GLUED WOOD PRODUCT AND A METHOD FOR MANUFACTURING A GLUED WOOD PRODUCT

The invention relates to a glued wood product that consists of finger-jointed lamel- las, which are glued to each other and placed so that the grain direction of the glued wood product is vertical; however, taking into account the angular deviation caused by the curvature of the annual ring of the log that is used as original material, the locations of the said finger joints being further placed so that the finger joints of at least adjacent lamellas are spaced apart in the longitudinal direction of the lamella.

The invention also relates to a method of manufacturing the glued wood product, wherein the finger-jointed lamellas are placed for gluing so that the grain direction of the glued wood product is vertical; however, taking into account the angular deviation caused by the curvature of the annual ring of the log that is used as original material; the locations of the said finger joints are further placed so that the finger joints of at least adjacent lamellas are spaced apart in the longitudinal direction of the lamella.

In the field of the invention, it is well-known to join, by gluing, lamellas, boards or laths, which have been sawn from a log, to provide the glued wood product. In conventional glued wood products, such as glued laminated beams or glued timber slabs, the required number of pieces that have been sawn from the log are joined so that the end product consists of the required number of joined pieces of wood. In connection with this invention, the board or the lath that constitutes the glued wood product is called a lamella.

In the prior art, publication US 5034259 is known, presenting a manufacturing method of glued wood products, and a product manufactured by the method. The publication defines and distinguishes, among others, the term "vertical grain" (vertical grain direction), referring to a board or a wood product, wherein the grains or the annual rings, i.e., growth rings of the wood are at an angle of 45° or more with respect to the surface of the board. Correspondingly, the publication defines and distinguishes the term "flat grain", which covers the grain directions of the grains or annual rings of the wood, which are less than 45° (a tangential grain direction). The publication teaches that the glued wood product is formed as follows. The log is first sawn into boards in the "flat grain" direction. After this, any defects are removed from the "flat grain" boards. The boards are further sawn into lamellas,

which are turned so that the lamellas are placed in the vertical grain direction. Thereafter, the lamellas are glued to each other. In practice, the glue surfaces between the lamellas can thus be faded out of sight.

In connection with the present invention, the same terms are used as in the publication presented above, i.e., the vertical grain direction and the tangential grain direction. Thus, the direction is defined in relation to the primary usable surface of the laminar product.

From the prior art, publication WO 97/48534 A1 is also known, wherein the vertical grain direction of the glued wood product is provided by star-sawing the log into triangular staves, which are further placed for gluing so that the grain direction on the surface of the wood product is vertical.

One objective of the invention is to provide a glued wood product, wherein the variations in dimensions caused by humidity are as controlled as possible. Another objective to provide a glued laminar wood product, its surface hardness and wear resistance being as high as possible, taking into account the raw material in question. A further objective is to also provide a glued wood product, which, when taking into account the raw material in question, has as even a colour, as uniform a pattern, and as homogeneous a structure as possible. This uniform quality further contributes to providing the glued wood product, which is manufactured from the raw material in question, with the highest possible structural strength. Naturally, the structural strength is a desirable property, in some cases even the primary desirable property.

The objective of the invention is to further improve the technology presented by the known publications and to provide an improved method by selectively applying the known methods, a glued wood product with top class properties being obtained by means of the method from a raw material, which conventionally is classified into the most inexpensive or low-cost category. A further objective is to develop a method, which can be used to maximize the added value of the Nordic pine (Pinus sylvestris), i.e., by means of the method, a raw material of the lowest value (which, accordingly, can be selected to be used, when desired) can be turned into a first class end product. A further objective of the invention is to provide a manufacturing method, which can extremely well and effectively be applied industrially on a large scale.

The glued wood product according to the invention is characterized in that the lamella comprises a selected side board of a log, which is cut next to each knot in the transverse direction, whereby a knotless lamella is formed from the knotless portions by means of finger joints, the glued wood product being arranged to be sawn along a splitting plane according to the thickness of the end product.

Correspondingly, the method according to the invention is characterized in that the side board of the log is selected as the said lamella, its said finger jointing being carried out so that the raw lamella is cut in the transverse direction next to each knot and the thus obtained knotless portions are further formed into knotless lamellas by means of the finger joints. In this way, it is possible to effectively utilize the relatively low-cost raw material, which is easy to obtain and which can be refined to the level of a product manufactured from top class raw materials.

Using the method and the glued wood product according to the invention enables the manufacturers of wood products not to be so dependent on the quality of the raw material respectively available. According to the method, for example, pine, which has grown in a non-suitable habitat and produced a lot of branches, can be converted into an end product that is fully competitive with knotless, top class pine logs, which have grown in the best habitats and have large diameters. Basically, the objective of the invention is to provide a fully knotless end product. In practice, however, wood material is a natural product that always contains more or less deviations of various types, which, however, do not have to be removed as knots without obscuring the purpose of the invention. Such extremely small deviations may include, for example, tiny healthy knots with a diameter of less than 7 mm; small but healthy inclusions or spots, etc. Regarding these, the final use of the glued wood product and the risk of the lamella damaging at the manufacturing stage, possibly caused by the deviation, define the minimum size of the knot to be removed.

In this context, the side board refers to sawn timber, which originates in the log in a spot that is at a distance of at least 50 mm from the core of the wood. In this way, the curvature of the wood grain is of a sufficiently large radius and, in particular, when the material is pine (Pinus sylvestris), the properties of the material are advantageous for the invention. In that case, the radius of curvature of the annual ring is approximately at least 50mm, respectively. This exemplary case is described in detail in connection with the embodiment of Fig. 5.

Furthermore, cutting in the transverse direction adjacent to each knot refers to points on both sides of the knot in the longitudinal direction of the lamella. To be more precise, the term "adjacent to" refers to a point that constitutes a tangent or nearly a tangent to the annual rings at the knot, but also so that, when the toothing required by the finger joints is made at the location of cutting, the grains at the outermost tips of these teeth correspond to the "knotless portion", or their deflection is only minor, at the most. On the one hand, the purpose of this is to keep the material loss as small as possible but, on the other hand, to ensure a successful toothing of the finger joint without a risk of chipping.

According to a feature of the invention, the grain direction is vertical; however, taking into account the deviation caused by the curvature of the annual ring of the log that is used as the original material. Of all the publications known by the applicant, US 5034259, which can be classified as being the closest to the prior art, determines that the limit value of the vertical grain direction is 45°. This is a suitable definition also for the present invention, but this starting point can further be improved according to the invention by limiting the deviation to a smaller value. On the other hand, the width of the widest possible side board, which can reasonably be obtained from the log, partly dictates the maximum value of this deviation. In some cases, it is preferable to select such a wide lamella that the deviation in the outermost board that is distinguishable even exceeds the said limit value in places.

In the following, the invention is described in more detail by means of examples and with reference to the appended drawings, wherein:

Fig. 1 shows a feasible sawing pattern of a log,

Fig. 2 shows the conversion of a single raw lamella into a lamella,

Fig. 3 is an end view of a glued wood product according to the invention, which is further processed into laminar products,

Fig. 4 shows some further embodiments of the products according to the invention,

Fig. 5 shows the density of pine as a function of distance from the core of the wood.

Fig. 1 shows a log 1 and a feasible sawing pattern of log 1 , disclosing the different sawn timber that is obtained from the log. A special concern of this invention com-

prises side boards 12, which according to the definition are thus at a distance of at least 50 mm from the core of the wood. In that case, the diameter of the log required is in the order of 150 mm, corresponding to the minimum diameter of pine logs generally used in Finland. Thus, the raw thickness of the side board is 25 mm. Fig. 1 also illustrates the occurrence of knots 22 along the length of the log.

Fig. 2 shows how a single side board or raw lamella 2, which has been sawn around, is converted into the lamella 2 that is used according to the invention. Accordingly, the side board 12 of the log is selected as the said lamella 2, the board being full-edged or waney edged, but any possible waney edges are removed at some process stage. The surface roughness needed for the gluing is provided by means of a tool suitable for the task. When the raw lamella 20 is ready for machining, knots 22 and any other defects in the longitudinal direction of the lamella 2 and, on the basis of them, the cutting point needed, are defined, as described above in connection with the term "adjacent to". Thus, the said finger jointing is carried out so that the raw lamella 20 is cut in the transverse direction adjacent to each knot 22. Furthermore, the knotless portions 21 that are provided at the following stage are shaped into the knotless lamella 2 by means of the finger joints 25. As the finger jointing technique, any well-known finger jointing method is used; especially taking account the dimensions of the lamella 2. The finger joint 25 is not allowed to weaken the strength of the lamella 2 too much; therefore, the parameters of the finger joint 25, such as the dimension of the toothing, must be selected accordingly.

Fig. 3 shows the end result of the subsequent process stage, wherein the lamellas 2 have been glued to each other at the front surfaces or the flat grain sides of the lamellas 2. The lamellas 2 are preferably glued to each other in such a number that their width corresponds to the dimensions of the actual end product with its tooling allowances. Thus, this width is the number of lamellas 2 times the thickness 202 of one lamella 2, including the thickness of the glue line.

Fig. 3 shows a glued wood product 10, wherein the lamellas 2 are located in the vertical direction so that the grain direction is vertical; however, taking into account the deviation caused by the curvature of the annual ring 15 of the log that is used as the original material. Furthermore, the figure shows the further processing stage that is planned, so that the splitting planes 4 are shown in the horizontal direction. This figure also shows in detail the purpose of the said deviation. The annual ring 15 of the wood provides a natural curved shape, wherein the grain direction 16 at some point in the middle area of the lamella is exactly vertical 17 and the

grain direction 16 in the edge zones begins to curve towards the tangential grain direction 16. When the objective regarding the vertical grain direction 16 is to stay as far as possible from the limit value, then, according to this classification, the further processed products, which have been separated from the middle area along the splitting plane 4 in the direction of the lamella's width 201 , have the best properties. One reason for this is that the change in the dimensions of the wood, such as pine, as a function of moisture is smaller in the radial direction of the wood than in its tangential direction. In this way, according to the invention, the hydro- instability of the glued wood product can mainly be directed in the thickness direction of the laminar product that has been separated along the splitting plane 4. In this way, for example, when the end product comprises floor boards, only minor gaps are formed between the individual boards, even if the moisture was decreased from the installation moisture to a relatively large extent. In the case of floor boards the hydro-instability would mainly affect the thickness of the board, which in most cases is not a considerable factor, especially when the length of the dimensional change is insignificant. According to a preferred embodiment of the invention, the said deviation is 35°, at the most, compared to the vertical direction. The said deviation is more preferably 25°, at the most, compared to the vertical direction. The said deflection is most preferably 15°, at the most, compared to the vertical direction.

The dimensional stability of the embodiment shown in Fig. 3 can even be slightly improved by reversing some lamellas so that the radius of curvature of the wood grain 15 points the other way. In that case, the core of the wood in some lamellas 2 is thus to the right of the lamella and in some lamellas 2, to the left of the wood. Every second lamella or half of the number of lamellas are most preferably turned this way.

The glued wood product 10 shown in Fig. 3 can preferably be formed, for example, so that the proportion between the width 201 and the thickness 202 of the said lamella 2 is 18:1 to 3:1. Furthermore, in typical Nordic applications, the width 201 of the said lamella 2 is preferably 70 to 150 mm. Correspondingly, the thickness 202 of the said lamella 2 is 8 to 32 mm. The thickness 202 of the lamella 2 is most preferably 8 to 16 mm. One objective of the lowest lamella thicknesses is to provide a visual effect, wherein the glue line between the lamellas 2 disappears among the annual rings of the wood.

The manufacture of the glued wood product comprises, among others, a drying stage, wherein the side board 12 is dried to a special dryness before the finger

joints 25 are made and the lamellas 2 are glued together to form the glued wood product 10. Furthermore, one stage comprises a pressing stage, wherein after gluing the lamellas 2 together, the glued wood product 10 is kept pressed for a time that corresponds to the drying time of the glue 3 that is used. In the field, special dryness generally refers to sawn timber that is dried to a percentage of moisture of 8 to 14%.

At the further processing stage, the glued wood product 10 shown in Fig. 3 can further be divided into smaller parts along the splitting plane 4. In that case, the glued wood product 10, or the blank of the wood product, which has a height defined by the width of one lamella 2, a width defined by the number of lamellas 2 glued to each other, and a length defined by the number of knotless portions 21 connected by means of the finger joints 25, is further processed into end products. The said end products provided by means of such a method can comprise boards, ceiling, wall or floor panelling, furniture laminates, small beams, blanks for the carpentry and joinery industries, and the like.

The wood product 10 that is glued by the method described above is thus not split into the thickness of the end product along the splitting plane 4 until at this stage. A special advantage of this order is that relatively large finger-jointed lamellas 2 can be handled effectively at high velocities. Among others, the drying time of such a thin and relatively wide lamella 2 to special dryness is relatively short. Correspondingly, any deformations caused by the drying are relatively easy to control; similarly, the risk of splitting is minor. Furthermore, the flat grain surfaces of the lamellas 2 that are glued are relatively large, whereby an even application of glue on the surface to be glued becomes easier, while the pressing stage included in the gluing stage is simple to carry out. Extremely true to size laminar pieces can be separated along the splitting plane 4 from the wood product 10 glued by the method presented above.

When further processing the glued wood products 10 according to Fig. 3, the locations of the finger joints with respect to each other are significant from the point of view of both the strength properties and the appearance of the product. Therefore, the said finger joints 25 in the glued wood product are located so that the finger joints 25 of at least adjacent lamellas 2 are spaced apart in the longitudinal direction of the lamella 2. This "spaced apart" preferably means that they are as far as possible; however, taking into account the entity. According to the method, the locations of the said finger joints 25 are correspondingly placed so that the finger joints 25 of at least adjacent lamellas 2 are spaced apart in the longitudinal direc-

tion of the lamella. According to a preferred embodiment, the finger joints 25 are "evenly" spread out in the area of the primary usable surface, whereby no clear fracture plane develops in the product, along which possible damaging in the case of overloading easily takes place. Secondarily, this influences the appearance of the product, when the finger joints of the lamellas, which are possibly observable, disappear evenly in the area of the surface or the splitting plane 4.

As a summary of the above, the following stages are thus carried out sequentially according to the method:

- the side board 12 of the log is selected as the raw lamella 20, which is dried to a special dryness,

- the raw lamella 12 is cut in the transverse direction adjacent to each knot 22,

- the knotless portions 21 thus provided are shaped into the knotless lamella 2 by means of the finger joints 25,

- the finger-jointed knotless lamellas 2 are placed for gluing with their flat grain sides against each other so that the grain direction 16 of the glued wood product 10 becomes vertical; however, taking into account the deviation caused by the curvature of the annual ring 15 of the log that is used as the original material,

- the locations of the said finger joints 25 are further placed so that the finger joints 25 of at least adjacent lamellas 2 are spaced apart in the longitudinal direction of the lamella,

- the number of lamellas 2 needed is selected according to the width of the desired end product, the lamellas 2 being glued to each other,

- after the glue has dried, the glued wood product 10 is sawn on a plane perpendicular to the lateral direction of the lamella along the splitting plane 4, according to the thickness of the desired end product.

Fig. 4 shows further possibilities of applying the glued wood product according to the invention. In the left-hand exemplary case of Fig. 4, the glued wood product can be used as such. In the right-hand exemplary case, the small beam thus formed is designed to be further split along the splitting plane 4.

Fig. 5 shows an explanation as to why the side board of pine mentioned previously is such an interesting raw material. The vertical axis in the figure shows a density

in units [kg/m ³ ]. Correspondingly, the horizontal axis shows the distance from the core of the wood in units [mm]. Furthermore, the graphs illustrate the densities at different heights of the wood, the uppermost H2 being at a height of 2 metres, H6 being at a height of 6 metres, and H10 being at a height of 10 metres. These graphs indicate that the density of the raw material changes radically in the vicinity of the core of the wood, whereby, at a distance of about 50mm from the core, the density has reached a value that remains quite constant. This density affects the operating properties and the machineability of the wood, and its surface hardness, which is a significant factor. The effect of the surface hardness can even be +15%, when moving from the core of the wood to the side board defined in this connection. Correspondingly, the grain direction also affects the surface hardness by about +15%, whereby the surface hardness of the glued wood product according to the invention, altogether, is about +30% harder that of a reference material. This reference material originates in the same wood in the vicinity of the core and it has been laid in the horizontal grain direction.

The invention can be applied to most kinds of wood, but an especially preferred embodiment comprises pine (Pinus sylvestris), which has already been mentioned several times. Another interesting northern application material comprises birch (Betula pendula), this material having a very strong tendency to defects in form. According to the invention, by making relatively thin lamellas, a glued wood product made of birch can be rendered very true to size and good in keeping its shape.