Micro Injection Moulding for Micro Fuel Cell Production

In this PhD-project, the development of injection moulded containers for Direct Methanol Fuel Cell (DMFC) systems is described. This developing work aims at DMFC-Systems for the application in hearing aid systems, which results in a variety of requirements and specifications for the design of the system as well as the manufacturing process.

Beginning with an introduction to different fuel cell systems and the suitability of the DMFC to power small portable electronical devices, the background knowledge is presented to provide an understanding of the challenges for the injection moulded container from both the fuel cell system and the manufacturing side.

The implementation of the transfer of the design of an existing fuel cell container made out of metal comprises the selection of a suitable process and of the materials, which are compatible to both the process and the functionality of the final part as well as the mould design. The experimental part deals with the suitability of the material combination of the chosen micro film insert moulding process.

A phenomenon which was found in this process is the occurrence of blisters in the surface-layer of the chosen dual layer film insert for certain combinations of the processing parameters. These blisters can deteriorate the functionality of the film insert in the application in the fuel cell container and even lead to fatal failure of the container in case of a complete delamination of the film insert.

A detailed investigation on the reasons for the blister formation was conducted. During these investigations, different techniques such as in-process monitoring, computed tomography (CT) scans, surface metrology and process simulations were employed. These investigations revealed that the blister creation is related to the shrinkage of the overmoulding material during the cooling phase of the injection moulding process cycle.

The biggest influence on the blister creation was found in the process parameters melt and mould temperature. A process window for micro film insert moulding with the chosen material combination was found, which led to the production of blister free surfaces. Therefore, the process was found to be suitable for the implementation of the chosen design.

The measurement routine developed to characterize the blistered surface can be applied to define a suitable process window for similar production with other combinations of materials than the one presented in this work. A mould based on a standard split-mould design was designed for the production of the fuel cell container.

This design contained features to create a venting structure in the sidewall of the methanol container and a vacuum system for the fixation of the film insert. Furthermore, injection moulding experiments for the fuel cell container were conducted. These experiments revealed problems with parts of the mould design, which were solved by a second iteration of the concerned mould part and by a change in the material of the film insert.

Finally, the successful production of the DMFC-container in a process suitable for cost efficient mass production could be achieved. A first test with the moulded containers showed insufficient methanol retention properties. Therefore, a redesign of the container will be necessary.