Investigation of anisotropic shrinkage in liquid silicone rubber injection molding: Process variables

Injection molded liquid silicone rubber (LSR) devices are used in high performance applications for the medical, transportation, and energy industries. Anisotropic shrinkage in LSR injection molding has been observed in practice but is generally not well understood. This research focused on anisotropic shrinkage based on flow and cross-flow orientation. LSR parts were molded at varying thicknesses, shear rates, fill times, and cure temperatures. Simulations accurately predicted the fill pattern and were used to predict the cure rate of the samples. LSR molding showed a significant range of linear shrinkage ratios between the flow versus crossflow directions shrinkage from above 1.30 to below 0.92. The underlying driver of anisotropic behavior is believed to be reaching a sufficient average cure through the cross section combined with flow-induced orientation. Shrinkage anisotropy was found to generally increase with increasing shear rate in regions where curing was significant. Additionally, dynamic mechanical analysis showed a connection between the polymer network using the modulus and anisotropic linear shrinkage.