LUUKKAINEN, Juha-Pekka (Pappilantie 7 A, Kerimäki, FIN-58200, FI)
NIEMINEN, Harri (Sänkikatu 2, Lahti, FIN-15300, FI)
LIIMATAINEN, Juhana (Myllytie 3 as. 1, Savitaipale, FIN-54800, FI)
KUKK, Kaido (Klooga mnt 2a-7, EE Tabasalu, 76901, EE)
SJÖGRÉN, Henrik (Tuulastie 15, Mikkeli, FIN-50190, FI)
KILJUNEN, Samantha (Lietokatu 12, Lappeenranta, FIN-53500, FI)
LUUKKAINEN, Juha-Pekka (Pappilantie 7 A, Kerimäki, FIN-58200, FI)
NIEMINEN, Harri (Sänkikatu 2, Lahti, FIN-15300, FI)
LIIMATAINEN, Juhana (Myllytie 3 as. 1, Savitaipale, FIN-54800, FI)
KUKK, Kaido (Klooga mnt 2a-7, EE Tabasalu, 76901, EE)
SJÖGRÉN, Henrik (Tuulastie 15, Mikkeli, FIN-50190, FI)
| CLAIMS 1. A method for manufacturing plywood, wherein the plywood comprising multiple veneer layers is formed from a number of veneers in such manner that the veneers are laid one above the other and combined by means of glue, c h a r a c t e r i z e d in that the veneers are pressed together by a continuous pressing containing a heating zone, a shearing zone and a cooling zone for joining the veneers together. 2. The method according to claim 1, c h a r a c t e r i z e d in that the veneers are treated in heating zone, shearing zone and cooling zone provided one after another during the pressing. 3. The method according to claim 1 or 2, c h a r a c t e r i z e d in that temperature in the heating zone is 130 - 160 °C. 4. The method according to any one of claims 1 to 3, c h a r a c t e r i z e d in that temperature in the shearing zone is 130 - 160 °C. 5. The method according to any one of claims 1 to 4, c h a r a c t e r i z e d in that temperature in the cooling zone is room temperature. 6. The method according to any one of claims 1 to 5, c h a r a c t e r i z e d in that the veneer is preheated prior to the pressing. 7. The method according to any one of claims 1 to 6, c h a r a c t e r i z e d in that the glue is heated so that the glue becomes adhesive prior to the pressing . 8. The method according to any one of claims 1 to 7, c h a r a c t e r i z e d in that the method comprises a pre-laminating for forming a pre-laminate by the continuous pressing. 9. The method according to any one of claims 1 to 8, c h a r a c t e r i z e d in that at least two pre-laminates are pressed together by the continuous pressing . 10. An apparatus for manufacturing plywood comprising multiple veneer layers formed from a number of veneers in such manner that the veneers are laid one above the other and combined by means of glue, c h a r a c t e r i z e d in that the apparatus comprises at least one continuous pressing unit (10) for press¬ ing veneers together in order to join the veneers to- gether with the glue, and the continuous pressing unit (10) comprises heating zone (1), shearing zone (2) and cooling zone (3) . 11. The apparatus according to claim 10, c h a r a c t e r i z e d in that the heating (1) , shearing (2) and cooling (3) zones are provided one after another so that the veneers convey through the heating zone (1), the shearing zone (2) and the cool¬ ing zone (3) in this sequence. 12. The apparatus according to claim 10 or 11, c h a r a c t e r i z e d in that the heating zone (1) comprises at least one heating stages. 13. The apparatus according to any one of claims 10 to 12, c h a r a c t e r i z e d in that the shearing zone (2) comprises at least one shearing stages. 14. The apparatus according to any one of claims 10 to 13, c h a r a c t e r i z e d in that the cooling zone (3) comprises at least one cooling stages . 15. The apparatus according to any one of claims 10 to 14, c h a r a c t e r i z e d in that the pressing unit (10) comprises at least one high fre¬ quency pressing device. 16. The apparatus according to any one of claims 10 to 15, c h a r a c t e r i z e d in that the apparatus comprises at least one heat-treatment unit for heat-treating the veneer prior to the pressing unit . 17. The apparatus according to any one of claims 10 to 16, c h a r a c t e r i z e d in that the apparatus comprises at least one heating device for heating the glue so that the glue becomes adhesive prior to the pressing. |
FIELD OF THE INVENTION
The invention relates to a method for manufacturing plywood as defined in the preamble of claim 1 and an apparatus for manufacturing plywood as defined in the preamble of claim 10.
BACKGROUND OF THE INVENTION
Known from prior art are various plywoods, veneer boards or the like. Known from prior art is the gluing of veneers by various glues, e.g. polyurethane or phenolic glue, for providing plywood.
Further, known from prior art is different types of methods and apparatus for manufacturing plywoods. In a standard plywood mill the pressing is performed in two stages to maximize output; using first a pre-pressing stage at ambient temperature and under pressure for a time just sufficient to consolidate the veneers into a unitary panel. The consolidated panel can be stored and handled without shifting or separation of the veneers and afterwards can be treated under heat and pressure to cure the thermosetting adhesive and form the final plywood panel.
Further, known from prior art is a continuous press with different temperature zones.
OBJECTIVE OF THE INVENTION
The objective of the invention is to disclose a new type of method for manufacturing plywood. Further, the objective of the invention is to disclose a new type of apparatus for manufacturing plywood .
SUMMARY OF THE INVENTION
A method and an apparatus according to the invention are characterized by what is presented in the claims.
The invention is based on a method for manufacturing plywood, wherein the plywood comprising multiple veneer layers is formed from a number of veneers in such manner that the veneers are laid one above the other and combined by means of glue. In accordance with the invention, the veneers are pressed together by a continuous pressing containing a heating zone, a shearing zone and a cooling zone for joining the veneers together.
Further, the invention is based on an apparatus for manufacturing plywood comprising multiple veneer layers. The plywood is formed from a number of veneers in such manner that the veneers are laid one above the other and combined by means of glue. In accordance with the invention, the apparatus comprises at least one continuous pressing unit for pressing veneers together in order to join the veneers together with the glue, and the continuous pressing unit comprises heating zone, shearing zone and cooling zone. Preferably, the apparatus is divided into at least three sections, such as heating, shearing and cooling zones.
Specially, the invention is based on the plywood mill production considerations for a continuous press utilizing modified thermoplastic to bond veneers in the production of plywood with focus on a shearing zone to reduce the viscosity of the film so that there is better penetration of the polymer into the veneers .
In this context, a veneer refers to any veneer of plywood. The plywood can be a wood panel product, plywood product, composite product, beam, pressed panel product or the like, formed of a number of layers, preferably veneer layers, and principally of wood-based materials, in which the layers are laid one upon the other and glued together. The veneer can be formed of any material, e.g. wood-based material, fiber material, composite material or the like. In this context, the veneer refers to any layer of the plywood. Typically the veneer is a thin layer of the plywood. The thicknesses of the veneer layers can vary.
In one embodiment of the invention the veneers are treated in heating zone, shearing zone and cooling zone provided one after another during the pressing. In one apparatus embodiment of the invention the heating, shearing and cooling zones are provided one after another so that the veneers convey during the pressing through the heating zone, the shearing zone and the cooling zone in this sequence.
In one embodiment the pressing unit comprises the heating zone for pressing the veneers together by means of glue, heat and pressure. In one embodiment the heating zone comprises at least one heating stages. In one embodiment the heating zone comprises at least two heating stages. Various heating and press devices can be used.
In one embodiment of the invention temperature in the heating zone is 130 - 160 °C. In one embodiment the pressing unit comprises the shearing zone for pressing the veneers together by means of glue and pressure. In one embodiment the shearing zone comprises at least one shearing stages. In one embodiment the shearing zone comprises at least two shearing stages. Various shear devices can be used. The shear can be provided e.g. by vibration pulses, oscillation, ultrasonic oscillation, pulsed pressure, melt vibration, high vibration frequency, shock pulse and their combinations.
In one embodiment of the invention temperature in the shearing zone is 130 - 160 °C.
In one embodiment the heating zone and the shearing zone form together one combined heating- shearing zone in the pressing unit.
In one embodiment the pressing unit comprises the cooling zone for pressing the veneers together by the pressure and cooling the veneers. In one embodiment the cooling zone comprises at least one cooling stages. In one embodiment the cooling zone comprises at least two cooling stages. Various press devices can be used.
In one embodiment of the invention temperature in the cooling zone is room temperature.
Preferably the hot-press is followed by the shearing and the shearing is followed by the cooling to get maximum adhesive strength.
In one embodiment of the invention the veneer is preheated prior to the pressing.
The veneers can be joined together using any adhesive or glue, e.g. phenol formaldehyde, melamine urea formaldehyde, bio-adhesive or other adhesive. The adhesive can be in form of liquid, molten, film, hot- melt adhesive, granulate, composite, powder, compounded granulate, conglomerate, extrusion coated and their combinations .
In one embodiment of the invention the glue includes maleic anhydride as a gluing component. Preferably the glue is self-adhesive glue after activation .
In one embodiment polymer based glue is used in the gluing veneers. The use of low molecular weight polymers, linear and low viscosity also result in better penetration into wood but the mechanical properties of these polymers are very poor. Poor strength properties of the thermoplastic adhesive will result in poor shear bond strength of the plywood glue-line. Therefore, in order to get excellent glue- line strength properties and good penetration into the wood medium to high molecular weight polymers (high strength and high viscosity) are utilized where the viscosity is reduced in the continuous press after the heating zone by shear.
In one embodiment a film is used as glue. In one embodiment the film is formed of a film containing glue material provided on the surface of the film. In one embodiment the glue material can be a hot-melt, resin adhesive, maleic anhydride composition or the like. The glue material can be in liquid or in molten form. In one embodiment the film is formed of polyolefin containing reactive groups with -OH groups of the wood for forming bonds, e.g. via esterification, between the veneer and the film. In one embodiment the film contains maleic anhydride grafted polyolefin. In one embodiment the film is 3-layer co- extruded modified polyolefin film. In one embodiment the adhesive film can be in the form of a 3-layer co- extruded film where the outer layers contain maleic anhydride grafted polyethylene or polypropylene. In one embodiment the film is the self-adhesive polyolefin film, as defined in patent applications
PCT/FI2009/050130 and PCT/FI2009/050662, to create the plywood that has strength properties and fibre breakage percentage that exceeds that of phenol formaldehyde bonded plywood.
In one embodiment the film is formed on-line in connection with the plywood manufacturing.
In one embodiment the glue is activated before the gluing. In preferred embodiment the glue is activated by heating so that the glue becomes adhesive. In one embodiment the glue is heated at temperatures of between 120 - 170 °C, e.g. by hot-pressing. In one embodiment the glue is heated by high frequency heating in which parameters can be 2 - 8 minutes and 12 - 14 MHz.
In one embodiment the activation of the glue can be made in the heating zone of the continuous pressing .
It is important that the glue is cooled under pressing at least below the polymer crystalline temperature .
The opening time between the hot and cold press should be kept to a minimum, preferably less than 15 minutes, more preferably less than 5 minutes, and the plywood should be kept as hot as possible, loss of 5 °C at the most, so that the glue penetration and strength is not compromised. Veneers contain moisture and while this is kept to a minimum, preferably less than 5 %, it still results in a build-up of steam pressure in the plywood panel during hot-pressing. Therefore, it is important that the strength of the glue after leaving the hot- press is greater than the steam pressure or the board will burst (blow) . In phenol formaldehyde bonded plywood this is also a problem at moisture contents o er 5~6 but at lower moisture contents it tends to be fine since the phenol formaldehyde cures to a hard material before leaving the hot-press. In the case of a thermoplastic glue-line maximum strength of the glue-line will not be reached until the temperature of the polymer is reduced below its crystalline temperature, e.g. 80 - 100 °C, and this depends on the thermoplastic grade, so that the polymer is solidified. Therefore, since steam pressure can not be totally avoided in the panel it is important to keep the pressure applied during cooling of the panel and if at all possible to not release the pressure between heating and cooling and continuous press.
In one embodiment of the invention the veneer is heat-treated prior to the pressing in order to remove moisture from the veneer. In one embodiment the veneers are dried, preferably to veneer moisture which is less than 5 %. In one embodiment the apparatus comprises at least one heat-treatment unit, e.g. a drying device, for heat-treating the veneer disposed prior to the pressing. The veneers can be fed directly to the lay-up system after drying as hot veneers. In fact hot veneers can be an advantage in speeding the adhesion process up by reducing the hot-pressing time. Also, using hot veneers is a cost saving.
Green veneers are dried in steam or gas heated dryers and the required moisture content of less than 5% is still necessary, for adhesion with modified thermoplastic glue.
In one embodiment of the invention the pressing unit comprises at least one high frequency press. In one embodiment high frequency pressing is followed by a cooling stage. Owing to the thermoplastic materials containing some polar material it is possible to use high frequency pressing to heat-up and melt the thermoplastic glue. High frequency pressing is usually applied for shorter times but a cold press is also needed.
The high-frequency and cold press can be multi-level stacked presses and this is also the case for the cold press. It is important that the glue is cooled under pressure at least below the polymer crystalline temperature. It is possible that a cold press, cold rollers or cooling under pressure of plywood own weight can also be utilized.
The heating and cooling can be controlled by air or spray nozzles which apply a spray of heated or evaporating liquids to the rolls. The latter has the advantage of intensive cooling. The press is designed to provide a continuous pressure to adhere dissimilar materials that require time under pressure in order to bond. The open time required depends on the time taken for the adhesive to cool below the crystalline temperature. This type of continuous press must be very long in order to heat and cool continuously moving plywood. The continuous press provides different temperature zones of treatment.
In one embodiment the pressing unit comprises at least one contact press for improving penetration and adhesion of the glue into the veneer.
In one embodiment of the invention the method and apparatus comprises a pre-lamination stage for forming a pre-laminate by the continuous pressing. In the pre-laminating at least two veneers are joined together with the glue for forming a pre-laminate. In one embodiment the pre-laminates are joined together with the glue to form the plywood.
In one embodiment the formed pre-laminates are joined together by the continuous pressing to form the plywood.
In one embodiment of the invention the apparatus comprises at least one heating device for heating the glue so that the glue becomes adhesive prior to the pressing.
In some cases adhesion can be improved by first pre-pressing or using hot veneers, e.g. straight from dryer. The different pre-presses can be a standard hot press or a microwave press. The pre- laminated plywood can then be processed into the final plywood product by utilizing any of the methods in or it can be sent directly to the customer for them to process further. Pre-laminated plywood would not involve total curing of the plywood totally. The plywood is heated just enough to create a sufficient bond to wood so that the panel is then completed by the customer.
The method and apparatus in accordance with the invention are suitable for various applications.
LIST OF FIGURES
In the following, the invention is described by means of detailed embodiment example with reference to accompanying figures 1 - 2, in which
Fig. 1 shows a continuous press with three zones according to the invention, and
Fig. 2 shows a continuous plywood process for making plywood product.
DETAILED DESCRIPTION OF THE INVENTION
In this example, the plywood is manufactured by the method and apparatus of the invention. In figures 1 and 2 there are shown a process for pressing the veneers together continuously to form the plywood product .
As figures 1 and 2 show, the pressing process comprises pressing unit 10 with three zones 1, 2 and 3.
Before the pressing the veneers are dried in steam or gas heated dryers and the required moisture contents of less than 5%. From the drying the hot veneers are fed automatically to the lay-up system. Also the glue is fed onto the veneers. The 3-layer co- extruded modified polyolefin film with maleic anhydride as a gluing component is used as glue. In the 3-layer co-extruded film the outer layers contain maleic anhydride grafted polyethylene. The glue is activated by heating at temperatures 120 - 170 °C so that it becomes self-adhesive. The veneers are laid one above the other and the glue is supplied between each two veneers to glue the veneers together.
After the laying-up the veneers are supplied to the pressing unit 10. In the pressing unit 10 the veneers are pressed together to form the plywood. The pressing unit comprises hot pressing 1, shearing 2 and cooling zones 3. The hot pressing zone 1 is followed by the shearing zone 2 and the shearing zone 2 is followed by the cooling zone 3. In the pressing unit 10 three veneer ply are pressed together to form a pre-laminate 11. The continuous press 10 where only three ply are made will cut down on the length of the press required. The length of the press required is a function of the time to cooling and this is a factor of plywood thickness.
The shearing zone 2 is to create a decrease in the polyethylene viscosity. Polyethylene and especially LDPE the viscosity properties arise from the entanglements between polymer chains and polymer chain length (molecular weight) . The chain entanglements affect greatly on the viscosity and therefore if the number of chain entanglements, are reduced the viscosity is reduced. If the rate of shear is increased the molecular chain contact time is reduced and consequently the number of entanglements and viscosity is reduced. In this case the film is applied to veneers, and reheated to 20-50 °C, above the melting point. It is slightly more difficult to apply the shear directly to the polymer and therefore a different type of shear, one that is not in direct cont with the polymer is required. Therefore this type of shear is possible in the following way: ultrasonic oscillations, vibration pulses, pulsed pressure, melt vibration, high vibration frequency, shock pulse and their combinations. These are only a few of the possible techniques that can be applied to the shear zone but the invention is not limited to these only.
After the pressing unit the pre-laminates 11 are joined to form the plywood product 13. The 3-plyes 11 are hot-pressed 12 to 9-ply 13 using e.g. standard hot-pressing .
The method and the apparatus of the invention are suitable to be used for manufacturing plywoods easily and cost effectively.
A method and an apparatus according to the invention are suitable in its different embodiments for different types of applications.
The embodiments of the invention are not limited to the examples presented rather many variations are possible within the scope of the accompanying claims .
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