Tools for ultrasonic hot embossing

Ultrasonic hot embossing is an emerging technology enabling molding of thermo-plastic polymers in seconds. A stack of polymer foils is heated by the friction between the foils and protruding microstructures on a tool when ultrasonic vibrations are generated by a sonotrode. The polymer is molten and adapts to the shape of the microstructures on the tool. Thus, a micro structure is generated in much shorter time than by injection molding or hot embossing. The objective of this work was investigating how tools for ultrasonic hot embossing can be fabricated and what is the influence of the tool on the process. How the embossing energy is distributed in the whole process is briefly discussed in this thesis. When ultrasound is generated, the induced energy is mainly transferred to and absorbed in sonotrode, polymer stack, tool and anvil. At the interfaces, ultrasound is reflected and, this way, re-contributing to heat generation. Ultrasonic energy distribution analyses are based on temperature measurements in the embossing tools and dimension measurements on the embossed polymer micro structures. Embossing tools from aluminum, nickel, steel, polycarbonate (PC) and polyether ether ketone (PEEK) were produced and embossed with high density polyethylene (HDPE). The cooling rates of different tools are compared while the energy distributed into embossed polymer and embossing tool was roughly calculated. Embossing tools with larger acoustic impedance compared to the embossed polymer reflect more ultrasound back to the polymer for melting. Embossing tools with higher heat conduction result in a faster cooling rate after embossing while with less heat capacity and density a higher peak temperature is achieved. The isolation at the interface between embossing tool and anvil affects the tool temperature and its cooling rate. Consequently, aluminum is more suitable for the sonotrode while nickel and steel are suitable for the tool. Embossing tools from PC and PEEK have shown limited lifetimes for ultrasonic hot embossing and, therefore, are no appropriate alternative.Various ways have been attempted to fabricate ultrasonic hot embossing tools. They are milling, nickel electroplating, lithography and silicon etching. New combinations of these ways were also developed to fabricate a tool with multi-level structures or enlarge the tool size. The operating steps, process parameters, challenges, precautions, advantages and disadvantages of these fabrication ways are discussed and compared. Among them, milling is the most promising way producing embossing tools allowing three-dimensional micro structures, up to more than several hundred micrometers in height and down to 50 µm in width. Besides, nickel electroplating was applicable to produce the tools with structure dimensions of less than 50 µm. The templates for electroplating were prepared by lithography and silicon etching. Liquid metals were also prepared to be employed as embossing tool. Their performance and the reasons of failure are discussed.Several micro devices were developed and fabricated demonstrating the fabrication of tools for ultrasonic hot embossing of three-dimensional micro structures. Design idea, production process and performance are introduced. A Fresnel lens was produced by ultrasonic hot embossing for the first time. Micro nozzles, 150 and 250 µm in diameter, which were used for gas/liquid dispersion, were produced by ultrasonic hot embossing and welding. The nozzle could sustain pressures as high as 29 MPa and 2.5 MPa at 20°C and 90°C, respectively. The multi-phase dispersing ability was also proven. An electronic spiral coil forming a molded interconnect device (MID) was fabricated by ultrasonic hot embossing. The surface of the coil was not flat but bowed and the coil line was distributed along a three-dimensional spiral curve. The calculated and measured resonance frequencies of this coil combined with a capacitor were 1.6 MHz and 1.65 MHz, respectively. Several polymer surfaces were embossed with tools fabricated by x-ray lithography and electroplating of nickel and milling of aluminum with different structures designs for hydrophobic surfaces. The wettability of the embossed surface was investigated by contact angle measurements. The polymer surface embossed from the x-ray lithography and electroplating of nickel tool turned out being more hydrophobic than the one embossed from the aluminum tool. The contact regimes of the embossed surface from the two embossing tools were also theoretically analyzed. The last application was the ultrasonic hot embossing of nano-structures, 600 nm in height, on PC and poly methyl methacrylate (PMMA) and poly (lactic acid) foils. The challenges and the solutions in ultrasonic hot embossing with this tool are introduced. Finally, the nanostructures were successfully embossed into these polymers