Note: Descriptions are shown in the official language in which they were submitted.
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Method of coating a building panel with digital printing/coating technique
Technical field
The disclosure generally relates to the field of panels with a decorative wear
resistant
surface, preferably floor and wall panels. The disclosure relates to building
panels
with such decorative surfaces and to production methods to produce such
panels.
Field of Application
Embodiments of the present invention are particularly suitable for use in
floors,
which may be formed of floor panels comprising a core, a decorative layer and
a
transparent wear resistant layer above the decorative layer. The following
description
of technique, problems of known systems and objects and features of the
invention
will therefore, as a non-restrictive example, be aimed at this field of
application and
in particular at floorings which are similar to traditional wood and laminated
floorings.
It should be emphasized that embodiments of the invention may be used to
produce a
surface layer in any type of panels for example in building panels such as
wall panels,
ceilings, and furniture components and similar.
Background
The majority of all laminate floors are produced according to a production
method
generally referred to as Direct Pressed Laminated (DPL). Such laminated floors
comprise a core of a 6-12 mm fibre board, a 0.2 mm thick upper decorative
surface
layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate,
plastic,
paper or like material.
The surface layer of a laminate floor is characterized in that the decorative
and wear
properties are generally obtained with two separate layers of paper, one above
the
other. The decorative layer is generally a printed paper and the wear layer is
a
transparent overlay paper, which comprises small aluminium oxide particles.
The printed decorative paper and the overlay are impregnated with melamine
resins
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and laminated to a wood fibre based core in large laminate presses where the
resin
cures under high heat and pressure and the papers are laminated to the core
material.
Laminate floor are produced as large boards, generally with a size of 2.1*2.7
m and
they are after pressing divided into several individual floor panels,
generally with a
size of about 1,3*0,2 m. The edges are thereafter machined and provided with
mechanical locking systems.
Laminated floors may also be produced with printing technology. One advantage
is
that the pressing operation may be avoided and that no printed papers are
needed to
provide a decorative wear resistance surface.
Floor panels with a Direct Printed Laminate surface comprise the same type of
HDF
core as DPL. The decor is printed directly onto the core. The production
process is
rather complicated and is only cost efficient in very large production
volumes.
In a first step a combination of pigmented, water-based primers and sealers
are
applied on the core in order to provide the adhesion to the core and a basic
colouration. Such coating is below referred to as adhesion coating.
Hydro printing inks are used to print the decor by a multicolour printing
press with
rollers that print directly onto the pre-sealed core. The printed decor layer
must be
protected against wear. In some cases a paper based overlay is used and the
board is
pressed in a traditional laminate press. The most common method to achieve
high
2 0 abrasive strength is to use, anti-abrasive UV sealers, which are then
applied on the
print by rollers and cured by UV. Structured and synchronised surfaces may
also be
produced by embossed rollers. Several layer are applied on the print. Base
coats are
used to build up the thickness of the protective layer and top coats are used
to provide
the final structure and scratch resistance. Embossed structures may be formed
by
embossed rollers that also may have a structure that is in register with the
printing
cylinders.
Direct printing technology may be replaced with digital printing technology
that is
much more flexible and small production volumes can be economically
manufactured. The difference between these two methods is mainly the printing
step
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where printing rollers are replaced by a digital non-contact printing process
and
where the desired image is directly applied onto the pre-finished core. The
final
transparent coating which protects the digital image and the structured
surfaces are
usually of the same type as used in direct printing. Any types or printed
images may
be created but the surface structure is always limited to the form of the
embossed
rollers or structured films that are pressed against the surface.
Digital printing may also be used to print on a paper sheet that is used in
conventional
laminate production. The digital print may be applied on the upper side of a
decorative paper comprising a base colour or on the lower side of the overlay
and
may be made prior or after impregnation. The print may also be made on a raw
paper,
with or without a base colour and the paper may be indirectly impregnated
during
pressing by resins from powder, a liquid layer or impregnated paper applied
under
and over the digitally printed paper. The raw paper may be connected to the
core prior
to the digital printing step and this allows that thinner raw paper may be
used and
positioning problems may be reduced.
Recently new "paper free" floor types have been developed with solid surfaces
comprising a substantially homogenous mix of fibres, binders and wear
resistant
particles. Such floor is referred to as WFF (Wood Fibre Floors).
The wear resistant particles are preferably aluminium oxide particles, the
binders are
2 0 preferably thermosetting resins such as amino resins and the fibres are
preferably
wood based. In most applications decorative particles such as for example
colour
pigments are included in the homogenous mix. In general all these materials
are
preferably applied in dry form as a mixed powder on a HDF core and cured under
heat and pressure to a 0.1 ¨ 1.0 mm solid layer.
Several advantages over known technology and especially over conventional
laminate
floorings may be obtained:
= The wear resistant surface layer, which is a homogenous mix, may be made
much thicker and a wear resistance is achieved, which is considerably higher.
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= New and very advanced decorative effects may be obtained with deep
embossing and by separate decorative materials, which may be incorporated
into the homogenous surface layer and coordinated with the embossing.
= An increased impact resistance may be reached with a homogenous surface
layer, which is thicker and has a higher density.
= The homogenous surface layer may comprise particles that have a positive
impact on sound and moisture properties.
= Production costs may be reduced since low cost and even recycled
materials
may be used and several production steps may be eliminated.
Powder technology is very suitable to produce a decorative surface layer,
which is a
copy of stone and ceramics. It is however more difficult to create designs
such as, for
example, wood decors. However, recently digital powder printing has been
developed
and it is possible to create very advanced designs of any type by injecting a
digital
print into the powder prior to pressing.
Floors with a surface of wood are produced in many different ways. Traditional
solid
wood floors have developed into engineered floors with wood layers applied on
a
core made of wood lamellas, HDF or plywood. The majority of such floors are
delivered as pre finished floors with a wood surface that is coated with
several
transparent layers in the factory. Wood floorings are one of the largest
applications
2 0 for the Roll-coat/UV-curing process that applies multi-layered coating
structures to
seal and protect the wood surface. Generally environmentally friendly water
based
UV cured polyurethane is used for the transparent protective layers. This
application
process generally comprises application of several types of transparent layers
- UV
primers, UV putties, UV sealers and UV topcoats. Each layer of coating is
given its
own specific function. Surface characteristics like abrasion resistance,
hardness,
scratch resistance etc. may be adjusted in order to meet different quality
requirements.
All such layers are applied by roller coaters and each individual layer is UV
cured. A
high quality wood floor may comprise a transparent protective layer of about
60 ¨
120 gr/m2 and there may be about 5 -10 layers or more applied in steps by
rollers
with intermediate UV curing.
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As a first step an adhesion coat is rolled on to form the bond between the
wood and
the subsequent finish coats. Depending on the wood species used in the floor
and the
desired final look, the following step may be a filler station, where clear UV
filler
may be applied and an UV oven cures the filler in one or several steps. The
next steps
5 are the application of the base coats that provides the main thickness of
the
transparent surface and finally the top finish comprising one or several top
coats is
applied with roller coaters. The number of coats used depends on the
manufacturer,
but each coat is followed by a trip through a UV curing oven.
Almost all producers of high quality wood floorings use wear resistant
particles such
1 0 as aluminium oxide particles in some or several layers in order to
increase the wear
resistance of the transparent surface. Transparent anti scratch topcoats
contain small
nano-particles, which offers high levels of scratch resistance.
Recently some laminate floors have been produced with a transparent UV based
topcoat above the resin impregnated surface, for example over a melamine
formaldehyde impregnated decor paper or overlay, in order to provide a
smoother
surface that is similar to wood. Even floors with a foil surface or a PVC
surface, so
called LVT (Luxury Vinyl Tile) floors, use a transparent polyurethane coating
as a
top coat in order to improve the stain resistance and wear resistance of vinyl
floors.
Transparent protective layers applied and structured by roller are very
important parts
of the floor surface technology and it would be a major advantage if such
applications
may be made in a more cost efficient and flexible way and with improved
properties
mainly related to design and quality.
Definition of Some Terms
In the following text, the visible surface of the installed floor panel is
called 'front
side", while the opposite side of the floor panel, facing the sub floor, is
called "rear
side". By "surface layer" are meant all layers which give the panel its
decorative
properties and its wear resistance and which are applied to the core closest
to the front
side covering preferably the entire front side of the floorboard.
By "coating" is meant a layer of a substance spread above a surface for
protection or
decoration.
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By "print" is meant a decor or image. By "adhesion coating" is meant an
application of layers or primers that are used to improve the connection
between a
core material and other layers applied on the core. By "base coat" is meant
layers that
are used to build up the thickness of transparent protective layers. By "top
coat" is
meant upper layers applied on the base coat in order to provide scratch
resistance and
final gloss levels
By "up" is meant towards the front side and by "down" towards the rear side.
By "pigments for digital print ink" is meant a material that changes the
colour of
reflected or transmitted light as the result of wavelength-selective
absorption.
1 0 By "dye ink" is meant a coloured substance that has an affinity to the
substrate to
which it is being applied. The dye is generally applied in an aqueous
solution, which
also may contain a binder, and may require a mordant to improve the fastness
of the
dye on the fibre. In contrast to pigments that are small insoluble particles,
a dye is
completely soluble like sugar in water.
By "aqueous or water based ink" is meant an ink where water is used as liquid
substance in the ink. The water-based liquid carries the pigments. A binder is
present
in the system as well to bind the pigments to the substrate.
By "solvent based ink" is meant ink that generally contains three major parts
such as
a fluid carrier, pigments and resins. Technically, solvent ink refers
generally only to
the oil-based carrier portion of the ink that keeps the other components in
liquid form
and once applied to a surface through jetting evaporates.
By "UV curable Inks or coating" is meant an ink or coating that after
application is
cured by exposure to strong UV-light in an UV oven.
Known Technique and Problems Thereof
A common feature for all such floors as described above, that comprise a
transparent
protective layer above the decorative layer, is that such transparent layers
are applied
by rollers. Problems related to application of transparent layers by roller
will be
described below with reference to wood flooring.
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Figure la shows a roll coating of a panel 1 that is transported under rollers
and UV
ovens on a conveyor 21. Transparent layers Li, L2, L3 are applied by rollers
22a-c
and cured in several steps by UV ovens 23a-c.
Figure lb shows roll coating of a floor panel 1 with a wood surface 2 and a
mechanical locking system comprising a tongue 10 and a tongue groove 9 for
vertical
locking of adjacent edges and a strip 6 with a locking element 8 that
cooperated with
a locking groove 14 in an adjacent edge for horizontal locking of the edges.
Roll
coating is complicated when the coating is applied on a surface that is not
completely
flat. It is difficult to avoid marks in the short edges of a floor panel where
a roll 22
meets the edge 15a and leaves the edge 15b. Such problems are especially
related to
floor panels which are coated as individual panels, i.e. not as large boards
that are
divided into several individual floor panels after coating, and to individual
floor
panels which are prior to the coating of the surface provided with a
mechanical
locking system that comprise an edge that is not completely supported
vertically by a
lower edge 16. Bevels 15a,b at long and short edges are also difficult to coat
and
overflow of the coating on the edges may change the locking geometry.
Individual
floor panels may be coated prior to the forming of the locking system and/or
the
bevels. This method gives very sharp edges that may comprise small cracks and
the
bevels must be coated in a separate operation where overflow of the coating
material
2 0 may create undesired visual edge effects.
The coating lines with roller stations and UV curing ovens are generally very
long
and may be 50-100 m. The rubber rollers have a limited lifetime and must be
cleaned
when new coating layers are applied. Structured rollers are expansive and only
a pre-
determined embossed pattern may be formed that is not possible to change
during
production unless the structured roller is replaced with another embossed
roller. It
would be a major advantage if the coating may be applied with improved
precision
and flexibility and with a non-contact method.
The general technology, which is used by the industry to provide a digital
printing, is
described below. The methods described below may be used separately or in
combinations to create a digital print or a digital application of a substance
in the
embodiments of this disclosure.
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High definition digital printers use a non-impact printing processes. The
printer has
print heads that "fire" drops of ink from the print heads to the substrate in
a very
precise manner.
Multipass printing, also called scanning printing, is a printing method where
the
printer head moves transverse above the substrate many time to generate an
image.
Such printers are slow but one small print head can generate a bigger image.
Industrial printers are generally based on a Single Pass printing method,
which uses
fixed printer heads, with a width that corresponds to the width of the printed
media.
The printed substrate moves under the heads. Such printers have a high
capacity and
1 0 they are equipped with fixed print heads that are aligned one after
each other in the
feeding direction. Each head prints one colour. Such printers may be custom
made
for each application.
Figure lc shows a single pass printer 35 comprising five digital print heads
30a-e,
which are connected with ink pipes 32 to ink containers 31 that are filled
with ink of
different colours. The ink heads are connected with digital data cables 33 to
a digital
control unit 34 that controls the application of the ink drops and the speed
of the
conveyor 21 that must be able to displace the panel under the print heads with
high
precision in order to guarantee a high quality image comprising several
colours. The
ink is generally circulated through the print head, the ink pipes and the ink
container
in order to avoid clogging of the nozzles.
A normal width of an industrial print head is about 6 cm and any lengths may
be
printed. Wide areas of 1-2 m may be printed with digital printers comprising
several
rows of print heads aligned side by side.
Recently print heads have been developed that may apply 5 different colours
and that
may be about 20 ¨ 30 cm wide. Digital printers are able to print a high-
resolution
image with a speed exceeding 60 m/min. Major improvements of speed,
resolution,
investment costs and ink costs are made continuously.
DPI is used to define the printing quality of a digital printer. It describes
the
resolution in number of dots per inch in the digital print.
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A relatively low resolution is typically below 100 DPI. This allows a high
printing
speed and a low ink content. A resolution of 200 DPI is generally sufficient
to
provide prints that may be used in flooring applications. 300 DPI is generally
sufficient to for example print wood grains structures of the same quality
presently
used in conventional laminate floorings.
Industrial printers can print patterns with a resolution of 300 ¨ 1000 DPI and
even
more.
The ink may be a "spot colour" ink. The ink has in such an embodiment been
specifically produced and adapted to a specific colour suitable for a specific
image.
1 0 Instead of creating the colour by mixing pixels of CMYK-colours the ink
creating the
spot colour pixel has a specific predetermined colour.
The print may be a "full print." This means that the visual printed decor is
mainly
created by the ink pixels applied into powder or on a paper surface. The
colour of a
powder layer or a base colour has in such an embodiment in general a limited
effect
on the visible pattern or decor.
The print may also be a "part print". The colour of another underlying layer
is one of
the colours that are visual in the final decor. The area covered by printed
pixels and
the amount of ink that is used may be reduced and cost savings may be obtained
due
to lower use of ink and increased printing capacity compared to a full print
design.
The print may be based on the CMYK colour principle. This is a 4-colour setup
comprising cyan, magenta, yellow and black. Mixing these together will give a
colour
space/gamut, which is relatively small. To increase specific colour or the
total gamut
spot colours may be added. A spot colour may be any colour. Additional colours
such
as orange, green, blue, red, light grey, and light colours of magenta and cyan
or white
may be used. These colours may be used separately or in combinations. The
colours
are mixed and controlled by a combination of software and hardware (print
engine/print heads).
New technology has been developed by Valinge Innovation AB that makes it
possible
to inject a digital print into a powder layer. A high definition print in a
powder layer
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requires a specific software program in order to control the printing hardware
and to
adopt it to the specific ink, printing method, distance from the printing head
to the
powder layer and the powder that is used. This software program is hereafter
called
"Digital Powder Raster Image Process" or DP-RIP and it is used to control
printing
5 speed, ink drop position, ink drop size and shape.
DP-RIP:s can be developed that allow that a print or decor may be positioned
in
several dimensions in for example powder based surfaces horizontally and
vertically
in different depths. This may be used to create 3D effects when transparent
fibres are
used and to increase the wear resistance. A specific advantage is that the
print is
10 extending from the upper part of the surface layer and downward. This
allows that a
part of the print will always be at the surface even when a part of the
surface is worn
down. No protective layers are needed that disturbs the original design.
This new type of "Digital Injection Print" or DIP is obtained due to the fact
that
printing is made into a powder that is cured after printing. The print is
embedded into
the cured layer and is not applied on a layer as when conventional printing
methods
are used.
The DIP method may be used in all powder based materials, for example moulding
compounds and similar plastic materials, which may be cured after printing.
However, the DIP method is especially suitable to be used when the powder
2 0 comprises a mix of wood fibres, and, optionally, for example, small
hard wear
resistant particles and a binder such as melamine formaldehyde resin. The
surface
layer may also comprise thermoplastic material, for example vinyl particles,
which
are applied in powder form on a substrate. This allows that the print may be
injected
in the vinyl powder particles. An improved design and increased wear
resistance may
be reached even in such materials.
A suitable printer head has to be used in order to obtain a high printing
quality and
speed in powder based layers and other layers as described above. A printer
head has
several small nozzles that can shoot droplets of inks in a controlled way
(Drop On
Demand ¨ DOD). The size of each droplet may vary, dependent on ink type and
head
type, between normally 1-100 picolitres. It is possible to design print heads
that may
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fire bigger drops up to 200 picolitres more. Some printer heads can shoot
different
droplet sizes and they are able to print a grey scale. Other heads can only
shoot one
fixed droplet size.
Different technologies may be used to shoot the drops out of the nozzle.
Thermal printer head technology use print cartridges with a series of tiny
chambers
each containing a heater, all of which are constructed by photolithography. To
eject a
droplet from each chamber, a pulse of current is passed through the heating
element
causing a rapid vaporisation of the ink in the chamber to form a bubble, which
causes
a large pressure increase, propelling a droplet of ink out through the nozzle
to the
1 0 substrate. Most consumer inkjet printers, from companies including
Canon, Hewlett-
Packard, and Lexmark use thermal printer heads.
Most commercial and industrial inkjet printer heads and some consumer printers
such
as those produced by Epson, use the Piezoelectric printer head technology. A
piezoelectric material in an ink-filled chamber behind each nozzle is used
instead of a
heating element. When a voltage is applied, the piezoelectric material changes
shape,
which generates a pressure pulse in the fluid forcing a droplet of ink from
the nozzle.
Piezoelectric (also called Piezo) inkjet allows a wider variety of inks than
thermal
inkjet as there is no requirement for a volatile component, and no issue with
kogation.
A lot of ink types may be used such as dye inks, solvent based inks, latex
inks or UV
curable inks.
Inks for flooring applications are generally individually mixed together by
using UV
stable colour pigments and several chemicals. Water based inks comprising
colour
pigments are especially suitable and may provide a cost efficient printing
method
with high quality in many different materials.
The above description of various known aspects is the applicants'
characterization of
such, and is not an admission that any of the above description is prior art.
Several of
the technologies described above are known and used individually but not in
all
combinations and ways as described above.
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The majority of conventional digitally printed floorings, except floors where
injection
print in powder is used, must have a transparent protective layer that is
applied by
rollers. The digital application technology is mainly used to obtain
advantages related
to the possibility to create a high-resolution image in a flexible way.
However, the
other aspects of the technology, mainly related to the possibility to apply a
substance
very precisely with a digital non-impact method, have not been fully utilized
or
developed, especially not in combination with substances that are not intended
to
create an image and that are intended to be applied in substantial quantities
to protect
an image, decor or surface structure such that they may be used for flooring
1 0 application or other applications where similar properties are
required.
It would also be an advantage if digital prints and digital embossing may be
combined
with decorative properties and structures provided by the basic decors and
structures
of a panel, for example a panel with a wood surface that may have a decor and
structure that may be improved by a partial print and/or embossing. This may
provide
a higher surface quality in a cost efficient way.
EP 2 108 524 (Flooring Industries Ltd) describes a method to produce coated
panels
with a surface comprising protrusions formed by two or more digital prints
provided
on top of each other.
WO 2011/064075 A2 (Theodor Hymmen Holding Gmbh) describes a method for
2 0 generating a three-dimensional structure on a surface. A panel with a
printed
decorative surface is coated with a radiation-curing transparent varnish that
is applied
with a digital ink jet method with the aim to create a structured layer. The
structure is
formed by using different drop sizes and several print layers. The structure
may be
synchronized with the printed decorative pattern and different gloss levels
may be
formed by the transparent ink. The printed decorative surface may be a paper,
a foil
or a coated board. A digital print may be applied on the digitally created
structure and
the print may be covered by a transparent protective layer applied by
conventional
rollers.
US 2011/0171412 (Dohring) describes a direct digital print applied on a HDF
board.
The optical characteristics of the board are at least partly visible and
incorporated in
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the design of the decor layer. The decor is roller coated with a transparent
protective
layer.
JP2008-265229 describes a method coating and providing a base material with a
pattern. The base material, such as timber, a resin material or a film
material, is
coated with a flexible resin layer. A decorative pattern is formed by a hard
protective
layer, which is applied by inkjet coating on the flexible resin layer.
These methods are not suitable to be used in for example wood floorings where
the
protective layers are applied on individual panels and they cannot be used to
provide
a digital print or a digital embossing that forms a part of the final decor or
surface
1 0 structure and that has to be adapted to basic designs and structures of
each individual
panel.
Objects and Summary
A general objective of certain embodiments of the invention is to provide a
building
panel, preferably a floor panel, and a method to produces such panels with a
transparent protective surface, which has better and/or different design
properties
and/or cost structure than the known building panels.
A specific objective of at least certain embodiments is to improve the surface
quality
and design of wood building panels, especially wood floor panels, that
generally are
coated as individual panels, where each panel comprises considerable design
2 0 variations and surface defects.
Another specific objective of at least certain embodiments is to provide an
improved
method for coating individual wood building panels such as wood floor panels,
especially individual panels already provided with a mechanical locking system
at
their edges and/or bevels.
The above objectives are exemplary, and the embodiments of the invention may
accomplish different or additional objectives.
A first aspect of the invention is a method of coating a building panel having
a
decorative surface of wood, comprising the steps of:
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= applying a UV curable coating layer with a digital print head on a
decorative
surface of wood of a building panel; and
= curing the UV curable coating layer with UV light, thereby forming a
transparent protective surface layer, wherein said decorative surface is
visible
through said transparent protective surface layer.
The building panel may be a floor panel.
The digital coating is a non-contact method that provide major advantages
compared
to the conventional roller coating method.
The building panel such as floor panel may be an individual panel having a
size,
1 0 which is essentially the same as the final building panel comprising
machined edges.
The building panel such as a floor panel may comprise a mechanical locking
system
at two opposite edges.
The building panel may comprise a bevel at an edge.
The UV curable coating layer may a liquid polyurethane substance.
The UV curable coating layer may be water based UV cured polyurethane.
The curable coating layer may a radiation curable coating layer. The radiation
curable
coating layer may be cured by UV light, heat radiation, electron beam etc.
The radiation curable coating layer such as a UV curable coating layer may
comprise
an acrylate or methacrylate monomer or acrylate or methacrylate oligomer. The
2 0 radiation curable coating may comprise an acrylic resin. The acrylate
or methacrylate
monomer or acrylate or methacrylate oligomer may be an epoxy acrylate, an
epoxy
methacrylate, an urethane acrylate, an urethane methacrylate, a polyester
acrylate, a
polyester methacrylate, a polyether acrylate, a polyether methacrylate, an
acrylic
acrylate, an acrylic methacrylate, a silicone acrylate, a silicone
methacrylate, a
melamine acrylate, a melamine methacrylate, or a combination thereof. The
above
examples are examples of monomer or oligomers polymerised by radical reaction.
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The above monomers or oligomers may form a component of a radiation curable
coating layer.
The decorative surface may comprise a print, preferably a digital print.
The digital print head may be a Piezo print head.
5 The digital print head may be designed to apply drops, preferably with a
size of about
60 -200 picolitres.
The UV curable coating layer may comprise wear and/or scratch resistant
particles.
The UV curable coating layer may comprise a structured surface with cavities
and
protrusions.
1 0 The structured surface may be in register with the decorative surface.
A second aspect of the invention is a floor panel having a core, a surface
layer
comprising a wood material, a print and transparent layers. A lower
transparent layer
may be located below the print, and an upper transparent layer maybe located
above
the print, the lower transparent layer may be UV cured polyurethane, a part of
the
15 wood material surface and the print form a part of the visible surface
layer. The print
may be at least partly synchronized with the visible design and/or structure
of an
individual floor panel.
Such wood floors may have a very attractive design even in the case that low
quality
wood is used as the core. The print which is applied with a digital print head
may be
adapted to each individual floor panels and the print may be used to form an
image
and/or an embossing that improves the basic design or structure of the floor
panel.
The upper transparent layer may comprise water based polyurethane.
The upper transparent top layer may be embossed.
The upper transparent layer may be embossed in register with the print.
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A third aspect of the invention is a method of forming a decor on a building
panel
with a digital vision control system that provides digital input to a digital
print head,
comprising the steps of:
= creating a digital image of a surface of a building panel by the digital
vision
control system;
= using the digital vision control system to provide digital input to the
digital
print head based on said digital image;
= digitally printing at least a part of said surface of the building panel
with the
digital print head and with a print that is at least partly adapted to the
digital
image of said surface of the building panel.
The major advantage is that a digital vision system may be used to analyse
each
individual panel and a computer program may guide print heads that apply a
print on
surface portions that need an improved design or structure.
The building panel may be a floor panel.
The print may comprise colour pigments.
The surface of the panel may comprise a transparent substance that is UV cured
and
that after curing forms an embossed structure.
The building panel and the production method according to embodiments of the
invention make it possible to produce very advanced decorative patterns with
high
2 0 wear and impact resistance in a cost effective way with the digital non-
impact coating
method. Even randomly formed in register embossed structured surfaces may be
formed with the digital coating method.
Embodiments and details of various aspects may be combined with embodiments
and
details of the other aspects.
A fourth aspect of the invention is a method of coating a building panel is
provided,
comprising the steps of:
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= forming a transparent protective surface layer by applying a UV curing
coating layer with a digital coating head on a decorative surface of a
building
panel;
= curing the coating layer with UV.
The building panel may be a floor panel. The decorative surface may be wood.
Brief Description of the Drawings
The invention will in the following be described in connection to embodiments
and in
greater detail with reference to the appended exemplary drawings, wherein,
Figs la-c illustrate known roll coating and digital printing method;
1 0 Figs 2a-b illustrate a digital coating method;
Figs 3a-c illustrate digital injection printing in transparent
layers;
Figs 4a-c illustrate digital printing and embossing.
Detailed Description of Embodiments
Figure 2a shows a floor panel 1 comprising a core 3 of solid wood and a
decorative
surface 5 that is the upper part of the solid wood core. The floor panel 1 may
comprise bevels 15a, 15b at edges of the floor panel 1. Several transparent
layers of
adhesion coat Ll, base coat L2 and a top coat L3 are applied on the wood
surface.
The adhesion coat and the base coat are applied by rollers and UV cured. The
topcoat
is in this embodiment applied digitally.
The main principles of a digital coating method and equipment are shown in
figure
2b. A digital coating unit 36 comprising a digital print head 30 used as a
digital
coating head. The digital print head 30 is preferably fixed. The digital print
head 30
applies a transparent topcoat L3, preferably on a base coat L2. The digital
application
may be made without any contact between the panel and the digital print head
30, and
the coating is applied as drops, which are fired from the digital print head
30 towards
the decorative surface that may comprise an adhesive coat Ll under the base
coat.
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A UV curing oven 23 with ultra violet light is located preferably after the
digital
coating unit 36 in the feeding direction and may provide a practically instant
curing
within a few seconds of the coating, especially if a UV cured polyurethane
coating
with an appropriate photo polymerization initiator is used. The digital print
head 30,
that preferably is a piezo head, has preferably a capacity to fire drops with
a drop size
of about 50 - 200 picolitres or more. The drops may have a diameter of 30 ¨
100
microns or more and may create a drop spot on the surface that exceeds 100
microns.
The drops are preferably positioned such that they overlap each other. The
overlapping may be obtained by a combination of drop size and drop position.
Several print heads located after each other in the feeding direction may also
be used
to create a continuous layer even when individual drops are applied in a
raster pattern.
The UV curing coating is preferably a water based UV curable polyurethane
substance with a viscosity that is adapted to the digital print head 30. Water-
based
polyurethane dispersions are preferred as coating used in the digital print
head. They
may be environmental friendly and technically superior to similar solvent-
based
coatings. They may be, for example, free of isocyanate and may have a zero or
a very
low volatile organic content. They have good properties related to hardness,
stain and
2 0 abrasion resistance, impact strength and temperature flexibility.
Polyurethane dispersions are fully reacted polyurethane/ polyureas of small
and
discrete polymer particles and such particles may be produced with a size of
about
0.01-5.0 microns and may therefore be handled in a digital Piezo print head or
other
similar heads. They may have 20 -70% solid content and a wide range of layers
with
different hardness may be produced with a digital coating method. Polyurethane
dispersions may be blended with for example acrylic emulsions in order to
reduce
costs in some applications. They may also comprise small wear and scratch
resistant
particles, for example aluminium oxide, that may be handled by the Piezo head.
Such
particles should be rather small since the nozzle opening in most digital
print heads is
about 10 microns. It is preferred that the wear resistant particles are
smaller than 5
microns and it is even more preferable that the wear resistant particles are
about 1
micron or smaller.
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Wear resistant particles may preferably be applied by scattering the particles
in dry
powder form on, for example, the base coat that preferably is in a liquid
state and not
yet cured. A second base coat or a top coat may be applied on the wear
resistant
particle layer. The advantage is that large particles that may clog the
nozzles are not
applied by the digital print head. Separate scattering allows that particles
with a size
of 10 ¨ 100 microns may be applied.
The coating is stored in liquid form in a coating container 31, which is
connected to
the digital print head 30 with a coat-feeding pipe 33. A digital control unit
34
1 0 connected to the print head and the conveyor with data cables 33 or
wireless controls
the drop size and the speed of a conveyor 21 that displaces the panel 1 in
relation to
the digital print head 30.
Such a digital coating unit is much more cost efficient than a conventional
digital
printer since much larger drops may be fired and this gives an increased
capacity and
less problems with the channels in the head that may be sealed by larger
particle in
the ink when the printer works with high resolution and small drops. Each
digital
coating head formed as a digital print head may be designed to apply one layer
only
and there is no need to coordinate the drop application of such different
print heads as
in conventional multi colour digital printing where drops aligned in a raster
pattern
2 0 side by side creates an digital image.
Special digital print heads may be design that allows applications of very
large drops
in the range of 200 ¨ 400 picolitres and more and the nozzle opening may
exceed 20
microns. Such digital print heads are generally not suitable to be used as
conventional
print heads aiming to create a high-resolution image. They may be designed to
apply
large overlapping drops in considerable quantities and in a cost efficient
way. Each
print head, that mainly is used as a digital coating head, may be designed to
apply 10-
20 gr/m2 or more in a single pass coating step.
The coating line may be very compact and the UV curing oven may be located
close
to the digital coating unit. The coating may be very precise and the non-
impact
method provides much better possibilities than roll coating to apply the UV
curing
coating on the edges, on bevels 15a, 15b formed at the edges, and on surfaces
that are
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not completely flat such as for instance brushed or hand scraped wood surfaces
or
embossed laminated paper based or powder based floor. UV cured protective
layers
may be applied by a digital coating method on practically all type of floors
in order to
improve the surface properties or designs. Some parts of the surface may
therefore,
5 according to an embodiment of the invention, be formed with known methods
to
obtain the basic strengths, designs or structures and the final layers may be
applied by
digital coating. Floor with surfaces comprising paper, powder, cork, vinyl and
even
stone and tiles and similar may be digitally coated in order to improve the
surface
properties.
1 0 UV cured liquid coating offers the advantage that the coating is liquid
until it is
exposed to the UV light. This increases the productivity of the digital
coating unit and
many problems related to for example solvent inks may be avoided.
The digital coating equipment may of course have several print heads. The
digital
coating may be applied on individual panels or on a large sheet that after the
coating
15 is divided into several panels.
The above described principles may preferably be used to apply a base coat L2
and
and/or the adhesion coat Li. It is possible to apply all transparent layers
that are used
to protect a wood surface or a printed image. Digital coating may be used to
apply
transparent layers with a film thickness that corresponds to 5-10 g/m2. Even
thicker
20 layers may be produced for example 10 ¨ 20 g/m2 and a total thickness of
about 100
¨ 120 g/m2 may be reached with 5 ¨ 10 coating stations. The panel may also
pass a
coating station several times.
Figure 3a shows that a digital print P1 may be applied on a transparent layer
and such
print is characterized in that a transparent layer L2 is located under the
print and
another transparent layer L3 may be located above the print Pl. The panel
surface,
that preferably is the upper part of a wood material 5 may be visible and may
together
with the print P form a part of the final surface decor and the decorative
surface 2.
The floor panel 1 may be provided with bevels 15a, 15b at its edges. At least
the
upper transparent layer is preferably applied by digital coating. A print P2
may also
be injected into the wet transparent coating prior to the UV curing.
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Such methods where a print P1 is applied on a transparent layer or when a
print P2 is
injected into a transparent layer may be used to obtain improved design
properties
since several images may be placed on top of each other with transparent
layers
between the prints and this may provide three dimensional images similar to a
stone
surface with transparent or semi-transparent crystals. Such three dimensional
images
may be partly or completely formed by conventional roller application methods
that
may be combined with digital printing or coating.
Figure 3b shows a first production step of a combined digital printing and
digital
coating line. A digital print P1, P2 is applied on a cured layer or on a wet
transparent
layer. The print may be cured by UV 23, if the print for example is injected
into a wet
polyurethane layer.
Figure 3c shows a digital coating unit 36 that may apply a transparent base
coat or top
coat on the digital print. The coat is UV cured in a UV oven 23. A transparent
or
semi-transparent adhesion coat may be applied digitally under the print.
It is an advantage if the digital coating unit 36 is connected, wireless or
with a data
cable, to a digital printing unit 35 as shown in figure 3c such that a digital
print and a
digital coating may be controlled and adapted to specific designs that are
based on a
combination of print and coating. A vision system 39 may also be digitally
connected
and digital coating and/or printing may be adapted to specific surface
structures and
designs. A vision system may for example be used in line with the printing and
coating steps to create a digital image of a surface of a panel that
preferably
comprises a wood based surface 2. The panel may be an individual floor panel
that
may be a solid wood panel or a panel comprising a core covered by a wood layer
with
at thickness of for example 0,3 -3 mm. Defects of the surface such as extreme
deviations in colour, cracks, knots, repetitive effects etc. may be detected
and
analysed by the vision system 39, preferably by analysing the digital image of
the
surface of the panel and comparing the image with images already stored in the
memory of the vision system. The system may be "intelligent" and new images
and
adjustment instructions may improve the efficiency. Specific software may be
made
for different wood species. The visible surface of the panel may be at least
partly
digitally printed based on an analysis of the digital image such that defects
are
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covered or adjusted with a digital print or that some surface portions are
printed in
order to create a more attractive wood design, such as creating an improved
wood
grain structure. As an alternative or complement to digitally printing, a
transparent
coating may be applied based on an analysis of the digital image. For example,
the
gloss grade of the surface of the panel may be changed and an increased amount
of a
liquid substance may be applied to cover cracks or cavities detected in the
wood
surface. The surface may be exposed to a various types of lights when a
digital
camera takes a picture and this may improve the possibilities to detect
specific defects
The Vision Controlled Digital Printing (VCDP) method as described above may
also
be used in other floor than wood floors. Conventionally printed paper or foil
surfaces
comprise repetition effects from the printing cylinders. Such effects may be
partly
eliminated by the vision controlled digital printing method. The method is
preferably
combined with a production step where the printed substrate is connected to a
core
prior the final digital printing step. A major advantage is that it is not
necessary to
position the already printed substrate in a precise manner on a panel since
the exact
position and/or the specific design may be detected by the vision system 39
and the
digital printer may apply the complementary design with great accuracy. If
paper is
used, it may be advantageous if such paper is a raw paper without any resins.
The
necessary thermosetting resins may be injected from a base layer under the
substrate
2 0 and/or from a top layer applied on the printed surface. The top layer
may be a powder
layer comprising wear resistant particles and thermosetting resins or a
conventional
overlay and the resins may be cured by heat and pressure. Digital coating as
described
above may also be used as top layer.
The VCDP method may also be used to create a decor in a WFF floor with a
powder
based surface. Powder comprising one or several colours may be scattered on a
board
and a basic design may be created. A vision system and a computer system may
analyse the basic design and give necessary digital input to a digital printer
that in a
second step may adjust or improve the basic design. The advantage is that the
basic
design may provide the major part of the pigments and the amount of ink
applied by
the digital printer may be reduced considerably, preferably to a few g/m2, for
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example, 1-5 g/m2. In some applications ink content of 3 g/m2 or less may be
sufficient. This method may also be used to create a surface design on a
ceramic tile.
The VCDP method is very suitable to combine with a Binder And Powder (BAP)
printing method wherein a pattern or image may be formed digitally by an ink
head
that only applies a liquid binder on a surface without any pigments. The
binder may
be a water based substance comprising glycol that provides a suitable
viscosity. The
pigments are scattered randomly by a second device over the liquid pattern.
The
binder connects some pigments that form the same pattern as the binder while
other
non-bonded pigments are removed. This two-step process, where the pigments and
a
liquid binder are applied separately, may provide an image with the same
quality as
convectional digital printing technology and is a very cost efficient method
to form a
basic design that is adjusted or improved in a final digital printing step
comprising
pigment based ink, preferably water based inks. The powder may be a
transparent
substance, for example bleached wood fibres, and BAP may be used to create an
embossed transparent surface.
Figure 4a shows that the digital print P may be applied to, or injected in,
the top coat
L3 and/or the base coat L2. The decorative surface 5 may be a wood layer, for
example solid wood, a wood sheet or a wood veneer that may be glued to a core
3, for
example a lamella core, HDF or plywood. The digital print is used to improve
the
design of the decorative surface.
Figure 4b shows that digital coating may be used to provide a Digitally
Embossed
Surface, hereafter referred to as DES. A first top layer with a discontinuous
cross
section L3a ¨ L3d may be provided that for example imitates a wood grain
structure.
The layer is applied in the form of a transparent image having a vertical
extension.
The wet three-dimensional structure may be UV cured and preferably a second or
a
third coating with a discontinuous structured cross section may be applied on
the first
layer. The discontinuous layers may be used to build a vertically extending
structured
surface with similar or different gloss levels. Embossed structures with for
example a
depth of about 0,1 mm and more may be formed with any shapes.
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This method may be used to form advanced structures in a very flexible way and
the
embossing may be precisely coordinated with a printed surface, preferably a
digital
print as shown in figure 4c. The print P is coordinated with the structure L3a
- c and
individual parts of a digital image may be provided in well-defined surface
structures
where for example a particular grain is printed in a part of the surface that
forms
small cavities 37 and other parts of the image may be formed in the surface
that
comprises small protrusions 38. Such a Digital Embossed In Register Surface,
hereafter referred to as DERIS technology makes it possible to create
individual
panels that are unique and without any repetition effect related to design or
structures.
Since no embossed cylinders or matrix films are needed, as in conventional
technology, a wide variety of practically unlimited structured designs may be
produced in a very flexible and cost efficient way.
DERIS is preferably used together with a digitally printed image. Such image
may be
printed on a paper, foil, on a board material or injected into powder-based
surfaces as
described above. The digital printer 35 and the coating unit 36 that forms the
top
layers L3 are preferably digitally connected to a computer that coordinate the
surface
design and the surface structure.
DERIS technology may even be used to coordinate an individual structured
surface
coating with individual wood panels that have different designs or with any
other
panels where a surface design or structure is already formed. This is an
advantage
especially in application where digital printing is used to improve the design
of a
wood surface. Wood species of lower quality may be improved by digital
printing as
described above. For example a digital picture of a wood surface of a panel
may be
made by the vision control system comprising a digital camera or scanner. This
is
preferably made in line prior to the application of the transparent coating
that
provides the final digital embossed surface. The vision control system may be
used
together with a computer program that may analyse the digital picture and may
adjust
the digital coating such that it is coordinated with the wood grain structure
and design
of each individual panel.
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Such Vision Controlled Digital Embossing (VCDE) methods are not used in
flooring
applications. VCDP and VCDE may be combined and very cost efficient advanced
decors and structures may be formed.
VCDE is especially suitable to form embossed structures on panels where parts
of the
5 decor are not formed digitally in line in a prior printing step. Even
when this is the
case, production efficiency and quality may be improved since there is no need
to use
high quality transportation unites that moves the panel with high accuracy
between
different printing, coating and application steps that all must be coordinate
in order to
give a high final surface quality. The vision system that preferably also may
be
1 0 combined with a position system between different production steps may
be very cost
efficient compared to known methods.
All described embodiments may be combined and all transparent layers may
comprise colour pigments.
The digital coating method may also be used to apply other chemicals on a
panel, for
15 example glue, preferably a water based glue.
The method to apply a UV cured polyurethane digitally in order to obtain 3D
structures may also be used for other application and not only building panels
as
described above. A digital printer and an UV oven may be used to apply and
cure
many layers of various types of UV cured polymers vertically in order to
create
20 complicated structures and models with a vertical extension of 1-10 mm
and even
more. A first layer may be applied on a substrate that is moved in an UV oven
and
back again thereafter a new layer is applied. Two digital Piezo heads may for
example also be positioned with a UV oven in between and the substrate may
move
from a first print head through the UV oven and into the second print head and
back
25 again. This sequence may be repeated many times and each layer may be
0.1 ¨ 0.3
mm thick or even more depending on the viscosity of the liquid polymer that
for
example may be polyurethane. In general no additional substances are needed to
cure
the polyurethane and the two printer heads may use the same type of UV cured
liquid
polyurethane. The two printers may also use different substances, which may be
used
to decrease the curing time. This production method may be combined with a
visions
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system and partial 3D structures may be formed on specific well-defined
portions of
various objects that may comprise various materials. 3D structures may have
different
colours injected digitally into the UV cured polyurethane.
Such digital 3D methods may be used to provide a matrix that may be used as a
sheet
that provides an embossed structure when laminate and powder based floors are
pressed in a press in order to cure the surface.
It is contemplated that there are numerous modifications of the embodiments
described herein, which are still within the scope of the invention as defined
by the
appended claims.
1 0 It is for example contemplated that curing of the curable coating layer
with UV may
be replaced by another radiation curing method. It is also contemplated that
the
present invention may be used for other types of radiation curing coating
layers.
As apparent from the description, the coating layer may be applied with the
digital
print head directly on the surface of the building panel forming the
decorative layer,
and may be applied on an intermediate layer arranged on the surface of the
building
panel.
In the above disclosure, it is contemplated that by transparent is meant
essentially the
same transparency as commonly used in coatings for conventional wood floors.
The
transparent protective surface layer is transparent to such an extent that the
underlying
decorative surface of wood is visible through the transparent protective
surface layer.
EXAMPLES
EXAMPLE 1:
A digital image was applied on a panel comprising a HDF board. The image was
created with a single pass printer comprising 5 fixed Piezo print heads. The
ink was a
water-based ink comprising colour pigments.
A piezo print head with a drop size of 50 picolitres was used to apply a
transparent
layer of water based UV cured polyurethane that corresponds to a film of 10
g/m2.
The above material was cured in a UV oven and a digital image with a digitally
coated transparent layer was obtained.
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EXAMPLE 2:
A digital image with a wood design was applied on a panel comprising a HDF
board.
The image was created with a single pass printer comprising 5 fixed Piezo
print
heads. The ink used was a water-based ink comprising colour pigments.
A piezo print head with a drop size of 50 picolitres was used to apply a
transparent
layer of water based UV cured polyurethane that corresponds to a film of 10
g/m2.
The above material was cured in an UV oven.
1 0 A second similar transparent layer with a weight of 5g/m2 was applied
with a piezo
print head with a drop size of 50 picolitres. The layer was applied as a
transparent
image coordinated with the digital image.
The above material was cured in an UV oven and a wood grain design with a wood
grain structure in register with the wood grain design was obtained.
Digital coating may also be used to seal the edges and/or the locking system
9, 10
against moisture, to eliminate squeaking sound, or to change friction
properties of
active surfaces in the locking system. The major advantage is that a liquid
substance
may be applied with high precision and unwanted over spraying on for example
the
surface may be avoided. A print head is generally used to apply a liquid
substance
vertical with a distance to the surface of a few mm. Print heads may be used
to apply
liquid substances from a distance of up to 10 mm and more and the application
may
be made in various angles against the surface for example 0-10, 10-20, 20-45
degrees
or even more then 45 degrees from above or from below. This allows that
locking
systems with advanced geometries may be coated with combination of several
print
heads positioned in several angles. The liquid substance may be liquid wax
that after
cooling to room temperatures becomes a soft layer.
