Abstract:
A process for the rapid bonding of lumber by surface heating is provided, which involves surface heating of a first piece of lumber to be bonded and applying an adhesive to the non-heated surface of another piece of lumber and then contacting the thus-prepared pieces of lumber and applying pressure with a cold press.

Description:
BACKGROUND OF THE INVENTION 
     1. Industrial Field 
     This invention relates to a method for the high speed bonding of lumber by surface heating. More specifically, this invention is very effective for rapid bonding of laminated wood or wood architectural members such as panels, I-beams, trusses or the like. 
     2. Description of the Prior Art 
     Wood has a good appearance, workability, availability and is a renewable resource. In light of these merits, new uses for wood are now being considered. 
     When manufacturing laminated wood structures or wood architectural members such as panels, I-beams, trusses or the like, suitable chemical adhesion is an effective bonding means. 
     Conventionally, when bonding laminated wood with decorative veneers, several alternative bonding processes have been employed, as shown in FIG. 10. 
     [1] Cold Press System 
     In accordance with this system, both surfaces of a dried lamina are planed with a planer and a lamina having 15 mm to 40 mm in thickness is prepared. One surface or both surfaces of the lamina are then coated with a glue. In this manner, a plurality of laminae in a vertical relation, as shown in FIG. 10, are assembled. The assembled laminae are then clamped by a screw clamping means and pressed by a cold press means (oil pressure). For a time period of from several hours to 24 hours, the assembled laminae are heated and clamped. Then, pressure is released. 
     Curing in this system requires a full day in many cases, so that productivity is too low. To solve this disadvantage, the following two systems have been adopted. 
     [2] High Frequency Through Heating System 
     In accordance with this system, a plurality of laminae assembled one above another with an adhesive are clamped and pressed using a cold press. When the required pressure is obtained, high frequency energy is applied to the assembled laminae by way of two parallel electrode plates disposed on upper and lower surfaces of the assembled laminae. When the adhesive layers reach a curing temperature, the high frequency energy application is ceased and pressure is released. 
     [3] Hot Press System 
     In a third system, both surfaces of each wood strip are planed with a planer and finished smoothly. Then, both surfaces of or both sides of each strip are spread with glue. Subsequently, a dried veneer strip 0.5 to 2 mm in thickness is placed on the laminated wood, and all strips are pressed using a hot press. When the glue layer between the laminated wood and veneer strips has reached the required curing temperature, the pressure is released. 
     As mentioned above, in the hot press system the materials used are limited to thin materials. In the high frequency through heating system, high energy must be supplied, so that production cost is increased, and further, when employing the high frequency, a large power output must be supplied, so that production cost is increased. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore a general object of this invention to provide a process for bonding strips of lumbers by surface heating, which comprises a step of heating a surface of a first lumber strip, a step of spreading a surface of a second lumber strip with an adhesive, and a step of contacting closely the heated surface of the first lumber strip and the glue-coated surface of the second lumber strip to assemble the strips and then clamping the thus-prepared strips. 
     It is another object of this invention to provide a process for bonding lumber by surface heating, which comprises a step of jetting steam to a surface of a first piece of lumber prior to heating to equalize the surface temperature thereof and improve high frequency absorption. 
     It is another object of this invention to provide a process for bonding pieces of lumber by surface heating, in which the curing time as well as the power output required is reduced. 
     Other features and advantages of this invention will be apparent from the following description taken in connection with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING 
     FIG. 1 is a flow chart of a process according to a first embodiment of this invention. 
     FIG. 2 is a flow chart of a process according to a second embodiment of this invention, for which a laminated plate is applied. 
     FIG. 3 is a side view of a high speed bonding device according to the first embodiment. 
     FIG. 4 is a view of a laminated wood structure. 
     FIG. 5A is a view of a laminated box-beam. 
     FIG. 5B is a view of an I-beam. 
     FIG. 6 is a view of a wooden panel. 
     FIG. 7A is a view of a parallel chord truss. 
     FIG. 7B is a view of a roof truss. 
     FIG. 7C is a view of a curved truss. 
     FIG. 8 is a graph of a temperature distribution of a surface-heated lumber according to this invention wherein the first box represents surface heating using a hot press; the second box represents surface heating by high frequency and the third box represents surface heating with far infrared radiation. 
     FIG. 9 is a graph of comparison of a curing time with a power consumption. 
     FIG. 10 illustrates a prior art process. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of this invention will be described with reference to the accompanying drawing. 
     FIG. 1 shows a basic process of a first embodiment and a second embodiment according to this invention. The first embodiment will now be discussed with regard to the basic process. 
     (1) A first piece of dried lumber is planed by a planer or the like, and its surface is finished flatly and smoothly. 
     (2) Steam is jetted to the surface of this first of two pieces of lumber to be bonded together. In case where the steam pressure of a steam jetting device is 2 kg/1 cm 2 , the diameter of the jetting holes is about 1 mm, the spacing is about 10 mm×about 10 mm and the steam jetting time is 10 to 15 seconds, the temperature of the lumber at a distance of from 5 to 10 mm from the jetting holes is raised to 50° to 60° C. 
     (3) Subsequently, high frequency energy is applied to the steam-jetted surface of the first piece of lumber. The required high frequency energy is calculated suitably in accordance with the properties of the lumber being used. In the case of a high frequency heating device in which a plurality of parallel electrode grids are positioned relative to each other with a space of 20 mm, the required time is 30 to 45 seconds, and the surface temperature of the lumber is raised to 120° to 150° C. 
     (4) A surface of the other one of the two pieces of lumber to be bonded together is coated with a glue at a thickness of 200 to 300 g/m 2 . 
     (5) The steam-jetted and high frequency heated surface of the first piece of lumber is assembled to the adhesive-coated surface of the other piece of lumber. 
     (6) Within one minute from this assembling, the assembled pieces of lumber are clamped together and pressed for 1 to 2 minutes under the pressure of 5 kg to 15 kg/cm 2  by use of a cold press means. When releasing the cold press means, the two pieces of lumber are firmly bonded to each other. 
     It is to be understood that the heating of the lumber can be carried out by a hot press means or a far infrared radiation, in addition to the foregoing high frequency heating. 
     A second embodiment of this invention will be discussed with reference to FIG. 1. 
     (1) A finished surface of a first piece of lumber is coated with a glue. 
     (2) A surface of the second piece of lumber is heated to the temperature of 150° C. with a hot press means. 
     (3) Subsequently, the glue-coated surface of the first piece of lumber is assembled with the heated surface of the second piece of lumber. 
     (4) The thus-assembled pieces of lumber are clamped and pressed as soon as possible. 
     The foregoing clamping and pressing means is a cold press means in this embodiment. 
     The heating time of test of the foregoing hot press means was varied in three cases, with heating conducted for 360 seconds, 480 seconds and 600 seconds. In each case, the clamping and pressing time was the same. (The clamping conditions were as follows: cold press system, a cold plate, temperature: 20° C., clamping and pressing time: 3 minutes). 
     As shown in Table 1 below the bonding performance was the best where the heating time exceeded 480 seconds. Needless to say, the foregoing hot press means may be replaced with a far infrared radiation means. 
     The following are the test results of (a) a high frequency application means, (b) a hot press means, (c) a far infrared radiation means (none of (a), (b) and (c) employed a steam jetting system) and (d) a steam jetting high frequency application system. 
     
         ______________________________________(a) High frequency application means for surface heating:1)   MaterialSpecies:        Japanese larchSpecific gravity:                0.45 to 0.60Moisture content:                10 to 15%Dimension:      20 mm thickness × 150 mm                width × 300 mm length2)   AdhesionResolcynol resin adhesiveRatio of contents:            100 parts of adhesive,            15 parts of paraformaldehyde,            10 parts of fillerGlue coating:    200 to 300 g/m.sup.23)   Conditions of high frequency heatingHigh frequency power:                1.25 kw                (6.5 kv voltage,                0.38 A current)Type of electrode:                parallel electrode grid                (150 mm grid length, 20 mm                spacing between grids and 16                pieces in the number of grids)Heating time:   60 to 120 seconds4)   Conditions of cold pressTemperature of cold                20° C.plate:Clamping pressure:                7.5 to 10.0 kg/cm.sup.2Clamping time:  1.0 to 2.0 minutes(b) Hot press means for surface heating1)   Species:        Japanese larchSpecific gravity:                0.45 to 0.60Moisture content:                10 to 15%Dimension:      20 mm thickness × 150 mm                width × 300 mm length2)   AdhesiveResolcynol resin adhesiveRatio of contents:            100 parts of adhesive            150 parts of paraformaldehyde            10 parts of fillerGlue coating:    200 to 300 g/m.sup.23)   Condition of Hot press means for surface heating:Temperature of hot                150° C.plate:Clamping pressure:                2 to 3 kg/cm.sup.2Clamping time:  6 to 10 minutes4)   Conditions of cold pressTemperature of cold                20° C.plate:Clamping pressure:                7.5 to 10.0 kg/cm.sup.2Clamping time:  3 minutes(c) Far infrared radiation for surface heating1)   MaterialSpecies:        Japanese larchSpecific gravity:                0.45 to 0.60Moisture content:                10 to 15%Dimension:      20 mm thickness × 150 mm                width ×  300 mm length2)   AdhesiveResolcynol resin adhesiveRatio of contents:            100 parts of adhesive,            15 parts of paraformaldehyde            10 parts of fillerGlue spread:     200 to 300 g/m.sup.23)   Condition of far infrared radiationPower of heater:                0.75 kwDistance of radiation:                40 mmRadiation time: 60 to 120 seconds4)   Conditions of cold pressTemperatures of cold                20° C.plate:Clamping pressure:                7.5 to 10.0 kg/cm.sup.2Clamping time:  2 minutes(d) Steam jetting high pressure application for surfaceheating1)   MaterialSpecies:        Japanese larchSpecific gravity:                0.45 to 0.60Moisture content:                10 to 15%Dimension:      20 mm thickness × 150 mm                width ×  300 mm length2)   AdhesiveResolcynol resin adhesiveRatio of contents:            100 parts of adhesive,            15 parts of paraformaldehyde            10 parts of fillerGlue coating:    200 to 300 g/m.sup.23)   Conditions of steam jettingSteam pressure:  2 kg/cm.sup.2Jetting area:    150 mm ×  300 mmJetting hole:    1 mm diameter            10 mm × 10 mm spacingJetting time:    10 to 15 seconds4)   Conditions of high frequency heatingHigh frequency power:                2.5 kw (6.5 kv voltage,                0.77 A current)Type of electrode:                parallel electrode grid                (150 mm grid length, 20 mm grid                spacing, 16 pieces in the                number of grid)Heating time:   30 to 45 seconds5)   Conditions of cold pressTemperature of cold                20° C.plate:Clamping pressure:                7.5 to 10.0 kg/cm.sup.2Clamping time:  1.5 to 2 minutes______________________________________ 
    
     Table 1 sets forth the bonding performance obtained in each case under the foregoing bonding conditions as well as that obtained in a conventional case. 
     Table 1 (Bonding conditions and bonding performance in the first and second embodiments) 
     
         ______________________________________    Heating      Heating    conditions 1 conditions 2Bonding    Heating  Time      Heating                                Timesystem     system   (sec.)    system (sec.)______________________________________Cold       none               nonepressThrough    none               noneheatingby hotpressThrough    none               noneheatingby highfrequencySurface    none               hot    360heating                       press  480by hot                        15° C.                                600pressSurface    none               high   60heating                       fre-   90by high                       quency 120frequency                     1.25 KWSurface    none               heater 60heating                       0.75   90by far                        KW     120infraredradiationSteam      steam    10        high   30jetting/   jetting  12.5      fre-   37.5high                15        quency 45frequency                     2.5 KWappli-cation______________________________________Clampingconditions                 Heat     Power  Clamp-             Total  amount amountBonding  ing       Time     time   (°C.,                                   (KW,system system    (min)    (min)  min)   min)______________________________________Cold   Cold      1 day    1 daypress  press  at  20° C.Through  hot       18.0     18.0   1800heating  press     21.0     21.0   2100by hot at        24.0     24.0   2400press  100° C.Through  Cold      6.0      6.0           14.6heating  press/    7.5      7.5           18.3by high  high      9.0      9.0           21.9frequen-  frequen-cy     cy  1.25 KWSurface  Cold      3.0      9.0     900heating  press     3.0      11.0   1200by hot 20° C.            3.0      13.0   1500pressSurface  Cold      1.0      2.0           2.5heating  press     1.5      3.0           3.8by high  20° C.            2.0      4.0           5.0fre-quencySurface  Cold      2.0      3.0           0.7heating  press     2.0      3.5           1.1by far 20° C.            2.0      4.0           1.5infraredradiationSurface  Cold      1.50     2.2     20    2.5heating  press     1.75     2.4     25    3.2by     20° C.            2.00     3.0     30    3.8steamjetting/highfre-quencyappli-cation______________________________________Test Results of Bonding Performance            Delami-            nation            boiling  Reduc-            hot      tion ofBonding Knife    water    thick- Bonding                                   Woodsystem  test     soak     ness   strength                                   failure______________________________________Cold    ⊚            ⊚                     ⊚                            ⊚                                   ⊚pressThrough ∘ Δ  x            ⊚ ⊚ ⊚                     ⊚ ⊚ ∘                            ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ⊚heating ∘ ∘ ∘            ∘ ⊚ ∘                     ⊚ ⊚ .circleincircl                     e.     ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ∘by hot  ⊚ ∘ ∘            ⊚ ⊚ ⊚                     ⊚ ⊚ .circleincircl                     e.     ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ∘pressThrough ∘ Δ Δ            ∘ Δ Δ                     ⊚ ⊚ .circleincircl                     e.     ⊚ ∘ .circleinc                            ircle. ⊚ Δ .smallcir                                   cle.heating ∘ ∘ ∘            ⊚ ∘ ⊚                     ⊚ ⊚ .circleincircl                     e.     ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ⊚by high ⊚ ∘ ⊚            ⊚ ⊚ ⊚                     ⊚ ⊚ .circleincircl                     e.     ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ⊚frequencySurface Δ Δ Δ            ⊚ ⊚ ∘                     ∘  ∘ ∘                            ∘ ⊚ .smallcirc                            le.    ∘ ∘                                   ∘heating ∘ ∘ ∘            ⊚ ∘ ∘                     ⊚ ⊚ ∘                            ⊚ ∘ .smallcirc                            le.    ⊚ ∘                                   ∘by hot  ⊚ ∘ ∘            ⊚ ∘ ∘                     ⊚ ∘ ∘                            ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ⊚pressSurface x x x    x x x    ⊚ ⊚ .circleincircl                     e.     Δ ∘ ∘                                   Δ Δ ∘heating Δ ∘ Δ            Δ ∘ ∘                     ⊚ ⊚ ∘                            ∘ ⊚ .circleinc                            ircle. ∘ ⊚                                   ⊚by high ∘ ∘ ∘            ∘ ∘ ∘                     ∘ ∘ ∘                            ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ⊚frequencySurface x        x        ⊚                            x      xheating Δ  ⊚                     ⊚                            ⊚                                   ∘by far  ∘            ∘                     ⊚                            ∘                                   ∘infraredradiationSurface Δ Δ Δ            Δ ⊚ ∘                     ⊚ ⊚ .circleincircl                     e.     ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ⊚heating Δ Δ Δ            ⊚ ∘ ⊚                     ⊚ ⊚ .circleincircl                     e.     ⊚ ⊚                            ⊚                                   ⊚ ∘                                   ∘by steam   ∘ ∘ ∘            ⊚ ∘ ∘                     ⊚ ⊚ .circleincircl                     e.     ⊚ ⊚                            ⊚                                   ⊚ ⊚                                   ∘jettingand highfrequency______________________________________ 
    
     
         ______________________________________Remarks:           per-     per-          per-  per-     centage  centage       centage  centage  of dela- of     bonding                                  of wood  of wood  mination reduction                           strength                                  failureSymbol (%)      (%)      (%)    (kg/cm.sup.2)                                  (%)______________________________________⊚  60˜            ˜1                     ˜1                           90˜                                   70˜Δ  30˜60            1˜5                    1˜2                           70˜90                                  50˜70∘  10˜30            5˜10                    2˜5                           50˜70                                  30˜50x       0˜10           10˜                    5˜                            0˜50                                   0˜30______________________________________ 
    
     In the knife test, the degree of bonding immediately after the clamping pressure was released was examined. In the block shearing test, the bonding strength was examined. In the boiling water soak delamination test, durability of adhesion was examined. Further, the degree of dimensional reduction was examined. 
     When using the bonded pieces of lumber in practical applications, it is necessary that the following criteria be satisfied. In the knife test, the percentage of wood failure must be more than 30%. In the block shearing test, the bonding strength must be more than 70 kg/cm 2 , in the boiling water soak delamination test, the percentage of delamination must be no more than 5%, in the dimensional test, the percentage of reduction of thickness must be no more than 2%. 
     The bonding conditions for satisfying the foregoing criteria are as follows. The cold press system requires one day in a clamping time, the high frequency through heating system: 7.5 minutes in the high frequency heating time, the hot press system: 21 minutes in a hot press time and the steam jetting high frequency surface heating system: 3 minutes in the high frequency heating time. 
     Accordingly, the curing time in the steam jetting high frequency surface heating system is significantly less than that in the cold press system, 1/2.5 times less than that in the high frequency through heating system by and 1/7 times less than that in the hot press system. Further, the calorie consumption in the former is remarkably less than that in the hot press system and the consumed power supply in the former is 1/4.8 times less than that in the high frequency through heating system. 
     Accordingly, it has been shown that the steam jetting high frequency surface heating system is significantly more efficient than the prior art in terms of the required curing time and power supply consumption. 
     In addition, the steam jetting high frequency surface heating system is more effective in case of e.g. thicker pieces of lumber. Namely, for such pieces, the heating time in the high frequency through heating system must be increased by a ratio more than the ratio of increment of lumber mass, and the press time in the hot press system must be increased 1.5 power times more than that. The curing time of the steam jetting high frequency surface heating system according to this invention is the same even though the pieces of lumber being employed are thicker. 
     The second embodiment of this invention relating to a process for manufacturing laminated wood will now be described with reference to FIG. 2. 
     (1) Both finished surfaces of each of two laminae are coated with a glue. 
     (2) Both finished surfaces of each of three different laminae are heated using the hot press system. 
     (3) The two adhesive coated laminae and the three heat-treated laminae are layered upon each other such that the glue-coated laminae are sandwiched between the heat-treated laminae, as illustrated in FIG. 2. 
     (4) The five laminae thus-assembled are then clamped and pressed as soon as possible. 
     The foregoing clamping process may utilize a clamping means for a straight laminated wood as well as for a curved laminated wood, as shown in FIG. 2. 
     The clamping means for the straight laminated wood employs a cold press system by oil pressure, and that for the curved laminated wood employs a screw clamping system. 
     The test conditions for the straight laminated wood were as follows. 
     (a) The heating temperature by the hot press was 150° C. and the heating time was 7.5 min., 10 min., and 12.5 min. respectively. 
     (b) Assembly time: None. 
     (c) The working time for clamping was 2 to 3 minutes. The pressure was maintained for 7.5 min., 10 min., and 12.5 min. respectively likewise in the foregoing heating time. As shown in Table 2 as listed hereinafter, the bonding performance was the best in case where the heating time as well as the pressure maintenance time were each more than 10 minutes. 
     The test conditions for the curved laminated wood were as follows: 
     (a) The heating temperature by the hot press was either 120° C. or 150° C. and the heating time was varied from 6 to 12 minutes. 
     (b) Assembly time: 0 to 2.5 minutes. 
     (c) The working time for clamping varied from 1.5 to 3.5 minutes and the pressure maintenance time varied from 5 to 7.5 minutes. 
     As shown in Table 2 below, the bonding performance was the best in the following case: a heating temperature of 120° C. with a heating time of 10 to 15 minutes or a heating temperature of 150° C. with heating time 6 to 12 minutes, assembly time: zero, working time for clamping: with 3.5  minutes, clamping maintenance time: 7.5 minutes. 
     The following are the test data of the foregoing two tests for the straight laminated wood and the curved laminated wood respectively, for which the surface heating system by the hot press was adopted. 
     
         ______________________________________(a) In case of the straight laminated wood:1.   MaterialSpecies:        Douglas firSpecific gravity:                0.05 to 0.60Moisture content:                10 to 15%Dimension:      35 mm thickness × 150 mm                width ×  2,000 mm lengthNumber of laminations:                5 layers2.   AdhesiveResolcynol resin adhesive agentRatio of contents:            100 parts adhesive            15 parts of paraformaldehyde            10 parts of fillerGlue coating:    250 g/m.sup.23.   Heating conditions by the hot pressTemperature of heat            120° C., 150° C.disc:Clamping pressure:            2 to 3 kg/cm.sup.2Clamping time:   7.5 min., 10 min., 12.5 min.4.   Conditions of the cold pressTemperature of cold            20° C.plate:Clamping pressure:            10 kg/cm.sup.2Clamping time:   7.5 min., 10 min., 12.5 min.(b) In case of the curved laminated wood1.   MaterialSpecies:        Douglas firSpecific gravity:                0.50 to 0.60Moisture content:                10 to 15%Dimension:      35 mm thickness × 150 mm                width ×  2,000 lengthNumber of laminations:                5 layers2.   AdhesiveResolcynol resin adhesive agentRatio of contents:            100 parts of adhesive agent            15 parts of paraformaldehyde            10 parts of fillerGlue coating:    250 g/cm.sup.23.   Heating conditions by the hot pressTemperature of hot disc:            120° C., 150° C.Clamping pressure:            2 to 3 kg/cm.sup.2Clamping time:   6 to 15 minutes4.   Conditions of screw clampingClamping space:  250 to 300 mmRadius of curvature:            2500 mm, 3000 mm, 3500 mmWorking time for 1.5 to 3.5 min.clamping:Pressure maintenance            5 min., 7.5 min.time:______________________________________ 
    
     
                       TABLE 2______________________________________               [ 3][1]      [2]             [10][4]  [5]     [6]    [7]  [8]  [9]  [11]  [12]  [13]______________________________________150  7.5                 2.00 7.5  ⊚⊚.circle                              incircle.                                    ⊚⊚.                                    smallcircle.                                          Δ⊚.DEL                                          TA.150  10.5    [14]   [15] 3.00 10.5 ∘⊚.smallcirc                              le.   ∘⊚.sma                                    llcircle.                                          ∘.circleincircl                                          e.∘150  12.5                1.75 12.5 ⊚⊚.smallc                              ircle.                                    ⊚⊚.                                    circleincircle.                                          ⊚.circleinci                                          rcle.∘120  10.0           3000 3.50 7.5  ∘⊚.circleinc                              ircle.                                    ⊚⊚.                                    circleincircle.                                          ∘∘.                                          smallcircle.        [16]120  15.0           3000 2.00 7.5  ∘∘∘                                    ⊚⊚.                                    circleincircle.                                          ⊚.smallcircl                                          e.⊚150  6.0            3000 1.50 7.5  ⊚⊚.circle                              incircle.                                    ⊚⊚.                                    circleincircle.                                          ⊚.circleinci                                          rcle.⊚150  8.0     [16]   3000 1.50 7.5  ∘∘∘                                    ⊚⊚.                                    circleincircle.                                          ⊚.circleinci                                          rcle.∘150  10.0           3000 1.50 7.5  ∘⊚.circleinc                              ircle.                                    ∘⊚.cir                                    cleincircle.                                          ⊚.smallcircl                                          e.∘150  12.0           3000 1.50 7.5  ⊚∘.circleinc                              ircle.                                    ∘⊚.cir                                    cleincircle.                                          ∘.circleincircl                                          e.∘150  12.0           2500 1.75 7.5  ∘∘∘                                    ⊚⊚.                                    circleincircle.                                          ∘.circleincircl                                          e.∘        [16]150  12.0           3500 1.25 7.5  ⊚∘.smallcirc                              le.   ⊚∘.DEL                                    TA.   ⊚.circleinci                                          rcle.∘150  12.0    [16]   3000 2.75 5.0  x x x x x x x x x        [16]--   --             3000 3.00 (1   ⊚⊚.circle                              incircle.                                    ⊚⊚.                                    circleincircle.                                          ⊚.circleinci                                          rcle.∘                         day)______________________________________Remarks:           Percentage                     Percentage  Bonding  of wood   of  strength failure   delaminationSymbol (kg/cm.sup.2)           (%)       (%)  100˜           70˜  ˜5   75˜100           50˜70                      5˜10   50˜75           30˜50                     10˜30   0˜50            0˜30                     30˜100______________________________________[1]  Heating conditions by the hot press[2] Clamping conditions by the cold press or screw clamping[3] Test Results of the bonding performance test[4] Heating temperature (°C.)[5] Heating time (min.)[6] Clamping system[7] Radius of curvature (mm)[8] Working time for clamping (min.)[9] Pressure maintenance time (min.)[10] Block shearing test[11] Bonding strength[12] Percentage of wood failure[13] Delamination due to boiling water soak[14] Cold press[15] In case of the straight laminated wood______________________________________ 
    
     The bonding strength was examined by the block shearing test, while the bonding durability was examined by the boiling water soak delamination test. 
     If in the block shearing test, the bonding strength is no less than 75 kg/cm 2  and the percentage of wood failure is no less than 50%, it has been confirmed that there is no obstacle for practical use. If the percentage of delamination is less than 10% in the boiling water soak delamination test, it has been confirmed that there is no obstacle for practical use. 
     The bonding conditions by the hot press for satisfying the foregoing criteria (Japanese Agricultural Standard for structural laminated timber) were as follows. In case of a straight laminated wood having five layers, with each layer having a thickness of 35 mm, the heating time and pressure maintenance time were 10 minutes and 12.5 minutes respectively at a temperature of 150° C. In case of a curved laminated wood having five layers, each layer having 20 mm thickness, the heating time was 10 or 15 minutes at a temperature of 120° C., while that was 6 to 12 minutes and the pressure maintenance time was 7.5 minutes at a temperature of 120° C. and 150° C. 
     Accordingly, when the surface heating system by the hot press is applied to the straight and curved laminated wood, the curing time is shorter than a conventional system (requiring one day) at a room temperature. 
     Further, the bonding performance of the laminated wood produced by the foregoing process is the same as that by the conventional curing system. 
     Since the working time for clamping is very short, e.g. 1.25 to 3.5 minutes, it is required to make it more efficiently in case of a large or long laminated wood. 
     An example of a rapid bonding device that can be used in accordance with the present invention will be discussed with reference to FIG. 3. 
     The rapid bonding device comprises a glue spreading means 1, a steam jetting means 2, a high frequency heating means 3 and a continuous clamping means 4, which device is suitable for producing continuously in particular a 3-layer laminated wood structure or I beam. Each component of the system is linked with the others by a large number of feeding rollers R. When the pieces of lumber pass through a space between upper and lower rollers R, the glue is spread, the stem in jetted, the high frequency is applied for surface heating and finally the clamping is carried out. 
     More specifically, both surfaces of a piece of lumber W2 are coated with a glue in the glue-spreading means 1. In addition, one surface of a piece of lumber W1 and one surface of a piece of lumber W3, both of which face both the glue-coated surfaces of lumber W2, are steam-jetted by steam jetting means 2, surface-heated by high frequency heating means 3 and then all the lumber pieces W1, W2 and W3 are firmly clamped by continuous clamping means 4 comprising upper and lower caterpillars C rotatable by a plurality of driving wheels M. During the time when lumber pieces W1 and W3 pass through a space of the continuous clamping means 4, both lumber pieces W1 and W3 are bonded with the piece of lumber W2, and a finished product W is thereby obtained. 
     FIGS. 4 to 7C are various examples of the bonding process according to this invention, in which FIG. 4 is a view of a wood laminate. FIG. 5A is a view of box-beams wood. 5B is a view of an I-beam wood, FIG. 6 is a view of a wooden panel, FIG. 7A is a view of a parallel chord truss, FIG. 7B is a view of a roof truss and FIG. 7C is a view of a curved truss. The bonding of the truss is carried out on each gusset (not illustrated) in a joint portion between components. 
     FIG. 8 is a graph of a temperature distribution from a heated surface (A) of a piece of lumber to a rear surface (E) thereof at the time of surface heating. As the heating time is increased, the temperature of the surface and interior of the lumber is raised. In case of a suitable heating time (a central curve line in each graph), it has been found that the surface temperature was 120° to 150° C. and the temperature of an area 5 mm away from the surface was 70° to 80° C., which is the curing temperature of Resolcyl resin adhesive. 
     Further, the temperature slope from the surface is more moderate in the hot press and high frequency heating and far infrared radiation in that order. Further, heat accumulation is more and bonding performance is more stable in that order. 
     As discussed previously, a surface of another lumber to be bonded with a glue-coated surface of one lumber is preheated, so that the bonding and curing time is shortened greatly and only the surface heating is required. Therefore, any waste heating energy is completely removed. 
     Prior to the surface heating, a steam is jetted to a surface of a lumber, so that the surface is observed by the steam. Thus, a density difference between spring wood and summer wood is reduced, thereby the heating process by use of high frequency application becomes more efficient and uniform. 
     Further, since the steam is jetted prior to the high frequency application, any cup of lumber that may occur in connection with the high frequency surface heating is eliminated. 
     Still further, as shown in FIG. 9, the bonding and curing time is greatly shortened and the power supply is also reduced on a full-scale. In addition, a rapid continuous production is realized, and its productivity is very high.