Investigation of the Viscosity of Liquid Silicone Rubber for Molding Microstructures in the Injection Molding Process

Abstract

The processing of liquid silicone rubber (LSR) in the injection molding process is becoming increasingly important. The reasons for this are its outstanding properties and economic processability. The functionalization of surfaces through nano- or microstructures introduced directly in the injection mold opens up new fields of application. For example, antibacterial, self-cleaning or tribologically and haptically optimized surfaces can be produced directly during injection molding. To ensure process-reliable molding of the microstructures, the influence of various process settings on moldability is investigated in this work. The viscosity of the LSR plays a decisive role. If it is too high, the LSR cannot lay into the nano- and microstructures and the structures will not be completely imprinted. The viscosity of LSR is influenced by several overlapping factors during processing. While the shear stress and the initial temperature input reduce the viscosity, the crosslinking reaction starts after the incubation time at a certain temperature, which abruptly increases the viscosity. Dynamic differential scanning calorimetry (DSC), oscillating rheometer tests and dielectric thermal analysis (DEA) measurement methods are used to investigate the properties of LSR during processing. Incubation times and crosslinking reactions are determined and compared for rheometer and DSC measurements. An attempt has also been made to define a gel point. Here, a clear distinction must be made between physical gel, consisting of entanglements and intermolecular interactions, and chemical gel, consisting of covalent bonds due to crosslinking. Since the physical gel recedes very quickly after destruction by shear stress, it is difficult to determine the chemical gel point using rheometry. Here DSC and DEA measurements help to determine the chemical crosslinking behavior. In addition to material characterization, microstructures are produced in an injection molding process with varying mold temperatures and injection speeds. Afterward, the microstructures are characterized for their degree of molding. It is shown that LSR molds the microstructures very reliably, even with poorly selected process parameters. This can be justified by the additional volume dilation, which is not taken into account in any of the measurement methods investigated. The viscosity of the injected, cold LSR has already decreased due to the shear stress. In addition, the temperature input initially reduces the viscosity. While the incubation time prevents immediate crosslinking as the temperature rises, the low-viscosity LSR expands due to thermal expansion and pushes into the nanostructures or microstructures. This is when the crosslinking reaction begins, during which the viscosity increases abruptly. The structures were not completely formed only at very high mold temperatures and low injection speeds. Here, it is helpful to cool the mold surface briefly and locally using variothermal tempering before injection.