Process and system for the production of resin-bonded mineral grit slabs

Abstract

The process for the production of resin-bonded mineral grit slabs comprises at least the phases of: - supply of at least one finished slab (L) obtained by means of at least the steps of: •supply of a mixture of mineral grits and resins; •compaction of the mineral grits and resins to obtain a compacted slab (C); •hardening of the compacted slab (C) to obtain the finished slab (L) provided with at least one exposed surface (S) and with one laying surface (P), the finished slab (L) having an inherent degree of porosity; decoration with ink, after the hardening, of the exposed surface (S) according to at least one predefined design (D); wherein the decoration comprises a phase of hot stamping with sublimation ink and wherein it comprises, subsequently to the hot stamping, a phase of cooling the finished slab (L) so decorated.

Description

[0001] PROCESS AND SYSTEM FOR THE PRODUCTION OF RESIN-

[0002] BONDED MINERAL GRIT SLABS

[0003] Technical Field

[0004] The present invention relates to a process and a system for the production of resin- bonded mineral grit slabs.

[0005] Background Art

[0006] In some manufacturing sectors, such as but not limited to construction and furniture, mineral grit slabs bonded with resins are now widely used.

[0007] In terms of aesthetic and functional characteristics, such slabs are quite comparable to slabs directly obtained from natural materials such as marble, granite or the like.

[0008] Compared to the latter, however, resin-bonded mineral grit slabs also have additional important advantages.

[0009] First of all, they can be made in numerous variants distinguished by colors and / or designs and / or decorative effects that cannot be obtained using the natural materials mentioned above: in other words, therefore, their production is more versatile and customizable to meet customers’ requirements.

[0010] Another advantage consists in the fact that since the resulting material is less porous than, e.g., marble, it is substantially impervious to the most commonly used liquids and, in addition, is more resistant to staining.

[0011] For these reasons, these slabs are widely used, for example, in the manufacture of countertops for kitchens, bathrooms or other similar applications.

[0012] The aforementioned slabs are produced from a mixture typically comprising minerals in granular form (e.g., marble, granite, glass, mirror fragments and / or others) as well as quartz powder and, indeed, resins that serve as binders.

[0013] Once prepared, this mixture is deposited within a mold, in which the molding cavity is the size of the item to be obtained (unless any shrinkage that may occur in subsequent production steps).

[0014] The mixture is then decorated in order to recreate the special aesthetic effects found in natural rocks.

[0015] For example, decorations are made of a pass-through material so as to obtain so- called “grains”, which are streaks of a different color than the base material, having an irregular pattern and running through its entire thickness.

[0016] The mold is then introduced within a compaction appliance in which the mixture is pressed into the cavity, and simultaneously subjected to a vibrating action, so as to obtain a compacted slab.

[0017] In addition to this, slab compaction is expected to take place in a depressurized environment, i.e. at a lower pressure than atmospheric pressure: in fact, the elimination of air from the forming area further promotes compaction of the mixture materials.

[0018] Next, the compacted slab undergoes a final stage of hardening in order to give it the desired mechanical strength to obtain a finished slab.

[0019] However, the processes and systems of known type do have some drawbacks mainly related to the difficulty of obtaining mineral grit slabs provided with valuable decorative effects that remain unchanged throughout the machining cycle.

[0020] In fact, as a result of pressing, the slabs have an uneven surface and need to be polished.

[0021] This operation involves the removal of material which, although partial, can lead to the alteration of the surface decorations applied to the slab in the previous phases, thus invalidating the entire process.

[0022] In the process of known type, it is also found to be difficult to reproduce the surface appearance of natural rocks or to vary the surface finish of the slabs to achieve the desired aesthetic effect.

[0023] Description of the Invention

[0024] The main aim of the present invention is to devise a process and a system for the production of resin-bonded mineral grit slabs which enable simple and reliable production of mineral grit slabs with decorative effects that remain unchanged throughout machining and as a result of the finishing operations.

[0025] A further object of the present invention is to devise a process and a system for the production of resin-bonded mineral grit slabs which enable a wide variety of decorative effects to be achieved in an easy and reliable maimer. Another object of the present invention is to devise a process and a system for the production of resin-bonded mineral grit slabs which can overcome the aforementioned drawbacks of the prior art within the framework of a simple, rational, easy and effective to use, as well as inexpensive solution.

[0026] The aforementioned objects are achieved by this process for the production of resin-bonded mineral grit slabs having the characteristics of claim 1.

[0027] The aforementioned objects are achieved by this system for the production of resin-bonded mineral grit slabs having the characteristics of claim 13.

[0028] Brief Description of the Drawings

[0029] Other characteristics and advantages of the present invention will become more apparent from the description of a preferred, but not exclusive, embodiment of a process and a system for the production of resin-bonded mineral grit slabs, illustrated by way of an indicative yet non-limiting example in the accompanying tables of drawings in which:

[0030] Figure 1 is a schematic side elevation view of a system according to the invention, in a first embodiment;

[0031] Figure 2 is schematic perspective view of a system according to the embodiment of Figure 1;

[0032] Figure 3 is a schematic side elevation view of a system according to the invention, in a second embodiment.

[0033] Embodiments of the Invention

[0034] With particular reference to these figures, reference numeral 1 globally denotes a system for the production of resin-bonded mineral grit slabs.

[0035] In the context of this disclosure, mineral grits and resins are defined as minerals in granular form, e.g., marble, granite, glass, mirror fragments and / or others, as well as quartz powder, while resins can be, e.g., of the thermosetting type, especially for construction and furniture applications.

[0036] The system 1 comprises at least one supporting surface 2 movable along one direction of forward movement A, distribution means for distributing at least one mixture of mineral grits and resins on the supporting surface 2, compaction means 11 of the mineral grits and resins to obtain at least one compacted slab C and hardening means 12 of the compacted slab C to obtain a finished slab L.

[0037] The distribution means, not shown in detail in the figures, are of the type of a hopper which, as a result of the movement of the supporting surface 2 along the direction of forward movement A, deposits the mixture of mineral grits and resins in such a way as to obtain a downward-facing laying surface and an upward- facing exposed surface S.

[0038] In the embodiment shown in the figures, mixture distribution is carried out within a forming mold. It cannot, however, be ruled out that the mixture distribution is carried out directly on the supporting surface 2.

[0039] The compaction means 11 are of known type and, in particular, allow compaction by vibro-pressing under vacuum.

[0040] Through this operation, the mineral grits and resins are pressed within the forming mold, and simultaneously subjected to a vibrating action, which facilitates the adhesion of the grits themselves.

[0041] At the same time, a suction operation is carried out aimed to create a depressurized environment so as to remove the air in the gaps between the mineral grits and to further optimize compaction.

[0042] The hardening means 12 comprise a heated chamber within which catalysis takes place of the resins in the grit mixture in order to obtain the finished slab L provided with the desired mechanical and physical properties. The hardening means 12 then operate a first heating of the compacted slab C. Such first heating is preferably carried out at a temperature of between 100°C and 150°C.

[0043] The finished slab L is thus provided with at least one exposed surface S intended, in use, to face outwards and with a laying surface P arranged to rest on the supporting surface 2.

[0044] Therefore, the supporting surface 2 is adapted to hold the finished slab L made of resin-bonded mineral grits with the exposed surface S facing upwards.

[0045] The supporting surface 2 is, for example, a conveyor belt, a roller conveyor or the like.

[0046] The supporting surface 2 can be of the continuous or discontinuous type.

[0047] More particularly, the various means making up the system 1 can be arranged in succession with each other along the same supporting surface 2.

[0048] Alternatively, the supporting surface 2 may be of the discontinuous type, that is, it may have interruptions.

[0049] The term “finished slab” as used in this description therefore means a slab that is ready for use, i.e., that does not require further machining operations before being used. The finished slab is, therefore, a slab that has already undergone at least the phases of compaction and hardening and, possibly, has also undergone one or more surface finishing treatments such as, e.g., sanding. The system 1 may therefore be provided with at least one surface machining station 13 adapted to exert a mechanical machining action on the exposed surface S, arranged downstream of the hardening means 12 with respect to the direction of forward movement A.

[0050] The finished slab L also has an inherent degree of porosity.

[0051] In the context of this disclosure, the expression “degree of porosity” is intended to mean the ratio of the volume defined by the gaps between the grit particles to the total volume of the finished slab L. The inherent degree of porosity of a finished slab L can be very low, even close to zero.

[0052] The finished slab L may have a decorative effect, such as grains, affecting the exposed surface S or the entire thickness of the slab itself.

[0053] The decorative effect is given, e.g., by the use of two or more mixtures of mineral grits and resins, differing from each other in at least one of color, composition and particle size, suitably distributed to form the grains.

[0054] The system 1 then comprises decoration means 3 of the exposed surface S according to at least one predefined design D.

[0055] Preferably, the system 1 comprises treatment means 4 of the finished slab L adapted to increase the inherent degree of porosity and placed between the hardening means 12 and the decoration means 3.

[0056] The treatment means 4 have, therefore, the function of increasing the volume of the gaps in the finished slab L with respect to its total volume.

[0057] In particular, the treatment is carried out at least on the exposed surface S.

[0058] The treatment of the finished slab L aimed at increasing the degree of porosity is, therefore, carried out on the already hardened slab.

[0059] The function of increasing the degree of porosity is to facilitate the penetration of ink into the finished slab L during the next phase of decoration.

[0060] The ink is thus able to infiltrate between the pores on the exposed surface S in the thickness of the finished slab L.

[0061] In this way, the decorative effect given by the predefined design D is preserved even after any sanding of the exposed surface S.

[0062] Advantageously, the treatment means 4 comprise at least one heating device adapted to heat at least the exposed surface S.

[0063] Therefore, the treatment means 4 and the hardening means 12 carry out two separate heating operations and in particular, while the hardening means 12 carry out a first heating on the compacted slab C, the treatment means 4 carry out a second heating on the finished slab L.

[0064] In the path between leaving the hardening means 12 and entering the treatment means 4, the finished slab L has a chance to cool down, only to be heated again by the treatment means themselves.

[0065] Specifically, the treatment means 4 comprise a kiln within which the finished slab L is inserted during the movement along the direction of forward movement A. More particularly, the heating of the finished slab L is carried out with the exposed surface S free, that is, without the aid of covers or other covering elements.

[0066] Conveniently, the second heating carried out by the treatment means 4 is performed at a temperature of between 30 and 180°C.

[0067] Preferably, the second heating of the finished slab L is carried out at a temperature of between 60° and 100°.

[0068] According to the invention, the decoration means 3 comprise hot stamping means by means of sublimation ink and the system 1 comprises, downstream of these stamping means with respect to the direction of forward movement A, forced cooling means 14 of the finished slab L so decorated.

[0069] In a first embodiment, shown in Figures 1 and 2, the hot stamping means 3 comprise at least one digital printing device 23 adapted to dispense at least one sublimation ink on the exposed surface S according to the predefined design D and heating means 24 of the finished slab L so decorated arranged downstream of the digital printing device 23 with respect to the direction of forward movement A. As a result of the heat generated by the heating means 24, sublimation of the previously applied ink and its attachment to the finished slab L is achieved.

[0070] More particularly, the heating means 24 comprise at least one heated upper plate 24a, adapted to contact the exposed surface S.

[0071] Advantageously, the upper plate 24a is vertically movable close to / away from the supporting surface 2 so as to exert a pressure of predefined intensity on the exposed surface S.

[0072] The fact of exercising such a pressure allows increasing the quality and definition of the obtained design since it prevents the ink during the hardening phase from deviating from the predefined design thus jeopardizing the quality of the final product.

[0073] Preferably, the heating means 24 comprise at least one heated lower plate 24b adapted to contact the laying surface P. Heating the laying surface P allows the finished slab L to be treated evenly, thus avoiding thermal shock and deformation. The lower plate 24b can be placed below the supporting surface 2 or it can define a length of the supporting surface itself.

[0074] Appropriately, the heating means 24 comprise both the upper plate 24a and the lower plate 24b so that they can interact simultaneously with both the exposed surface S and the laying surface P.

[0075] In an alternative embodiment, the heating means 24 comprise one or more infrared lamp(s) arranged above and / or below the supporting surface 2 so as to irradiate the exposed surface S and / or the laying surface P of the finished slab L. In a second embodiment, schematically shown in Figure 3, the hot stamping means 3 comprise at least one transfer device 25 for transferring the predefined design D from a supporting element F, onto which the predefined design itself is applied by means of a sublimation ink, onto the exposed surface S. The supporting element F consists, e.g., of a so-called “sublimation paper”. In this embodiment, the transfer device 25 comprises heating means and pressing means of the set consisting of the finished slab L and of the supporting element F. More particularly, the heating means and the pressing means comprise at least one heated top plate 25a movable close to / away from the supporting surface 2 so as to exert a pressure of predefined intensity onto the supporting element F. Preferably, the heating means and the pressing means comprise at least one heated bottom plate 25b and adapted to serve as an abutment for the pressing action exerted by the top plate 25a.

[0076] As a result of the action of the transfer device 25, the sublimation ink passes from the supporting element F to the finished slab L, so once the top plate 25a has been lifted, the supporting element F is removed.

[0077] Advantageously, the cooling means 14 comprise at least one cooling chamber 14a adapted to receive the decorated finished slab L.

[0078] In more detail, the cooling means 14 comprise air blowing means within the cooling chamber 14a.

[0079] In an alternative embodiment, the cooling means 14 comprise at least one supporting plate 14b arranged within the cooling chamber 14a and adapted to hold at least one finished slab L and comprise refrigeration means of the supporting plate 14b, such as of the type of a coil within which a cooling fluid flows which is chilled by an external refrigeration unit.

[0080] Appropriately, the cooling means 14 comprise at least one pair of supporting plates 14b arranged on opposite sides of the finished slab L, of which one is adapted to interact with the laying surface P and one with the exposed surface S. In one possible embodiment, the cooling means 14 may comprise a plurality of supporting plates 14b (or pairs of supporting plates 14b) arranged vertically overlapping each other to define a plurality of planes adapted to support a corresponding number of finished slabs L.

[0081] The operation of this system 1 in the execution of the process according to the invention is as follows.

[0082] The process covered by the present invention first involves a supply of at least one finished slab L made of resin-bonded mineral grits provided with at least one exposed surface S and having an inherent degree of porosity.

[0083] In other words, the process comprises at least the following phases of: supply of at least one mixture of mineral grits and resins; compaction of said mineral grits and resins to obtain a compacted slab C; hardening of said compacted slab C to obtain said finished slab L.

[0084] The mixture of mineral grits is distributed directly on the supporting surface 2 or, alternatively, within a forming mold placed on the supporting surface 2.

[0085] This phase is carried out through the distribution means.

[0086] At this point, the process involves compacting the mixture to obtain the compacted slab C, through vibro -pres sing under vacuum, carried out by the compaction means 11.

[0087] Next, the process comprises the hardening phase which is carried out by the hardening means.

[0088] More particularly, the hardening phase is carried out by means of a first heating of the compacted slab C. Such first heating is carried out at a temperature of between 100°C and 150°C.

[0089] The finished slab L is arranged on the supporting surface 2 which is moved along the direction of forward movement A to allow the various phases of the process to be carried out.

[0090] Specifically, the finished slab L is arranged on the supporting surface 2 with the exposed surface S facing upwards.

[0091] Next, the process comprises a phase of decoration with ink the exposed surface S according to at least one predefined design D.

[0092] The finished slab L may have one base decoration, such as grains, affecting the exposed surface S or the entire thickness of the slab itself.

[0093] Base decoration is achieved, e.g., by the use of two or more mixtures of mineral grits and resins, differing from each other in at least one of color, composition and grain size, suitably distributed to form the grains.

[0094] Advantageously, the predefined design D is applied in a coordinated maimer to the base decoration. In other words, the term “coordinated” means that the predefined design D can resume the underlying base decoration (in the sense that the predefined design D matches the underlying base decoration), creating a so- called “matching” effect, e.g. so as to reproduce a natural or similar stone, or it can be complementary thereto, thus creating a so-called “mismatching” effect.

[0095] According to the invention, the decoration phase comprises a phase of hot stamping by means of sublimation ink and, subsequently to this stamping, a phase of cooling the finished slab L so decorated.

[0096] In a first embodiment, the hot stamping phase comprises the sub-steps of: application by digital printing of the sublimation ink according to the predefined design D; sublimation, subsequently to the application phase and prior to cooling, of the sublimation ink by heating the finished slab L.

[0097] More particularly, the sublimation phase is carried out by heating the finished slab L to a temperature of between 120°C and 200°C.

[0098] Appropriately, such heating is carried out for a time interval of between 5 minutes and 15 minutes.

[0099] In a second embodiment, hot stamping comprises the sub-steps of: supply of a supporting element F of the sublimation ink reproducing the predefined design D; transfer of the predefined design D from the supporting element F to the exposed surface S.

[0100] More particularly, transfer is carried out by placing the supporting element F in contact with the exposed surface S and carrying out sublimation of the ink by heating to a temperature of between 120°C and 200°C.

[0101] Preferably, in both the first and second embodiments, sublimation by heating the ink is carried out by also exerting a pressing action on the exposed surface S. More specifically, such pressing is carried out by exerting a pressure on the finished slab L of between 0.3 kg / cm2and 0.6 kg / cm2.

[0102] Advantageously, the phase of ink sublimation is carried out by heating both the exposed surface S and the laying surface P.

[0103] The sublimation phase is carried out, e.g., by means of the plates 24a, 24b, 25a, 25b of the system 1 described above. The subsequent phase of cooling is preferably carried out for a time interval of between 30 minutes and 60 minutes.

[0104] Appropriately, the cooling phase allows the temperature of the finished slab L to be raised to about 40°.

[0105] Advantageously, the process comprises, subsequently to hardening and prior to decoration, at least one phase of increasing the inherent degree of porosity of the finished slab L, which phase is adapted to promote the penetration of the ink into the finished slab L.

[0106] In more detail, the phase of increasing the degree of porosity is then carried out on the already hardened finished slab L, that is, once the finished slab L has cooled down as a result of the first heating mentioned above.

[0107] The fact of increasing the degree of porosity allows the ink to infiltrate between the pores on the exposed surface S, within the thickness of the finished slab L.

[0108] The phase of increasing the degree of porosity is carried out by the treatment means 4.

[0109] Appropriately, the phase of increasing the degree of porosity is carried out by performing a second heating on the finished slab L at a temperature of between 30° and 180°.

[0110] Thus, the phase of increasing the degree of porosity of the finished slab L is separate from and subsequent to the phase of hardening the compacted slab C.

[0111] More particularly, the second heating for increasing the degree of porosity is carried out at a temperature of between 70° and 100°.

[0112] Advantageously, the phase of increasing the degree of porosity is carried out by heating at least the exposed surface S.

[0113] In more detail, such heating involves the entire finished slab L.

[0114] It has, in practice, been ascertained that the described invention achieves the intended objects, and in particular, the fact is emphasized that the process and system according to the invention make it possible to obtain mineral grit slabs provided with a wide variety of decorative effects.

[0115] This is made possible thanks to the use of a sublimation ink, the attachment thereof to the finished slab by heating and the subsequent cooling of the finished slab thus decorated.

[0116] In addition, the presence of the treatment means adapted to increase the degree of porosity of the compacted plate enables the ink to penetrate within the slab itself The process and the system according to the invention enable simple and reliable production of mineral grit slabs with decorative effects that remain, thus, unchanged during the entire machining and following finishing operations.

Claims

CLAIMS1) Process for the production of resin-bonded mineral grit slabs comprising at least the phases of: supply of at least one finished slab (L) obtained by means of at least the steps of:• supply of a mixture of mineral grits and resins;• compaction of said mineral grits and resins to obtain a compacted slab (Q;• hardening of said compacted slab (C) to obtain said finished slab (L) provided with at least one exposed surface (S) and with one laying surface (P), said finished slab (L) having an inherent degree of porosity; decoration with ink, after said hardening, of said exposed surface (S) according to at least one predefined design (D); characterized by the fact that said decoration comprises a phase of hot stamping with sublimation ink and by the fact that it comprises, subsequently to said hot stamping, a phase of cooling said finished slab (L) so decorated.2) Process according to claim 1, characterized by the fact that said hot stamping comprises the sub-steps of: application by digital printing of said sublimation ink according to said predefined design (D); sublimation, subsequently to said application and prior to said cooling, of the ink by heating said finished slab (L).3) Process according to claim 2, characterized by the fact that said sublimation is carried out by heating to a temperature of between 120°C and 200°C.4) Process according to claim 3, characterized by the fact that said heating is carried out for a time interval of between 5 minutes and 15 minutes.5) Process according to claim 1, characterized by the fact that said stamping comprises the sub-steps of: supply of a supporting element (F) of the sublimation ink reproducing said predefined design (D); transfer of said predefined design (D) from said supporting element (F) tosaid exposed surface (S).6) Process according to claim 5, characterized by the fact that said transfer is carried out by placing said supporting element (F) in contact with said exposed surface (S) and carrying out sublimation of the ink by heating to a temperature of between 120°C and 200°C.7) Process according to one or more of claims 2 to 4 and claim 5 or 6, characterized by the fact that said sublimation is carried out by exerting a pressing action on said exposed surface (S).8) Process according to claim 7, characterized by the fact that said pressing is carried out by exerting a pressure on said finished slab (L) of between 0.3 kg / cm2and 0.6 kg / cm2.9) Process according to one or more of claims 2 to 4 and claim 5 or 6 or according to claim 7 or 8, characterized by the fact that said sublimation is carried out by heating both said exposed surface (S) and said laying surface (P).10) Process according to one or more of the preceding claims, characterized by the fact that said cooling is carried out until the finished slab (L) is brought to a temperature of about 40°.11) Process according to one or more of the preceding claims, characterized by the fact that it comprises, prior to said decoration and subsequently to said hardening, at least one phase of increasing said inherent degree of porosity of the finished slab (L), which phase is adapted to promote the penetration of said ink.12) Process according to claim 11, characterized by the fact that said phase of increasing the inherent degree of porosity is carried out by heating at least said exposed surface (S) to a temperature of between 60°C and 100°C.13) Process according to one or more of the preceding claims, characterized by the fact that said finished slab (L) has a base decoration running through the entire thickness thereof and by the fact that said predefined design (D) is applied to said base decoration in a coordinated maimer.14) System (1) for the production of resin-bonded mineral grit slabs, comprising: at least one supporting surface (2) movable along one direction of forward movement (A);distribution means of at least one mixture of mineral grits and resins on said supporting surface (2); compaction means (11) of said mineral grits and resins to obtain at least one compacted slab (C); and hardening means (12) of said compacted slab (C) to obtain a finished slab (L) provided with at least one exposed surface (S) and of at least one laying surface (P), said finished slab (L) having an inherent degree of porosity; decoration means (3) of said exposed surface (S) according to at least one predefined design (D); characterized by the fact that said decoration means (3) comprise hot stamping means by means of sublimation ink and by the fact that it comprises, downstream of said stamping means with respect to said direction of forward movement (A), cooling means (14) of said finished slab (L) so decorated.15) System (1) according to claim 14, characterized by the fact that said hot stamping means (3) comprise at least one digital printing device (23) adapted to dispense at least one sublimation ink on said exposed surface (S) according to said predefined design (D) and heating means (24) of the finished slab (L) so decorated arranged downstream of said digital printing device (23) with respect to the direction of forward movement (A).16) System (1) according to claim 15, characterized by the fact that said heating means (24) comprise at least one heated upper plate (24a) adapted to contact said exposed surface (S).17) System (1) according to claim 16, characterized by the fact that said upper plate (24a) is vertically movable close to / away from said supporting surface (2) so as to exert a pressure of predefined intensity on said exposed surface (S).18) System (1) according to one or more of claims 15 to 17, characterized by the fact that said heating means (24) comprise at least one heated lower plate (24b) adapted to contact said laying surface (P).19) System (1) according to claim 14, characterized by the fact that said hot stamping means (3) comprise at least one transfer device (25) for transferring the predefined design (D) from a supporting element (F) onto which the predefineddesign itself is applied by means of sublimation ink on said exposed surface (S). 20) System (1) according to claim 19, characterized by the fact that said transfer device comprises heating means and pressing means (25a, 25b) of the set comprising said finished slab (L) and said supporting element (F). 21) System (1) according to one or more of claims 14 to 20, characterized by the fact that said cooling means (14) comprise at least one cooling chamber (14a) adapted to receive said decorated finished slab (L).22) System (1) according to claim 21, characterized by the fact that said cooling means (14) comprise air blowing means within said cooling chamber (14a). 23) System (1) according to claim 21 or 22, characterized by the fact that said cooling means (14) comprise at least one supporting plate (14b) of at least one finished slab (L) arranged within said cooling chamber (14a) and comprise refrigeration means of said supporting plate (14b).24) System (1) according to one or more of claims 14 to 23, characterized by the fact that it comprises treatment means (4) of said finished slab (L) adapted to increase said inherent degree of porosity and arranged upstream of said decoration means (3) with respect to said direction of forward movement (A).

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