Breviary Technical Ceramics


      From Powder to Part




4.1.3 Forming

The powder particles are compacted to form a coherent shape with sufficient strength for subsequent handling. If necessary, this shaped, unsintered mass of powder (known as a green body) can be machined economically before firing, since corresponding steps are much more expensive after sintering. When applying the various forming processes, care must be taken to avoid significant density gradients and textures in the green body, since these can be amplified during sintering, leading to distortions and internal mechanical stresses. The choice of a suitable forming process is usually determined by economic factors (efficient manufacturing).

Methods of shaping ceramic parts can be divided into the following basic types:

  • pressing (0 – 15 % moisture)
  • plastic forming (15 -25 % moisture)
  • casting (> 25 % moisture)

Dry pressing

is used to manufacture mass-produced precision products. Non-clumping granulates are compressed in steel dies designed appropriately for the part to be manufactured. The high cost for the dies (sometimes made of carbide) can only usually be justified for large runs.

Figure 53: Dry pressing


Figure 54: Single axis dry pressing, single and double ended, with regions of different compression (grey levels)

Dry pressing is the most economic process for large production runs, and is suitable for both simple and complex geometries. Depressions and holes are normally only designed in the pressing direction.

Depending on the design of the dry pressing machine, components ranging in size from tiles down to match heads can be manufactured. Small discs or plates can be pressed with thicknesses of around 0.8 or 1.0 mm. The tape casting process is more suitable for even thinner, flatter components. It is still possible to manufacture fine ridges or similar structures on the component if the granulate being pressed can effectively fill hollows in the pressing tool, and provided it is possible to create the necessary tool.

Isostatic pressing

is suitable for the manufacture of uniformly compressed blanks and large parts that are appropriate for machining in the green state. Simple rubber moulds determine the initial form.

Figure 55: Isostatic pressing with regions of different compression (grey levels)

This type of forming is well-suited to the manufacture of exacting prototypes and small series, but for some products can also be fully automated (spark plugs, grinding balls, small pistons, welding nozzles).

Wet pressing / moist pressing

allows the manufacture of parts with complex geometries such as screw threads, side holes, recesses and undercuts. The unfired material used for this purpose usually has moisture levels in the range of 10 to 15%. Compressing with a single axis makes these materials able to flow freely, so that relatively even compression can be achieved.
The disadvantage of this, however, is that wet pressing materials can accept only low compressive strains. This also means that the degree of compression is limited. It depends heavily on the moisture content of the unfired material, and is lower than in the case of dry pressed parts. In some circumstances, moreover, it is necessary to dry the pressed parts before sintering. Mean tolerances in accordance with DIN 40 680-1 are based on this.


is carried out using piston extruders or vacuum screw presses. The homogenised mass of material is pressed through a nozzle, so forming endless billets. Optimum compression of the material is important. Extrusion is particularly suitable for manufacturing rotationally symmetric parts such as axles or pipes. Complex profiles can also be made with the aid of appropriate nozzle design. The lengths of the billets to be manufactured depend to a large extent on the tendency of the material being processed to warp.

Figure 56: Extrusion


Injection moulding

is principally suited to the mass production of complex products. It is limited by relatively high die costs and the complex burnout of organic additives. The conveying capacity ("shot weight") of large injection moulding machines is typically up to about 70 g. Generally, the part should be designed so that thicknesses are as consistent as possible, having an upper limit of approx. 12 mm.

Slip casting

is a simple method for the manufacture of prototypes, parts with complex geometries and relatively large items. It can be used to manufacture both thin-walled and solid objects. Ceramic slip casting involves a stable suspension, referred to as the slip, being poured into a porous, absorbent plaster mould. Extraction of the suspending liquid causes a layer of particles to develop on the mould wall. This layer develops, in solid casting, to create the fully moulded body. In the case of hollow casting, the superfluous slip is poured out once the desired wall thickness has been achieved.

Tape casting

Here, a ceramic slip containing various organic additives is poured onto an endless steel strip carried by rollers. The slip flows continuously from a reservoir through an adjustable slot onto the strip. Hot air is blown over the strip in the opposite direction to dry it, so that at the end of the strip, thanks to the organic additives, a flexible tape of green ceramic is obtained. This can either be wound up and stored for further processing at a later time, or maybe processed immediately through cutting, punching, stamping or other similar methods. Tape casting is typically used to manufacture ceramic parts with thickness ranging from 0.25 to 1.0 mm. The formed products are suited for the manufacture of substrates, housings, capacitors and multi-layer transducers.

Figure 57: Tape casting

Figure 58: Tape casting machine

The choice

of the forming process to be used in any particular case depends, from a technical point of view, on the geometry and size of the part and the needs of the application. The piece count, raw material consumption and process costs determine the most economic choice.
Further extensions to the forming processes initially introduced here are possible.

Table 3: Summary of ceramic forming processes


Table 4:
Advantages and disadvantages of common forming processes


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