PANEL FORMING

Abstract

A method for forming of a building panel with a surface including a thermosetting resin such that tension created during curing of the surface is reduced or eliminated. Method for producing a panel with a wood based core and a surface layer including a thermosetting resin wherein the method including: curing and connecting the surface layer to the core while applying heat and pressure in a first main pressing step, thus raising a temperature of the surface layer above an initial temperature; applying a bending force on the panel after the first main pressing step to bend the panel such that an uppermost surface of the panel is convex an a lowermost surface of the panel is concave while the panel is still above the initial temperature; and releasing the bending force such that the panel springs back to an essentially flat shape.

Claims

1-22. (canceled)

23. Method for producing a panel comprising: providing a wood-based core and a surface layer, wherein the surface layer comprises a thermosetting resin; curing and connecting the surface layer to the core under heat and pressure for forming a panel, thereby raising a temperature of the surface layer above an initial temperature; bending the panel by applying a bending force on at least a portion of the panel while the panel is still above the initial temperature, wherein the bending force bends the panel by lifting the at least a portion of the panel; and releasing the bending force such that the panel springs back to an essentially flat shape.

24. The method according to claim 23, wherein the at least a portion of the panel is lifted by vacuum cups.

25. The method according to claim 24, wherein the vacuum cups comprise adjustable cylinders.

26. The method according to claim 23, wherein the at least a portion of the panel is lifted immediately after pressing.

27. The method according to claim 23, wherein the bending is made within 10-20 seconds after the pressing when the panel surface still has a temperature above 140° C.

28. The method according to claim 23, wherein the panel is bended under cooling of the panel.

29. The method according to claim 23, wherein the panel is kept in a bended position until it cools down to below 100° C.

30. The method according to claim 23, wherein the method further comprises decreasing the surface temperature during bending.

31. The method according to claim 30, wherein the surface temperature decrease is more than 20° C.

32. The method according to claim 23, wherein the thermosetting resin is a melamine formaldehyde resin.

33. The method according to claim 23, wherein the surface layer comprises wood fibres and wear resistant particles.

34. The method according to claim 23, wherein the surface layer comprises a powder-based layer, the powder-based layer comprising wood fibres and wear resistant particles and colour pigments.

35. The method according to claim 23, wherein said bending is performed after said curing.

36. The method according to claim 23, wherein the panel undergoes a plastic non-reversible deformation.

37. The method according to claim 23, further comprising providing a backing layer and connecting the backing layer to the core.

38. The method according to claim 23, wherein the panel is bended such that an uppermost surface of the panel is convex and a lowermost surface of the panel is concave while the panel is still above the initial temperature.

39. The method according to claim 23, wherein the core is a high-density fibreboard, HDF, core.

40. The method according to claim 23, wherein the bending force is applied to at least two opposite edge portions of the panel.

41. The method according to claim 23, wherein the panel is rectangular and wherein the bending force is applied to both sets of opposite edge portions of the panel.

42. The method according to claim 23, wherein the panel is a building panel or a floor panel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The disclosure will in the following be described in connection to exemplary embodiments and in greater detail with reference to the appended exemplary drawings, wherein:

[0037] FIGS. 1a-1d show known production of a floor panel according to known technology.

[0038] FIGS. 2a-2d show production of a panel according to a first embodiment of the disclosure.

[0039] FIGS. 3a-3d show production of a panel according to a second embodiment of the disclosure.

[0040] FIGS. 4a-4c show a method to measure internal tension in a panel.

[0041] FIGS. 5a-5b show the panel temperature during pressing and cooling.

[0042] FIGS. 6a-6d show a method to obtain plastic deformation in a panel.

DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

[0043] FIG. 2a shows a panel 1 that has been pressed with only a surface layer 2 applied on a core 3. The panel may be a DPL panel or a WFF panel. The shrinking of the surface layer 2 during pressing, when the thermosetting resin has cured, causes the panel edges 1 to bend upwards. The bending may increase further when the panel 1 is cooled down to room temperature. The bending is such that the panel 1 is not possible to use for flooring production. The reason is that there is no backing layer that balances the shrinking of the surface layer 2 during pressing and cooling.

[0044] The panel has essentially the same surface temperature as the press table when it leaves the press. A normal pressing temperature is about 170° C. The core has generally a much lower temperature of about 80-100° C. This means that the surface layer will cool down very quickly after the pressing. After 10 seconds, the surface temperature may be 120° C. and after 15-20 seconds only about 80° C. The cooling of the surface and the core is effected by the pressing time. A longer pressing time will increase the core temperature more than a short pressing time.

[0045] FIG. 2b shows that the panel 1 is pressed in a second pressing step between curved pressing tables 5, 7 when the surface layer 2 is still hot after the first pressing step. If such post pressing is made at a temperature of about 70-90° C., it will deform the hot HDF core and create a counter pressure that is sufficient to produce a flat panel without a backing layer. A plastic non-reversible deformation will take place.

[0046] A post pressing at higher surface temperatures will also stretch and deform the surface layer 2 and eliminate the tension caused by the shrinking in the first main pressing step. Such surface stretching is preferably made at temperatures around 140-160° C.

[0047] The second pressing and surface stretching step is preferably a cold pressing and it is an advantage if the panel is moved to the second pressing and bending station as soon as possible in order to maintain a high initial surface temperature when the bending starts. For example, the peak surface temperature may be in the range of 160-200° C., for example at about 170° C. during the first pressing, where the surface layer is connected to the core. When the bending step starts the surface temperature may be at least at 140° C., or at least 120° C. For example, when the bending may step starts the surface temperature may be within 30° C., or within 40° C., or within 50° C., of the peak surface temperature during the first pressing, where the surface layer is connected to the core. A decrease of the temperature in bended and stretched position will generally be obtained automatically when the core cools down the surface. This will stabilize the surface layer in a stretched position and a permanent deformation may be obtained. Internal tension in the panel may be reduced considerably or completely eliminated and the panel may be completely flat when it is cooled to room temperature as shown in FIG. 2c.

[0048] The production balancing and the cooling balancing that is made with a backing layer may be replaced partly or completely with plastic deformation.

[0049] The bending may be such that the panel is convex and bends backwards when it leaves the second pressing station. The panel may be slightly bended in a third strep upwards towards the surface such that a pre-determined shape is obtained. This type of overstretching and double bending may reduce the internal tension completely. The temperatures, bending time and bending magnitude should be adapted to the quality and specification of the surface and the core.

[0050] Heat may be added in the second surface-stretching step in order to improve and control the stretching and to adapt the production method to various surface thickness and resins. The surface may also be cooled in order to stabilize the panel in a pre-determined condition.

[0051] The second post pressing operation and the bending of the panel may be made in several ways. It is preferred that the bending and stretching is three-dimensional such that the bending is made along and across the panel. The press tables are preferably curved in two perpendicular directions like a top of a ball.

[0052] The backward bending may also be made with pressing cylinders, frames, pressing against a rubber or silicone matrix, with rollers and in many other ways where the panel is bended in hot condition after the initial main pressing operation where the thermosetting resin is cured.

[0053] The thermosetting resin may be adapted such that it is easy to stretch. Resins similar to the types used in so-called post-forming laminate qualities may facilitate surface stretching by bending.

[0054] The surface layer may also be stretched by lateral pulling forces applied at the panel edges.

[0055] It is also possible to heat up the panel again after cooling in a separate production step with for example a hot press table, infra-red lamps and similar in order to soften and stretch the surface layer in order to obtain a flat panel. A first cylinder may bend the panel backwards along the length and a second cylinder may, after preferably an intermediate heating, bend the panel along the width. The panels may also be held in a bended position in the cooling part after the pressing.

[0056] FIG. 2d shows that a moisture barrier 4 may be used of for example a plastic foil, treated paper, foam etc. in order to prevent moisture to penetrate into the panel from the sub floor. Such a backing layer may be used to obtain the climate balancing. In some application the bending of the HDF core may be sufficient even for the climate balancing.

[0057] The Surface Stretching Process (SSP) according to embodiments of the disclosure may offer one or more or all of the following advantages: [0058] a more cost efficient board with lower internal bonding strength may be used; [0059] topping that causes high wear at the edges and decreases locking strength due to internal tension may be eliminated; [0060] splitting of the board in the locking system may be avoided; and [0061] cost savings may be obtained by eliminating the backing or replacing the paper/melamine backing with a thinner or more cost efficient material.

[0062] FIG. 3a shows an embodiment with a thin backing layer 4 that is not able to balance the panel 1 after initial pressing. FIG. 3b shows bending and preferably also stretching of the surface layer 2. The result is a flat panel as shown in FIG. 3c with a backing layer 4 and reduced internal tension. The backing layer prevents moisture to penetrate into the backside of the panel 1 and will stabilize the panel when humidity changes. FIG. 3d shows three-dimensional bending of a panel 1. The magnitude of the bending B is preferably about 3-6 cm/m or more.

[0063] FIGS. 4a-4c show a method to measure internal tension in a panel. A groove 11 is formed in the panel edge. A high internal tension will separate the edges as shown in FIG. 4b. A large difference between the inner panel thickness T1 and the outer panel thickness T2 means that the panel 1 has a large internal tension that may cause topping and cracks in the locking system, especially in dry conditions. FIG. 4c shows a panel 1 with a low internal tension. For example, a panel with a reduced internal tension will have a T2 that is lower than a T2 for the same panel that did not have method, such as the bending step, performed on the panel.

[0064] This measuring method may be used to adjust the parameters of the backwards bending in the second pressing step by modifying initial pressing temperature, final pressing temperature, pressing time and the magnitude of the bending.

[0065] FIG. 5a shows the temperature curve during pressing of a WFF floor panel with a 0.5 mm surface layer. The temperature A is measured in ° C. and the time B in minutes. The upper curve C is the temperature of the surface layer and the lower curve D is the temperature of the core measured 3 mm below the surface. The surface temperature is increasing during pressing to 170° C. and the core temperature to about 110° C. The cooling of the surface layer takes place rather quickly as may be seen from FIG. 5b that shows the same temperature curves C and D during the first 60 seconds. For example, the bending step may start within 60 seconds or less, or 90 second or less, from when the first pressing starts. For example, the bending step may start within 30 seconds or less, from when the first pressing step ends. The plastic deformation is preferably made when the surface temperature is at least about 140° C. and when the core temperature, measured a few mm below the surface, is at least about 100° C. It is possible to modify the resins in the core and in the surface layer such that plastic deformation may be made at lower temperatures for example at 120-140° C. for the surface layer and 80-100° C. for the core.

[0066] FIGS. 6a-6d show that vacuum cups 15 with adjustable cylinders 16 may be used to bend the pressed panel 1 after pressing. The vacuum cups may lift the panel immediately after pressing and form it to a pre-determined shape as shown in FIGS. 6b and 6c. Such forming may be made within for example 10-20 seconds after the pressing when the panel surface still has a temperature around and above 140° C. The panel is kept in a bended position until it cools down to a suitable temperature, preferably below 100° C., and a permanent non reversible plastic deformation is obtained such that the panel is essentially flat at room temperature as shown in FIG. 6d.

Prophetic Example: Second Pressing and Stretching Step

[0067]

TABLE-US-00001 Panel size 1200 * 2400 * 9 mm Surface 500 gr. powder mix/m2 Initial pressing temperature 140-160° C. Second pressing temperature  80-140° C. Pressing time  5-15 sec Bending-width  5 cm Bending-length 14 cm