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Verwertbare Ergebnisse
The miniaturization of manufactured complex micro-components, devices and equipment is increasing significantly. Such emerging needs include tubular micro-components (diameter < 1.0mm), as those used in electrophysiological devices, micro-fluidic-devices and heat-management systems. However, shaping of polymer micro-tubes cannot be achieved simply by scaling down a large scale process and equipment to the micro-scale, due to many size-factors relating to the material, process, tool and machines. To address this issue, specific micro-shaping technologies, which are able to convert small tubes and thin sections into the required functional structures, and corresponding machine systems, have been developed for mass production. The hot-embossing process and the machine are one of the developments within the FP7 POLYTUBES project for the shaping of micro-tubes. Other developments include Laser-Drilling/Trimming, Blow Forming and Cross-Rolling of micro-tubes and the manufacturing platform.
The Hot Embossing Process
Comparing to other processes, hot-embossing has less system-complexity, shorter production cycle-time, lower processing temperature, etc. Nevertheless, the hot embossing process is often used for the moulding on the plain surfaces such as polymeric sheets and thin films or foils, to produce, mostly, 2.5D features. Hot-embossing of polymeric micro-tubes is to form 3D features (both outer and inner features), which requires specific tool-design and process control, including considerations on the stiffness of tubular structures.
Tools and accesories
To enable high-quality hot-embossing of polymeric micro-tubes, tools and coatings are designed and constructed to enable: modular insertion of the core dies; precision guiding of the micro-tube inside the dies; heating and cooling units with a temperature control system; heat-insulation for an improved heating-efficiency; precise alignment of the upper and lower die-sets, etc. The tool design also took the requirement for automated handling into account.
Swerea IVF has built a polymer extrusion unit specially designed for the stable manufacturing of precision polymer microtubes. The system has been built in the frame of the project POLYTUBES NMP2-SE-2009-229266 of the FP7. The equipment has a large degree of set-up flexibility, enabling production of polymer microtubes of different dimensions and polymer types. The principal design consists of an extruder feeding a polymer melt to a gear pump which in turn delivers an exact and steady melt flow through a ring shaped die. The extruded tubular formed melt is drawn and cooled to achieve the desired tube dimensions.
The extruder and gear pump
The extruder ensures the condition of the polymer melt (e.g. temperature, pressure and flow) fed to the gear pump. The gear pump in turn delivers a precise melt flow to a ring shaped die inside the extrusion tooling forming the micro-tubes. A gear pump offers better "flow control" at these small flows than a corresponding extruder. Fittings have been specially designed and machined to connect the extruder to the pump and the pump to the crosshead. These are fitted with pressure gauges before and after the gear pump in order to control the state of the melt.
Extrusion crosshead and dies
To produce tubing with minimum tolerances it is important to ensure a balanced extrusion flow and to use a precise tooling geometry designed with the polymer's rheological properties in mind. The chosen crosshead system is specifically engineered to provide minimum residence time and pressures but at the same time give upstream thermal balancing. Inside the extrusion crosshead an appropriate die is fitted. The dies are designed to shape steady melt flow into its desired ring shape. Different die heads and die tips are designed in order to produce the different sizes of micro-tubes desired. The dies mainly set the ratio between the outer and inner diameter of the tube. The exact dimension is achieved by setting other parameters.
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