Lightweight PCW kiln furniture for processes up to 1.600 °C

The following article is a piece of writing that is published on the SCHUPP??-?#Ceramics?website and is an intangible property of the company.

M.E. SCHUPP Industriekeramik GmbH

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Innovative, dense, and light PCW kiln furniture for high temperature sintering processes up to 1.600 °C (application temperature).

The #hightemperature #sintering processes, which normally require temperature between 1.400 °C and 1.700 °C, are associated with several technical and economic challenges.

Efficient use of your sintering furnaces and kilns requires ensuring process reliability, constant optimization of operating costs as well as increasing productivity on the other side. Therefore, suitable #kiln furniture is just as important for the processes as for the firing material. Depending on the process conditions, substrates based on alumina (Al2O3), mullite (72% Al2O3 and 28% SiO2) or silicon carbide (SiC) are usually used today. These products are used in form of e.g. setter, plates, skillet systems, trays, or cassettes to meet a wide variety of requirements of the firing material, and to use the furnace volume as optimally as possible.

The requirements for kiln furniture are high, for example:

• heat resistance at high temperatures up to 1.750 °C,
• good thermal shock resistance (TSR),
• low heat capacity,
• mechanical stability,
• the highest possible creep resistance,
• low reactivity and high chemical resistance to the firing goods and gases in the atmosphere,
• variable geometries at low manufacturing cost to meet customer requirements etc.

Classic kiln furniture based on Al2O3 (corundum) or mullite are generally made from many various ingredients. Manufacturing process and grain size distribution often differentiate these raw materials. The production basically differs in the temperature at which they are burned (calcined). In addition to calcination, they are commonly melted in an arc furnace and then milled to the desired grain size. These substances have a particularly low reactivity and are resistant up to very high temperatures (> 1.700 °C).

The grain size distribution ranges from a few μm to several mm. Additionally, the qualities can vary between sintered corundum and mullite and fused corundum and mullite.

Due to the components used and the manufacturing processes, such kiln furniture has a density of approx. 2.5 to over 3 g / cm3, which results in a very high heat capacity. Furthermore, due to the high weight, the underlays require complex handling when loading the furnace. Basically, these products have a relatively poor thermal shock resistance (TSR), which is very disadvantageous for rapid firing processes and negatively affects the durability of the kiln furniture.

An overriding goal in many industries is to shorten the time of burning cycles to improve cost-effectiveness. Saving energy costs is in fact an important issue, which can also be achieved by reducing the thermal capacity of the kiln furniture.

Usually, the weight ratio of kiln furniture to the products to be burned is 2:1 or even significantly worse. This means that most of the energy that goes into the process is consumed to heat up the kiln furniture and not to sinter the product itself.

Kiln furniture based on polycrystalline mullite / alumina wool (PCW).

A solution seems to be the development of kiln furniture underlays which are based on polycrystalline mullite / alumina wool (PCW). In contrast to refractory ceramic fibres (RCF), these PCW fibres are not classified as hazardous to health in the European Union and therefore often used as a substitute material for RCF, which have been banned in many countries.

SCHUPP? Ceramics produces customised and application-specific vacuum formed parts made of PCW. The company's own vacuum-forming line, which become operational in 2018, was an important milestone in increasing the vertical range of manufacture. Furthermore, the modern production offers flexibility in terms of shapes and recipes, fast deliveries and controlled high product quality. This makes it possible to develop innovative, high-dense, and light mullite-based kiln furniture for high-temperature sintering processes up to 1.600 °C.

The vacuum moulding is used for shaping, in combination with a special compression technique. If necessary, the product is then processed to achieve complex geometries.

Already during the development of the kiln furniture, high technical functionality and conscious economic aspects were aimed for. The dimensions and geometry of this product can vary significantly. Apart from small, round saggars with a diameter in the range of 80-120 mm, widely spread e.g. in the dental industry, larger cassettes (e.g. 150 mm x 250 mm) can be produced to sinter of ceramic beads. Additionally, very complex constructions can be manufactured, adapted to customer requirements.

From a chemical point of view, these products have a very similar composition to conventional kiln furniture, but PCW-products have a completely different structure, which makes them very suitable as insulation material. The major indicator here is the structure of high-temperature insulation materials based on polycrystalline mullite / aluminium oxide wool (PCW), which are used up to 1.600 °C furnace temperature, under oxidizing and slightly reducing atmosphere. The density of these substances is usually around 0.4 - 1.5 g / cm3, which is just a small fraction of the corresponding density of conventional kiln furniture.

Due to the high density of the regularly used, traditional kiln underlays (2.5 to over 3 g / cm3), their heat capacity is accordingly high. Of course, these also have a very high compressive strength and hot bending strength. The new materials from SCHUPP? Ceramics are not as strong but convince with densities between 0.4 and a maximum of 1.5 g / cm3, which can reduce the heat capacity by up to 60%.

Considering the firing goods and the furnace lining too, this could lead to a reduction in the total energy requirement of up to 25%.

In addition, the kiln furniture from SCHUPP? Ceramics has a greatly better thermal shock resistance (TRS) and a significantly longer durability than conventional products. Traditional underlays work around 100 to 200 cycles at temperatures of 1.600 °C, strongly depending on the application and the heating rate. However, they are limited to cycles with a “cold-cold” firing time of more than 10 hours. Depending on the process conditions, the PCW-based #insulation materials can be heated up to 1.600 °C for several hundred or even up to 2000 cycles and then cooled down again without the material being significantly damaged. It is already used as an insulation material and kiln furniture at heating and cooling rates of up to 100 K / min at temperatures up to 1.600 °C. #Dental industry is an excellent example for such application. This corresponds to a firing cycle of approx. 30 min. "cold-cold", in this case at max. temperature of 1.500 °C. As already mentioned, the material has a low resistance to compressive strength and hot bending strength, which makes it suitable only for relatively light products.

The chemical resistance is essentially determined by the level of the aluminium oxide content (Al2O3). The higher the Al2O3 amount, the better the chemical resistance. For this reason, an Al2O3 content has been specified for at least 80%.

With the own vacuum forming production line, SCHUPP? Ceramics has the ideal conditions to use the existing technology to develop further products that better meet the special requirements of the market. The aim is to manufacture kiln furniture that is also a custom-tailored solution.?The SCHUPP? Ceramics company looks forward to your inquiries.

Read more about lightweigt kiln furniture made of PCW!

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