A new continuous precompaction process for porcelain tile increases the speed and flexibility of production while providing enhanced technical and aesthetic features.

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 Process

The 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).

In each pressing cycle, the required powder is released from the hopper onto a grid below, which then conveys the powder to the die cavity. The grid must fit the colored veins in the hopper as closely as possible to arrange them appropriately in the tile. The colored veins and effects that are present in the feeding hopper thus reappear throughout the whole tile, which explains the term "whole body." This technology is particularly suitable for fabricating mixed products with different shades and veins.

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 Innovation

To 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 Precompaction

The 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):

V_R = w_R

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:

where d₀ is the bulk density of the non-compacted powder, and c₁ and c₂ are the characteristic constants to be determined from case to case. The mean values for porcelain tile bodies with a moisture content of 5.5%, which will be assumed as the basis for these considerations, are:

d₀ = 1.016 g/cm³

c₁ = 1.1753 (dimensionless)

c₂ = 0.0131 bar^-1

The variation of density vs. compaction pressure (assuming the above values for the constants) is plotted in Figure 4. As the graph shows, the density gradient decreases with increasing pressure. This indicates that the precompaction stage must be limited to low pressures to obtain intermediate densities relative to the normal final values. In the plot, point A marks the density of the non-compacted powder, point C the normal compaction value for porcelain tile (400 bar, density 2.02 g/cm³), and point B a reasonable precompaction target (70 bar, density 1.59 g/cm³).

After selecting the compaction model, it is possible to calculate the variation of the pressures that develop in compression by the cylindrical surface of the compacting roller. Figure 5 illustrates this concept, where h₀ is the powder input thickness, hf is compacted band output thickness, x the abscissa parallel to the powder direction of advance and R the outer radius of the compacting roller.

For a generic value of the x abscissa between 0 and x₀ (corresponding to the initiation of powder and roller contact), the corresponding thickness is (3):

so that the value of the abscissa x₀ in relation to h₀ is (4):

Bulk density rises during compaction, i.e., it moves along the abscissa from x₀ to 0 (which is equivalent to going from h₀ to h_f), and is inversely proportional to thickness h(x). It is therefore possible to set density as a function of abscissa x (5):

Inverting equation (2) and taking compaction pressure, p, as a function of bulk density, d, gives (6):

Introducing the density relation (5) in equation (6) yields (7) an expression for compaction pressure as a function of h(x):

Figure 6 plots the variation of the functions h(x), r(x) and p(x) with the assumption of precompaction in a single throughput (with a cylindrical roller), from 22 to 14 mm thick, at a theoretical output density of 1.59 g/cm³, corresponding to a maximum end compaction pressure of about 70 bar.

To avoid problems of spray-dried powder escaping at the entrance of the compactor (abscissa x₀) with the ensuing refusal of the powder to let itself be compressed, it is important to use high R/h₀ ratios (i.e., a much higher compaction radii to input thickness). The use of the roller minimizes friction resistance and allows the system to achieve total efficiencies, with reduced installed power. Based on the specific application, manufacturers can adjust the maximum pressing force to be applied onto the upper rolling group. Using a series of rollers with decreasing heights, sheathed in a very rigid material, creates the equivalent of a roller of infinite radius with evident advantages for compaction efficiency.

Figure 7 plots the variation of the characteristics h(x), d(x) and p(x) in the case of a constant inclined compaction plane, where the function h(x) adopts the form (8):

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, h_f = 14 mm and x₀ = 300 mm.

Continuous Operation

The 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.

As a last step in the process, the material exiting the precompaction system is subjected to transverse dynamic continuous cutting and side rectification to obtain a suitable rectangular size that can be placed directly in the traditional press cavity for the definitive pressing. Cutting is performed by high-speed rotating diamond discs and provides high dimensional accuracy. Figure 9 illustrates the difference between the profile obtained with a continuous cutting operation and the desired theoretical geometry for a final fired size of 600 x 1200 mm. The maximum error of the longest sides (continuous cut) is 0.35 mm in a length of around 1300 mm, which is quite acceptable in view of the subsequent repressing stage. The error on the shorter sides is practically negligible.

Applications

The 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.

Figure 10a shows a characteristic whole-body veined effect, where the stratifications appear similar to real stones. The tile is not polished, and the veined effect appears immediately without any material removal from the surface. Figure 10b is a ceramic slab where the body is divided into two parts. The thin upper layer contains the aesthetic effects, while the thick bottom layer is generally made in a single color base or mix. In this way, different ceramic materials can be used to reduce costs and/or simplify the firing process of highly fusible materials. Moreover, it would be also possible to use a certain amount of waste materials such as green cut-off scraps in such a thick bottom layer.

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 sacmi@sacmi.it; 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.

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