COVER STORY: Dynamic Pressing
The recent introduction of tile manufacturing systems based on double-pressing technology has aroused great interest, particularly in the porcelain tile field, which imitates natural products and requires a ceramic body with an appropriately decorated surface. These systems have broadened the range of attainable "in pressing" effects without affecting line productivity. The latest development in this technology involves the preparation of precompacted ceramic bodies through a continuous process. The constraints of the traditional charging and compaction systems are thus avoided, achieving total decoration, even inside the tile body.
The Traditional ProcessThe most widely used technique for ceramic tile forming is powder compaction with uniaxial hydraulic presses, in which the tiles are formed in one or more cavities inside the die. Dies with a single cavity consist of a bottom punch connected to a compact ejector that moves inside the die and whose dimensions are slightly larger (generally a few tenths of a millimeter) than the dimensions of the punch itself. The bottom punch with the die forms the cavity that is charged with spray-dried powder by the feeding system. The die is completed by the top punch and fixed to the mobile press frame, whose dimensions enable it to enter the die cavity or rest on its top plane. In the latter case, the die must be able to slide vertically under the press stroke. The tile is formed as the top punch descends until it rests gently on the powder, and the main press cylinder then exerts the successive pressure.
Over the years, ceramic tile manufacturers have increasingly focused on fabricating porcelain tiles that reproduce the aesthetic characteristics of natural stones such as marble and granite. These types of tiles are mainly made by two die filling technologies: the whole-body charge and the double charge. The whole-body technology uses various feeding systems to deposit a charge composed of dispersed veins of colored spray-dried powders in the hopper of the press filler box (see Figure 1).
As the term indicates, the double-charge technology consists of filling the die cavity in two different stages (see Figure 2). First, the base powder (i.e., a layer of a plain-colored material or, at most, a material with a salt-and-pepper effect) is deposited into the die cavity to form the main portion of the tile body. Subsequently, a dispenser enters through the die opening and applies a second layer, generally a few millimeters thick, which forms the actual tile decoration. For example, if tiles with characteristics similar to those of Travertine-type marble are required, the second layer can be composed of micronized powders. Alternatively, grits and flakes can be used to reproduce a granite look, or spray-dried glaze powders can be applied to achieve a rustic effect. The multiplicity of pigments fused together provides naturalness and depth to the effects.
However, both the whole-body and double-charge technologies have certain disadvantages. The whole-body charge is particularly limited when it comes to varying the aesthetic effects from one piece of tile to another. The overall design, in addition to the arrangement and variable quantity of the colors used, is strongly linked to the fixed geometry of the grid that conveys the powder to the die cavity. This feature generates repetition in the decoration, which is generally unattractive after the tile installation.
Another drawback of this technique is that the spray-dried powder first slides on a plate and then on the die in the die charging stages. This movement inevitably causes a remix of the powder at the expense of the definition of the pattern determined by the grid.
The main disadvantage of the double-charge technology is the low productivity of the line. The filling of the press die cavity takes place in two separate stages, with a considerable increase in pressing cycle time.
Double-Pressing InnovationTo eliminate these problems and respond to the growing need for greater creative freedom and more attractive aesthetic effects, a double-pressing production technology* was developed. The system allows manufacturers to form a tile in two pressing stages, with two different presses, incorporating both wet and dry multiple decoration systems (see Figure 3).
The tile is formed in the first press in a conventional manner but with a low forming pressure (50-80 bar). The precompacted tile is then conveyed through a line equipped with several dry and wet decoration systems, including possible applications of flakes and other semi-processed products. The second press is located at the end of the line. This press is fitted with an appropriate system for introducing the precompacted and decorated tile in the die cavity, and applies the final pressing at the traditional compaction pressure (400-500 bar). At this point, the tile is perfectly formed and ready to be sent to the subsequent manufacturing process stages.
The double-pressing technology provides freedom in the search for aesthetic effects, since the decoration is deposited in the wide space between the two presses. The limitations of the double-charge process are also eliminated (i.e., the need to use a system inside the die opening and the reduction of the pressing cycle rate due to the time lost in the deposition of the second charge layer inside the cavity).
Continuous Powder PrecompactionThe latest development in the double-pressing technology** involves the concept of continuous powder precompaction, which can be carried out by subjecting the layer of spray-dried powder to the action of a pair of rigid rollers that are both horizontal and perpendicular to the advancing powder. The powder is conveyed at a uniform velocity, VP, equivalent to the peripheral tangential velocity, VR, according to the equation (1):
VR = wR
where w is roller angle velocity and R its outer radius. As the powder advances through the gap between both rollers, it undergoes progressive compaction that increases its bulk density to the final output value. To evaluate the compacting action, density and pressure applied to the powder, a model needs to be derived for the compressibility of the ceramic powder. The Kawakita and Lüdde model1 has thus been adapted, where density, d, depends on compaction pressure, p. The resulting equation (2) follows:
d0 = 1.016 g/cm3
c1 = 1.1753 (dimensionless)
c2 = 0.0131 bar-1
where x0 is the initial abscissa of powder contact with the inclined plane, defined by the geometry of the system. In this case, h0 = 22 mm, hf = 14 mm and x0 = 300 mm.
Continuous OperationThe continuous precompaction technology deposits a continuous layer of spray-dried powder with the desired decoration throughout the entire thickness of the ceramic tile body as it passes through the system on a conveyor belt (see Figure 8). Dimensioning of the powder conveyor belt allows for increasing the complexity of the decoration at will (in terms of successive applications/elaborations), without compromising the productivity of the system, which remains at the maximum values allowed by the successive phases.
After crossing the application/elaboration stations, the belt conveys the powder into the continuous compactor, which puts out a continuous compacted band with values of density and mechanical strength comparable to those of the tiles precompacted with the new double-press technology. The powder deposited on the conveyor belt is put through the compactor without any movement or mixing, thus "freezing" the effects and preserving them for the subsequent forming stages.
ApplicationsThe typical products obtained by the continuous precompaction technology are square and rectangular porcelain tiles characterized by decorations applied through the whole thickness (shades and patches of color, inclusions and subtle veining) and obtained by different colored powder charges. The product surface may be natural, satin or polished.
In the panorama of new technologies for porcelain tile manufacturing, the system for continuous precompaction represents a radical change from traditional working methods; in fact, all the current powder compaction systems involve forming individual tiles and batch processing cycles. The system for continuous precompaction is not only an innovation from a conceptual point of view, but also from a practical standpoint, since it allows the decoration of a layer of powder throughout the whole thickness of the tile without any repetition.
The technology replaces the press feeding system, providing freedom in powder charges, while also solving all the problems of a traditional belt charge since it conveys not a powder but an inalterable, precompacted product to the press. The objective of this new technology is the industrial-scale fabrication of porcelain tiles characterized by an extraordinary aesthetic and technical nature.
For more information about tile pressing systems, contact SACMI Imola at Via Selice Provinciale, 17/A, 40026 Imola, Italy; (39) 542-607111; fax (39) 542-642354; e-mail firstname.lastname@example.org; or visit www.sacmi.it or www.sacmiusa.com.
SIDEBAR: Stages of the New Forming and Decoration Process
- Powder feed. The spray-dried powders are fed by the appropriate proportioning dosing systems directly onto the conveyor belt, using numerous automatic and robotized systems, to create various effects in the body (veins, shades, etc.).
- Continuous precompaction. The deposited powder layer is precompacted between two belts to a mechanical consistency that enables it to be transported and handled.
- Continuous cutting. The precompacted material is subjected to a transverse continuous cutting operation without interrupting its progress on the line. At the same time, the edges are also rectified to obtain a tile with a well-defined geometry.
- Subsequent decorations. The precompacted tile is conveyed on rollers for possible further surface decoration, combining the most varied techniques currently available.
- Final pressing. The precompacted and decorated tile is placed in the die and given its definitive pressing to reach density and mechanical strength values analogous to those of traditional porcelain tiles.