Micro size components from polymers have been conventionally produced by injection moulding process. A new and somewhat novel thermoforming process has been developed to manufacture micro size plastic components used as technical parts such as polymer chips.
Researchers at the Institute of Microstructure Technology at Forschungszentrum Karlsruhe GmbH, Germany, have developed a lab-scale thermoforming process that reportedly represents the industry's first move into micro-forming of structures with dimensions between 0.1 and 1000 nanometers (1000 nm = 1 micron). The process targets life-science applications such as single-use “lab on a chip” (LOC) micro fluidic devices that perform synthesis and analysis of biochemical agents for pharmaceutical research.
The new process is a microscopic adaptation of the trapped-sheet forming technique. The three-part forming system consists of a plate-shaped mold with micro-cavities, a counter plate with holes for vacuum and gas pressurization, and a seal between the mould and the counter plate. A thin (20-50 micron) thermoplastic film is inserted in the mold assembly, which is mounted in a heated laboratory press. The press is closed and vacuum sealing is achieved although the film is not yet clamped between the mould plates. The mould is then evacuated and completely closed, thus clamping and heating the film. When the material has reached forming temperature, the film is forced into the evacuated mould cavities with compressed gas pressure of about 50 bar. When the mould has cooled to about 20° C below the material's forming temperature, the gas pressure is released, the mould opens, and the micro-part is ejected.
To produce closed-container parts, a second film with a heat-activated adhesive is inserted into the press after the first film is formed but before it is ejected. In a process resembling twin-sheet forming, the second film is pressed against the formed microstructure to activate the adhesive and form the container. This method can produce liquid-tight products that compete with micro-injection moulded and vacuum hot-embossed products with features such as micro channels and reservoirs. Pre and post-processing of the sheet could include ion bombardment or UV based surface modification with appropriate masks before forming and ion-track etching after forming.
Moulds can be made by mechanical micro-machining, EDM, lithographic methods combined with electroplating, wet or dry etching, laser ablation or powder blasting. Metal, ceramic, glass, and silicon carbide have been used as mould materials. The process can use 50 micron thick film of polystyrene, polycarbonate or cyclic olefin copolymer (COC).
The first application for micro-forming is 10x10 mm cell-culture chips, each one formed into 675 micro pockets. The chips were previously micro-injection moulded in a labor intensive process that required hand removal of sprues and runners. The micro-thermoformed structures are a few microns thick and boast a smooth inside surface that is difficult to achieve with other methods.
Current limitations of the micro-forming process include manual operations that result in a 10 minute cycle time. Development efforts focus on automating pressure build-up and ejection systems. Work also continues on advanced heating systems that hold the mould temperature constant and preheat the film instead. The objective is to reduce cycle time and improve reproducibility.
The research group wants to license the patented technology and is in discussions with thermoformers in the U.S. and Europe . The technology would be most attractive to medical packaging manufacturers with clean room capabilities.
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