FIELD OF INVENTION The present invention refers to a process for production of laminated wood, specifi¬ cally so called edge glued panels, by gluing the vertical joint surfaces of elongate wood pieces using a curing adhesive system comprising at least a resin and a hard¬ ener, and by which the resin and hardener are separately applied to opposite verti¬ cal sides, respectively, of the wood pieces that are advanced through an apparatus used in the process. The invention also refers to the apparatus for separately apply¬ ing resin and hardener components of a curing adhesive system to opposite vertical sides of the wood pieces.

BACKGROUND OF THE INVENTION The term "adhesive system" as used herein refers to a curing formulation of a resin and a hardener component, and other suitable additives if appropriate. In the pro¬ duction of laminated structures using curing adhesive systems, curing time is a critical factor. It is common, in connection with fast-curing adhesive systems, to avoid pre-curing by separately applying resin and hardener to a joint surface on which the components mix before assembly. An even better utilization of fast curing adhesive systems may be obtained if the resin and hardener are kept separate until assembly of the joint, by applying resin to a first joint surface and hardener to the opposite joint surface.

In the production of edge glued panels by joining together wood pieces side by side, the adhesive system is applied to joints that are vertically oriented with respect to the major, horizontal dimension of the assembled panel. Conventionally, the wood pieces are individually conveyed through an applicator station where the adhesive system is applied, and further conveyed to be assembled on a horizontal press ta¬ ble. It is previously known in the production of edge glued panels to apply the resin and hardener separately. Typically, a hardener of comparatively low viscosity is ap- plied to the joint surface by spraying, and the resin being applied by spraying, or being released to the surface from a resin applicator roll. Spraying however gives rise to hygienic problems and losses due to splashing, and additionally the usually acid hardener component may cause corrosion on equipment, which constitutes a problem by itself. Also, spraying requires frequent cleaning of nozzles, and an even distribution of adhesive components may be difficult to obtain by spraying, due to clogging.

SUMMARY OF THE INVENTION The present invention aims to avoid these problems and to provide a process as well as means for the production of laminated wood, specifically edge glued panels, in which the advantages of fast-curing adhesive systems are better utilized.

One object of the present invention is to provide a process and an apparatus by which resin and hardener of a curing adhesive system are kept separated until as¬ sembly of the wood pieces to form an edge glued panel having vertical joints, avoid¬ ing the problem of pre- curing.

Another object of the present invention is to provide a process and an apparatus by which resin and hardener are released from rolls to the opposing vertical joint sur¬ faces of wood pieces to be included in an edge glued panel, avoiding the problems of splashing and airborne spray particles.

Yet another object of the present invention is to provide an apparatus of compact structure for release and non-release, respectively, of resin and hardener from ver¬ tical rolls to the opposite vertical joint surfaces of a wood piece to be included in an edge glued panel.

Still another object of the present invention is to provide a process in which the hardener is adapted to be released from a vertical roll to a joint surface in the pro¬ duction of edge glued panels.

The method and the apparatus of the invention are defined in the appended claims. Briefly, the process and the apparatus of the present invention are concerned with the production of laminated wood, specifically edge glued panels, by gluing the ver¬ tical joint surfaces of elongate wood pieces using a curing adhesive system compris¬ ing at least a resin and a hardener, and in which wood pieces are individually con¬ veyed through an apparatus wherein resin is applied separately to a first joint sur¬ face and hardener is applied separately to the opposite joint surface such that resin and hardener are mixed at the joint upon assembly of the wood pieces. The process further comprises the steps of releasing resin from a resin applicator roll which is driven to rotate about a first vertical axis, and releasing hardener from a hardener applicator roll which is driven to rotate about a second vertical axis, while control¬ ling the resin and hardener applicator rolls between alternate positions for release and non-release, respectively, in cooperation with counter-pressure rolls that are associated with the applicator rolls.

The apparatus of the present invention comprises a conveyor arranged to feed indi¬ vidual wood pieces longitudinally through the apparatus; a resin applicator roll driven to rotate about a first vertical axis and operative to release resin to a first joint surface of the wood piece; a hardener applicator roll driven to rotate about a second vertical axis and operative to release hardener to the opposite joint surface of the wood piece; counter-pressure rolls associated with the resin and hardener applicator rolls, respectively, and control means for shifting the applicator rolls and counter-pressure rolls between release and non-release positions, respectively.

Embodiments of the process and apparatus of the invention are further defined in the subordinated claims.

SHORT DESCRIPTION OF THE DRAWINGS The invention is more closely explained below with reference to the diagrammatic drawings. In the drawings,

Fig. 1 illustrates a process for production of edge glued panels by joining together the vertical sides of elongate wood pieces, viewed from above;

Fig. 2 is a sectional view through an applicator station, transversely to a feed direc¬ tion of the wood pieces; Fig. 3 shows alternative positions of applicator rolls and counter-pressure rolls in the applicator station of fig. 2;

Fig. 4 is a phantom drawing showing a roll assembly, and

Fig. 5 is a longitudinal section through a roll.

DETAILED DESCRIPTION OF THE INVENTION An adhesive system suitable for the present process may be, for example, a urea- formaldehyde (UF) resin gluing system, a melamine-formaldehyde (MF) resin gluing system, a melamine-urea-formaldehyde (MUF) resin gluing system, a phenol- resorcinol-formaldehyde (PRF) resin gluing system, a polyvinyl acetate gluing sys¬ tem, and various combinations of two or more of these gluing systems.

A hardener typically comprises an aqueous solution or dispersion of an acid or acid-forming substance as curing agent. Suitable curing agents are phosphoric acid, trichloroacetic acid, maleic acid, citric acid, formic acid, aluminum sulfate, and ammonium chloride, e.g. Selection of curing agent is made with respect to de¬ sired curing time or rapidity of the hardener. In combination with the resin, the curing time can vary within wide limits, for example, from about 1 minute to about 5 hours at room temperature (about 20°C).

Conventionally, in a process wherein an adhesive system is applied to vertical sur¬ faces of a glue joint, the viscosity of the hardener is kept low to allow application through a spraying nozzle.

In one embodiment of a process according to the present invention the hardener is modified in order to facilitate release of the hardener from a vertically oriented ap¬ plicator roll. Specifically, the viscosity of the hardener may be adapted to form a layer of continuous thickness about the cylinder periphery of the vertical roll. In this respect, the viscosity of the hardener may be adjusted to facilitate a complete and even coating of the hardener applicator roll also in an axial/vertical direction, while avoiding an ejection of the fluid hardener radially from the rotating roll. In operation, the peripheral velocity of the roll may range up to about 200 m/min or more while releasing hardener to the joint surface at an amount ranging from about 15 to about 70 g/m2. e.g. In the case of a very low viscosity the hardener may not be able to climb the vertical wall of the roll, whereas in case of a very high viscosity the hardener may be separated from the roll surface by centrifugal forces. In this con¬ nection, a thickener such as polyvinyl alcohol, polyvinyl acetate or mixtures thereof, may be added to provide the hardener a viscosity ranging from about 1 ,000 to about 7,000 mPa-sec, preferably within about 2,000 to about 4,000 mPa-sec (by Brookfield, LVT spindle 4, 12 rpm at 250C).

The resin may typically be applied to the joint surface at an amount of 30-300 g/m2.

A production line is diagrammatically illustrated from above in fig. 1, showing elon¬ gate wood pieces 1 being advanced in succession from an in-feed end at the left hand side of the drawing, towards an applicator station 2. The wood pieces are in¬ dividually conveyed through the applicator station, where the adhesive system is released to vertically oriented sides of the wood pieces. Typically, a band conveyor may be arranged for feeding the wood pieces through the applicator station 2. After gluing, the wood pieces are assembled to form a laminated wood board 3, or an edge glued panel, having glue joints of a vertical orientation with respect to a major horizontal dimension of the board and which is further advanced to a press (not shown), towards the right hand side of the drawing.

In the applicator station 2, wood pieces are guided longitudinally into a gap that is formed between a resin applicator roll 4 driven to rotate about a first vertical axis, and a hardener applicator roll 5 driven to rotate about a second vertical axis. The resin applicator roll 4 is associated with pump means 6, supplying the resin com¬ ponent. Correspondingly, the hardener applicator roll 5 is associated with pump means 7, supplying the hardener component.

Applicator rolls 4 and 5 are driven to rotate about their vertical axis, in contact with opposite joint surfaces of the wood pieces as these are conveyed through the gap, such that the resin applicator roll 4 releases resin to one vertical side and the hard¬ ener applicator roll 5 releases hardener to the opposite vertical side of the wood pieces. In fig. 1, each applicator roll 4,5 is associated with counter pressure rolls 8 and 9, respectively, cooperating with the applicator rolls to provide counter pres¬ sure for balancing the contact pressure that is applied from the applicator rolls upon release of resin or hardener to the joint surfaces. The counter-pressure rolls may be idling, or driven at speeds that are synchronized with the rotation of the applicator rolls. The applicator rolls, as well as the counter-pressure rolls, may be dimensioned to have a diameter of 100-150 mm, e.g., and driven at speeds ranging up to about 400 rpm.

In fig. 1, resin and hardener applicator rolls 4 and 5 are oppositely arranged at lon¬ gitudinally spaced positions, as seen in the feed direction F of the wood pieces. An alternative and more compact structure is shown in fig. 2, wherein the applicator rolls 4,5 are oppositely arranged on a line transversely to the feed direction.

In fig. 2 , showing a wood piece 1 from an end view, each applicator roll 4,5 is asso¬ ciated with a counter-pressure roll 8 and 9, respectively, jointly supported on a common axis 10 or 11. Drive means 12, 13 are provided to drive the applicator rolls and counter-pressure rolls for co-rotation about the axes 10 and 11, respectively. In this embodiment, the diameters of applicator and counter-pressure rolls on a com¬ mon axis are equally dimensioned.

Returning to fig. 1, the applicator rolls 4,5 and counter-pressure rolls 8,9 are sepa¬ rately journalled and displaceable laterally and transversely to the vertical roll axes and controllable in a horizontal plane, i.e. in a radial direction of the rolls, to be brought in contact with the vertical sides of the wood pieces for release of resin/hardener, or out of contact for leaving either side uncoated (non-release) as desired with respect to the outermost members included in an edge glued panel.

In fig. 2, the assemblies of applicator rolls and counter-pressure rolls coaxially sup¬ ported on common axes are vertically displaceable transversely to the feed direction and controllable in axial direction of the rolls for release and non-release of resin or hardener.

Fig. 3 illustrates the possible operative positions of the assemblies of fig. 2. In the uppermost position, resin alone is released to a left vertical side of the wood piece 1 from a resin applicator roll 4 of the left hand assembly, and counter pressure is provided from the counter-pressure roll 9 of the opposite assembly which is lowered with respect to the wood piece. In the centre position both assemblies are leveled with the wood piece, such that resin is released from the applicator roll 4 of the left hand assembly while hardener is simultaneously released from the applicator roll 5 of the opposite assembly. In the lowermost position, the left hand assembly is low¬ ered to provide counter pressure while the applicator roll 5 of the opposite assembly releases hardener alone to the right side of the wood piece.

A linear, lateral or vertical displacement of the rolls or the roll assemblies may be driven pneumatically, hydraulically or electrically, and guided by structural ele¬ ments included in a frame structure defining the applicator station 2. Advanta¬ geously, pneumatic or hydraulic cylinder units may be arranged in the frame struc¬ ture and controllable for shifting movements between release and non-release posi¬ tions, by operatively engaging a rotary mounting for the roll axis and which is sup¬ ported to slide on guides arranged on the frame.

Programmable control means may be arranged to operate the cylinders and thereby to shift the applicator and counter-pressure rolls between the release and non- release positions in response to an output signal from a counter which monitors the number of wood pieces being conveyed through the applicator station.

Fig. 4 illustrates the feed of adhesive component to the cylinder surface of applica¬ tor rolls 4,5. The adhesive component, such as resin or hardener which are kept separate until assembly of the wood pieces, is supplied via pump means and con¬ duits to a bottom region of a vessel 14. The vessel 14 has a vertical wall 15 sub¬ stantially enveloping the applicator roll 4,5, a clearance in the wall leaving a portion of the roll surface free for contact with the wood piece 1. The lower end of the appli¬ cator roll reaches down into a volume of adhesive component, resting on the bottom of vessel 14. Level gauging means preferably are arranged in the vessel to control the level of adhesive component received by the vessel. A scraper 16 runs vertically adjacent to the applicator roll, an edge of the scraper sliding on the roll surface as the roll rotates about its axis 10, 11. The lower end of scraper 16 reaches down into the volume of adhesive component. The vessel 14 is rotatable but axially affixed relative to the applicator roll or roll axis, and movable with the applicator roll be- tween release and non-release positions, respectively. The cylindrical surface of ap¬ plicator roll 4,5 has a rifling 17 rising helically about the roll surface. As the roll is driven for rotation, the adhesive component is captured and forced by the sliding scraper edge to climb the helical rifling, filling the roll surface with adhesive compo¬ nent that is subsequently released to a vertical joint surface. An agitator 18 may be arranged in the lower end of applicator roll 4,5, co-rotating with the roll near the bottom of the vessel for mixing the substances included in the adhesive component and this way maintaining the homogeneity of the component.

Alternatively, and preferably in connection with more fluid hardeners at lower vis¬ cosities, hardener may be supplied to the roll surface at an upper region of the hardener applicator roll 5. In this embodiment, not shown in the drawings, hard¬ ener is supplied at the upper margin of the cylinder surface. For example, the hard¬ ener may be supplied directly or indirectly via a conduit or other distributor means that mouths at the upper end of the vertical hardener applicator roll. Excessive hardener may be collected below the roll, and pump means are suitably arranged for circulation of the hardener. A scraper 16 may be arranged to enhance distribu¬ tion of the hardener in a descending coating of the roll surface.

The applicator rolls may be cylinders made of metal such as steel, or made of syn¬ thetic materials, elastic or non-elastic in a radial direction of the rolls. Preferably the applicator rolls are structured to release the adhesive components radially un¬ der an elastic contact with the wood pieces to ensure continuous wetting and filling of the joint surface, also in the event of possible irregularities in the surface. Elas¬ ticity in the radial direction may be provided through an elastic support of the roll axes, or by designing the rolls to have an inherent elasticity. For example, a steel or metal cylinder may be surrounded by an elastic envelope. Alternatively, and illus¬ trated in fig. 5, a composite roll has a rigid jacket 19 enveloping a flexible core 20. For example, the jacket may be a steel cylinder and the core a rubber or polymer composition, the core having a Shore A hardness of approximately 30-70°, e.g. Al¬ ternatively, the rolls may be assembled or integrally formed entirely from nature rubber, synthetic rubber or polymer material, such as nitrile rubber, diene rubber (EPDM), Hypalon®, etc. The elasticity may be provided through corresponding struc¬ ture of the applicator rolls 4,5, or/and of the counter-pressure rolls 8,9. Thus there has been disclosed an improved process and means for applying a cur¬ ing adhesive system to vertical joint surfaces, avoiding the drawbacks of spraying. The claimed invention is defined through the features listed in the appended claims.