Further development of vacuum hotcasting

Weiterentwicklung des Vakuumheißgießens

Vacuum hot casting is a versatile process for the production of technical ceramics. In contrast to ceramic injection moulding, which only becomes economically relevant in the context of large series, vacuum hot casting is particularly suitable for small series, prototypes and feasibility studies. The reasons for this lie on the one hand in the cost-effective production of the silicone moulds used and their long lifespan, and on the other hand in the re-meltability of the thermoplastic mass used, the so-called feedstock.

Usually, these feedstocks are produced with the help of intensive mixers, kneaders or dissolvers. Since these preparation processes usually use steel mixing tools, metal abrasion can occur during the production of the feedstock. This metal abrasion can then appear as a slight or even stronger grey colouration of the feedstock. In addition, metal abrasion primarily represents a chemical contamination of the ceramic, which decisively reduces the electrical and mechanical properties and can also become visible as red dots on the surface of the sintered components. Despite these negative effects on the quality of ceramic end products, this topic is rarely discussed in the technical literature.

Due to this issue, WZR has made major improvements to the production of the feedstocks so that no metal tools are used in the entire production chain. The process developed uses wet homogenisation, which is also commonly used for the production of ceramic slurries. Ceramic powder and binder system are homogenised in a plastic grinding drum by means of ceramic grinding balls in a ball mill. In this way, metallic abrasion cannot occur in at all.

In additional, the agglomerates of the ceramic raw material used are destroyed by the falling grinding balls during this process, whereby the entire powder surface can be wetted with the binder system. This leads to a very homogeneous feedstock, which is evacuated just again before the casting process in the vacuum hot casting machine in order to avoid pore formation in the later component.

For internal processes, the newly developed process offers advantages in terms of production time, cleaning effort and personnel deployment.

So far, WZR has implemented the new preparation process for alumina ceramics. After sintering, a pore-free microstructure and a parts surface without optical impurities are obtained. Manufactured samples could achieve a 4-point bending strength of up to 400 MPa.

With these properties, WZR can now offer components for high-quality applications in the mechanical engineering and electrical sectors that could not be served previously.

Since all relevant 3D printing processes for ceramics are used at WZR, the direct comparison can be made in terms of material properties, surface quality and dimensional accuracy. This can be summarised as follows:

  • Advantages of vacuum hot casting
    • The material properties of 99.7% alumina correspond to those of components from series production. Components from the VPP process show comparable properties; alumina parts produced by means of binder jetting cannot be densely sintered at present; as a result, the strength is quite low.
    • The material properties in vacuum hot casting are homogeneous. In contrast, 3D printing processes are frequently found to have lower strength and higher shrinkage in z-direction.
    • Vacuum hot casting has a surface quality that cannot be achieved with any 3D printing process at present. Roughness (Ra value) of less than 1µm is easily achieved, details of less than 10µm can be achieved.
  • Disadvantages of vacuum hot casting
    • It works with a mould. The mould is cast from a master model, which means a lead time of approx. 2 weeks in total. There are additional costs for the master model and mould, which are charged to the number of parts to be produced. For a quantity of 1, this massively increases the delivery time and costs.
    • The geometrical freedom of the ceramic is limited, as the demouldability must be given

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Binder jetting is one of the 7 3d printing processes, depicted in the standard DIN EN ISO/ASTM. In addition to vat photo polymerisation (VPP), material extrusion (MEX) and material jetting (MJT), binder jetting is used to produce ceramic parts as well.

For whom are microstructural investigations interesting?

The microstructure of a material is one of the most important parameters – if not THE most important – to be used to assess the quality of a newly developed material or component. Therefore, information about the microstructure can be obtained by porosity measurements or strength tests, since these properties are greatly influenced by the microstructure. Directly, the microstructure of e.g. technical ceramics can be studied using a wide range of methods that have been available to industry and the end-user market since the last century to complement conventional optical microscopy.

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