Improved opposed belt drier for wood veneers

Abstract: In a wood veneers drier (10) of the type in which the veneers are fed between two belts (12, 13) one above the other along a path, which is generally sinusoidal, formed by a plurality of guide rollers (16) around which the said belts partially wind, the said guide rollers are disposed along the said generally sinusoidal path followed by the pair of belts. In consequence, the said path comprises a plurality of rectilinear sections connected one to the next by very short arcuate sections proximal to the pitch points between the said pair of belts and each guide roller, where each arcuate section preferably subtends an angle of proximately 40°-50°.

Description: The present invention relates to a certain number of improvements made to a drier for wood veneers, of the type featuring overlaid belts. As is known to persons with ordinary skill in the art, wood veneers are dried by the combined effect of pressure and heat inside driers designed for the purpose. The pressure can be applied either by passing the wood veneers through successive pairs of opposed pressure rollers, which also move the veneers forwards, or by disposing the veneers between two overlaid belts which move along a rectilinear path within the drier. The driers featuring pressure rollers produce a more pronounced flattening than the belt-type driers. However, because of the elevated pressures entailed, roller-type driers do not lend themselves satisfactorily to the treatment of thin veneers made from delicate, superior woods. In addition, on account of their scant solidity, the thin veneers have to be guided on their path from one pair of rollers to the next, which fact prevents them from shrinking freely and causes flaws to occur in them. The driers with overlaid belts, on the other hand, with a simple rectilinear path, have the disadvantage that the weight of the upper belt may not suffice to ensure that the veneers are satisfactorily flattened. To overcome the aforesaid disadvantage of belt-type driers, it has already been proposed to cause the belts to take a generally sinusoidal path instead of a rectilinear one, and a drier of this type is for example described and illustrated in German patent 12 66 233. As a result of the sinusoidal path they follow, the belts are enabled to bring an adequate flattening pressure onto the veneers; however, such pressure is permanent throughout the drying cycle and does not permit the veneers to shrink freely without risk of causing flaws or cracks. For the veneers are at every instant obliged to follow a curvilinear path during which they are practically always in contact with the guide rollers on which the overlaid belts containing the veneers wind. A proposal for overcoming this drawback is contained in European patent 0152576, and it provides, between one guide roller and the next -the axes of rotation of which are all in one plane- for a rectilinear section of length equal to at least half of the maximum width of the veneers. In this manner the veneers follow ample curvilinear sections around the relatively large-diameter guide rollers alternated with rectilinear sections between one guide roller and the next. Such proposal is not however satisfactory; for if it is true that the veneers are free to shrink in the appropriately sized rectilinear section between one guide roller and the next, in the curvilinear sections having a relatively high width, the pressure brought to bear, at least in respect of certain very thin special and also superior wood veneers, can in any case be such as to cause micro-fissures which cause the surface porosity of the veneers to vary, thus creating problems in the subsequent lacquering operations. In addition, the arrangement of all the guide rollers in one and the same plane represents a very considerable limitation as regards mounting nozzles able to blow warm air directly onto the veneers contained between the overlaid belts. Because of this, the drying process becomes relatively lengthy, unless costly heating elements are provided within each roller. The overall object of the present invention is to obviate the aforesaid drawbacks of the known art by embodying a drier for wood veneers, of the type with overlaid belts, in which the path of the belts is such as to allow application of an adequate flattening pressure also to thin, high-quality wood veneers, to allow a shrinkage without fissuring, and to permit a straightforward and economical mounting of nozzles able to blow warm air directly onto the veneers, so as to speed-up the drying process. In accordance with the invention, the said object is achieved by a wood veneer drier of the type disclosed in claim 1. The structural and functional characteristics of the invention and its advantages over the known art will become more apparent from an examination of the following description, reffered to the appended drawings in which: Figure 1 is a longitudinal section illustrating an example of a drier embodied according to the invention; and Figure 2 is an enlarged particular thereof. With reference to the drawings, the drier in question is indicated overall by 10 and consists structurally of a heated tunnel 11 through which the veneers (not shown) are moved forward between a pair of overlaid conveyor belts 12, 13, upper and lower respectively, each moved continuously along the endless paths shown diagrammatically in Figure 1 of the drawings. The overlaid conveyor belts 12, 13 can for example consists of metallic nets, and can comprise a first rectilinear section 14 followed by a second generally sinusoidal shaped section 15. A drier of such type is well-known to persons with ordinary skill in the art and is for example described and illustrated in European patent application 0152576. According to the present invention, instead of being formed as in the known art of a series of some few large-diameter guide rollers all lying in one and the same plane (with the disadvantages previously mentioned), the said sinusoidal path of the overlaid belts is embodied in the form of a plurality of small-diameter rollers 16 which are disposed in different planes, following the same sinusoidal path as the overlaid conveyor belts. Thus, as is clearly shown in Figure 1, the sinusoidal path of the belts 14, 15 consists of a broken line comprising a plurality of rectilinear sections connected by very short arcuate sections proximally to the pitch points of the belts vis-à-vis the rollers. The forward movement of the said belts 14, 15 can be controlled by motorizing either the four rollers at the lower apices or the four rollers at the upper apices of the sinusoid. If necessary, the non-motorized rollers can be adequately frictioned. It will therefore be evident that, in a drier in accordance with the invention, the disposition of the guide rollers is such as to permit applying to the overlaid belts a tension sufficient to transmit the requisite flattening pressure to the veneers, although with the arcuate path around the said guide rollers minimized, and such as to permit the veneers to shrink even if, during the passage inside the drier, they are always in contact with at least one roller. For the plurality of the curvilinear sections proximal to the pitch points of the guide rollers brings to bear a non-permanent flattening pressure so as to make it possible to regulate shrinkage without causing micro-fissures in the veneers even in the case of very concentrated loads. In addition, as the travel of the overlaid belts around the guide rollers is minimized, the intactness of the veneers, even if delicate and very thin, is assured. To such end the overlaid belts can on each roller 16 go through an arc subtending an angle of about 40°-50°. Moreover, with the guide rollers disposed in the manner described it becomes possible to mount on each side of the belts nozzles 17 for blowing warm air directly onto the belts, the which cannot be done in the case of driers of known type because of the bulk of the guide rollers all lying in the same plane. The drier according to the invention also provides, downstream, for a cooling chamber 18 where cold air is blown onto the veneers coming from the drier, through a series of nozzles 19. Characteristically, within the said chamber 18, the veneers are moved forward along an at least partially sinusoidal path equal to that of the drier. For such purpose the veneers are fed between a pair of overlaid belts 20, 21 guided by guide rollers 22. It is in this way advantageously assured that the veneers remain in the same condition as when they left the drier. As clearly shown in Figure 2, two further rollers 23 of even smaller diameter can optionally be provided between two guide rollers 16, in order to increase the number of points where a slight pressure is applied to the veneer.

Claims: Wood veneers drier, of the type in which the veneers are caused to move forward between two overlaid belts along a path, generally sinusoidal, formed by a plurality of guide rollers around which the said belts partially wind, characterized in that the said guide rollers are disposed along the same generally sinusoidal path followed by the pair of belts, so that the said path comprises a plurality of rectilinear sections connected to each other by very short arcuate sections proximal to the pitch points between the said pair of belts and each guide roller, each loop of the sinusoidal path being formed by a plurality of rollers, said arcuate sections subtending an angle less than 50°. Drier as described in claim 1, wherein each of the said arcuate sections subtends an angle larger than about 40°. Drier as described in claim 1, wherein on both sides of the said generally sinusoidal path, also in the spaces between one guide roller and the next, there are disposed nozzles for blowing warm air only directly against the said pair of belts. Drier as described in claim 1, wherein the movement is transmitted to the said overlaid belts only by the rollers proximal to the upper and lower apices of the sinusoid. Drier as described in claim 1, wherein the plurality of the curvilinear sections proximal to the pitch points of the guide rollers exerts a flattening pressure in a non-permanent form so as to allow the veneers to shrink normally without giving rise to micro-fissures even in the case of very concentrated loads.

Inventors: CREMONA LORENZO, CREMONA, LORENZO
Applicants: CREMONA ANGELO & FIGLIO, ANGELO CREMONA & FIGLIO S.P.A.
Application year: 1988
Main classification: F26B15
IPC codes: F26B13, F26B15