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CRC/TR 39: Production technologies for light metal and fiber reinforced composite based components with integrated piezoceramic sensors and actuators
Sub-project A6

Subproject A6

Manufacturing technology for Piezo-moduls with integrated ceramic-fibre-composites and functional polymers for the integration in active metallic devices

Project Manager:

Prof. Dr.-Ing. habil. Lothar Kroll
Technische Universität Chemnitz
Institut für Allgemeinen Maschinenbau und Kunststofftechnik
Professur Strukturleichtbau und Kunststoffverarbeitung
Reichenhainer Str. 70
D-09126 Chemnitz

Telephon: +49-(0)371 / 531 - 38081
Telefax: +49-(0)371 / 531 - 23129
E-Mail: lothar.kroll@mb.tu-chemnitz.de

Presenting the Research Program

Abstract

Purpose of the subproject is research and development of a novel production technology suitable for series production of piezo transducer modules with a high shape freedom for use primarily in active metallic components. Based on the large series two-component injection moulding technology, piezo-ceramic elements are electrically conducted and connected by electrically conductive thermoplastics. Afterwards, the isolation, the packaging and the close-contoured finish of the piezo-ceramic elements take place in an in-situ injection moulding process.

Motivation

The increasing miniaturisation of electronic products and mechatronic assemblies leads to a maximum of function integration and development of manufacturing technologies for small multi-component parts. For this, the micro-injection moulding (µIM) technology is best suitable for plastic-based micro-parts. By micro-injection technology plastic components can be manufactured with high degree of shape freedom and free of post-processing in large quantities. The flexibility of the method allows the combination of electrically conducting and insulating materials and the integration of piezo-ceramic inserts for the formation of electromechanical transducer modules. The studies from the first application period of the TP A6 show the basic functional capability of a piezo-ceramic generator showing embedded in a plastic matrix, the electro-mechanical behavior of the example of an active bending structure. Moreover, it was demonstrated that the electrical contacting of the piezo fiber composites (PFCs) and forwarding of the generated charges by electrically conductive carbon-fiber-reinforced plastics (CFRP) with thermoset matrix are possible in principle.
 

    
a)       b)
Figure 1: a) Active bending structure with integrated piezoelectric ceramic fiber composites (PFCs)
               b) Design of the active bending structure; GRP - glass reinforced plastic, CFRP - carbon fiber
                   reinforced plastic, PFC - piezo-ceramic fiber composites

Aim

Based on the results from the first phase in the sub-project A6, the main target of research is the development of a high-volume capable polymer processing technology for the production of complex piezoelectric modules using a two-component micro-injection moulding machine (see Figure 2). The piezoelectric elements should be supplied and fixed at the mould cavity by suitable pick-and-place systems. In the subsequent injection process, the modular mould cavity is filled with an electrically conductive thermoplastic. The so electrically and mechanically connected piezoelectric ceramics will be covered and packaged with insulating plastic by subsequent releases of defined cavities in the injection process. This results in particular challenges for technological implementation, such as the selection of electrically conductive thermoplastic polymers and filling material, the positioning and fixing of the piezo elements and the moulding tool design. The so produced µIM piezo modules will be embedded in a thermoplastic coating in another injection step to prepare them between metal layers for further processing in the process chain of forming. A major research focus is the simulation of the injection moulding process and the hybrid module structure resulting from thermal stress due to manufacture. Based on the simulation results, a sensitivity analysis is performed to set optimized process windows for the two-component micro-injection moulding.

Figure 2: Functional Principle of the two-component injection-molding machine "Microsystem 50"  (Wittmann/Battenfeld)

Methods

The planned development of manufacturing technologies for piezoelectric modules with integrated piezo-fiber composites (PFCs) and functional polymers for additional contacting and connection of piezoelectric components it is necessary to clarify material technical, structural mechanical and process-related issues and the supply of appropriate handling and tooling technology. The specific scientific and technical challenges in the technological implementation of the new µIM piezo modules include: selecting the best possible thermoplastic material systems and electronic additive fillers, the positioning of piezo elements in the cavity and the design and production of suitable modular tooling systems. The establishment of appropriate electrically conductive plastic materials requires basic technical studies of the fillers and their effect of the conductivity related to the filler content, particle size and shape. For the modification of the injection moulding process the process steps of tool positioning and fixation of the piezoelectric ceramics, back and extrusion coating of the thermoplastic electrically contacting and connection as well as the injection moulded insulation considering the heat conduction have to be adjusted to each other. The favored design variants (see, illustrative embodiment in Figure 3) are structurally implemented and adapted to the µIMP tool.
 
Figure 3: Bending converter module as embodiment of a piezo module µIM

The injection moulding two-component manufacturing of the first piezo modules is used to determine optimal process parameters and the adjustment of all in-situ process steps. In terms of theory-based approaches to the development of new µIM piezo modules is the simulation of the injection moulding processes of the form filling and the temperature profile as well as the design of the hybrid module structure of particular importance. For the structural mechanical and process-oriented design of µIM piezo modules should be determined based on simulation results, detailed production studies, the resulting sensitivity analysis and the optimized process window as well as the associated process-related properties. Furthermore, the mechanical characteristics of the individual components of the bending transducer module as a function of its structural design shall be determined and the piezo module as well as its function should be optimized to the metallic structure. To avoid a large deformation due to cooling especially symmetrical piezoelectric modules should be considered, where internal stresses do not induce fail critical plate bending (see Figure 4).
            
    
a)   b)
Figure 4: Von Mises stress in MPa of the electrically conductive polymer component and the electrically insulating polymer
               components for the construction of a) one top layer (asymmetric) and b) two top layers (symmetric)

To ensure the piezoelectric functions the lead capacity of the µIMP modules, numerical calculations of the associated injection mould-based stresses must be carried out. The necessary electrical, mechanical and rheological characteristics should be identified in material specific stress tests. Of particular interest are the mechanical properties of the different thermoplastic components as well as the interlaminar strength of the polymer combinations and the piezoelectric ceramic/thermoplastic compound in relation to the failure relevant tensile and shear stress caused by shrinkage.
 

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