Process for producing a surface covering with an embossed printed surface

Abstract

A process for producing a surface covering with an embossed printed surface is described. A substrate (16) is continuously moved through a production line, and this substrate (16) is first provided, in a printing equipment (12), with a printed pattern and thereafter, in an embossing equipment (14), with an embossed pattern, which is registered with the printed pattern. The printing equipment (12) produces the printed pattern in-line with the production of the embossed pattern. During printing in the printing equipment (12), the printed pattern is stretched or compressed, dynamically responsive to indicators of misalignments between the printed pattern and the embossed pattern, so as to correct or prevent the misalignments. A production line for carrying out this process is also proposed.

Claims

1. A process for producing a surface covering with an embossed printed surface, wherein a substrate is continuously moved through a production line, and said substrate is first provided with a printed pattern and thereafter with an embossed pattern, which is to be registered with said printed pattern; characterized in that a printing equipment is used in said production line to produce said printed pattern in-line with the production of said embossed pattern; and during printing in said printing equipment, said printed pattern is stretched or compressed, dynamically responsive to indicators of misalignments between said printed pattern and said embossed pattern, so as to correct or prevent said misalignments.

2. The process as claimed in claim 1, wherein said printing equipment is a digital printing equipment, and said printed pattern is stretched or compressed by controlling said digital printing equipment.

3. The process as claimed in claim 2, wherein said printed pattern is stretched, respectively compressed, by up-scaling, respectively down-scaling, in real-time at least a part of a digitally recorded image of said pattern to be printed.

4. The process as claimed in claim 3, wherein said up-scaling and/or down-scaling is a unidirectional scaling, the scaling direction corresponding to the longitudinal direction of said printed pattern on said moving substrate.

5. The process as claimed in claim 1, wherein said printed pattern is stretched or compressed by reducing or increasing the velocity with which said substrate passes through said printing equipment.

6. The process as claimed in claim 4, wherein an accumulation loop, which is arranged between said printing equipment and an embossing equipment, allows to control the velocity of said substrate in said printing equipment independently from its velocity in said embossing equipment, while maintaining the tension in said moving substrate substantially constant.

7. The process as claimed in claim 1, wherein: a rotating embossing cylinder is used for providing said embossed pattern in the moving surface covering; and an angular encoder is used for continuously measuring the angular position of said rotating embossing cylinder; and said angular position signal is used as a real-time process parameter for continuously determining the position of said embossing pattern relative to said moving substrate.

8. The process as claimed in claim 1, further comprising the steps of: comparing a dimension of said printed pattern, which is measured in real-time on said moving substrate, with a reference value said dimension should have to properly match with a corresponding dimension in said embossed pattern; and during printing, stretching or compressing said printed pattern to reduce the difference between said measured dimension and said reference value.

9. The process as claimed in claim 1, further comprising the steps of: comparing in real-time a detected position of said printed pattern on said moving substrate and a computed position of embossed pattern on said moving substrate; and temporarily stretching or compressing said printed pattern during printing, to reduce an offset between these positions.

10. The process as claimed in claim 1, further comprising the steps of: predicting, on the basis of measurements made on said moving substrate upstream of said embossing equipment, an offset between said embossed pattern and said printed pattern; and during printing, stretching or compressing said printed pattern to reduce said offset.

11. The process as claimed in claim 1, further comprising the steps of: measuring, at at least one position between said printing equipment and a downstream embossing equipment, the passage times of reference marks printed on said substrate; or measuring at regular time intervals, at at least one position between said printing equipment and a downstream embossing equipment, the longitudinal offsets between the detected positions of reference marks printed on said substrate and the computed position of said reference marks at said time intervals; and using said measured passage times or said measured offsets to predict a misalignment between said printed pattern and said embossed pattern at said embossing equipment; and using said predicted misalignment to compute a required stretching or compression of said printed pattern.

12. The process as claimed in claim 11, wherein: said passage times or offsets of said reference marks are measured at several positions along the path of movement of said substrate between said printing equipment and said embossing equipment; and said measured passage times or said measured offsets are used to predict an evolution of the alignment of said printed pattern with said embossed pattern at said embossing equipment; and said predicted evolution of said alignment is used to compute a preventive stretching or compression of said printed pattern.

13. The process as claimed in claim 1, further comprising the step of: applying between said printing equipment and a downstream embossing equipment a transparent wear layer onto said printed pattern; and embossing said embossed pattern into said transparent wear layer in said embossing equipment.

14. The process as claimed in claim 1, wherein said printing equipment is an inkjet printer.

15. The process as claimed in claim 1, wherein the surface covering is a heterogeneous resilient floor or wall covering.

16. A production line for producing a surface covering with an embossed printed surface, said production line comprising: a printing equipment for applying a printed pattern onto said moving substrate; an embossing equipment downstream of said printing equipment, for providing said substrate with an embossed pattern, which is registered with said printed pattern; and control equipment for correcting or preventing misalignments between said printed pattern and said embossed pattern, by stretching or compressing said printed pattern, dynamically responsive to real-time indicators of said misalignments.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0018] The afore-described and other features, aspects and advantages of the invention will be better understood on the basis of the following description of an embodiment of the invention and upon reference to the attached drawings, wherein:

[0019] FIG. 1: is a schematic illustrating a first embodiment of a section of a production line used for implementing the present invention;

[0020] FIG. 2: is a schematic illustrating a second embodiment of a section of a production line used for implementing the present invention; and

[0021] FIG. 3A, 3B, 3C: are schematics illustrating a stretching and compressing of the printed pattern during pritting.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0022] It will be understood that the following description and the drawings to which it refers describe and illustrate, by way of example, preferred embodiments of the claimed subject matter. They shall not limit the scope, nature or spirit of the claimed subject matter.

[0023] FIGS. 1 and 2 schematically illustrate a section 10 of a production line used for producing a flexible surface covering with a printed and embossed decorative surface. This surface covering is for example a polymer-based flexible flooring, such as for example a heterogeneous or homogeneous vinyl flooring. The embossed printed surface simulates, for example, the natural appearance of hardwood, stone or tile flooring, by adding depth and texture in alignment with a printed pattern. However, the printed pattern may also comprise any other decorative design or pictures, wherein the embossed pattern is registered with this decorative design or these pictures to confer a 3D aspect to the design or the pictures.

[0024] It will be understood that FIGS. 1 and 2 only show equipment that is directly relevant for the present invention. Thus, the section 10 shown in FIGS. 1 and 2 starts with a printing equipment 12 and ends with an embossing equipment 14. The complete production line may be more than 100 m long. The distance between the printing equipment 12 and the embossing equipment 14 shall normally be less than 5 m, preferably less than 2 m and, if possible, even less than 1 m, but may exceptionally also be greater than 5 m.

[0025] Reference number 16 points to a substrate that is continuously produced in other sections of the production line, which are arranged upstream of the printing equipment 12. This substrate 16 travels on carrier belts 18′, 18″, 18′″ through the section 10 of the production line, in the direction of arrows 20, i.e. from the printing equipment 12 to the embossing equipment 14. The terms “upstream” and “downstream” are used herein with reference to the conveying direction of the substrate 16, which is identified by arrows 20. The term “longitudinal” is generally used herein to refer to a direction that is parallel to the conveying direction. The term “width” is generally used herein to refer to a dimension that is transversal to the conveying direction.

[0026] The substrate 16 can be a homogeneous substrate or a multilayer substrate. It is for example a homogeneous or multilayer plastic foil, made for example of the following materials: PVC, vinyl, polyethylene, polypropylene or polyamide. Its thickness is generally in the range of 2 mm to 5 mm. The width of the substrate 16 is typically in the range of 2 m to 5 m.

[0027] At the entrance of the printing equipment 12, the top-surface 22 of the substrate 16 is advantageously formed by a dimensionally relatively stable print support layer. A suitable print support layer is for example a veil of glass fibres or a film that is applied onto the substrate 16 upstream of the printing equipment 12. This print support layer forms the surface onto which the printed pattern is applied in the printing equipment 12. The print support layer can be coated or impregnated with a primer that is selected, amongst others, in function of the base material of the substrate 16, the printing technology and the inks used in the printing equipment 12, and in function of the background colour to achieve. Alternatively, the primer may be directly coated onto the substrate. In exceptional cases, it may even be possible to dispense with the print support layer and the primer, such that printing is effected directly on the substrate.

[0028] Instead of printing the printed pattern onto the substrate 16, it is also possible to print, within the production line and synchronously with the production of the substrate 16, the printed pattern onto an auxiliary film, which is applied onto the substrate 16 downstream of the embossing equipment 14. In most cases, it will however be of advantage to print the printed pattern with the printing equipment 12 directly onto the substrate 16, as described in the previous paragraph.

[0029] The printing equipment 12 is advantageously an industrial inkjet printer, preferably working with water-based colours, while the use of solvent-based colours is however not excluded. The inkjet printing heads are individually controllable and are distributed over the whole width of the substrate. A drying and/or curing equipment is advantageously arranged in the outlet section of the printing equipment 12, for drying and/or curing the printed pattern. The inkjet printing equipment 12 may include one line of printing heads for each of the four colours generally used in inkjet printing (i.e. cyan, magenta, yellow, and black), wherein two successive lines of printing heads are advantageously separated by a drying equipment for drying (partially or completely) the first colour before the next colour is applied. The drying equipment comprises for example hot air dryer units and/or infrared dryer units. Alternatively, each printing head may be conceived for simultaneously printing more than one colour. In the printing equipment 12, the substrate 16 is advantageously free of substantial mechanical tension, i.e. the substrate simply lies in a substantially tension-free manner on the carrier belt 18′.

[0030] Between the printing equipment 12 and the embossing equipment 14, a transparent wear layer 24 is applied onto the upper surface of the moving substrate 16 bearing the printed pattern. This application is for example achieved in a lamination equipment 26 located between the printing equipment 12 and the embossing equipment 14. The transparent wear layer 24 protects the printed pattern on the substrate 16 and forms the layer that is embossed in the embossing equipment 14. A suitable transparent wear layer 24 is e.g. a film of PVC with a thickness in the range of 0.05 to 1 mm. A heater (not shown) is generally arranged between the lamination equipment 26 and the embossing equipment 14, to mollify the wear layer 24 before the substrate 16 enters into the embossing equipment 14.

[0031] The embossing equipment 14 comprises for example a rotating embossing cylinder 28 and a backing cylinder 30. The embossing cylinder 28 has a cylindrical surface 31, bearing a negative of the pattern to be embossed into the transparent wear layer 24. The backing cylinder 30 allows to press the substrate 16 with its transparent wear layer 24 against the cylindrical surface 31 of the rotating embossing cylinder 28. Thereby, a positive copy of the negative pattern, which is borne by the embossing cylinder 28, is continuously embossed into the transparent wear layer 24. It will be noted that the pitch of the embossed pattern (i.e. the length of a repeating motif) cannot be greater than the circumference of the embossing cylinder 28. However, the circumference of the embossing cylinder 28 can be a multiple of the pitch of the embossed pattern, i.e. during one revolution, the embossing cylinder 28 the embossing cylinder embosses several repeating motives into the transparent wear layer 24.

[0032] In the final surface covering, a 3D feature of the embossed pattern shall occupy a well-defined position relative to a corresponding 2D feature of the underlying printed pattern; i.e. the embossed pattern shall be properly aligned with the underlying printed pattern. However, imperfect initial registration of the 3D pattern with the 2D pattern, imperfect control of the velocities of the carrier belts 18′, 18″, 18′″, of the rotation speed of the lamination rolls or of the embossing and backing cylinder 28, 30, but also changes in the linear expansion/shrinking of the substrate 16, as well as other process disturbances, may result in misalignments between the printed pattern and the embossed pattern (i.e. 3D features of the embossed pattern are no longer aligned with the corresponding 2D features of the underlying printed pattern). It will consequently be appreciated that the present invention proposes to correct or prevent such misalignments, by stretching or compressing the printed pattern, relative to the substrate, during printing, and to this dynamically responsive to real-time indicators of such misalignments.

[0033] In accordance with FIG. 1, a controller 32 sends modified printing data to the digital printing equipment 12, thereby instructing the printing equipment 12 to stretch or compress the printed pattern. In practice, this is e.g. achieved by digitally up-scaling respectively by digitally down-scaling a digitally recorded pattern image, wherein the up-scaling will result in a stretching and the down-scaling will result in a compressing of the printed pattern. This scaling may concern the whole digitally recorded pattern image or only part thereof, e.g. one or more details or one or more sections of the digitally recorded image. As misalignments are normally in the direction of movement of the substrate 16, the up-scaling and/or down-scaling are normally a unidirectional scaling, the scaling direction corresponding to the longitudinal direction of the printed pattern on the substrate 16. Such a unidirectional scaling allows to preserve the original width the printed pattern, while it is stretched or compressed in the longitudinal direction.

[0034] In accordance with FIG. 2, the printed pattern is stretched or compressed by reducing or increasing the velocity with which the substrate 16 passes through the printing equipment 12. To achieve this aim, a controller 32′ modifies for example the speed with which the carrier belt 18′ moves the substrate 16 through the printing equipment 12, controlling for example the speed of a drive motor 34 of the carrier belt 18′. An increase of this speed will result in a stretching, whereas its decrease will result in a compressing of the printed pattern. While the velocity with which the substrate 16 passes through the printing equipment 12 is decreased or increased, the velocity with which substrate passes thereafter through the embossing equipment 14 remains basically unchanged. To allow this difference in velocities, an accumulation loop 36 is arranged between the printing equipment 12 and the embossing equipment 14, or more specifically, between the carrier belt 18′ carrying the substrate 16 through the printing equipment 12 and the carrier belt 18″ transporting it to the embossing equipment 14. The accumulation loop 36 allows to maintain the tension in the moving substrate 16 substantially constant.

[0035] FIGS. 3A, 3B and 3C are now used to illustrate the effects of stretching/compressing of the printed pattern. FIG. 3A shows printed pattern consisting of an array of circular surfaces 40, which have a different colour than the background 42. To further fix the ideas, it is assumed that the embossed pattern, which is created by the embossing cylinder 28, comprises an array of raised circular surfaces, which array is an exact image of the array of coloured surfaces 40 shown in FIG. 3A. In the embossed printed surface, the coloured surfaces 40 should be aligned under the raised circular surfaces as precisely as possible.

[0036] FIG. 3B shows the printed pattern of FIG. 3A, wherein after the first row of circles 40, the pattern is stretched parallel to the direction of arrow 44. Such a temporary stretching allows to compensate for an offset between the printed pattern and the embossed pattern, in which the printed pattern is, at the embossing equipment, in advance of the embossed pattern. Indeed, in the stretched pattern, the distance L′ between the first row of (still undeformed) circles 40 and the third row of circles 40′ is greater than the corresponding distance L in the pattern of FIG. 3A. The temporary stretching may be limited to one pattern motif or be distributed over several pattern motifs, so as to make it less apparent. It will further be noted that a stretching of the printed pattern is also an adequate solution if—for any reason—the substrate 16 longitudinally expands less than expected or shrinks more than expected between the printing equipment 12 and the embossing equipment 14.

[0037] FIG. 3C shows the printed pattern of FIG. 3A, wherein after the first row of circles 40, the pattern is compressed in the direction of arrow 42. Such a temporary compressing allows for example to compensate for an offset between the printed pattern and the embossed pattern, in which the embossed pattern is in advance of the printed pattern. Indeed, in the compressed pattern, the distance L″ between the first row of (still undeformed) circles 40 and the third row of circles 40″ is smaller than the corresponding distance L in the pattern of FIG. 3A. The temporary compressing may be limited to one pattern motif or be distributed over several pattern motifs, so as to make it less apparent. A compressing of the printed pattern is also an adequate solution if—for any reason—the substrate 16 longitudinally expands more than expected or shrink less than expected between the printing equipment 12 and the embossing equipment 14.

[0038] It remains to be pointed out that the stretching in FIG. 3B and the compressing in FIG. 3C are strongly exaggerated. Most often, the misalignment to compensate is only a few millimetres, and the stretching or compressing of the printed pattern can be limited to similar amounts, i.e. to a range not exceeding several millimetres.

[0039] According to a first aspect, the proposed process comprises the steps of: (1) comparing in real-time a detected position of the printed pattern on the moving substrate and a computed position of embossed pattern on the moving substrate; and (2) temporarily stretching or compressing the printed pattern during printing to reduce an offset between these positions.

[0040] According to a second aspect, the proposed process comprises the steps of: (1) predicting, responsive to real-time indicators, an offset between the embossed pattern and the printed pattern; and (2) stretching or compressing the printed pattern to prevent or reduce the predicted offset.

[0041] According to a third aspect, the proposed process comprises the steps of: (1) comparing a dimension of the printed pattern, which is measured in real-time on the moving substrate, with a reference value this dimension should have to properly match with a corresponding dimension in the embossed pattern; and (2) stretching or compressing the printed pattern to reduce any difference between the measured dimension and the reference value.

[0042] Referring again to FIGS. 1 and 2, the control circuit used for controlling the stretching/compressing of the printed pattern will now be further described. It will first be noted that the embossing cylinder 28 is equipped with an angular encoder 50 continuously measuring the angular position of the rotating embossing cylinder 28. This angular position signal is then used as a real-time process parameter for continuously determining the position of the embossing pattern relative to the moving substrate 16 in the embossing equipment 14. Reference numbers 50, 52 point to sensors (typically optical sensors) capable of detecting reference marks printed onto the substrate 16, synchronously with the printed pattern. In a first embodiment, the sensors 50, 52 are used to measure the passage times of these reference marks at predetermined positions between the printing equipment and the embossing equipment. In a second embodiment, the sensors 50, 52 are used to measure, at regular time intervals, the longitudinal offsets between the detected position of a reference mark and the computed position this reference mark is supposed to have at that time. The measured passage times or the measured offsets are then used to predict a misalignment between the printed pattern and the embossed pattern at the embossing equipment, wherein this predicted misalignment is used to compute a required stretching or compression of the printed pattern. If the passage times or offsets of the reference marks are measured at several positions along the path of movement of the substrate 16 between the printing equipment 12 and the embossing equipment 14, the measured passage times or the measured offsets can be used to predict an evolution of the alignment of the printed pattern with the embossed pattern at the embossing equipment 14. The predicted evolution of the alignment may then be used to compute a preventive stretching or compression of the printed pattern. Reference number 54 points to an additional sensor arranged downstream of the embossing equipment 14, which is mainly used to detect an offset that has its origin only closely before or within embossing equipment 14.

LIST OF REFERENCE NUMBERS

[0043] 10 section of production line [0044] 12 printing equipment [0045] 14 embossing equipment [0046] 16 substrate [0047] 18′, 18″, 18′″ carrier belt [0048] 20 direction arrows [0049] 22 top surface of 16 [0050] 24 transparent wear layer [0051] 26 lamination equipment [0052] 28 embossing cylinder [0053] 30 backing cylinder [0054] 31 cylindrical surface [0055] 32, 32′ controller [0056] 34 drive motor of 18′ [0057] 36 accumulation loop [0058] 40 circular surface [0059] 40′ surfaces 40 when the printed pattern is stretched [0060] 40″ surfaces 40 when the printed pattern is compressed [0061] 42 arrow [0062] 48 angular encoder [0063] 50, 52, 54 optical sensor