Drying

Subsequent to the forming, drying is usually necessary, before the shaped bodies can be debindered in the furnace. The moisture content must thereby be reduced from 10 to 50% by volume to approximately 1% by volume. At higher moisture contents, the rapid heating of the shaped bodies during the removal process would lead to the boiling of the liquid component and to damage to the moldings. Forming processes which usually require drying are slip casting, film casting, wet pressing and plastic molding processes, such as extrusion. Organic solvents such as alcohols and increasingly water are used as liquids. Even with slower drying, mechanical or thermal stresses are created as a result of the dry shrinkage, which can lead to the deformation of the mold parts or to cracks. A gentle and even drying process takes a lot of time, sometimes many weeks, and correspondingly lots of space. The aim is to identify drying parameters which do not yet cause damage to the molded parts but which enable the most economical drying.

The drying parameters, which can be adjusted in the drying process - depending on the degree of drying - are the relative humidity of the surrounding atmosphere, the temperature and the gas convection on the surface of the molded body. The drying rate and the forming stresses also depend on the vapor pressure of the solvent, the solvent concentration at the liquid / gas interface and capillary effects, and the geometry of the drying material. Drying parameters must therefore be optimized for each product.

At Fraunhofer-Center HTL, measuring instruments are available with which the drying rate can be measured under controlled conditions in the laboratory or in the industrial drying units. The dry shrinkage, the dry strength and the viscoelastic properties of the shaped body are determined experimentally at the HTL. The measurement data is used as input for a finite element (FE) simulation of the drying process. For this purpose, a special FE model for drying was developed at the HTL. In the FE model, temperature effects are generated by heating the shaped bodies to drying temperature as well as by the evaporation chill combined with mechanical stresses by the drying shrinkage. By means of FE simulation, the drying parameters are optimized in such a way that stresses and deformation in the moldings are minimized. In addition to moisture, gas velocity and temperature, the influence of the arrangement and holding of the moldings during drying is also simulated. As a rule, during the drying process, moisture and temperature must be adapted to the degree of drying.

Fraunhofer-Center HTL supports the transfer of the optimized drying parameters to the production scale. For this purpose, on-site measurements are carried out on the moisture content and the flow conditions in the industrial drying units. By means of FE simulations, improvement possibilities in the drying units can be evaluated, before they are tested on the production scale. The influence of volumetric heating methods on the material to be dried can also be determined by means of the FE method. Volumetric heating of the shaped bodies can be performed by different methods: microwave, medium or high frequency and alternating current heating. They enable a quicker heating of the drying material to the drying temperature, but are, generally, also associated with higher investment or operating costs.

Fraunhofer-Center HTL can also help with the selection of suitable additives for optimal drying results. In addition, numerous specific measuring methods are available for evaluating the dried shaped bodies.

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