation of separation force for bottom-up stereolithography process from mechanics perspective

Relative to the free surface stereolithography (SLA) process, the bottom-up process has several advantages that include better vertical resolution, higher material filling rate, less production time, and less waste of photopolymer materials. However, one of the major concerns of the bottom-up SLA process is that the built-up part may break due to the resultant force generated during the pulling up process. This resultant force may become significant if the adhesive mechanism between the two contact surfaces (i.e., newly cured layer and the bottom of the resin vat) produces a strong bonding characteristic. In this work, the traction force is monitored using FlexiForce® force sensors. The experimental data are analyzed in order to obtain the initial guess for the fracture properties of the separation process. Then the separation process has been modelled based on the concept of Cohesive Zone Model (CZM) in order to study crack propagation behavior in the field of fracture mechanics. The classic bi-linear traction-separation law is adopted in the present work as the nominal constitutive law that relates the resultant traction stress and the separation distance between the two contact surfaces. The results from simulation are compared with experimental data, a good agreement for maximum traction force is found, and the discrepancy is discussed.