The processes for manufacturing and forming paper and board packaging, such as creasing, embossing and compression moulding, are used millions of times a day. Nevertheless, the design of tools, process control and optimisation are mostly based on empirical knowledge. Extensive scientific studies of the processes and the application of modern methods and simulations are still an exception in these applications. The informative value of simulative studies has so far been limited due to the complex deformation mechanisms in the fibre material and due to the intrinsic dynamic system of temperature and moisture that interacts with these deformation mechanisms. The low level of research is contrasted by constantly increasing demands on packaging. Shelf-life periods are being extended and product safety must be guaranteed. At the same time, packaging forms are becoming increasingly complex. In addition, consumers are placing higher demands on the environmental compatibility of packaging3 (Innventia AB 2013). The inadequate state of research is also an obstacle with regard to shifting process control and optimisation to the machines’ software. Algorithms for automated process control are based on empirical, semi-empirical, statistical or FE models. The latter are not yet sufficiently available for the forming processes in packaging production and processing of cardboard packaging. The basis for modelling, systematic troubleshooting in production and optimisation are reliable characteristic values for the mechanical and thermal characterisation of the packaging material. For metals and plastics, basic values can be taken from table books, as guide values have been measured and published for defined material compositions. In contrast, data sheets for paperboard usually only give values for thickness, bending stiffness, whiteness, moisture content after paper production, surface roughness and extensibility4 (cf. StoraEnso 2017). For further characteristic values, such as modulus of elasticity or shear modulus in the respective directions, there is usually no information and also no table books or databases that users could refer to.
The basis for this is provided by a systematic characterisation of the material properties for selected board grades. This material characterisation as a function of moisture and temperature is still a great challenge today and is associated with a high level of effort that cannot be borne by SMEs. Within the framework of the present project, two complementary strategies are pursued for characterisation. On the one hand, experimental investigations are carried out under controlled humidity and temperature conditions in order to record the material behaviour under the various boundary conditions. On the other hand, based on the hypothesis that moisture and temperature transport processes are determined by the microstructure, these experimental investigations are supplemented by microstructure simulations on fibre networks. In this way, the necessary in-depth understanding of the transport processes at the level of the fibre networks can be gained, and parameters for the heat and moisture transport can be derived, which in part cannot be determined experimentally. At the same time, the research risk associated with the experimental set-ups can be minimised. The material characterisation is carried out with regard to the requirements of an FEM material model to be developed, but should also serve users as a basis for empirical and semi-empirical models. Currently, the combined influences of temperature and humidity and their interactions cannot be represented in FEM models. An essential sub-goal of the project is therefore the development of an FEM material model for the simulation of the mechanical behaviour of cardboard in manufacturing processes with consideration of the combined influences of temperature and humidity on the mechanical properties. For the calibration, the experimentally and simulatively obtained results of the material characterisation are used. The validation of the model is done by calculating different load scenarios containing combinations of in-plane and out-of-plane loads with different moisture and temperature values. Finally, the FEM material model developed and calibrated in this way is used in process simulations of forming processes. This not only ensures the applicability of the approach to problems with industrial relevance, but also allows initial indications to be derived as to how the processes under consideration can be optimised in the future. Through an improved understanding of material behaviour, the project results should enable material manufacturers, mechanical engineers, toolmakers and material refiners to use the material in a targeted and efficient manner and to already include it in the development of new packaging and machines and to make the associated challenges but also development potential visible at an early stage. The associated minimisation of the use of plastics in optimal functional synthesis with paper or cardboard contributes to solving the challenge of resource and disposal problems for society as a whole.
Aim of the project:
The overall objective of the project is the process optimisation and targeted process control of manufacturing processes with cardboard and paper, taking into account material moisture and material temperature. Furthermore, the THERMO-FIBRE-BASE project is to contribute to providing a model basis for forming and joining processes of paper and cardboard in order to be able to simulatively support the development of new moulded parts, especially packaging, in the future. It specifically addresses the influence of the material’s own moisture content, which varies volatilely in transport and storage processes, and its influence on the properties in the manufacturing process, which is difficult to control in production. Microstructure simulation is used to characterise the course of the temperature-moisture development in the dynamic process and to provide conditions for the structural-mechanical experiments, as well as characteristic values for integration into the macroscopic FE modelling. A model basis is created as well as a method for the adaptation of its parameterisation by modified experiments, which take the moisture and temperature influence into account. The aim is to make the influence of moisture in the processing process, e.g. with regard to heat conduction, vapour development and reduction of strength and stiffness, more tangible for the industry partners, so that process regimes (processing times, temperatures) can be used in a more targeted manner and already taken into account in the development of new products (materials, packaging and machines).
The IGF project presented here by the Research Association of the Industrial Association for Food Technology and Packaging (IVLV e.V.) is funded by the Federal Ministry for Economic Affairs and Energy via the AiF as part of the program for the promotion of industrial community research (IGF) based on a decision of the German Bundestag.