EP0532376A2 - Process for the manufacture of cylindrical pipes from wood veneer and product thereof - Google Patents

Abstract

The invention relates to a process for the manufacture of cylindrical pipes from wood veneer in which, in a first phase, a first pressing operation is carried out in a flat press on at least one assembly of veneers in order to form a multi-ply structure, this multi-ply structure is allowed to deform into a tile shape at the outlet from the press, and in a second phase, this multi-ply structure is pressed in an assembly press in order to form the cylinder, the multi-ply structure comprising at least three plies, its structure being asymmetric, one face of the multi-ply structure consisting of a veneer (1) in which the direction of the wood grain is parallel to the axis of the cylinder to be formed, the other face consisting of a veneer (2) in which the direction of the grain is perpendicular to the axis of the cylinder, the tile-shaped deformation at the outlet of the flat press being such that it corresponds to a bending whose radius is substantially equal to the radius of the pipe to be manufactured, the pressing of the second phase essentially consisting in connecting the ends of at least this multi-ply structure. <IMAGE>

Description

The present invention relates to a method of manufacturing cylindrical pipes based on wood veneers.

In the current state of the art, by pressing around a cylindrical mandrel using two semi-cylindrical molds, two stacks of glued veneers are joined to each other by their longitudinal edges so as to no longer form only one cylinder.

A method currently used consists in stacking several veneers glued wire on wire, that is to say with their respective wood wire parallel to that of the others, and so that this direction of the wire is parallel to the axis of the mandrel. central. This double parallelism, on the one hand of the wires between them, on the other hand of the wires with the central axis, is necessary to allow sufficient flexibility for forming. The two heated semi-cylindrical molds are brought together and tighten the two stacking assemblies around the central mandrel: at the start the forming is done cold, before the calories from the molds have hardened the glue, then the hardening takes place while the assembly forms only one cylinder. The two stacking assemblies were made in a size such that the sum of their two widths is equal to or slightly greater than the circumference of the central mandrel. The pointing of the edges parallel to the axis is then done either by simple docking, or by overlap.

The cylinder thus obtained, after opening the two half-molds and releasing the central mandrel, has a high resistance to bending which makes the fibers parallel to their yarn, that is to say in elongation where the value is maximum, but a low tensile strength which makes the fibers work perpendicular to their yarn, that is to say in tearing where the value is minimal. In particular, the cylinders thus produced have poor resistance to internal pressures during use.

Document US-A-2 449 526 discloses the manufacture of cylindrical pipes from several veneers with two plies of different wood grain direction which can bend naturally, the thickness of the double ply veneers being such that it allows the additional folding of the veneer using a press to give it the curvature of the cylinder to be manufactured by using several double ply overlapping plies. During this additional bending, the veneers are subjected to stresses which ultimately prove to be detrimental to the compressive and tensile strength of the pipes produced.

The invention provides a method of manufacturing cylindrical pipes based on wood veneers which takes advantage of the anisotropy characteristics of wood without creating additional bending constraints.

According to the method of the invention, as starting material is used at least one assembly intended to form a multiply with at least three plies, of asymmetrical structure, an outer face of the multiply consisting of a wooden veneer whose direction of the wood fiber wire is parallel to the axis of the cylinder to be formed, the other outer face of the plywood being made of a wooden veneer whose direction of the wood fiber wire is perpendicular to the axis of the cylinder, a pressing is carried out in a flat press of at least this assembly, then the multiply obtained is allowed to deform into a tile at the exit of the press so that it takes a bending of radius substantially equal to the final radius of the cylinder to be manufactured, and the ends of at least this multiply are connected in an assembly press. Of course, all the faces of the wooden veneers intended to come into contact with an element used in the composition of the plywood are glued.

The bending of the plywood according to the invention is therefore essentially carried out by the phenomenon of natural bending of the asymmetrical plywood, and it does not require additional bending constraints. The product obtained by this process then has very good mechanical properties, in particular very good tensile and compressive strength.

When the starting plywood is an assembly of three plies, the third ply, intermediate, can be a glued wooden veneer similar to one of the two external plies of your choice according to the desired bending, knowing that an intermediate ply presenting a fiber thread parallel to the axis of the cylinder, that is to say parallel to the largest dimension of the cladding, will accentuate the bending. On the contrary, the choice of an intermediate fold in the direction of the perpendicular wire will lead to a reduction in bending. In a variant, the third ply is an intermediate ply which is not a wooden veneer. This fold subsequently designated as a separator promotes the relative sliding of the two outer folds during bending.

This separator is made of a material different from wood, it can be a non-woven or needled plastic film such as polyester.

The radius of curvature of the natural bending obtained from the asymmetric plies according to the invention depends on different parameters, among which: the thickness of the veneers, the material or quality of the veneer, the number of veneers, the distribution of these veneers according to the orientation of the wood fiber wire in the plywood, knowing that in the plywood used in the context of the invention, the veneers having the same orientation of the wood fiber wire are preferably adjacent to each other.

The veneers used as constituent elements of the plywood according to the invention have a thickness of between 0.5 and 3 mm and preferably between 0.5 and 2.5 mm. The length and width of the veneers are directly related to the size of the cylinder to be manufactured as explained below.

Bending takes place naturally around an axis parallel to the largest dimension (length) of the veneers and with a concavity oriented on the side of the exterior veneer having the grain of the wood fiber parallel to the length of the veneers. So that the width chosen for the cladding then corresponds to the circumference of the cylinder to be produced when the cylinder is formed from a single plywood.

ou $\frac{\text{1}}{\text{3}}$

ou $\frac{\text{1}}{\text{4}}$

, etc, de la circonférence du cylindre.When the cylinder is formed from 2 or 3 or 4, etc ... plywood arranged one next to the other to form a circle, the width of the plywood is then a fraction equal to the inverse of an integer, that is to say, equal to $\frac{\text{1}}{\text{2}}$

or $\frac{\text{1}}{\text{3}}$

or $\frac{\text{1}}{\text{4}}$

, etc, of the circumference of the cylinder.

The number of wood fiber veneers oriented in the direction of the cylinder axis is preferably greater than the number of perpendicular wire veneers.

As indicated above, the bending radius obtained naturally depends on different parameters and therefore on the structure (constitution, arrangement, etc.) of the starting assembly.

As an example, a plywood made up of two rectangular veneers of 2.5 mx 1.25 m with direction of wire perpendicular to each other, 1 to 1.5 mm thick, made of wood soft, separated by a 1 mm separator made of a polyester nonwoven, bends at the exit of the flat press with a final bending radius between 75 and 150 mm approximately, while a multiply made of two veneers adjacent to the wood wire parallel to the length and a perpendicular wood wire veneer separated from the other two by a polyester non-woven separator bends at the outlet of the flat press with a bending radius of about 200 mm , this for softwood veneer thicknesses between 1 to 1.5 mm approximately.

When we pass to an assembly of 3 adjacent softwood veneers 1.5 to 2 mm thick with a wood fiber thread parallel to the length and two veneers with a perpendicular wire, separated from the other 3 by the same separator as previously, the bending radius obtained is approximately 250 mm. This same hardwood assembly gives a bending radius of around 500 mm.

The natural tendency to bend asymmetric plywood according to the invention is accentuated according to whether, in the stacking of veneers considered, the thickness or the density of the essence of one or more of them is modified in one direction or in the other.

According to a characteristic of the process, any synthetic film such as for example a paper impregnated with formo-phenolic resin can be added to the composition of the stack either on the side intended to become the concavity of the tile, or on the other where it will constitute a tight and hard skin by having participated, if the direction of its own constituent fiber has been well implemented, in the desired deformation in tile. This sheet is not considered to be a fold according to the invention.

During the second phase, simply place the open cylinder obtained in an assembly press. This assembly press consists of a simple heated inner mandrel which need not be cylindrical but perhaps only flat with two reactive ends and an outer heated mandrel whose necessary and sufficient condition to be effective is 'be a circular arc of diameter equal to that of the cylinder to be manufactured: this outer mandrel will be accompanied by a possibly heating block of equal curvature placed symmetrically at the diametrically opposite end of the press. It will indeed suffice to bring the two outer mandrels together so that the two edges of the plywood parallel to the longitudinal axis of the tile, guided by the heating mandrel of adequate curvature, are brought together and come to weld to each other for that the assembly naturally takes a perfectly cylindrical shape aided in this by the simultaneous action of the opposite wedge. We will have taken care to place at the level of the curved heating mandrel, parallel to the generator of the final cylinder, inside or outside, either one or more plating (s) glued (s) of length equal to that of said final cylinder, ie a piece in the form of glued; a second identical plating may also be placed outside or inside if the end use of the cylinder does not prevent it. The hot pressing thus produced makes it possible to obtain a final closure of the desired cylinder. So that this cylinder is balanced, we can stick, on the diametrically opposite side, using the shim which is there for this purpose, inside or outside an assembly identical to the previous one (s) ).

A cylinder is thus obtained which has the advantage, while having sufficient flexural strength thanks to the plating (s) whose wire direction is parallel to the main axis, of having a high tensile strength thanks to the (X) cladding (s) whose direction of the wire perpendicular to the axis of the cylinder is such that an internal pressure will make the fiber work in elongation, that is to say with maximum resistance values.

, soit environ 1 mètre.In a variant of the method, it is possible to use the heating wedge placed on the side diametrically opposite to the first external mandrel, not for bonding a balancing, but for associating between them two asymmetric plies deformed in tiles. This has the advantage, when one is limited in width of flat press, to be able to manufacture cylinders of larger diameter. For example, if the flat press has a width of 1500 mm, it is only possible to produce cylinders whose maximum diameter is equal to 1500 mm / 3.1416, therefore a little less than 50 centimeters;
on the other hand, if we can associate two plies, we can realize in this case cylinders with a maximum diameter of $$\frac{\text{2 x 1500}}{\text{3.1416}}$$

, about 1 meter.

• La figure 1 représente la première phase de fabrication, c'est-à-dire celle du multipli asymétrique plan.
• La figure 2 représente, en coupe, la seconde phase de fabrication, c'est-à-dire celle du cylindre lui-même dans une presse d'assemblage spécialement conçue à cet usage.
• La figure 3 représente, en coupe, une variante du procédé.

Other characteristics and advantages of the invention will appear in the description of examples given with reference to the figures.

• FIG. 1 represents the first manufacturing phase, that is to say that of the plane asymmetric plywood.
• Figure 2 shows, in section, the second manufacturing phase, that is to say that of the cylinder itself in an assembly press specially designed for this purpose.
• Figure 3 shows, in section, a variant of the process.

With reference to FIG. 1, glued wood veneers 1 whose direction of the grain of the wood is parallel to their largest dimension in the plane are associated with a smaller number of glued veneers 2 whose direction of the grain is perpendicular to that of the previous two.

According to a preferred variant, a separator 9 1 mm thick in a polyester nonwoven is placed between the veneers 1 and the veneers 2, for example the product sold under the name BIDIM by the company Rhône-Poulenc. This separator allows better relative sliding of the veneers 1 vis-à-vis the veneers 2 and therefore facilitates subsequent natural bending.

According to another variant, it is also possible, in the two preceding cases, to combine a paper impregnated with synthetic resin 3, the direction of the fiber, if known, will preferably be placed parallel to that of the fiber of the veneers 1.

The whole is placed in a hot flat press, at a sufficient pressure and the time necessary for the resins to be well polymerized. When the flat press is opened, the plane parallelepiped obtained is taken out and placed in the ambient atmosphere. The variations in temperature and humidity, and, in general, the release of internal stresses consecutive to the asymmetry of its constitution and to the large difference in tendency to dimensional variations thus caused between the two axes of its plane cause the natural deformation of the parallelepiped into a tile 4 here in the shape of a semicircle. The deformation of the multiply can go as far as forming a complete circle, this deformation being more pronounced if the thicknesses and / or densities of some of the veneers 1 are greater than those of the veneers 2.

Referring to Figure 2, we place a tile 4 forming a circle in an assembly press so that the open part of the tile 4 is in face of the external heating mandrel 5 which is in the form of an arc of a circle whose radius of curvature is equal to half the diameter of the final cylinder desired.

One or more glued plating (s) or a glued shaped piece 6, possibly lined with impregnated paper identical to the previous one, are placed along the mandrel 5, preferably with the direction of its fibers perpendicular to the axis of the cylinder. A wedge 7 of equal curvature is located symmetrically, that is to say at the diametrically opposite end of the press.

An internal mandrel 8, of dimension at most equal to the value of the desired final diameter, is designed in such a way that it can react to the pressures of the external mandrels 5 and 7.

When the press is closed, the action of the curved mandrel 5 naturally causes the multipli 4 to close according to a perfect cylinder of diameter equal to the value of the ratio of its width divided by the number π = 3.1416; the assembly being maintained by the mandrel 7. The pressure obtained by the contact between the mandrel 5 and the mandrel 8 and the corresponding temperature are maintained for the time necessary for the polymerization of the bonding of the assembly 6: this causes sealing the plywood closed under press.

When the press is opened, a perfectly cylindrical pipe is released.

If the cylinder is to be balanced around its main axis, a second identical assembly 9 is placed at the start of the operations and in a position diametrically opposite to the assembly 6.

In a variant of the method, and with reference to FIG. 3, one can no longer place a single tiled plywood, but two tiled plywoods, of equal widths and such that their sum is itself equal to the developed of the final cylinder desired. Their edges will be welded using an assembly 9.

As a first non-limiting example and with reference to FIG. 2, a cylinder with a length of 2500 mm, a diameter of 400 mm and a wall thickness of 3.3 is manufactured. mm with laminated phenolic interior surface. For this purpose, an asymmetric plywood panel with two veneers 1 and a veneer 2 separated by a polyester nonwoven separator, for example, is produced in a flat press having plates of a little more than 2500 mm long by 1300 mm wide. the product sold under the name BIDIM, the veneers are 1.15 mm thick each, 2500 mm long and 1256 mm wide and a sheet of phenolic impregnated paper 3 with a grammage per m² adapted to the need and dimensions in the plane of 2500 mm and 1256 mm. The cylinder will be produced in the special press by means of a mandrel 5 with a radius of curvature equal to 200 mm using a pointing assembly 6 of 2500 mm long having a suitable thickness and width. if necessary, associated with a sheet of impregnated paper of the same dimensions in the plan. An assembly 9 identical to assembly 6 will preferably be used.

As a second nonlimiting example and with reference to FIG. 3, a cylinder with a length of 3100 mm, a diameter of 900 mm and a wall thickness of 5.3 mm with a laminated phenophenic outer surface is produced. For this purpose, two asymmetrical 5 + 1 ply asymmetric plywood panels with two veneers 2 and three veneers 1 of 1.15 mm are produced in the flat press having plates of just over 3100 mm long and 1300 mm wide. each thick and 3100 mm long and 1413 mm wide, separated by a separator made of non-woven polyester. These two plies will then be assembled in the special press using assemblies 6 and 9 of suitable pointing.

• coffrage perdu de cylindres creux,
• coffrage perdu ou restant en place et participant à la décoration de piliers pleins,
• mandrin de bobines, par exemple bobines à papier,
• coeur de touret pour enroulage de câble, fil, etc...
• gaine d'aération,
• tuyauterie de transport de fluides (grains, poudres diverses, etc...)

The method according to the invention makes it possible to manufacture cylindrical pipes usable in different applications such as:

• lost formwork of hollow cylinders,
• formwork lost or remaining in place and participating in the decoration of solid pillars,
• reel core, for example paper reels,
• reel core for winding cable, wire, etc ...
• air duct,
• fluid transport piping (grains, various powders, etc.)

Claims (14)

Method of manufacturing cylindrical pipes based on wood veneers, in which, in a first phase, a first pressing is carried out in a flat press of at least one assembly of veneers to form a multiply, this multiply is allowed to deform in tile at the exit of the press, and in a second phase we press this plywood in an assembly press to form the cylinder, characterized in that the plywood has at least three plies, that its structure is asymmetrical, one side of the plywood consisting of a veneer (1) whose direction of the wood fiber thread is parallel to the axis of the cylinder to be formed, the other face consisting of a veneer (2) whose direction of the wire is perpendicular to the axis of the cylinder, that at the outlet of the flat press the deformation in tiles is such that it corresponds to a bending of radius substantially equal to the radius of the pipe to be manufactured, and in that the pressing of the second phase consists essentially to connect the ends of at least this multiply. Method according to claim 1, characterized in that the number of veneers (1) is greater than the number of veneers (2). Method according to one of claims 1 or 2, characterized in that a separator (10) is placed between the veneers (1) and the veneers (2). Method according to claim 3, characterized in that the separator is made of a non-woven or needled polyester. Method according to one of claims 1 to 4, characterized in that all the veneers of the same type (1) are adjacent. Method according to one of claims 1 to 5, characterized in that in the second phase the tile (s) (4) are placed in an assembly press allowing, thanks to at least one external heating mandrel (5), to at least one internal heating mandrel (8) and at least one heating wedge (7), to bring the tile (s) to a perfect cylinder shape and to block it by means of at least one assemblies (6) and (9) consisting either of one or more glued veneers or of a piece in the form of glue and arranged inside or outside. Method according to claim 6, characterized in that the heating external mandrels (5) and (7) are in the form of an arc of a circle of diameter equal to that of the cylinder to be manufactured. Method according to one of claims 6 or 7, characterized in that the internal heating mandrel (8) has a length equal to the internal diameter of the desired final cylinder and two convex ends rounded in an arc of a circle of smaller diameter and at most equal to that of cylinders to be manufactured. Method according to one of claims 1 to 8, characterized in that the asymmetric multiply produced in the first phase has its width, or smaller dimension in the plane, equal to the development of the desired final cylinder. Method according to one of claims 1 to 8, characterized in that the asymmetric plywood produced in the first phase have their width or smallest dimension in the plane, each equal to the inverse of an integer of the developed of the desired final cylinder. Method according to one of claims 1 to 10, characterized in that all or part of the veneers (1) have a thickness greater than that of the veneers (2). Method according to one of claims 1 to 11, characterized in that all or part of the veneers (1) are of essence (s) of density greater than that (s) of the veneers (2). Method according to one of claims 1 to 12, characterized in that at least one film impregnated with synthetic resin is located on the internal surface and / or the external surface of the cylinder. Cylindrical pipe made according to one of claims 1 to 13.

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