On preferential debonding during demolding of a sandwiched elastomeric layer

Abstract

Separation of an elastomeric layer sandwiched between two flexible molds is a common processing step in the fabrication of soft articles such as ophthalmic lenses. The demolding process is typically engineered for the interfacial separation to preferentially occur at a desired interface by applying displacements at suitably selected locations. This study addresses the potential to predispose debonding along the desired interface by exploring roles of (i) the relative flexibilities of the two molds, (ii) differential preheating of the two molds and the interlayer, and (iii) the interlayer curvature. These objectives are accomplished by numerically analyzing the axisymmetric deformations of an elastomeric layer of uniform thickness sandwiched between the flexible molds, the flanges of which are pried apart by applying displacements along the axis of symmetry. The debonding at each interface between the molds and the interlayer is modeled by using a bilinear traction-separation (TS) relation. It is found that (i) debonding initiates preferentially from the edge of the interface between the elastomer and the mold of lower bending rigidity; (ii) preferential debonding can be engineered by differentially preheating the assembly; (iii) edge debonding is inherently biased to the interface with the smaller radius (inner) mold interface, and (iv) the occurrence of the preferential edge debonding at an interface can be attributed to a bias in the opening shear towards that interface. However, when the overall confinement of the system is large enough as dictated by the flexural rigidities of the molds for an elastomeric layer of given modulus and thickness, debonding may nucleate internally and complicate the progression of the separation process. Internal debonding may also occur when the temperature differential between the two molds is large enough at a given absolute level of temperature increment. Sensitivities of the predicted debonding mechanisms to prescribed biases in the TS parameters are assessed. These results could prove useful in design of demolding processes to achieve desired separation characteristics.