Microstructuring Polycarbonate Films by Variothermal Extrusion Embossing

The variothermal extrusion embossing process enables a high quality replication of microstructures in plastics films. In this advancement on regular flat film extrusion a polymer melt is shaped by a flat film die and cast on an embossing roll. The surface of the embossing roll has the negative form of the desired microstructure. A counter pressure roll is used to apply the line force needed for embossing, thereby transferring the microstructure into the polymer melt. The melt stays in contact to the embossing roll allowing for a rapid cooling of the film and a solidification of the microstructures inside the molds on the embossing roll's surface. At the haul-off point the cooled down film is removed from the roll. The governing process parameters for successful embossing are the melt temperature, the viscosity, the surface temperature of the embossing roll and its rotation speed. By tempering the embossing roll to a low base temperature while using an electrical inductor or a high-power infra-red laser as an additional local heat source the temperature profile along the circumference of the embossing roll is finely tuned to the process. This allows for a high surface temperature in the embossing zone improving the molding quality. Simultaneously the additional heat quantity is low enough to be removed before the film reaches the haul-off point allowing for a stable process. In this paper the variothermal extrusion embossing process is examined for polycarbonate (PC) films. The replicated microstructures vary in size (30 - 200 mu m) and shape (mountains, waves) and aim for the optical functionalization of the film. A major challenge is the damage-free hauling-off of the film due to PCs brittleness. Systematic investigation on the process parameters is conducted and reveals a process window in which accurate reproduction of the microstructures can be achieved. Depending on the microstructure at hand the process has to be further adjusted for best quality reproduction.