Isostatic die for pressing ceramic tiles and method for manufacturing the tiles

Abstract: The isobaric die for pressing ceramic tiles comprises a semi-die (1) having a plate (6;39;42;48,48a) delimiting a cavity (7) of the semi-die (1) in which is contained an incompressible fluid (F) and anchored to the said semi-die (1) by means of an elastic component (2;68) sealing the incompressible fluid inside the said cavity, the said plate (6;39;42;48,48a) being free to perform limited displacements closer to, and further from, the bottom of the cavity (7), the plate (6;39;48,48a) being provided with openings (22a;40;43) passing through it. This affords the particular advantage of obtaining uniform pressing whilst maintaining a substantially homogeneous thickness of the tile, due to the discontinuities in the plate (6;39;48;48a) acting in a differentiated manner on the various areas of the surface of the membrane (2;68) in function of the loads transmitted locally by the powders during pressing.

Description: The invention concerns a die for pressing ceramic tiles and a relative manufacturing method. IT-A-1104511 discloses a method for manufaturing ceramic tiles in which tiles are pressed under the action of an elastic modelling membrane, for example, made of rubber, delimiting a chamber in a respective semi-die in which an incompressible fluid, for example, oil, is introduced; by means of such a die it is possible to make the density of the pressed tiles homogeneous so as to limit undesired dimensional variations during subsequent firing. IT-A-1104511 also proposes the use of stiffening plates applied to the external surface of the elastic modelling membrane to limit excessive deformation in the membrane. IT-A-1104511, however, whilst introducing the fundamental concept of homogeneous pressing of ceramic tiles, has been found to be subject to a tendency towards a non uniform thickness of the pressed items. With a view to obtaining a substantially uniform thickness, at least in the areas of the supports of the tile, the adoption of a grill, or lattice, has been proposed, for example, in EP-A-0556163, occupying the cavity in which the incompressible fluid is placed, and which is rigidly fixed to the bottom of said cavity, in order to enable the elastic membrane to be anchored to it at least in the areas which have to form the support appendanges on the base of the tile. However, having anchored the membrane to a fixed grill prevents, in the anchorage areas, possibility of displacement of the membrane that is necessary in order to achieve homogeneous pressing, thereby giving rise to variations in the density of the pressed tile which cause, on the opposite face, an aesthetically unpleasant motif underlying grill. In particular, if, in an area of the matrix, the level of the loaded powders, or their density, is less than the predetermined amount, the areas corresponding to the support ribbing of the tile are not as well pressed and consequently the corresponding areas on the opposite face have a greater porosity, with an unacceptable variation in the surface finish and a considerable reduction, in these areas, in the mechanical strength of the support. The visibility of the lattice where the elastic membrane is anchored to the underlying grill is due, in particular, to the manner in which the elastic membrane deforms, that is, with bulging surfaces, substantially spherical, with considerable lack of homogeneity of pressing between the areas of the membrane close to the grill and the central areas further from the said grill. On the other hand, in the case whereby, locally, the level of the powders loaded in the matrix is greater than the predetermined amount, bulging can occur between one rib and the next on the reverse side of the tile, the size of the bulge being such as to protrude beyond the theoretical support plane of the tile as defined by the extremities of the ribs, or support appendages in general: this gives rise to corresponding surface defects of the top face of the support during firing, also making it difficult to transfer the tiles along the motorised roller conveyor lines. IT-U-214739 proposes a die for ceramic tiles in which the body of a punch has a plurality of intercommunicating seats, filled with oil or other incompressible fluid, in which are inserted, with wet seal, pistons sliding in them in order to impart differentiated degree of compaction in the various zones of the mass of powders during pressing. Between the active ends of the said pistons and the powders to be pressed there is an elastically deformable membrane acting as a coating for the surface of the punch. However, such a die requires a high degree of precision in machining the seats and the relative pistons, and as a result, is very expensive. Furthermore, with such a die, there is the risk that, during operation, a certain amount of oil may get past the seals placed between the said pistons and the relative seats and infiltrate between the active faces of the said pistons and the portion of membrane facing it: this causes the formation of pools of liquid which set up permanent deformations in the membrane and defects in the pressed tiles. It is also to be noted that the areas of the membrane that interact with the edges of the underlying pistons are subject to high loads that can rapidly cause the said membrane to rupture. Finally, during pressing, the powders exert a high pressure on the membrane which tends to cause the membrane to extrude between the radial gap between the piston and the relative seat: this produces a progressive blocking of the pistons in their relative seats with, consequently, a loss of mobility of the pistons. The prior art also comprises a die described in IT-A-MO93A000068 in which the elastic membrane, on the side opposite to that in contact with the powders for forming the tile, is equipped with appendages, for example, each consisting of a cover made of an elastic material, preformed and co-operating with the walls of the alveolus to which the said cover is attached and on whose base its lower rim rests: during vulcanisation the said elastic cover forms an integral part of the said membrane, closing off the alveolus and so enabling the chamber for the incompressible fluid to remain free, delimited by the internal surface of the same cover. However, given that the lower rim of the cover has to rest on the base of the relative alveolus, the corresponding part of the surface of the said cover cannot be subjected to the pressure of the incompressible fluid, with loss, therefore, of active surface as regards the homogenising action of the membrane on the powders being formed. Such prior art may be subject to considerable improvements with a view to eliminating the said drawbacks. From the foregoing emerges the need to resolve the technical problem of inventing an isobaric die for ceramic tiles in which it is possible to obtain a homogeneous degree of pressing whilst avoiding the generation of surface defects in the pressed items and maintaining a substantially constant thickness. Another aspect of the same technical problem being that of reducing the manufacturing costs of the isobaric dies. The invention resolves the said technical problem by adopting die means for isobaric pressing of ceramic tiles by means of an active surface, comprising a die body having a plurality of intercommunicating cavities containing an incompressible fluid, each cavity accomodating a respective tablet element, characterized in that, between each tablet element and the die body a joint is defined having a looped form, said joint having one internal side in contact with said incompressible fluid and one external side forming a part of said active surface, the joint extending between said internal side and said external side leaving substantially no gaps between said internal side and said external side. The tablets allow displacements of the membrane only in planes substantially parallel to the forming plane: this achieves, with a given displacement of the tablets, the compaction of a greater volume of powder than is possible with a membrane that deforms in curved configurations; conversely, with a given volume of powder displaced in compensating for the non homogeneous loading of the matrix of the die, the displacement of the tablets in planes parallel to the forming planes is much less than that which would be required with curved deformations of the parallel to the forming plane afford a better compensation of pressing, also in the areas close to the ribbing, or appendages, where significant lack of homogeneity can be found when using prior art dies. Further characteristics and advantages of the invention will emerge from the following description, given purely by way of example, of some embodiments illustrated in the drawings attached in which: Figure 1 is a partially sectioned and interrupted front view of a die according to the invention; Figure 2 is a section taken along line II-II of Figure 1, but during vulcanisation; Figure 3 is a section as in Figure 2, but during pressing; Figure 4 is a partial and interrupted plan view as in Figure 1, but with anchoring elements for the membrane having "T"-shaped sections; Figure 5 is a section taken along line V-V of Figure 4 during pressing; Figure 6 is a partial and interrupted vertical section of an isobaric die, but in a version with tablets inserted in respective seats, during vulcanisation; Figure 7 is a section as in Figure 6, but prior to pressing; Figure 8 is a section as in Figure 6, but in a further version with stepped seats for housing sealing rings co-operating with the tablets; it also shows a variant of the die with tablets emerging at the pressing surface; Figure 9 is a section as in Figure 8, but with cavities, or grooves, for the formation of support ribbing, or appendages, of the ceramic tile; Figure 10 is a section of the die as in Figures 8, 9, but during the pressing phase. As shown in Figure 2, a die for ceramic tiles 41 comprises at least one semi-die 1, that is, a punch insertable into a matrix M (Figure 3), having an elastic membrane 2, anchored peripherally to a frame 3 of the semi-die 1, and having, on its external face, a surface 4 having the profile of one of the faces of tile to be formed, for example, the rear face. The tiles 41 are pressed in the matrix M between a first punch 1 a a second punch PZ. In this case grooves 5, or indents, may be provided to form the projections in the rear face of the tile, to enable it to be laid and/or transported during the various phases of the production cycle. A plurality of tablets 48, for example, quadrangular, is inserted so as to be floating in corresponding seats 49, or cavities. The tablets 48 have any plan view shape, for example, polygonal with rounded vertices, for example, rectangular, or square, or circular, or whichever other convenient shape. The seats 49 are intercommunicating by means of channels 60 and 61 for the incompressible fluid: the seats 49 are preferably flared upwards and in any case such as to enable the formation, during vulcanisation, of lips 50a in the membrane 2 wrapping round each tablet 48 so as to incorporate it into the relative membrane. Anchorage appendages 19c may be provided in the membrane 2 opposite to the grooves 5 for insertion into corresponding seats 44: the anchorage appendages 19c give the membrane further possibility of adjusting its trim in equilibrium with the incompressible fluid when the pressing load is applied, in such a way as to guarantee an isostatic pressing also in the areas corresponding to the supports of the tiles. Figures 4, 5 show how in this version the membrane 2 is anchored to a plurality of anchoring elements 51 having a "T"-shaped cross-section, for anchoring the said membrane in undercut: these elements also being provided with transverse through holes 52 to further improve anchoring. In Figures 6, 7, the tablets 48 are of considerable thickness and have smooth lateral surfaces 53, so as to co-operate with sealing rings 54 intended to prevent, during vulcanisation, leakage of the elastomer material the membrane 2 is made of towards the underlying seats 55, said seats constituting integral parts of the chamber 7 for the incompressible fluid; the sealing rings 54 are inserted in stepped grooves 56 in the side walls of the seats 55. On the other hand, should it be preferable to have smooth side walls for the seats 55, stepped grooves 57 for said sealing rings may be made in the side walls of the tiles 48. During forming, the tablets 48 can be positioned so that they are in contact with the bottom of the corresponding cavity 55 or raised from it, for example with the positioning between them of a fluid material, for example, granular, such as sand S, that is expelled after forming. In this way it is possible to assign the tablets a maximum predetermined stroke H during pressing, with an amplitude advantageously less than the depth R of the grooves, or cavities, 5 so as to guarantee the formation of ribs or, respectively, support appendages for the tile in all working conditions. In the versions shown in Figures 8, 9, 10, the sealing rings 54, in order to allow a greater flexibility and to improve sealing against oil leakage from cavity 7, can be inserted in stepped seats 56a having a diameter which is essentially greater than the external diameter of the corresponding sealing element 54. It is to be noted that the membrane 2 can have a smooth external surface 59 (Figure 8) during the vulcanisation phase with matrix 23, the grooves, or cavities, 5 being obtained with the deformation of the membrane only with the introduction of the pressurised fluid in the cavity 7. The seats 55 for the tablets 48 are an integral part of the cavity 7 in which the incompressible fluid F is contained and can be made intercommunicating by means of channel 62 which has an axis bent in the shape of a "V" to connect one seat directly with the one adjacent to it. Between the side walls of the tile 48 and the side walls of the seat 55 there is an annular joint 68 made of the material of the membrane 2 that favour the floating movement of each tile 48. The membrane 2 can be limited to the ensemble of annular joints 68, that is, tablets 48a do not have to be lined with the elastomer material on its active face in pressing.

Claims: Die means for isobaric pressing of ceramic tiles by means of an active surface, comprising a die body (1) having a plurality of intercommunicating cavities (7) containing an incompressible fluid, each cavity accomodating a respective tablet element (48, 48a), characterized in that , between each tablet element (48, 48a) and the die body a joint (68) is defined having an annular form, said joint (68) having one internal side in contact with said incompressible fluid and one external side forming a part of said active surface, the joint (68) extending between said internal side and said external side leaving substantially no gaps between said internal side and said external side. Die means according to claim 1, wherein said tablet elements (48) have side surfaces convergent inwardly of said die body (1). Die means according to claim 1, wherein each joint (68) includes an elastic ring (54) receivable in a respective seat (56a; 57). Die means according to claim 3, wherein said seat (57) is provided for in said tablet element (48). Die means according to claim 3, wherein said seat (56a) is provided for in said cavity (7). Die means according to claim 1 or 3, wherein said joint (68) is anchored to the respective tablet element (48, 48a) and to the die body (1). Die means according to any preceding claims, wherein said joint (68) is part of a membrane (2) defining said active surface. Die means according to any preceding claims, wherein said tablet element (48, 48a) is substantially rigid with respect to said joint (68).

Inventors: ALGERI MARIS, ALGERI, MARIS
Applicants: ALGERI MARIS, ALGERI, MARIS
Application year: 1995
Main classification: B28B3
IPC codes: B30B11, B28B3, B30B15, B30B5