Breviary Technical Ceramics


      From Powder to Part




4.1.5 Dimensions and Shrinkage

The primary goal when forming the green body from unfired ceramic material is to come as close as possible to the final dimensions and shape of the product to be manufactured, since machining after sintering is costly due to the very high hardness of the material.
In contrast to metals, the forming of ceramic parts is not the last step in the process chain, but rather almost at the beginning.

The green body for the ceramic product must be formed larger than the final dimensions, since one of the particular properties of ceramic technologies is that almost all materials experience a reduction in volume during the manufacturing process. This is due to volume loss during process steps such as moisture removal, drying and firing.
The actual ceramic material and its characteristic microstructure are not created until firing, making the desired material from the mixture of raw materials.
The high temperatures lead to a reduction of the specific surface area of the particles (diffusion processes, the creation of liquid phases, phase changes) and thus to compaction of the microstructure. This is associated with a reduction in volume, and is called shrinkage. For this reason, moulds or dies must be "over-dimensioned" with respect to the desired geometries, in order to compensate for the shrinkage that takes place during processing.

Table 6: Longitudinal shrinkage of some materials

After the mould has been filled with a measured quantity of material, even very slight variations in the density of the loose granulate being pressed have the effect that, in pressing, the unfired parts compact to a variable degrees, with the result that the degree of shrinkage occurring during sintering varies.
Tolerances in the linear dimensions of sintered parts using standard processes are therefore approximately ± 2 %, despite density variations and high shrinkage. These standard tolerances are specified in DIN 40 680 . The use of optimised, synthetic powders or working materials along with highly developed process control nowadays permit tolerances of < ? 0.5 % to be guaranteed.

Figure 61: Dimensional tolerances after sintering and after hard machining


In contrast to the usual procedure with metallic materials, all dimensions should be given the widest possible tolerance ranges in order to avoid increased manufacturing costs. Above all, particular stress must be placed on the need for the user to limit close tolerances to whatever is absolutely necessary for the function of the ceramic part.


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