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Defects and problems related to uniaxial pressing
Dr. José Luis Amorós Albaro

Some of the problems and defects that most frequently appear during the manufacturing process of ceramic tiles formed by uniaxial pressing, owing to inappropriate performance of this operation, are:

    i) Inappropriate compaction of the piece
    ii) Wear by mould abrasion
    iii) Crack formation
    iv) Inconsistent compaction in the tile and/or among tiles

i) Inappropriate compaction of the piece. This defect is usually due to lack of control in pressing powder preparation. In fact variations in pressing powder moisture content, caused by alterations in spray dryer or granulator operating conditions, substantially modify pressed tile compaction.

ii) Wear by mould abrasion. This problem progressively changes tile size and deteriorates its surface texture.

iii) Crack formation. The appearance of cracks in the pressed piece can be due to different causes. The most frequent are: inadequate mould design, excessive quantity of air trapped in the compaction step, excessive tile expansion during ejection from the mould, high friction between tile and mould wall during ejection, etc.

Generally, crack formation starts at the top edges of the piece, either on removing the applied load (Figure 12a) or on ejecting the piece from the die (Figure 12b). In the first case (Figure 12a), the top centre surface of the piece, which is in contact with the top punch, expands axially while the punch is being withdrawn. However, at the top edge this expansion is hindered by friction between the die wall and the tile surface. This produces tensile stresses at the top edge of the piece, which can cause cracks to appear if the value of these stresses exceeds the mechanical strength of the material in this region.

Figure 12: Crack formation mechanisms

The formation of cracks by this mechanism, known as « endcapping » can be avoided by implementing one or more of the following measures:

  • Reducing the coefficient of friction of the piece/mould system, by using an appropriate lubricant.
  • Increasing green mechanical strength of the compact, by selecting the most suitable quantity and nature of the binder.
  • Reducing after-pressing expansion.
  • Withdrawing the top punch without completely removing the load to avoid axial expansion of the piece during ejection.

The second crack formation mechanism (Figure 12b) is also related to after-pressing expansion of the piece. In fact as the piece leaves the mould, its cross section increases in size. This expansion produces tensile stresses in the piece, just above the top edge of the die, which cause laminar cracks (laminations) to appear if the magnitude of these stresses exceeds the material’s mechanical strength. To avoid the appearance of this defect, the tile’s green mechanical strength must be increased and after-pressing expansion must be reduced to a minimum. The best way of doing this is by appropriate binder selection.

iv) Inconsistent compaction in the tile and/or among tiles. Excessive variations in compaction inside a body cause strains, distortions and even fractures in the body during firing. On the other hand, if these alterations in compaction occur between pieces, they cause lack of uniformity in the fired product.

One of the causes of variations in density inside a tile is friction between particles, and between particles and the die, as set out above. However, in general, the principal cause of this type of defect is irregular filling of the mould cavity. The area of the mould cavity that contains more powder after filling will produce higher compaction in the compaction step. If a mould contains different cavities for producing several pieces in a single pressing, lack of uniformity in filling will lead to inconsistent compaction among the pieces. In order to reduce or eliminate these defects, the mould filling programme must be optimised and pressing powder flowability increased.

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