Entwicklung eines In-situ Feuchtemesssystems für Schlickergussformen in der Sanitärkeramikindustrie

摘要

The production of ceramics such as wall and floor tiles, roofing tiles or sanitaryware involves an enormous expenditure of energy, not only in form of gas and electricity but also other primary raw materials such as water. In the subsequent process steps the water is removed from the product by the use of energy. Drying ceramic products is a very slow process which is extremely critical above all in the case of complex geometrical shapes that are prevalent in sanitaryware. In most cases the products are dried at initially very low temperatures which permit gentle dewatering in order to prevent the formation of drying cracks due to different shrinkage. As a consequence, drying plants are very large and require a lot of space and energy. Compared to heavy clay products, sanitaryware is produced by means of a complex moulding process. Slip casting is a very old but still common procedure for the manufacture of sanitaryware. Slip casting moulds are made of absorbent plaster by which the dewatering of the slip is effected. The moulds are grouped in so-called slip-casting lines. The casting time depends upon the size of the mould and varies between some minutes and several hours. During this time water is drawn from the slip and stored within the moulds. After demoulding, the water thus stored has to be “expelled” from the mould again in order to restore the absorptive capacity of the plaster mould. In plaster moulds for the production of sanitaryware this is achieved by opening the two halves of the mould and blowing them with warm dry air passing guiding plates on the slip-casting line. The drying time varies depending on the process; the moulds are mostly dried over night. However, it is not always possible to remove the complete quantity of water stored in the moulds with this method. The residual “water nests” within the mould may cause varying wall thickness of the products when the mould is used again, and consequently stress within the product during the drying process. In the course of later production stages, during the sintering process, this stress may have disastrous consequences such as the destruction of the actual ceramic piece, but also the demolition of neighbouring products and possibly even damages in the firing aggregate. At present, the outcome of drying is determined only exemplarily on single moulds by measuring the total weight and deducing the resulting total moisture of the slip-casting mould.However, this method does not allow determining the moisture exactly and, above all, in a space-resolved way. The development of the sensor used in the experiments described here renders this type of measurement possible. The spots in plaster moulds for sanitaryware which cause problems in drying were equipped with sensors; thus, it was possible to detect the “water nests” and to document the drying result. Alternative measuring methods such as optical and contactless systems were tested within the scope of this dissertation and compared with the newly developed system. Being used in practice, the alternative measuring methods were not able to determine the space-resolved moisture of the moulds.In the industrial application the newly developed sensor showed that this is possible and entails a considerable improvement by optimizing the drying parameters. This improvement does not only comprise an optimization of the drying process and hence a reduction of the primary energy used for the production of the drying air, but also an enhancement of the casting quality due to a minimization of casting defects caused by moist moulds. The improved casting quality brings about an increase of the first-rate quality ratio, and thus an augmentation of the utilization level of energy. Subsequently to the moisture measurements on industrial scale, the moulds used in the process were examined for common ageing processes. A typical sign for aged plaster moulds is a decrease of their absorptive capacity which is not caused by too much moisture but by the clogging of plaster capillaries. The moulds used were examined by means of an ESEM device with a connected EDX.