Method and Apparatus for Measuring a Mechanical Property of Decorating

Rollers

The invention relates to a method and apparatuses for measuring a mechanical property of a decorating roller, in particular a decorating roller for decorating ceramic objects. The measured mechanical property can be the elasticity of the decorating roller, or alternatively the deformability, or resistance to deformation, which is correlated to hardness.

In the field of decoration of ceramic tiles a roller is often used comprising a rigid central core on which a layer of spongy material is applied. The layer of spongy material confers the roller a degree of deformability, allowing it to adapt to surface irregularities on the tiles, thus permitting decoration of the tiles without causing damage.

Applied to the spongy layer is an elastomeric material, generally siliconic, forming an external layer on the roller and in which incisions can be made, forming a decorative pattern for reproduction on ceramic tiles being decorated. In order that tiles can be decorated repeatedly to an adequate level of precision and quality, the decorating roller must exhibit particular mechanical properties of elasticity and/or hardness.

In order to establish whether a decorating roller can be successfully used for decoration, currently an operator touches various points on the surface of the roller with a finger, thereby establishing, entirely empirically, the elasticity or hardness of the roller.

This approach exhibits numerous limitations of repeatability of results, because the assessment of quality of a roller depends on the subjectivity of the operator, making it impossible to obtain accurate measurements. JP 2001074573 discloses a device for measuring the distribution of elasticity of a roller shaped brush intended for installation on a lithographic machine. JP

2001074573 thus belongs to a technical field which is very remote from the field of decoration of ceramic objects.

An object of the invention is to improve known methods and apparatuses for measuring mechanical properties, in particular elasticity and/or hardness, of decorating rollers for decorating ceramic objects, in order to easily assess whether a decorating roller is suitable to be used for decorating ceramic tiles, and so that any defective decorating rollers can be disposed of.

Another object of the invention is to provide apparatuses and methods for measuring with high precision and in a substantially repeatable manner the elasticity and/or hardness of decorating rollers.

A further object of the invention is to provide methods and apparatuses capable of measuring, also continuously and point by point, values of elasticity and/or hardness of a decorating roller for decorating ceramic objects without altering or damaging the surface of the roller. Another object of the invention is to provide apparatuses capable of measuring values of elasticity and/or hardness in relatively small areas of the surface of a decorating roller.

In a first aspect of the invention, a method is provided comprising the steps of: bringing measuring means into contact with an external surface of a decorating roller for decorating ceramic objects, such that the measuring means apply a pressure force on the external surface, the external surface producing a reactive force in response to the pressure force; using motor means in order to mutually move the decorating roller and the measuring means, such as to measure the reactive force and then numerically process the reactive force at a plurality of zones of the external surface with which the measuring means successively come into contact.

Owing to the first aspect of the invention, it is possible to obtain high precision and substantially repeatable measurement of the mechanical properties of decorating rollers including elasticity and/or hardness. The measuring means, by measuring reactive force, provide a more objective and detailed assessment of decorating rollers compared with known methods, in particular compared to methods that involve manual assessment by an operator. Furthermore, by moving the decorating roller using the motor means, it is possible to check the quality of the decorating roller in a plurality of zones, thereby enabling identification of imperfections even of minor dimension, which might escape a manual check by an operator.

In a second aspect of the invention, an apparatus is provided for assessing a decorating roller for decorating objects, comprising measuring means suitable for contacting an external surface of the decorating roller in order to apply to the external surface a pressure force, characterized in that it comprises means for promoting mutual movement suitable for aiding mutual movement between the measuring means and the external surface, when the measuring means and the external surface are in mutual contact.

The apparatus of the second aspect of the invention enables analysis of the mechanical behaviour of a decorating roller even continuously along adjacent zones of the roller and without altering or damaging the external surface, due to the presence of the means for promoting mutual movement. In an embodiment, the means for promoting mutual movement comprise a rounded end formed on the measuring means, which can be shaped to act on a substantially punctiform zone of the external surface. The rounded end of the measuring means, in addition to reducing the friction generated during contact with the decorating roller, enables more spatially limited and localized measurements to be made of the elasticity and/or hardness of the decorating roller.

In another embodiment the means for promoting mutual movement comprise a rolling body included in the measuring means, the rolling body permitting the measuring means and the external surface to reciprocally slide over each other with considerably reduced friction. In a third aspect of the invention, an apparatus is provided comprising: first pressing means for deforming a portion of a decorating roller for decorating objects, such that said portion generates a first force on the first pressing means; second pressing means for deforming said portion after said portion has interacted with the first pressing means, such that said portion generates a second force on the second pressing means, the second force being comparable with the first force in order to assess the elastic behaviour of the decorating roller.

The apparatus of the third aspect of the invention provides information on the elastic behaviour of the decorating roller. In particular, if the second force is significantly less than the first force, the external surface of the decorating roller has not recovered its original dimensions while moving from the first pressing means to the second pressing means. This implies that the decorating roller is behaving anelastically and might decorate objects incorrectly. Assessments of this type were not possible in the prior art.

The invention will be better understood and carried out with reference to the accompanying figures of the drawings, which illustrate non-limiting embodiments by way of example, and in which:

Figure 1 is a schematic side view of an apparatus for measuring the hardness and elasticity of a decorating roller for decorating objects, in particular ceramic objects;

Figure 2 is a front view, enlarged and cut-away, of the apparatus of Figure 1 in a first embodiment;

Figure 3 is a front view, enlarged and cut-away, of the apparatus of Figure 1 in a second embodiment;

Figure 4 is a partial view in cross-section of measuring means of the apparatus of

Figure 1 ; Figure 5 schematically shows zones of a decorating roller assessed using a first operating mode;

Figure 6 schematically shows zones of a decorating roller assessed using a second operating mode;

Figure 7 is a table of the results of the measurements conducted on a decorating roller using the apparatus of Figure 1 ;

Figure 8 is a three-dimensional graph of the data in the table of Figure 7;

Figure 9 is a table showing the results of measurements conducted using the apparatus of Figure 1 on a decorating roller of cylindrical shape;

Figure 10 is a three-dimensional graph obtained from the data in the table of Figure 9;

Figure 11 is a table containing the results of measurements conducted using the apparatus of Figure 1 on a defective decorating roller, exhibiting a frustum- conical external surface;

Figure 12 a three-dimensional graph obtained from the data in the table of Figure 11;

Figure 13 is a table containing the results of measurements conducted using the apparatus of Figure 1 on a decorating roller exhibiting a central portion of lesser diameter and two end portions of greater diameter;

Figure 14 is a three-dimensional graph obtained from the data in the table of Figure 13.

Figure 1 shows an apparatus 1 for measuring mechanical properties of a decorating roller 2 for decorating objects, in particular ceramic objects, for example tiles.

The decorating roller 2 can comprise a central core 10, which can be internally hollow, on which an intermediate layer 11 of spongy material is applied, as shown in Figures 2 and 3. The intermediate layer 11 gives the decorating roller 2 a degree of deformability, enabling the roller 2 to conform to the surface irregularities of tiles being decorated without damaging the tiles.

Applied to the intermediate layer 11 is an external layer 12 in elastomeric material, for example a polyurethane or a silicone material, on which incisions can be formed defining an ornamental pattern for reproduction on the tiles being decorated. The external layer 12 is substantially continuous and, before being engraved, is delimited by a smooth surface.

As will better emerge herein below, the apparatus 1 permits determination of whether a decorating roller 2 is in an acceptable condition for decorating objects, or whether the decorating roller 2 exhibits one or more defects which exclude use of the roller for creating good quality decorations. The apparatus 1 can measure decorating rollers not yet engraved, such that, if the decorating roller 2 is not of good quality, the roller 2 can be rejected before sustaining the costs of engraving. The apparatus 1 comprises a base 3 to which a vertical upright 4 is connected. A support shaft 5 is connected to the vertical upright 4, the support shaft 5 being suitable for supporting for the decorating roller 2. The support shaft 5 can be rotated about an axis of rotation R, by motor means, which can comprise a motor 6, for example an electric motor. Consequently the support shaft 5 causes the decorating roller 2 to rotate about the axis of rotation R. The axis of rotation R is substantially horizontal, such that the decorating roller 2 is mounted on the apparatus 1 in conditions similar to the conditions occurring when the decorating roller 2 is fitted on a decorating machine. This allows measurements to be taken in a context that is as realistic and accurate as possible.

The apparatus 1 comprises a cross-member 7 extending parallel to the axis of rotation R. The cross-member 7 supports measuring means, comprising a measuring element 8 that serves to measure a mechanical property, for example the elasticity and/or hardness of the decorating roller 2. The term "elasticity" here refers to the capacity of the decorating roller 2 to recover its initial shape after being deformed. The term "hardness" here refers to the resistance that the external surface of the decorating roller 2 offers to deformation by a penetrating body. Consequently, both hardness and elasticity are related to the deformability of a decorating roller 2. However, while hardness refers to an instantaneous behaviour of a decorating roller 2, i.e. to the behaviour of a decorating roller 2 at the moment of interaction with a penetrating body, elasticity enables as assessment to be made of the behaviour of a decorating roller 2 over a given interval of time. The measuring means, and hence the measuring element 8, function as pressing means or penetrating means, which act on a zone of the external layer 12, deforming said zone by applying a pressure force. In response, the deformed zone of the external layer 12 exerts a reactive force on the measuring element 8, the reactive force being useful for an assessment of mechanical behaviour of the decorating roller 2. The measuring element 8 is supported by a slide 23, which can be moved along a direction of longitudinal advancement A parallel to the support shaft 5, for example by means of a further motor (not illustrated).

The measuring element 8 can comprise a so-called "loading sphere", acting to contact an external surface 9 of the decorating roller 2. The measuring element 8 will be described in detail herein below.

The support shaft 5 can be moved vertically in a direction F. This makes it possible to approach the decorating roller 2 to the cross-member 7, or distance the decorating roller 2 from the cross-member 7.

In an embodiment, instead of moving the support shaft 5, it is possible to move the cross-member 7 along the direction F, such that the cross-member 7 can be approached to the decorating roller 2, or distanced from the decorating roller 2.

In this case the cross-member 7 can be driven by a respective motor (not illustrated).

In a further embodiment, both the cross-member 7 and the support shaft 5 can be moved along the direction F.

The apparatus 1 comprises a control unit, for example a computer 16, enabling the setting of operating modes for the apparatus 1, for example adjustment of the movements of the support shaft 5 and of the slide 23. The computer 16 also interacts with the measuring means 8 in order to numerically process the measured reactive force and assess mechanical properties of the decorating roller

2.

Figure 4 shows a measuring element 8 in detail, the measuring element 8 extending along an axis J. The measuring element 8 comprises a body 13 provided with a seating 15, which in the illustrated example is delimited by a surface 17 of semi-spherical shape.

The measuring element 8 includes means for promoting mutual movement 30 comprising a contact element 14 housed in the seating 15 and suitable for contacting the decorating roller 2. The contact element 14, which in the illustrated example comprises a spherical body, contacts the decorating roller 2 in a contact zone of very limited area.

In this way the measuring element 8 and the decorating roller 2 can move very easily relative to each other. Owing to the contact zone of limited area, the contact element 14 acts as a friction reduction element, reducing the friction generated by the contact between the measuring element 8 and the external surface 9 of the decorating roller 2.

The means for promoting mutual movement 30 further comprise rolling elements, for example spherical elements 18, interpositioned between the surface 17 and the contact element 14. The spherical elements 18 reduce the friction generated between the contact element 14 and the surface 17. A lubricating fluid can be introduced into the seating 15, the lubricating fluid further reducing friction.

The measuring element 8 also comprises a cover 21 which extends around the contact element 14 and is fixed to the body 13. The cover 21 serves to prevent any dust or waste material, for example deposited on the surface 9 of the decorating roller 2, from penetrating inside the seating 15.

The spherical elements 18 can be substituted with other types of elements capable of reducing the friction generated during the relative movement between the contact element 14 and the seating 15. For example, it is possible to interpose a material having a low coefficient of friction between the contact element 14 and the surface 17, such as a film of polytetrafluorethylene (PTFE), known commercially as Teflon. Alternatively, it is possible to make the contact element 14 and/or the seating 15 in various types of self- lubricating materials. The body 13 is provided with a connection rod 19 by which the measuring element 8 can be connected to a transducer, for example a load cell 20, shown in Figures 2 and 3. The transducer serves to transform the reactive force exercised on the measuring element 8 into an electrical signal that can be transmitted to the computer 16.

Alternatively, the contact element 14 can comprise a rounded end or tip which is fixed relative to the body 13, without the interpositioning of rolling elements. The rounded geometry of a tip of this type also reduces friction generated between the external surface 9 and the decorating roller 2. The contact element 14 can also be shaped in different ways.

For example, in one embodiment, the contact element 14 can be a contact cylinder. Between the contact cylinder and the seating 15 rolling elements can be provided, also of cylindrical or spherical shape, thus significantly reducing friction between the contact cylinder and the decorating roller 2. Alternatively, between the contact cylinder and the seating 15 a low frictional coefficient material can be interposed, as described above, for example a film in polytetrafluorethylene (PTFE). The contact cylinder and the seating 15 can also be made of various self- lubricating materials. The measuring element 8 is installed on the apparatus 1 such that the axis J intersects the axis of rotation R. In this way it is certain that the load cell 20 receives a correctly oriented force, this force being substantially perpendicular to the external surface 9 of the decorating roller 2.

Before taking measurements on decorating rollers by using the apparatus 1, a calibration procedure is carried out on the apparatus 1 in order to ensure that the geometric configuration of the apparatus 1 is correct, and in particular that the axis of rotation R is parallel to the cross-member 7. The calibration procedure is foreseen during the installation of the apparatus 1 and is repeated periodically in order to compensate any deformations that the apparatus 1 might assume. The calibration procedure is conducted by mounting a sample roller on the support shaft 5, for example a solid roller made of precisely rectified rubber designed to simulate a decorating roller exhibiting high precision of dimensions and shape. Subsequently, the support shaft 5 is approached to the cross-member 7, or vice-versa, until a reference position is reached in which the load cell 20 starts to measure a reactive force produced by the sample roller on the measuring element 8. The reference position can also be referred to as the "zero position", in the sense that it is the relative position between the roller and the measuring element 8 at which a force begins to be detected.

At this point, the measuring element 8 is approached to the sample roller, or vice- versa, by a predefined distance, such as to deform the external surface of the roller. The slide 23 is now translated along the direction of longitudinal advance A, such that the load cell 20 measures the force in a plurality of zones arranged along a generatrix of the decorating roller 2. If the force readings measured in this way are equal to each other, or exhibit differences less than a predefined tolerance, the axis of rotation R is parallel to the cross-member 7. If the force readings differ, the axis of rotation R and the cross-member 7 are inclined relative to each other and, before starting measuring, it is necessary to carry out appropriate adjustments.

The calibration procedure can also comprise a series of measurements taken while applying known forces, in order to establish the correlation between the applied force and the electrical signal measured, as in known procedures in the field of the calibration of weighing devices. When the mechanical properties of a decorating roller 2 need to be measured, the decorating roller 2 is mounted on the support shaft 5. Decorating rollers 2 for decorating ceramic tiles are normally provided with reference means, comprising for example a reference key, which can be positioned on a flange of the decorating roller 2. The decorating roller 2 is mounted on the support shaft 5 such that the reference key is in a predefined position relative to the measuring element 8.

This ensures that, if the same decorating roller 2 is measured more times, the results obtained can be usefully compared because the decorating roller 2 is mounted in a repeatable manner on the apparatus 1. Subsequently, a calibration step is carried out in order to ensure that measurements conducted on different rollers produce results that can usefully be compared. The calibration step makes it possible to ensure, for example, that the contact element 14 exerts a predefined initial force on the decorating roller 2, or

that initially the contact element 14 penetrates into the external surface 9 to a predefined extent.

During the calibration step, the support shaft 5 is approached to the cross- member 7, or vice-versa, such as to bring the decorating roller 2 into contact with the measuring element 8. This enables determination of the reference position, or "zero position", this position being the reciprocal position of the decorating roller 2 and the measuring element 8 at which the load cell 20 starts to register an applied force. At this point the support shaft 5 is moved towards the cross-member 7, or vice- versa, such that the contact element 14 penetrates the decorating roller 2 to a predefined extent, for example 5 mm, or to an extent sufficient in order to apply a predefined force, for example 2 N.

The apparatus 1 is now ready to begin actual measuring operations. The support shaft 5, together with the decorating roller 2, is rotated in a direction of rotation W, shown in Figure 2. In this way the contact element 14 interacts progressively with different zones of the external surface 9. In each of the zones the load cell 20 measures a specific force transmitted by the contact element 14, the force being converted into an electrical signal proportional to the force, and then transmitted to the computer 16. The speed of rotation of the support shaft 5 is chosen such as to resemble the speed at which the decorating roller 2 rotates when fitted on a decorating machine. In this way the measurements are conducted in similar conditions to the operating conditions of the decorating roller 2. The measured force is indicative of the resistance that the decorating roller 2 offers to penetration, i.e. of the superficial hardness of the decorating roller 2.

In a first operative measuring mode, shown in Figure 5, the contact element 14 is moved along circumferences. In particular, the contact element 14 begins interacting with the decorating roller 2 along a first measuring circumference 22,

located near to an end of the decorating roller 2. The computer 16 records the reactive force measured in a plurality of equidistant zones, located along the first measuring circumference 22 with an interval selectable by the operator, as shown in Figure 5. After travelling around the entire first measuring circumference 22, the slide 23 advances by a certain quantity Q along the directional of longitudinal advance A. While the slide 23 is moving along the direction of longitudinal advance A, the contact element 14 can remain in contact with the decorating roller 2. Subsequently, the contact element 14 can interact with a second measuring circumference 24, located parallel to the first measuring circumference 22.

Proceeding in this way, the measuring element 8 interacts with further circumferences until completing the entire length of the decorating roller 2. Figure 6 shows a second operating mode, which differs from the first operating mode shown in Figure 5 in that the contact element 14 does not interact with circumferences of the decorating roller 2, but instead moves along a helical measuring path 25 on the external surface 9. In this case the slide 23 and the support shaft 5 are moved in synchrony such that the movement of the contact element 14 relative to the external surface 9 exhibits a circular movement component and a longitudinal movement component, the longitudinal movement component being parallel to the axis of the decorating roller 2.

The second measuring mode, wherein the measurements are conducted along a helical measuring path 25, is of reduced duration relative to the first measuring mode wherein the measurements are conducted along successive circumferences. The movement of the contact element 14 along a helical path eliminates periods of dead time that occur when the contact element 14 is moved from one circumference to the next. However, measurements conducted in zones located along subsequent circumferences are very precise, while using a helical

measurement path the behaviour of the decorating roller 2 in a circumferential direction can only be assessed with a degree of approximation. It is also possible to use further operating modes, wherein the measuring element 8 moves relative to the decorating roller 2 along paths of different forms, as desired.

In all cases, the contact element 14 is moved continuously along the respective paths, even if the hypothesis of moving the contact element intermittently is not excluded.

The signals transmitted from the measuring element 8 to the computer 16 at the successive measuring points are processed by the computer 16 in order to establish values for hardness, or deformability, of the decorating roller 2. For example, these values can be presented in a table, illustrated in Figure 7, which represents a complete mapping of the behaviour of the decorating roller 2. On each circumference, exhibiting a length of 720 mm (or 72 cm), the hardness value was measured at points spaced by 20 mm (or 2 cm) from each other, as shown in the first column of the table of Figure 7. The circumferences analyzed were spaced 20 mm from each other, and chosen such as to study the behaviour along the entire length of 640 mm (or 64 cm) of the decorating roller 2, as shown in the first row of the table of Figure 7. A predetermined force of 2 N was applied to the decorating roller 2, through the contact element 14. The table reports the values measured at the points studied. As can be observed, at some points a higher reactive force was measured, for example of 2.2 or 2.4 N. These points correspond with zones of greater hardness, or zones that offer higher resistance to penetration. Contrarily, points at which lower forces were measured, for example 1.95 N, correspond with softer zones in which the contact element 14 penetrates more easily.

By using the measured values in the table of Figure 7, a three-dimensional graph can be obtained, shown in Figure 8, exhibiting a first axis of the abscissas, the

points of which correspond to different points on the roller, measured along a same circumference, and a second axis of the abscissas, the points of which refer to successive circumferences, and an axis of ordinates relative to the measured force values. The graph of Figure 8 provides an immediate visual assessment of any anomalies in the surface hardness of the decorating roller 2.

Two peaks can be identified in the graph of Figure 8, corresponding with defective areas of relatively high degrees of hardness. The leftmost peak, of minor entity, may correspond with an acceptable defect, while the most rightward peak, considerably higher than the other, may correspond to an excessively hard area of the decorating roller 2, which would compromise the quality of the decorations.

The force measurements obtained using the apparatus 1 and the method described herein above provides not only a knowledge of the hardness of the decorating roller 2, but also a qualitative assessment of its geometric properties. For example, a decorating roller of regular cylindrical shape and uniform hardness, if assessed using the apparatus 1, provides reactive force measurement values, for all the examined zones, which differ slightly from zone to zone, as illustrated in the table of Figure 9. These values, if used to create a three- dimensional graph as described herein above with reference to Figure 8, identify an approximately horizontal plane, as shown in Figure 10.

Alternatively, a defective decorating roller of truncated cone shape, if subjected to a force of 2 N applied by the contact element 14, produces the reactive force values listed in the table of Figure 11. These values produce the three- dimensional graph shown in Figure 12 and are distributed approximately along an inclined plane. The lowest-lying zones of the inclined plane, corresponding with the lowest reactive force values, are located along circumferences of the decorating roller of smallest diameter. In these zones, under a same force applied by the contact element 14, the interference between the decorating roller and the

contact element 14 is lower. Consequently, the contact element 14 deforms the decorating roller relatively little, resulting in smaller reactive forces. Contrarily, the highest positioned zones of the inclined plane, corresponding with greater reactive force values, are positioned along circumferences of the decorating roller of greater diameter. In these zones the contact element 14 deforms the decorating roller to a greater extent.

If the decorating roller comprises a central region of lesser diameter and two end regions or greater diameter, a table is produced of the type shown in Figure 13 and a graph of the type shown in Figure 14. In this case the measured reactive force is distributed in the three-dimensional graph so as to form a sort of wave, wherein the lowest zones of the wave correspond with the lower reactive forces and consequently a lesser compression of the decorating roller, resulting from its minor diameter. By contrast, the highest zones of the wave correspond with the highest reactive forces, resulting from the greater compression experienced by the region of the roller of greater diameter.

In general, apparatus 1 makes it possible to establish if a decorating roller being assessed exhibits zones in which reactive force values are measured outside of a predefined range of tolerance. This can occur for numerous reasons, in particular because the decorating roller examined is of non-uniform hardness or of an irregular geometric shape. Regardless of the cause, if the measured reactive force is outside the predefined range, the pressure exerted by the decorating roller on the object being decorated will not be uniform. This would make it impossible for the decorating roller to evenly transfer the ink, for example the glaze, onto the object, for example a ceramic tile. The measured force values can be processed using software installed on the computer 16 to assess if the decorating roller 2 exhibits excessive surface hardness anomalies, for example anomalies outside a predefined range of tolerance. If the computer 16 establishes that there are anomalies of this category,

the decorating roller 2 is not suitable for decorating ceramic tiles and the computer 16 signals this fact by generating an appropriate message. The apparatus 1 can comprise signalling means, comprising for example a spray device or a marking device like a marking pen, operable by the computer 16 in order to generate a graphic indication on the external surface 9 of the decorating roller 2, thereby permitting operators to immediately identify defective zones of the external surface 9.

Figure 3 shows an apparatus 101 according to an alternative embodiment, the apparatus 101 including a further measuring element 27 in addition to the measuring element 8. The further measuring element 27 is conformed analogously to the measuring element 8 and comprises a further contact element 28. Analogously to the measuring element 8, the further measuring element 27 exhibits a respective axis Z which intersects the axis of rotation R. The measuring element 8 and the further measuring element 27 are arranged in sequence along a transversal direction to the axis of rotation R, at a distance D from each other. The measuring element 8 and the further measuring element 27 can be positioned such that the contact element 14 and the further contact element 28 act on the same circumference of the decorating roller 2. In all cases, the measuring element 8 and the further measuring element 27 are arranged such that, when the decorating roller 2 is moved, the same zone of the external surface 9 contacted by the measuring element 8 subsequently interacts with the further measuring element 27.

During operation, after mounting a decorating roller 2 on the apparatus 1, the first measuring element 8 and the second measuring element 27 are positioned such that the corresponding contact elements 14 and 28 penetrate toward the interior of the decorating roller 2 by a predefined quantity, equal for both contact elements.

Subsequently, the decorating roller 2 is rotated in the direction of rotation W. The contact element 14 interacts with a zone 26 of the external surface 9 and pushes the zone 26 against the internal layer 11, which is locally compressed by a certain quantity. The load cell 20 thus measures a first force value Fl, which corresponds to the reactive force produced by the zone 26 of the external surface 9 when the external surface 9 is deformed by the predefined quantity. Subsequently, the zone 26 of the external surface 9 previously deformed by the contact element 14, as a result of the rotation of the decorating roller 2, is moved away from the measuring element 8 and moved into proximity to the further measuring element 27, while a successive zone is brought into contact with the contact element 14.

When the zone 26 of the external surface 9 is released from the measuring element 8, this zone, which is no longer compressed by the contact element 14, tends to recover its original shape due to the elastic return of the materials making up the intermediate layer 11 and the external layer 12. However, a certain period of time is required before the zone 26 completely recovers its original shape, the time depending on the local properties of the specific decorating roller 2 being measured. On the basis of the specific local properties, it can occur that the time elapsing before the zone 26 is again compressed by the further contact element 28 is sufficient, or insufficient, for the zone 26 to entirely recover its initial shape after interacting with the contact element 14.

This behaviour can be verified by measuring, using the further measuring element 27, a second force value F2 measured in the moment at which the zone 26 is compressed by the further contact element 28. If the further measuring element 27 measures a second force value F2 lower than the first force value Fl, this indicates that the force that the further contact element 28 must apply to the zone 26 in order to deform the zone 26 by the required quantity is less than the force that the contact element 14 previously

applied to the zone 26 in order to deform the zone 26 by the same quantity. This implies that the zone 26, when interacting with the further contact element 28, had not recovered its initial shape, but was still partially compressed as a consequence of the previous interaction with the contact element 14, thereby making it easier to deform the zone 26 by the further contact element 28. In other terms, the zone 26 behaves anelastically.

If instead the second force value F2 does not differ significantly from the first force value Fl, the contact element 14 and the further contact element 27 must have applied substantially the same force to deform the zone 26 by the same quantity. This indicates that, after interacting with the contact element 14, the zone 26 completely recovered its initial shape before interacting with the further contact element 28. Consequently the zone 26 behaves elastically. The less is the difference between the first force value Fl and the second force value F2, the more the decorating roller 2 behaves elastically. It is stressed that, by using two measuring elements configured as are the measuring element 8 and the further measuring element 27, two distinct mechanical properties can be measured. In other terms, while the first force value Fl obtained by the measuring element 8 indicates to what extent the decorating roller 2 is deformable under a given force of pressure, thus indicating the hardness of the decorating roller 2, the comparison of the first force value F 1 with the second force value F2 indicates to what extent the decorating roller 2 is capable of recovering its original shape after being deformed, thus providing information regarding the elasticity of the decorating roller 2. The apparatus 101, like the apparatus 1, is capable of analyzing decorating rollers of various types, also exhibiting more layers and a more complex structure than the illustrated decorating roller 2.

As described previously with reference to the apparatus 1, the apparatus 101 also permits, for each point on the external surface 9, assessment of the elasticity and

hardness of the decorating roller 2, thereby generating numeric values that can be processed to produce a table of the type contained in Figure 7 and graphs of the type illustrated in Figure 8.

Using either the apparatus 1 or the apparatus 101, it is possible to vary the numeric resolution, or the capacity to measure small variations in the mechanical properties under investigation when measuring. This capacity depends on the dimensions of the contact element 14 and of the further contact element 28, if present, and further depends on the number of measurements conducted per unit of surface, The numeric values obtained in this way make it possible to assess whether a decorating roller 2 satisfies specific quality requirements and is therefore usable for decorating ceramic tiles.

Decorating rollers judged to be suitable for decorating tiles by the apparatuses 1 and 101 exhibit a grade of quality not established in a subjective and poorly repeatable way, and instead the rollers can be certified on the strength of an objective numeric procedure of assessment.