Debinding

Suitable in-situ measurement systems are available at the HTL for all relevant furnace atmospheres. Inert gases, oxidizing or reducing gases, and even 100% hydrogen can be used as furnace atmospheres. The furnace atmosphere in gas-heated furnaces can also be precisely replicated. Matching the furnace atmospheres between the production furnace and the TOM system being used is important, as this enables optimized debinding conditions determined using TOM to be transferred to the production furnace.

The degree of debinding can be measured with very high reproducibility measuring weight of the samples. Reproducibility is 0.1% [1]. This allows for the creation of a meaningful database for calculating debinding kinetics. Several debinding runs with different temperature-time cycles are carried out on the same green samples at the HTL, and a robust numerical method is used to calculate a kinetic model from the measurement data. This enables prediction of the degree of debinding for any temperature-time cycle falling within the measured range [2]. Simple optimizations of the debinding cycle can already be achieved using this kinetic model. For example, temperature-time cycles can be calculated at which the debinding rate is nearly constant, resulting in lower stresses on the components than constant heating rate temperature-time cycles [3]. The maximum safe debinding rate is then experimentally determined using appropriately calculated temperature-time cycles, with debinding carried out on larger samples or small components in the TOM systems and any sample damage occurring during debinding being registered in situ. Sound or gas emission measurements are primarily used at the HTL to register damage since they can detect even slight damage.

For more detailed investigations, such as scaling up to other component geometries and considering effects in the industrial furnace, further in-situ measurements are necessary. The endo- and exothermic effects during pyrolysis and binder burnout must be quantified, which is done at the HTL by means of dynamic scanning calorimetry (DSC) in a controlled atmosphere. The thermal conductivity of the green parts is determined during debinding by means of laser flash method. In addition, the permeability of gases through the pore channels of the green body is measured. Together with the kinetics model, these measurement data are used in a coupled finite element (FE) model, which has been developed at the HTL for the optimization of debinding processes. With the model, the temperature distribution in the green body during debinding is calculated for each time step, taking into account the heat of reaction. The local debinding rate is calculated from the local temperature and the locally available oxygen with the kinetics model. The resulting gas-phase reactions lead to concentration and pressure gradients, which are reduced by diffusion and flow processes in the pore channels. These processes are also simulated by means of FE. Finally, for each time step, the mechanical stresses resulting from temperature differences and gas overpressure are calculated. The simulation is then repeated for the next time step until debinding is completed. The debinding conditions are varied with the FE model so that the mechanical stresses in the green body are minimized. In this way, debinding conditions can be targetedly optimized for individual components. Debinding cycles can be drastically shortened compared to empirically optimized cycles.

[1] Raether, F. (ed.): Energieeffizienz bei der Keramikherstellung, ISBN 978-3-8163-0644-3, VDMA-Verlag, Frankfurt, 2013.

[2] Raether, F.: The kinetic field - a versatile tool for prediction and analysis of heating processes, High Temperatures-High Pressures, 42.4, 2013, pp. 303-319.

[3] Raether, F.; Klimera, A., Herrmann, M., Clasen, R. (eds.): Methods of measurement and strategies for binder removal in ceramics. Special edition of Ceramic Forum international: Thermal process engineering in the ceramics industry, Göller Verlag, Baden Baden, 2008, pp. 5-11.

Implementation in the production furnace can raise further questions. For example, the temperatures of the green parts can significantly deviate from the oven temperature, which requires adjustments in the setting plan or in the temperature-time cycle. Additionally, the debinding process is influenced by the flow conditions in the furnace. The measurement methods and FE models for production furnaces are available at the HTL for these adjustments.

Another problem is the accumulation of smoldering gases in the production furnace, which can ignite in an oxygen-containing atmosphere. In an inert or reducing atmosphere, these smoldering gases can deposit on colder areas in the furnace, which can cause quality problems, especially in continuous ovens, if deposition occurs on other, colder green parts. The carbon generated during debinding can also lead to problems in subsequent sintering or deteriorate product properties in many cases. Special detection methods and debinding conditions are helpful, which are available at the HTL.

The HTL can also examine and optimize the suitability of binders for the respective product. Many binders become liquid when heated before pyrolysis or combustion begins. Depending on the wetting properties to the ceramic, the liquid binder can then redistribute in the pore channels, which has disadvantages for the homogeneity of the green parts. The melting behavior of the binders and their wetting of the ceramic are measured at the HTL with TOM systems under controlled atmosphere. Suitable binders are selected in screenings.