A Method of Manufacturing A Veneer Article by Hot Pressing, And A Press Tool For Carrying Out The Method.

Background of The Invention This invention relates to a method of manufacturing, by hot pressing, a polygonal veneer article consisting of one or more adhesively bonded veneer plies and having a flat or substantially flat edge zone and a central panel which are connected by means of a strongly profiled transition zone.

It is known from Danish patent specification No. 138 628 to manufacture plywood articles by hot pressing of a plurality of superposed veneer plies coated with adhesive, and it is explained in the patent specification that a smooth surface of the finished article can be obtained by previously- apply¬ ing onto one of the outer plies an external coating capable of absorbing surface irregularities -in the veneer ply. The method has, however, proven unsuit¬ able for manufacturing strongly profiled articles having sharp bends and abrupt transitions between successive sections of the profile because folds and/or cracks occur in the profiled portions of the article after the lapse of shorter or longer time. Summary of The Invention It is an object of the present invention to remedy the deficiency of the prior art pointed out above, and according to the invention there is provided a method of manufacturing a polygonal article consisting of a single veneer ply or of a plurality of adhesively bonded veneer plies and having a flat or substantially flat, circumferential edge zone, a central panel and a strongly profiled transition zone -connecting the edge zone and the central panel, which method comprises subjecting a

flat workpiece consisting of the required number of veneer plies to the combined action of heat and transverse compression forces, characterized in that at least one veneer ply, which in the finished article constitutes an external ply, is formed with an incision at each corner of the ply, said incision comprising a wedge-shaped portion which extends through the transition zone with its apex located at the junction between that zone and the central panel, and which continues in a portion having parallel edges extending through the edge zone to the outer periphery of the ply.

The invention is based on the insight that the- above mentioned folds and cracks , v/hich sooner or later occur in the finished article, are due to the relief of internal stresses in the article ^" resulting from the pressing operation because the moulding or reshaping of the profiled transition _^ zone requires veneer material to be pulled from the edge zone inwardly towards the central panel. Since that material cannot be compressed and shortened in the plane of the veneer ply it will instead tend to fold. It is true that the pressing force will smooth out the folds, but this results in said internal stresses which are relieved at a later time.

The invention eliminates in advance said folding tendency of the veneer ply or plies in which incisions are made prior to the pressing because the edge and transition zone of the veneer ply is thereby divided into individual and mutually separated sections each of which is capable, during the moulding operation, of moving inwardly towards the central panel independent of the adjoining sec¬ tions . It will be understood that the gap width in the outer, rectilinear portion of the incision and,

hence, the maximum width of the inner wedge-shaped portion is determined by the geometry of the finished article and that it should preferably be chosen such that the incision has only just closed when the moulding operation is terminated. During the pressing of a multi-ply article the adhesive between the veneer plies and, where applicable, the previously mentioned coating on the outer ply will flow into the gap, and after solidification it will not only bond the juxtaposed gap edges to one another but it will also make the joint virtual¬ ly invisible.

In several cases, in particular when the finished article is to be used as a facing sheet of a sandwich element, such as a door for cabinets or other furniture, or a wall panel, it has i ^' n ^" practice been found sufficient to make incisions solely in one outer ply since cracks and folds formed in the other, invisible plies can be filled up by the adhesive during the pressing operation. Evidently, it is nevertheless possible to effect incisions in several or all veneer plies and it is then advisable, with regard to the strength properties of the finished article, to ^' stagger the incisions in the individual plies such that they do not overlap.

In a preferred embodiment of the invention the line bisecting the angle of the wedge-shaped portion of each incision coincides with the line bisecting the angle included between the contiguous sides of the central panel.

A further embodiment of the invention is characterized in that the veneer ply is composed of a central panel and separate edge panels joined to the edges- of the central panel and each

corresponding to a polygon side of the transition and edge zones of the finished article, and in that the incisions in the veneer ply are formed by pre¬ paring the opposed edges of the edge panels with corresponding clearances in the flat state. By cutting the edge panels such that the grain extends by and large in the longitudinal direction of each panel it is possible to manufacture an article which, when used as a facing sheet of a sandwich element, illudes as a frame of solid wood surrounding a veneered panelling formed by the central panel.

The invention also relates to a press tool for carrying out the method and comprising an upper tool and a lower tool guided for movement towards and away from another. The tool is characterized in that the lower tool has upstanding stops for -position¬ ing the flat veneer ply or plies by abutment against the veneer' edges. This feature ensures an exact location of the incisions in the veneer ply or plies relative to those areas of the tool v/hich mould the strongly profiled transition zone.

According to a feature of the invention at least that part of the upper tool, which moulds the transition zone, may be a separate part which is displaceable, relative to the remainder of the upper tool, towards and away from the lower tool and spring biased towards the latter. In this way it is ensured that the reshaping or moulding of the strongly profiled transition zone has been terminated and correspondingly the incisions have closed before the remaining areas of the veneer workpiece are clamped between the upper and lower tools.

Brief Description of The Drawings The invention will be described in more detail below with reference to the schematical drawings in which

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Fig. 1 is a plan view of a cabinet door made as a sandwich element with a facing sheet manufactured by a method according to the invention,

Fig. 2 is a section on a larger scale along line II-II of Fig. 1,

Fig. 3 is a plan view approximately on the same scale as Fig. 2, showing the outermost veneer ply of the facing sheet of the sandwich element prior to the pressing of the facing sheet, and Fig. 4 is a section on a still larger scale through that portion of a press tool embodying the invention which serves for moulding the transition zone of the facing sheet.

Detailed Description In Figs. 1 and 2 there has been illustrated a sandwich element 1 consisting of a core 2, -edge strips 3 along the sides of 'the core, and two veneer facing sheets 4 and 5 in engagement with and bonded to the opposed surfaces of core 2 and to the edge strips 3 such that they extend to the edges of the element. Core 2 can consist of a foamed plastic material with or without a filler or it can be made as a slab of chipboard. In the drawing element 1 has been shown with a profiled facing sheet 4 and a planar facing sheet 5 but if desired both facing sheets could be profiled with the same or with different cross-section profile.

As shown in Fig. 4 facing sheet 4 is composed of three adhesively bonded veneer plies, viz. a ply 6 of noble veneer which in the finished sandwich element is outwardly oriented, and two so-called backing plies 7 and 8 which can be made of cheaper veneer. As shown in Figs. 1 and 3 outer veneer ply 6 is composed of a central panel 9 and separate edge panels 10 and 11 extending along the longer

and shorter sides, respectively, of the central panel and adhesively bonded thereto. The edge panels may be cut from the veneer in such a way that within each panel the grain extends by and large in the direction of the edges of the finished facing sheet 4 whereby the corresponding edge panels of the facing sheet of the finished sandwich element, which in Fig. 1 have been designated by 10 ^l and ll ¹ , respectively, illude as solid edge strips. The press tool for compression-moulding facing sheet 4, as shown schematically in Fig. 4, comprises • a lower tool 12 and an upper tool 13 which both in¬ clude means (not shown) for heating the tool. Each of the two tool parts is mounted on an associated platen 14 and 15, respectively. The pressing surfaces of tools 12 and 13, which are oriented towards-one another, have been profiled corresponding to the desired shape of the finished facing sheet 4 with a flat circumferential edge zone 16 comprising panels 10' and 11', the weakly profiled central panel 9' and a strongly profiled transition zone 17 (see in particular Fig. 2) intermediate the edge zone and the central panel.

It will be understood from Figs. 2 and 4 that during the pressing operation the veneer material within the transition zone 17 is subjected to some elongation and this elongation is accompanied by a displacement of the veneer within edge panels 10 and 11 inwardly towards central panel 9. In order to permit this inward displacement without the detrimen- tal effects mentioned above the edge panels 10 and

11 of the initial flat veneer ply 6 are shaped as shown in Fig. 3. This implies that at each corner of the flat veneer ply there is a gap which from the periphery of the veneer ply extends inwardly with

parallel edges in a rectilinear portion 17 by and large to that point which in the finished article constitutes the exterior corner of transition zone 17. From there the gap continues as a wedge-shaped portion 19 terminating in an apex at the interior corner of transition zone 17, i.e. at the corner of central panel 9.

As shown in Fig. 4 that part of upper tool 13, which moulds the strongly profiled transition zone 17 and the adjoining, less strongly profiled area of the central panel 9' of the facing sheet, has been made as a separate, frame-like member composed of bars 20 extending parallel to the edges of the workpiece and guided relative to the remainder of the upper tool in such a way thatthe frame is separately movable towards and away from the -lβ er tool. The frame is spring loaded towards the lower tool by means of schematically shown compression springs 21 operative between the frame and platen 15. For manufacturing the profiled facing sheet 4 the three veneer plies 6, 7, and 8 are inserted in flat state into the open press tool together with intermediate adhesive films (not shown) which become activated at the pressing force and temperature .so that the bonding and the compression moulding take place in one operation. The intermediate adhesive films could be replaced by an application of adhesive onto the veneer surfaces. In both cases it is advis¬ able to apply to the outwardly oriented surface ^" of the external ply 6 a thin film of a material, which by the application of heat and pressure becomes hard and tough, in order to ensure a smooth surface of the finished facing sheet.

In order to ensure that the incisions or gaps 18, 19 at the edges of veneer ply ^* 6 are-exactly

located relative to the profiled parts of tools 12 and 13, upwardly projecting stops 22 have been se¬ cured to platen 14 of the' lower tool for abutment against the precision-cut edges of the veneer ply. For obtaining a good result it is, furthermore, important that the tool parts 12 and 13 are moved towards one another at a relatively low speed so that the necessary time is obtained for the previous¬ ly mentioned inward displacement of veneer material from edge zone 16 and for the moulding of the material within the strongly profiled transition zone 17.

When manufacturing sandwich elements as those shown in Figs. 1 and 2 with a core of e.g. chipboard, the lower tool shown can be replaced by the pre- fabricated core with edge strips and one of the facing sheets glued thereto whereby the opposed facing sheet is pressed and bonded to the core in one operation. Especially in this variant the compres¬ sion moulded facing sheet may often consist of one veneer ply only.

As examples of other veneer articles , which can be manufactured by the technique described above, there may be .mentioned waiter's trays, side and gable members for bed frames, front members for drawers, ceiling and wall panels, side panels for loudspeaker cabinets and various decorative elements. For manufacturing finished articles with two visible surfaces, as opposed to the described facing sheets for sandwich elements, it may be advisable, with regard to the appearance of the finished article, to effect the described incisions at least in both outer plies. The polygonal articles in question may also be made with rounded corners.

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