Drying is usually necessary after shaping, before the green bodies can be debinded in the furnace. The moisture content must be reduced from initial values between 10 and 50 vol.-% to about 1 vol.-%. Higher moisture contents would cause rapid heating of the green bodies during debinding, leading to boiling of the liquid component and damage to the green bodies. Shaping processes that usually require drying are slip casting, tape casting, wet pressing, and plastic shaping processes such as extrusion. Organic solvents such as alcohols as well as increasingly water are used as liquids.

Even with slower drying, incorrect process management could lead to mechanical or thermal stresses, which can cause deformation of the green parts or cracking. Gentle and uniform drying often requires a lot of time, sometimes many weeks, and correspondingly a lot of space. Optimized process parameters are often not known. The aim of drying optimization at Fraunhofer Center HTL is to identify drying parameters that do not cause damage to the green parts, but enable the most cost-effective drying.

Optimization of Drying Parameters

The drying parameters that can be set in the drying process - depending on the degree of drying - are the relative humidity of the surrounding atmosphere, temperature, and gas convection at the surface of the green bodies. The drying rate and the stresses that form also depend on the vapor pressure of the solvent, the solvent concentration at the liquid/gas interface, and capillary effects, as well as the geometry of the drying material. Drying parameters must therefore be optimized for the specific product.

Measurement Data as Input for a Finite Element (FE) Simulation

The HTL has measuring devices that can be used to measure the drying rate under controlled conditions in the laboratory or in industrial drying units. The drying shrinkage, dry strength, and viscoelastic properties of the green body are also 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 due to heating the green bodies to the drying temperature are combined with mechanical stresses due to drying shrinkage and the heat of vaporization. The drying parameters are optimized by means of FE simulation so that stresses and distortion in the green bodies are minimized. In addition to humidity, gas velocity, and temperature, the influence of the arrangement and support of the green parts during drying is also simulated. In general, humidity and temperature must be adjusted during drying to the degree of drying.

Transfer to Production Scale
Drying scale
© Fraunhofer Center HTL
Drying scale

The HTL supports the transfer of optimized drying parameters to the production scale. On-site measurements of the moisture content and flow conditions in industrial drying units are carried out for this purpose. Improvement possibilities in the drying units can be evaluated by means of FE simulations before they are tested on a production scale. The influence of volumetric heating methods on the drying material can also be determined using FE methods. Volumetric heating of the green bodies can be done using different methods: microwave, medium or high frequency, and alternating current heating [1]. They allow for faster heating of the drying material to the drying temperature, but are generally associated with higher investment and operating costs.

The HTL can also help in the selection of suitable additives for optimal drying results. In addition, numerous specific measurement methods are available for evaluating the dried green bodies.

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

Service Offering:

  • Investigation of drying behavior
  • Design and optimization of drying processes
  • Investigation of temperature homogeneity and volume flows in the drying oven using sensor modules