Abstract:
The invention discloses a process for printing of panel-shaped workpieces, in particular in the region of an edge, in which a section of a panel-shaped workpiece to be printed is subjected first of all to one or a plurality of pre-treatment stages which are selected from a) pre-cleaning, b) degreasing, c) improvement of the adhesion and wetting properties and d) reduction of the electrostatic charge, and in which the pre-treated section to be printed is subsequently printed with a desired pattern, in particular by means of inkjet printing. The invention also provides an apparatus for performing said process.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a process and an apparatus for the printing of panel-shaped workpieces in which a section of the panel-shaped workpiece is printed with a desired pattern, in particular by means of inkjet printing. 
       PRIOR ART 
       [0002]    After being cut to length and having passed through the sizing or shaping process, panel-shaped workpieces, which are used, for example, as construction elements for furniture, are often provided on a narrow side with suitable materials such as edges, webs or foils made from technical polymers (ABS, PP, PVC) or wood veneer strips. This measure serves, for example, to achieve a deliberate appearance or haptic or to act as protection against mechanical stress or to prevent ingress of moisture. 
         [0003]    The panel-shaped workpieces considered here may be chipboard panels, plywood panels, fiberboard panels and other wooden material boards or, for example, gypsum plasterboard and gypsum fiber board or, for example, laminated panels and rigid foam panels or wood wool composite panels or composite boards. The narrow sides of panel-shaped workpieces considered here are the narrow sides of panel-shaped workpieces sealed with an appropriate material for the purpose, executed for example as plastic web, section or foil or as wooden strips or veneers or other natural materials or as a surface sealed with lacquer, UV varnish or sealing medium. 
         [0004]    A process and an apparatus for printing of such panel-shaped workpieces by means of inkjet printing is disclosed, for example, in DE 100 31 030 A1. However, it appears that the known printing process and the known printing apparatus lead to a limited quality and durability of the printed image applied. 
       PRESENTATION OF THE INVENTION 
       [0005]    The object of the present invention is, therefore, to provide a process and an apparatus for printing of panel-shaped workpieces, in particular by means of inkjet printing, which facilitates enhanced quality and durability of the printed image applied to the panel-shaped workpiece. 
         [0006]    This object is achieved according to the invention by a process according to claim  1  and an apparatus according to claim  9 . Particularly advantageous developments of the invention are stated in the dependent claims. 
         [0007]    The invention is based on the knowledge that, in panel-shaped workpieces of the type under discussion here, the machining process for sealing the narrow side or other sections of surface leads to a distinctive level of contamination on the surface which impairs the quality and durability of the printed image subsequently applied. Usually, dust particles, lubricants, adhesive residues, splinters, sweat from hands, finger marks, scratches and other types of dirt or damage are applied to the surface during and as a result of the manufacturing process when sealing the panel-shaped workpieces. In addition, the technical polymers which are used here, for example, are water-resistant in their surface properties and can thus only be wetted with difficulty. 
         [0008]    Against this background, it is provided in the process according to the invention that a section of a panel-shaped workpiece to be printed is subjected first of all to one or more pre-treatment stages which are selected from a) pre-cleaning, b) degreasing, c) improvement of the adhesion and wetting properties and d) reduction of the electrostatic charge. It is possible in this manner, using the process according to the invention, to create printed images with outstanding quality on the panel-shaped workpieces, said images having excellent adhesion on the printed section and thus having very good durability. 
         [0009]    In this regard, according to a development of the present invention, it is provided that the pre-treatment stages are chosen selectively depending on the material and/or the surface condition of the section to be printed. As a result, it is not always necessary to perform all the pre-treatment stages referred to since it may be sufficient to perform just one or some pre-treatment stages matched to the material or the surface condition in each case without having to make sacrifices in terms of the quality or durability of the printed image obtained. Although this selection may be made manually, by an operator for example, it is preferable within the scope of the invention for the selective choice to be controlled electronically, by means of an appropriate control device for example. 
         [0010]    In principle, the process according to the invention may be carried out on a workpiece disposed in a stationary position, in a processing centre for example, which is equipped to perform the various pre-treatment stages. With respect to a speedy process sequence with high throughput, however, it is provided according to a development of the invention that the pre-treatment stages and/or printing will be performed in through-feed operation. 
         [0011]    The relevant pre-treatment stages may be performed in the most varied manner within the scope of the present invention, whereby particularly advantageous developments of these pre-treatment stages are the subject-matter of dependent claims  5  to  8 . 
         [0012]    An apparatus according to the invention for performing the process according to the invention is the subject-matter of independent claim  9 . This apparatus is characterized in that it has one or more pre-treatment modules which are selected from a pre-cleaning module, a degreasing module, a module for improving the adhesion and wetting properties and a module for reducing the electrostatic charge. As a result of this it is possible to achieve, as in the process according to the invention, a printed image on the panel-shaped workpiece which has excellent quality and a high level of durability. 
         [0013]    According to a development of the apparatus according to the invention, it is provided that the pre-treatment modules may be operated selectively depending on the material and/or the surface condition of the section to be printed. This renders it possible to match operation of the apparatus according to the invention selectively to the material and/or the surface condition of the section to be printed so that the outcome is an optimum pre-cleaning result for each panel-shaped workpiece and thus also an optimum printed image. 
         [0014]    According to a development of the invention, the pre-treatment modules and preferably also the printing device and/or the handling device are provided in conjunction with a control device. This renders it possible for the apparatus according to the invention to be fully automated and thus to work quickly and efficiently without having to make sacrifices in terms of the quality or durability of the printed image obtained. 
         [0015]    The apparatus according to the invention may, for example, be what is known as a stationary machine (a processing center for example) or what is known as a through-feed machine. In the former case, the workpieces to be printed are stationary and the handling device moves the relevant pre-treatment modules or the printing device in relation to the workpieces to be printed. In the latter case, the pre-treatment modules and the printing device are stationary and the workpieces to be printed are moved past these pre-treatment modules or the printing device by means of the handling device. Within the scope of the present invention, however, combinations of both concepts are also possible, for example pre-treatment modules working in through-feed operation and a printing device working as a stationary machine. Particularly fast and efficient operation of the apparatus may, however, be achieved if the handling device is a through-feed handling device according to a development of the present invention, which transports the panel-shaped workpieces to be printed past or through the pre-treatment modules and/or the printing device. 
         [0016]    Furthermore, the apparatus according to the invention may be constructed of a plurality of different or separate machine units, for example by providing a separate machine unit in each case for each pre-treatment module and for the printing device, whereby these units may also be spatially separate. According to a development of the present invention, however, it is preferable for a plurality, preferably all, of the pre-treatment modules to be provided in one machine layout since this simplifies operation of the apparatus according to the invention and improves the pre-cleaning result. Moreover, it is particularly preferable that the printing device and/or the handling device, if necessary, are also provided in this machine layout. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
         [0017]    The FIGURE shows a schematic lateral view of a preferred embodiment of the apparatus according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    Preferred embodiments of the present invention will be described in detail in the following with reference to the accompanying FIGURE. 
         [0019]    The FIGURE shows a schematic lateral view of a pre-treatment and printing apparatus  1  as a preferred embodiment of the present invention. Apparatus  1  serves for the printing of panel-shaped workpieces  2 , which are made at least in part of wood, wood materials, wood replacement materials, plastics, lightweight construction materials or the like, such as are frequently used, for example, in furniture manufacture. 
         [0020]    As can be seen in the FIGURE, apparatus  1  in the present embodiment comprises a plurality of pre-treatment modules  10 ,  20 ,  30 ,  40  or submodules, which will be addressed in greater detail below. The present invention is not, however, restricted to the layout shown. In fact, an apparatus according to the invention may also have only one or a plurality of pre-treatment modules, for example if the apparatus is only designed for a specific type of panel-shaped workpieces to be printed. The order of the individual pre-treatment modules or submodules is also not restricted to the layout shown in  FIG. 1 , although in this case the disposal shown is a preferred layout or sequence. 
         [0021]    In addition, apparatus  1  comprises a printing device  50 , which in this case in particular is preferably an inkjet printing device. An inkjet printing device is understood to be an ink printing device in which ink droplets are ejected according to the drop-on-demand process, i.e. the printing device ejects a desired sequence of ink droplets in response to a print command. Print heads, which have piezoelectric elements or which work thermally (bubble jet), are frequently used in such printing devices. It should, however, be noted that other appropriate printing devices, such as laser printing devices, thermal printing devices or the like, may also be used within the scope of the present invention. 
         [0022]    The apparatus according to the invention additionally comprises a handling device  60 , which in the present embodiment is designed as a linear conveyor, for example a chain or belt conveyor. Panel-shaped workpieces  2  may be transported past or through pre-treatment modules  10 ,  20 ,  30 ,  40  and the corresponding submodules and printing device  50  on this handling device  60 . In this regard, a section  4  of panel-shaped workpiece  2  to be printed and pre-treated faces pre-treatment modules  10 ,  20 ,  30 ,  40  or printing device  50 , i.e. section  4  is disposed in  FIG. 1  on the side facing away from the observer. 
         [0023]    Pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules will now be explained in greater detail in accordance with the present embodiment. 
         [0024]    Apparatus  1  comprises first of all a pre-cleaning module  10 , which in the present embodiment has three submodules, that is to say a brushing module  11 , a grinding module  12  and a CO 2 -snow blast cleaning module  13 . The brushing module may function mechanico-chemically in that it has, for example, filament fiber brushes coated with diamond dust to which an anti-static and cleaning agent is added during operation. In this case, brushing simultaneously achieves a grinding, cleaning and anti-static treatment. This brushing module  11  is particularly well suited to hard surfaces or to surfaces where an increase in roughness is desired. These include, for example, wood veneers or lacquered surfaces. A further cleaning device (brush for example) may be provided downstream of brushing module  11  and grinding module  12  to ensure that abraded dirt is removed, although this is not shown in the FIGURE. 
         [0025]    Cleaning of the surface by means of CO 2 -snow blast cleaning module  13  is advisable in particular if greases, oils or other organic substances are to be found on surface  4  to be printed. Cleaning using CO 2 -snow blasting is based on a plurality of physical and chemical effects whereby their thermal, chemical and mechanical action is utilized. The process converts liquid CO 2 , which is provided, for example, in CO 2  cylinders, into compressed, solid CO 2  snow particles by means of thermodynamic and physical processes. These CO 2  snow particles are produced in a specific size with the help of an appropriate process and nozzle technology. At the same time, they are added to a compressed air flow in a multi-stage mixing chamber with the aid of a jet nozzle. In this regard, a homogenous free jet is generated with which surfaces may be cleaned and pre-treated. 
         [0026]    The thermal effect of CO 2 -snow blasting brings about shock freezing of the surface contamination. The use of CO 2 -snow blasting leads to embrittlement of the contamination due to the difference between the coefficients of thermal expansion of the contamination and the surface. As a result it is easily removed from the surface. In addition, the use of compressed air as a transport gas and the design of the blasting nozzle lead to mechanical impacts which remove the embrittled contaminations from the surface. The sublimation expansion of the CO 2  also contributes to this which contributes to detachment and removal of the contamination from the surface. The chemical effect of CO 2 -snow blasting also consists of processes, which detach the contamination from the surface, that are brought about by the carbon dioxide which becomes gaseous following the sublimation process. This has a further positive effect on detachment of the surface contaminations and assists with removal of the contamination. 
         [0027]    CO 2 -snow blasting modules and devices are available, for example, from Cryosnow GmbH, Zitadellenweg 20 E, D-13599 Berlin under the designation CS-4. 
         [0028]    With CO 2 -snow blasting it is also no problem to remove the loosened contaminations and the process gas by means of an appropriate extraction system (not shown in the FIGURE). Overall, CO 2 -snow blasting has the advantage of bringing about non-contact and thus non-abrasive cleaning. Thus CO 2 -snow blasting is particularly suitable for plastic-based surfaces for example. In addition, CO 2 -snow blasting is particularly well-suited to surfaces contaminated with greases, oils or other organic substances as it is possible to extract these contaminations, which shock freeze and become brittle due to the low temperatures of the CO 2 , and remove them by means of an extraction system (not shown). 
         [0029]    As can be seen in the FIGURE, apparatus  1  also comprises a degreasing module  20 . Degreasing module  20  is represented in the present embodiment by an electro-mechanical wiper module which is set up to apply a degreasing agent to the section to be printed  4  and to wipe it off. Acetone, methyl ethyl ketone (MEK), isopropanol, ethanol or the like, for example, may be used as cleaning and degreasing agents. Attention must be paid to compatibility with the surface to be degreased when selecting the agent; the agent used should not attack the surface. The wiper module comprises a wiping means, for example in the form of a suitable non-woven fabric, cloth or other medium which soaks up liquids, which is appropriately moistened by means of a spray nozzle. The wiping means is capable of being brought into direct contact with surface  4  in order to bring about the mechanico-chemical degreasing process. 
         [0030]    The use of degreasing module  20  and its wiper module is particularly suitable, for example, for plastic-based surfaces such as plastic edges or plastic foils. 
         [0031]    Apparatus  1  also comprises a module  30  for improvement of the adhesion and wetting properties, which in the present embodiment has a plurality of submodules, that is to say a grinding and roughening module  31 , a priming module  32 , a corona treatment module  33 , a plasma treatment module  34 , a flame treatment module  35  and an adhesion promotion module  36 . 
         [0032]    Grinding and roughening module  31  is designed as a conventional mechanical grinding apparatus which has, for example, a circular or rectangular grinding means or a grinding means designed as a belt product. This grinding apparatus may be disposed, together with a downstream cleaning unit (not shown), under an extractor hood (not shown) to pick up and discharge the contamination occurring during grinding. 
         [0033]    Grinding and roughening is preferably used, for example, on narrow sides to which natural materials such as wood veneer or cork are bonded or which have been provided with lacquers, UV varnishes, paints or sealing agents. Roughening of such surfaces facilitates enhanced meshing with additional layers to be applied on top. 
         [0034]    Priming module  32  may, for example, have a spray or roller application device, in order, for example, to apply a solvent- or water-based primer, which contains, for example, pore filler as the chemically active component. Such pore filler is taken up by the surface by means of adsorption and reduces the surface&#39;s absorbency which may be expedient, in particular in natural materials. In addition, the pore filler forms a sealed film after drying for accepting further finishing layers such as printing dyes or inks. Such priming is preferably carried out, for example, on natural materials such as wood strips, wood veneer or other natural materials such as cork or the like. In this regard, the primer should be matched to the natural materials, i.e. be chemically compatible, in order to achieve the best possible adhesion and wetting properties. 
         [0035]    Apparatus  1  preferably has a corona treatment module  33 , a plasma treatment module  34  and a flame treatment module  35 , which will be described subsequently, in order to facilitate improvement of the adhesion of layers such as printing dyes or inks to be applied to polymers or similar surfaces  4 . 
         [0036]    Corona treatment module  33  is set up to generate a high voltage discharge to surface  4 . The high-voltage or corona discharge generates an electron avalanche towards surface  4  by means of flow filaments. In this case surface  4  represents a lower voltage potential and a high-voltage discharge takes place via the surrounding atmosphere. The electrons generated in the discharge hit surface  4  with such high energy that the molecular bonds of the surfaces (in polymers) are split. A corresponding corona treatment is available, for example, from Tigres Dr. Gerstenberg GmbH, Muhlenstr. 12, D-25462 Rellingen. 
         [0037]    Plasma treatment module  34  is set up to generate a plasma spatially separated from a high-voltage discharge and to blow this plasma onto surface  4  by means of compressed air. A corresponding plasma treatment module  34  is available, for example from Tigres Dr. Gerstenberg GmbH, Muhlenstr. 12, D-25462 Rellingen. 
         [0038]    Flame treatment module  35  is set up to bring about the formation of chemically functional groups such as oxygen and hydroxyl radicals which act on a surface located within a flame. Propane/butane gas mixtures are used as the combustion gas whereby the gas flame is adjusted such that an excess of oxygen occurs. In this regard surface  4  is heated for a short time without melting it. In addition, lightly bonded combustible layers sitting on the surface are burnt and the base material is oxidized. As a result, the oxygen atoms incorporated in the polymer structures increase the surface energy and thus the wettability of the (plastic) surface. A corresponding flame treatment module is available, for example, from Arcotec GmbH, Rotweg 24, D-71297 Monsheim. 
         [0039]    Common to all three submodules  33 ,  34 ,  35  described is that they bring about incorporation of oxygen atoms in the topmost surface layers. Furthermore, any lightly bonded extraneous layers possibly present on the surface are removed by means of these submodules. In this regard, the properties of the surface in respect of appearance, consistency and geometry are only altered to a small extent which is not detectable to the naked eye. Associated with this is an increase in the physical surface energy to the values advantageous for further finishing. In this case, the level of surface energy is directly proportional to the oxygen concentration in the plastic surface. The result is an improvement in the chemical bond between the plastic molecules and the other finishing layers to be applied, for example the printing dye or ink. 
         [0040]    Corona treatment module  33  and plasma treatment module  34  have the advantage that they only heat surface  4  a little which prevents a deformation or even ignition of the plastics. In flame treatment module  35 , the gap between the flame and surface  4  should be adjusted appropriately in order to prevent excessive heating of surface  4 . In addition, flame treatment module  35  must be switched off when handling device  60  stops so as to rule out damage to surface  4 . 
         [0041]    Devices which provide what are known as cold, open, atmospheric plasmas have proven their value in respect of corona treatment module  33  and plasma treatment module  34 . In this case, the actual plasma (gas) is spatially separated from the flow filaments between the electrodes. In addition, normal ambient air is used as the surrounding gas. Thus it is possible to dispense with a particular atmosphere (vacuum or process gas). 
         [0042]    Apparatus  1  according to the invention in the present embodiment also comprises an adhesion promotion module  36 . Adhesion promotion module  36  is set up, for example, to apply a primer or a UV primer to surface  4 , which will frequently be performed as an alternative to physical pre-treatment by corona, plasma and flame treatment modules  33 ,  34 ,  35  described above. 
         [0043]    The aim of applying adhesion promoters is likewise to enhance the wetting and adhesion properties of surface  4 . Adhesion promotion module  36  may, for example, have spray or roller application equipment and, if necessary, an appropriate drying device (hot air fan for example), which is connected downstream of the application equipment. In the case of a UV primer, adhesion promotion module  36  may also, if necessary, have a UV light source with appropriate emitter. 
         [0044]    The primer may, for example, be a solvent-containing primer, which contains a chemically active component dissolved in the solvent. This chemically active component should be matched in each case to the plastic surface and to the finishing layer simultaneously. The chemically active component remains on the surface after evaporation of the solvent. The primer then forms a chemical bridge between the surface and a subsequent finishing layer thus creating chemical compatibility between both materials. 
         [0045]    On the other hand, for example, what is known as a UV primer may be used as has already been mentioned above. This usually does not contain any solvent and curing takes place rather by irradiation with UV light of an appropriate wavelength. In this case, polymerization of unsaturated monomers located in the liquid primer is activated by means of photo-initiators (radicals) sensitive to UV light. When the chain reaction has finished (completion of polymerization), the UV primer&#39;s carrier material remains as another additional plastic layer on the (plastic) surface lying beneath it in which (plastic) surface the chemically active component is embedded and which here provides a chemical bridge to further finishing layers in the same manner. 
         [0046]    Apparatus  1  according to the invention also comprises a module  40  for reducing the electrostatic charge. Some of the pre-treatment modules  10 ,  20 ,  30  and their submodules which are connected upstream of module  40 , have by their nature the characteristic of causing an electrostatic charge of surface  4  of panel-shaped workpiece  2 . This is primarily the case in materials which have a lower electrical surface conductivity such as, for example, plastics or lacquered surfaces. In order to reduce the electrostatic charge in these surfaces, module  40  may, for example, have an ionizing device, which may, for example, be rod-shaped, to reduce the electrostatic charge. The electrostatic charge of surface  4  may be brought to values which are no longer critical for subsequent printing by means of this rod-shaped ionizing device. 
         [0047]    As can also be seen in the FIGURE, apparatus  1  according to the invention in the present embodiment also comprises a control device  70  which is represented, for example, by an appropriate control computer or similar. Control device  70  is connected, as shown by dotted lines in the FIGURE, to pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules and also additionally to printing device  50 . Although not shown in the FIGURE, control device  70  may also be appropriately connected to handling device  60 . In this manner, operation of individual pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules may be selectively and individually controlled by the control device. In this regard, it is preferable for control device  70  to be supplied with information by way of the material and/or the surface condition of sections  4  to be printed in each case so that pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules can be operated selectively depending on these or other suitable parameters of the section to be printed or even depending on other parameters. 
         [0048]    Pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules are represented in the FIGURE in such a manner that they are provided on a common machine layout (not shown in detail). Although the printing device is represented at a distance from these components in the FIGURE, the printing device may also be provided on the same machine layout, as may handling device  60  and control device  70  also. As already referred to initially, the present invention does, however, also comprise apparatuses in which individual pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules and the remaining components of the apparatus are provided on separate machine layouts or are even disposed so as to be spatially separate (for example in different halls or even different sites). 
         [0049]    Operation of the apparatus according to the invention takes place, for example, as follows. First of all, information about the type, number, condition, etc. of panel-shaped workpieces  2  to be printed is routed to control device  70 . This information may be specified externally or may, if necessary, be detected and passed onto control device  70  by sensors, measuring devices or the like, not shown in greater detail here, which are disposed upstream of the apparatus. 
         [0050]    Based on this information, control device  70  controls handling device  60  such that panel-shaped workpieces  2  are transported along handling device  60  whereby surface  4  of relevant panel-shaped workpieces  2  to be printed faces towards pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules and subsequently towards printing device  50 . During the through-feed of each panel-shaped workpiece  2 , the control device controls the individual pre-treatment modules and the printing device such that, depending on the material and/or the surface condition of section  4  to be printed or, if necessary, on other parameters, at least one or a plurality of pre-treatment modules are operated in each case in order to carry out the appropriate pre-treatment on section  4  to be printed, before said section is subsequently printed with a desired pattern by printing device  50 . 
         [0051]    The selection criteria for individual pre-treatment modules  10 ,  20 ,  30 ,  40  and their submodules are not particularly restricted within the scope of the present invention although basic approaches for an advantageous selection of the individual modules have been given in the description above.