METHOD FOR MANUFACTURING A WOOD MOLDED PART

Description

[0001] The invention relates to a method for manufacturing a wooden molded part, in particular with a three-dimensional component geometry according to claim 1. A wood-based semi-finished product and a method for manufacturing components therefrom are provided, which enables the production of high-strength, yet very lightweight and ecologically sustainable components in three-dimensional component geometries.

[0002] In current technology, wooden components with high mechanical requirements are manufactured exclusively from veneers, which are pressed and bonded under pressure and temperature in molds. The veneers are approximately 0.5 to 8 mm thick sheets of solid wood, separated from the log using various sawing and cutting processes. In this technology, the wood veneers are compressed in a single pressing step, with the remaining cavities being filled with resin. This produces a so-called resin-bonded wood (or "armored wood") with a strength of approximately 160 to 210 MPa and a density of approximately 1.1 to 1.3 g / cm³.

[0003] The production of components from such resin-impregnated wood is subject to significant geometric limitations resulting from the low strength of the veneers used perpendicular to the grain. This means that only components with relatively large radii of curvature can be manufactured. Furthermore, combining multiple radii of curvature, for example to create so-called "case corners," is only possible to a very limited extent. Complex three-dimensional structures, such as flat components with ribbing on the back, cannot be produced using this approach. Another disadvantage lies in the use of (usually thermosetting) plastics, which at least partially negates the ecological advantages of wood as a raw material.

[0004] German patent DE 10 2013 111 393 A1 discloses a process for manufacturing molded parts from a vulcanized fiber material. EP 3 178 622 A1 discloses a process for manufacturing wood-based panels, i.e., semi-finished wood products. In this process, wood chips are first extracted from wood chips and then pressed into a wood-based panel. The heat treatment described is applied to the wood chips. EP 3 170 635 A1 discloses an OSB wood-based panel with improved properties and a process for its manufacture. EP 3 396 063 A1 discloses a process for manufacturing a compressed cellulose composite material. WO 2012 / 168563 A2 discloses a generic process for manufacturing a molded wood part.

[0005] The object of the invention is to provide a method with which wood-based lightweight components can be manufactured in large series, which, compared to the prior art, offer high component strength, low density and high geometric degrees of freedom.

[0006] The problem is solved by the features of claim 1. Preferred embodiments of the invention are disclosed in the dependent claims.

[0007] According to claim 1, the wood molded part is manufactured in a process sequence in which wood chips are first provided. These are at least partially delignified in a delignification process step. In a subsequent forming process step, the delignified wood chips are placed into a tool cavity of a forming tool and pressed under pressure, heat, and optionally moisture to form the wood molded part. Preferably, the wood chips bond together during thermomechanical compaction in the forming tool solely by the wood's own binding forces, and in particular without the addition of plastic binders.

[0008] A new approach involves producing a flat semi-finished product from so-called wood strands. In a preparatory process step, the wood strands have been at least partially delignified. In a shaping process step, the flat semi-finished product is formed into the component geometry and compacted in the process. Wood strands are defined as large-area wood chips with optimized chip geometry (length-width-thickness ratio). The production of delignified wood strands with defined or deliberately modified chip geometry and a defined ratio of the wood components cellulose, hemicellulose, and lignin enables a shaping manufacturing process for producing complex, three-dimensional wooden molded parts with defined local strand orientation and strand length.

[0009] At the process level, the new manufacturing method enables the production of wood-based components with complex 3D structures, such as ribbing and high strength. By potentially orienting the wood strands within the flat semi-finished product being formed, or by combining it with other materials such as wood veneers, fiber-reinforced plastics, and metals, a wide range of component requirements can be met.

[0010] For example, semi-finished products can be manufactured using a number of component-specific panels or from a combination of veneers and wood strands of varying lengths and orientations within a single semi-finished product. The panels can be bonded together using process-effective adhesion promoters or pre-compacting.

[0011] The forming process step can be implemented as a temperature-controlled forming and consolidation process (extrusion), possibly in conjunction with superheated steam (compare particle foaming). Wood-compatible adhesion promoters can also be used if necessary.

[0012] A key aspect concerns the fact that wood strands are used in the process sequence. If the process sequence is carried out using wood veneers made from solid wood instead, the following disadvantages arise: Delignification of solid wood involves very long process times, as the wood components to be removed must be detached and flushed out of the wood structure over greater distances. Furthermore, the unidirectional fiber orientation results in a preferred direction of the mechanical properties (anisotropy) in the pressed composite, suggesting that complex geometries are also not possible.

[0013] The main advantages are listed below in bullet points: a high degree of geometric freedom through the use of wooden strands, allowing for the formation of ribs and case corners; as little or no [constraints] as possible.Minimal use of plastic means the finished component consists almost entirely of natural and renewable materials; no cavities that would promote moisture absorption, resulting in good dimensional stability and resistance to environmental influences; very good mechanical properties and strength, comparable to simple steels but at one-fifth the weight; due to the random orientation of the wood strands across the workpiece surface, quasi-isotropic mechanical properties in the surface direction; however, defined anisotropy, for example with simple load paths, is also possible by adapting the flat semi-finished product and the manufacturing process; furthermore, easy combination with other semi-finished products and materials (e.g., (delignified) veneer, fiber-reinforced plastics (FRP), metals) is possible.

[0014] An ecologically sustainable lightweight construction based on renewable raw materials is provided. It offers the possibility of manufacturing structurally relevant components with high strength and simultaneously low density, and thus high lightweight potential, entirely from renewable and ecologically sustainable materials. At the same time, the invention allows for a high degree of geometric freedom, so that, for example, ribbing can be incorporated to improve component stiffness.

[0015] As an example, a first process step can involve at least partial delignification of the wood strands. Delignification of the wood strands is possible using various methods, such as the sulfite method, which is used to produce cellulose fibers in papermaking. To shorten the process time and reduce the process temperature, different concentrations of chemicals (Na₂SO₃ and NaOH) are used compared to papermaking. A continuous process is possible here.

[0016] To reduce the amount of chemicals used, partial delignification can alternatively be achieved using ethanol under overpressure. This process can only be partially continuous. A favorable surface area to volume ratio of the wood strands allows for rapid pretreatment.

[0017] The fact that wood possesses its own inherent bonding forces can be utilized. These bonding forces allow the wood strands to be glued together. Partial delignification positively influences the chemical structure of the wood with respect to these inherent bonding forces, thus reducing the amount of additional binding agents required or even eliminating their use entirely. The use of moisture, temperature, and pH value is particularly important in achieving cross-linking. Therefore, pretreating the wood strands in the delignification process step enables at least partial delignification of the solid wood, allowing stable hydrogen bonds to form again between the compressed cell walls during the subsequent thermomechanical compression.

[0018] In a second process step, the component is manufactured by compression molding. Four process variants are conceivable for creating and processing the flat semi-finished product in this second step, and these variants can be combined with one another: The first and second variants are based on conventional extrusion processes. Here, compound masses of defined and undefined shape and strand orientation are compressed close to the final contour. Examples of extrusion semi-finished products are sheet-molding compound (SMC) and bulk-molding compound (BMC).

[0019] The flexible handling of the flat semi-finished products from the first two variants allows for combination with similar and dissimilar materials. Similar to the manufacturing processes for multilayer composites, combinations with other semi-finished products and other manufacturing technologies are also possible. These combinations achieve synergistic effects in terms of improved mechanical properties and functional integration.

[0020] A third option involves combining the material with other semi-finished products, such as veneers for optical applications, fiber-reinforced plastics, metals, fabrics, nonwovens, and fleeces. Furthermore, functional integration is possible (for example, the insertion of cables, etc.).

[0021] A fourth option involves combining it with other manufacturing technologies, such as simultaneous metal forming.

[0022] In summary, the new process enables the production of high-strength, complex components that are potentially suitable for all structurally relevant applications in the automotive industry and other sectors (for example, rail vehicles, aerospace, construction, wind and energy plants). There, they can be used as an alternative to currently used plastic or plastic-metal hybrid components (potentially also as part of new wood-metal hybrid composites).

[0023] The following are further details of the invention: The wood chips can each have a large-area flat profile geometry with opposing flat sides and opposing narrow sides. The flat profile geometry is designed with a favorable length-to-width-to-thickness ratio for the definement and forming processes.

[0024] To provide a flat semi-finished product with a unidirectional fiber orientation, the wood fibers of the wood chips forming the semi-finished product can be aligned unidirectionally with each other.

[0025] In contrast, if the flat semi-finished product has a multidirectional fiber orientation, the wood fibers of the wood chips forming the flat semi-finished product are provided in an unoriented, random arrangement relative to each other.

[0026] According to the invention, the wood molded part is constructed as a multi-layer composite. To produce the multi-layer composite, the flat semi-finished product is inserted into the mold cavity of the forming tool along with at least one additional insert. For example, the additional insert can be at least one other flat semi-finished product. Alternatively or additionally, the additional insert can be a wood veneer, a fiber-reinforced plastic insert, a metal insert, and / or a woven fabric, a non-woven fabric, or a fleece. Alternatively and / or additionally, the insert can consist of loose wood chips.

[0027] In another variant, the additional insert can be a functional element, for example a cable, which is to be integrated into the wooden molded part to be manufactured.

[0028] To simplify the process, the forming process step can involve not only pressing the flat semi-finished product into its shape, but also additionally forming the insert part. For example, a metal insert part can be subjected to metal forming.

[0029] In a comparative embodiment not covered by the invention, the delignified wood chips can be loosely placed into the tool cavity of the forming tool, for example, as bulk material. According to the invention, the delignified, and in particular still loose, wood chips are pressed into a flat semi-finished product in a press tool in a preparatory process step. The flat semi-finished product is then placed into the tool cavity of the forming tool to prepare for the forming process step.

[0030] Exemplary embodiments of the invention are described below with reference to the accompanying figures.

[0031] They show: Figure 1 shows a side-section view of a finished wooden molded part; Figure 2 shows a block diagram illustrating the process steps for manufacturing the part. Figure 1 illustrated by the wooden molded part shown; and Figures 3 and 4 in views corresponding to the Figure 2 further embodiments of the invention; and Figure 5 in a view corresponding to the Figure 1 a wooden molded part according to one design variant.

[0032] In the Figure 1 Figure 2 shows a wooden molded part 2 with a three-dimensional component geometry. The wooden molded part 2 is formed from large-area wood chips 1, which are bonded together by the wood's own bonding forces through thermomechanical compression. The wooden molded part 2 is in the Figure 1 exemplified by a unidirectional fiber orientation.

[0033] The following will be based on the Figure 2 a process sequence described by means of which the in the Figure 1The wooden molded part 2 shown can be manufactured as follows: Accordingly, wood is first shredded into wood chips 1 or cut to a defined chip size in a cutting station (not shown). The wood chips 1 each have a length-width-thickness ratio (l, b, d) that is favorable for subsequent processing steps I, II, III, as shown in the Figure 2 As shown, the wood chips 1 each have a large-area flat profile geometry with opposing flat sides and opposing narrow sides. The chip length l is, for example, in the range of 5 to 10 mm, while the chip width b can be in the range of 4 to 7 mm. The chip thickness d is in the range of 0.5 to 1.5 mm. These dimensions are merely examples and are in no way intended to limit the invention.

[0034] Subsequently, a delignification process step I takes place, in which the wood chips 1 are at least partially delignified. The wood chips 1, thus pretreated, are then placed in a press tool 3 in a prepared pressing process step II and pressed into a flat semi-finished product 5. With regard to a unidirectional fiber orientation, the following are carried out in the Figure 2 the wood fibers 5 of the wood chips 1 forming the flat semi-finished product 5 are oriented unidirectionally towards each other.

[0035] In delignification process step I, the wood chips 1 undergo at least partial delignification. Delignification process step I is designed such that stable hydrogen bonds re-form between the wood chips 1 during the subsequent thermomechanical compaction in pressing process step II and forming process step III. Therefore, the wood chips 1 can bond together during thermomechanical compaction solely through the wood's own bonding forces, preferably without the addition of plastic binders.

[0036] The flat semi-finished product 5 produced in pressing process step II is placed into the tool cavity 9 of a forming tool 11. In the forming tool 11, thermomechanical compression also takes place under pressure, heat and moisture, during which the flat semi-finished product is pressed to form the wooden molded part 2.

[0037] Based on the Figure 3An alternative process sequence for the production of the wooden molded part 2 is described, the process steps of which are essentially identical to those based on the Figure 2 explained process steps. In contrast to Figure 2 This indicates that in Figure 3 The manufactured wooden molded part 2 does not exhibit a unidirectional fiber orientation, but rather a multidirectional fiber orientation. For this to occur, the delignified wood chips 1 in the flat semi-finished product 5, with their wood fibers 7, are no longer unidirectionally aligned, but rather positioned in a random, unoriented manner within the flat semi-finished product 5. Otherwise, process steps I, II, and III up to the completion of the wooden molded part 2 are identical to those already described based on the Figure 2 explained process steps I, II, III.

[0038] Based on the Figure 4 Another alternative process sequence for manufacturing the wooden molded part 2 is described. In contrast to the previous embodiments, in the Figure 4Pressing process step II is omitted. Therefore, the wood chips 1 pretreated in delignifying process step II are no longer pre-pressed into a flat semi-finished product 5, but rather placed directly into the tool cavity 9 of the forming tool 11 as bulk material. In this way, a multidirectional fiber orientation results in the wood part 2 to be produced.

[0039] In the Figure 5 A wooden molded part 2 is shown according to a further design variant. In the Figure 5 The wooden molded part 2 has a multi-layer composite 16, consisting of a wood chip layer and a metal layer. For the production of the part in the Figure 4In the multi-layer composite 16 shown, the flat semi-finished product 5, in combination with a metal insert 15, is inserted into the tool cavity 9 of the forming tool 11 during forming process step III. The forming process then takes place, in which not only the flat semi-finished product 5 is shaped, but the metal insert 15 is also formed. Reference symbol list

[0040] 1 Wood chips 2 Wood molded part 3 Press tool 5 Flat semi-finished product 7 Wood fibers 9 Tool cavity 11 Forming tool 13 Wood chip layer 15 Metal insert 16 Multi-layer composite I Wood chip length b Wood chip width d Wood chip thickness I Delignifying process step II Preparatory process step III Forming process step

Claims

1. Method for producing a wooden molded part (2), in which wood chips (1) are provided that are at least partially delignified in a delignification process step (I), and in which, in a forming process step (III), the delignified wood chips (1) are inserted into a tool cavity (9) of a forming tool (11) and are pressed under pressure and heat to form the wooden molded part (2), characterized in that in a preparatory process step (II), the delignified wood chips (1) are pressed to form a flat semi-finished product (5) in a press tool (3), and in that the flat semi-finished product (5) is inserted into the tool cavity (9) of the forming tool (11) for the forming process step (III), and in that the wooden molded part (2) has a multi-layer composite (16), and in that to produce the multi-layer composite (16), the flat semi-finished product (5) is inserted into the tool cavity (9) of the forming tool (11) together with at least one further insert (15).

2. Method according to claim 1, characterized in that the wood chips (1) are bonded together during thermomechanical compaction in the forming tool (11) and / or in the press tool (3) solely by the wood's own bonding forces, without the addition of plastics binders.

3. Method according to claim 1 or 2, characterized in that the wood chips (1) each have a large-area flat profile geometry comprising opposite flat sides and opposite narrow sides.

4. Method according to any of the preceding claims, characterized in that to provide a flat semi-finished product (5) having a unidirectional fiber orientation, the wood fibers (7) of the wood chips (1) forming the semi-finished product (5) are oriented unidirectionally to one another, or in that to provide a flat semi-finished product (5) having a multidirectional fiber orientation, the wood fibers (7) of the wood chips (1) forming the flat semi-finished product (5) are unoriented and in random orientation to one another.

5. Method according to any of the preceding claims, characterized in that the insert (15) is a further flat semi-finished product (5), a wood veneer, a fiber composite plastics insert, a metal insert and / or a woven fabric, nonwoven fabric or fleece, or the insert (15) consists of loose wood chips.

6. Method according to any of the preceding claims, characterized in that the additional insert (15) is a cable that is to be integrated into the wooden molded part to be produced.

7. Method according to claim 5 or 6, characterized in that in the forming process step (III), a forming process of the insert (15) also takes place simultaneously, namely a metal forming process of a metal insert (15).