Porous mould for injecting natural fibres in suspension

Abstract

A mould for injecting natural fibres in aqueous suspension, the mould defining an outer surface (Se) and an internal cavity (C) having an inner surface (C1), the mould comprising at least three distinct zones (Zs, Zf, Zd) of different porosities, namely: a) a support zone (Zs) of low porosity forming the outer surface (Se) and a part of the inner surface (C1), this support zone (Zs) defining a fibre inlet (E) and a water outlet (S) which are connected to the internal cavity (C), b) at least one filtration zone (Zf) of moderate porosity forming another part of the inner surface (C1), allowing water to be discharged but retaining the natural fibres, and c) at least one drainage zone (Zd) of high porosity in contact with the filtration zone (Zf) and support zone (Zs), but away from the internal cavity (C1), characterized in that the filtration zone (Zf) occupies 5 to 50% of the inner surface (C1) of the internal cavity (C), advantageously 7 to 30% and preferably 9 to 20% of the inner surface (C1) of the internal cavity (C), and the support zone (Zs) occupies 50 to 95% of the inner surface (C1) of the internal cavity (C), advantageously 70 to 93% and preferably 80 to 91% of the inner surface (C1) of the internal cavity (C).

Description

[0001] Porous mold for injecting natural fibers in suspension

[0002] The present invention relates to a mold for injecting natural fibers, in particular cellulosic fibers, in aqueous suspension. The mold defines an external surface and an internal cavity having an internal surface intended to come into contact with the natural fibers. The mold may sometimes consist of a single shell, but in general, it comprises several shells, which are assembled along parting planes which pass through the internal cavity. Thus, after molding, the shells are separated to release the molded part. The mold of the present invention can be used in many technical fields, such as for example packaging, decoration, electrical insulation, furniture, and more generally in applications which are not in contact with a liquid.

[0003] In the field of plastics processing, molds are used for the pressure injection of molten plastics, which behave like a highly viscous liquid. To allow the escape of air present in the internal cavity of the mold, it is possible to provide vents, which are in the form of small channels, through which the air can escape, but not the molten plastic, due to its viscosity.

[0004] Plastic molds cannot be made with natural fibers in aqueous suspension. The difficulty arises from the two-phase nature, as the fibers (solid material) are mixed with water (liquid). During molding, the fibers must remain in the internal cavity of the mold, while the water must be removed. The vents in plastic molds are specially designed for air removal, but not for water. Indeed, the fibers would quickly clog the vents and the water could no longer be removed, leading to a molded object with too much water.

[0005] The present invention provides a mold that is specially designed for the injection of natural fibers in aqueous suspension. The mold aims to evacuate the maximum amount of water from the internal cavity of the mold to obtain a molded object made of natural fibers that is particularly dry and therefore solid.

[0006] Another object of the invention is to define a method of manufacturing the mold, which makes it possible to produce each shell (or the single shell) in a single-material monoblock manner.

[0007] To achieve these aims, the present invention proposes a mold which comprises at least three distinct zones of different porosities, namely: a) a support zone of low or zero porosity, i.e. dense, forming the external surface and a part of the internal surface, this support zone defining at least one inlet for natural fibers in aqueous suspension and at least one water outlet connected to the internal cavity, b) at least one filtration zone of medium porosity forming another part of the internal surface, allowing the evacuation of water but retaining the natural fibers, and c) at least one drainage zone of high porosity in contact with the filtration zone and the support zone, but away from the internal cavity, this drainage zone being directly connected to the water outlet, so that the water present in the internal cavity can be evacuated by the water outlet through the filtration zone and the drainage zone.

[0008] In other words, the support area serves as a framework that gives the mold its support and strength, the filtration area serves as a sieve that retains the fibers and lets water pass through, and the drainage area serves as a pathway between the sieve and the exterior of the mold.

[0009] Such a mold is already known from documents EP0559491A1, EP3985170A1 and WO2022096888A1.

[0010] According to the invention, the filtration zone occupies 5 to 50% of the internal surface of the internal cavity, advantageously 7 to 30% and preferably 9 to 20%, and the support zone can occupy 50 to 95% of the internal surface of the internal cavity, advantageously 70 to 93% and preferably 80 to 91%. The support zone, at the level of the internal cavity, can extend continuously or on the contrary form several zones separated by a filtration zone. Similarly, the filtration zone, at the level of the internal cavity, can extend continuously or on the contrary form several zones separated by a support zone. For example, it is possible to have a continuous support zone and several separate filtration zones. It can then be said that the support zone comprises inclusions of filtration zones.It should be borne in mind that the drainage area does not extend to the internal cavity, which is only formed by the support area, the filtration area, the natural fiber inlet(s) and the water outlet(s).

[0011] The use of a partial filtration zone (which only extends over a part of the internal surface of the internal cavity) makes it easier to fill the mold by creating preferential flows in the tooling: this makes it possible to balance the flow of the material, which improves the homogeneity of the transformed material. This facilitates the injection of the material onto all the molding surfaces.

[0012] The use of a partial filtration zone also makes it easier to eject the part. Once the liquid has been sucked out, it is possible to detach the molded part by blowing air into the channels thus created. This facilitates the ejection of the part and at the same time allows the filtration elements to be cleaned, which limits the clogging of the drainage elements.

[0013] Finally, the use of a partial filtration zone helps limit the deterioration of the surface condition of the molded parts. Indeed, the filtration zones mark the parts, because they have a surface roughness due to their porous nature. Using local porosity allows this marking to be limited to finer areas, which can be located in parts of the product not visible to the consumer and where aesthetics are less important.

[0014] According to an interesting characteristic of the invention, the filtration zone may comprise several superimposed layers with an angular offset, from 1° to 89° and advantageously 67°, each layer being formed by spaced parallel cords, which may be made of metal, such as stainless steel. Advantageously, the cords have a spacing of 0.2 mm to 1.5 mm, preferably approximately 0.7 mm. Furthermore, the spaced parallel cords may have a width of 0.08 mm to 0.15 mm, preferably approximately 0.1 mm, and a thickness of 0.02 mm to 0.08 mm, preferably approximately 0.04 mm.

[0015] According to a practical embodiment, the cords can each have a half-cylinder shape, with a flat face and an opposite curved face.

[0016] In other words, each layer, with a thickness of 0.02 mm to 0.08 mm, comprises or is made up of half-cylinders, 0.08 mm to 0.15 mm wide, which extend parallel with a gap of 0.2 mm to 1.5 mm. The layers are superimposed and bonded, with any angular offset, which is preferably 67 °, in order to avoid any phenomenon of alignment repetition. With this angle of 67 °, the randomness of the structure formed by the stacking of layers is maximum, so that the porosity of the filtration zone is uniformly sinuous, with an absence of rectilinear pores.

[0017] In practice, it is common for the mold to comprise at least two mold shells assembled along a joining plane passing through the internal cavity, each mold shell being produced in a single piece by a selective laser melting process of a metal powder bed, advantageously of stainless steel, using a laser to melt particles of metal powder, layer by layer.

[0018] Thanks to this selective laser melting process of metal powder bed, the three zones (support, filtration and drainage) can be produced in a single block by varying the spacing of the beads, and possibly their thickness and / or width. It is easy to understand that beads that are closely spaced, below 0.2 mm, or even contiguous, will lead to a very slightly porous, or even impermeable, structure. This can thus produce the support zone. Conversely, beads that are widely spaced, beyond 1.5 mm, will lead to a very porous structure. This can thus produce the drainage zone. As for the filtration zone, it can be carried out with this selective laser melting process of metal powder bed respecting the indications of spacing (0.2 mm to 1.5 mm, preferably about 0.7 mm), width (0.08 mm to 0.15 mm, preferably about 0.1 mm) and thickness (0.02 mm to 0.08 mm, preferably about 0.04 mm) already mentioned above.

[0019] Thus, a mussel shell is made up of a stack of flat layers, each of which comprises at least one support zone sector and / or one filtration zone sector and / or one drainage zone sector: the sectors can simply be differentiated by the spacing of the beads, which can have the same width and the same thickness. It can also be said that each layer is made up of a single layer (for example of support zone) or by an assembly of layers (for example of support, filtration and drainage zone) in the manner of a patchwork.

[0020] Generally speaking, it should be noted that the mold of the invention is a single piece, while it comprises three distinct zones. This characteristic can advantageously be achieved with the selective laser melting process of metal powder bed, but other processes can also be used.

[0021] The invention also defines a method of manufacturing a mold as defined above, implementing a method of selective laser melting of a bed of metal powder, advantageously stainless steel, using a laser to melt particles of metal powder, layer by layer.

[0022] The invention also defines a molding method using a mold as defined above, comprising the following steps: i. injecting under pressure the natural fibers in aqueous suspension through the natural fiber inlet of the mold so as to fill its internal cavity, ii. while maintaining the injection pressure, simultaneously sucking the water through the water outlet and heating the mold to a drying temperature.

[0023] Advantageously, the injection pressure is 500 to 1500 bars, preferably about 800 bars, the suction depression is -0.1 to -0.8 bars, preferably about -0.6 bars and the drying temperature is 100 to 180 degrees, preferably about 150 degrees. It should be noted that this molding method can be implemented in any mold comprising the three distinct zones of different porosities defined in points a), b) and c) above, namely a support zone of low porosity, a filtration zone of medium porosity and a drainage zone of high porosity.

[0024] The spirit of the invention lies in the fact of producing, advantageously in a single piece, a mold having distinct porosity zones to ensure the support, filtration and drainage functions. The structure based on parallel cords or strips, with different spacings, is particularly advantageous, especially since it can be obtained with the selective laser melting process of metal powder bed.

[0025] The invention will now be described in greater detail, with reference to the attached drawings, giving by way of non-limiting example, an embodiment of the invention.

[0026] In the figures:

[0027] Figure 1 is a perspective view of a mold according to the invention,

[0028] Figure 2 is a perspective and transparent view of a shell of the mold of Figure 1,

[0029] Figure 3 is a very schematic sectional view illustrating the internal structure of the mold shell of Figure 2,

[0030] Figure 4 is a perspective view of the cords forming the layers,

[0031] Figure 5 is a schematic view showing several layers of cords superimposed, and

[0032] Figure 6 is a schematic view illustrating the method of manufacturing the mold according to the invention.

[0033] The mold M which serves to illustrate the present invention comprises two shells M1, M2, identical or not and attached to each other along a joining plane Pj. The two shells M1, M2 are assembled in a sealed manner by any suitable means, such as for example screws or bolts. A mold gasket can be inserted between the two shells M1 and M2. The mold M thus formed comprises an inlet E for the injection under pressure of natural fibers in aqueous suspension. These natural fibers can be chosen from cellulosic fibers, flax and hemp, jute or kenaf. The mold M here comprises two water outlets S, each being formed by a mold shell M1, M2. Alternatively, a single water outlet or more than two are also conceivable. The mold M of course defines an external surface Se.

[0034] Referring to Figure 2, we can see not only the interior of a shell M1, but also its structure, since the representation is transparent. We can thus notice that the shell M1 defines an internal cavity C which defines an internal surface C1, intended to come into contact with the natural fibers in aqueous suspension. The cavity C extends in a hollow from the junction plane Pj. Of course, we must imagine that the other part of the cavity is formed by the shell M2, which once assembled to the shell M1, constitutes the entire cavity C, through which the joint plane Pj passes. The natural fiber inlet E is formed jointly by the two shells M1, M2: we can notice that the inlet E is located on the joint plane Pj. The natural fiber inlet E communicates directly with the interior of the cavity C, whereas the water outlet S of the mold M1 does not communicate directly with the cavity C, as does the natural fiber inlet E.The water outlet S is connected to a drainage zone Zd, which itself communicates with the cavity C through two filtration zones Zf. The rest of the shell M1 is formed by a support zone Zs.

[0035] As can be seen in Figure 2, the cavity C has a general configuration in the shape of a bone or a diabolo, with two heads located at the ends and connected by a thinned passage. This particular shape for the internal cavity C should not be considered as limiting: other shapes are of course conceivable. However, in this embodiment with this cavity C in the shape of a bone or a diabolo, the two filtration zones Zf are located at the two end heads. All the rest of the cavity is formed by the support zone Zs, which also forms the external surface Se of the shell M1 and the mold M. We can thus say that the drainage zone Zd is an intermediate zone between the filtration zones Zf and the support zone Zs.

[0036] Referring to Figure 3, it is easier to understand the particular arrangement of the support zones Zs, drainage Zd and filtration Zf. The internal cavity C dug in the junction plane Pj defines an internal surface C1, which is partially formed by the surface zone Zs and by the filtration zone Zf. It can be noted that a part of the support zone Zs separates the cavity C from the drainage zone Zd. The natural fiber inlet E, formed in the junction plane Pj, communicates directly with the cavity C and the water outlet S, which passes through the support zone Zs, communicates directly with the drainage zone Zd.

[0037] In terms of percentage of the internal surface C1 of the internal cavity C, the filtration zone occupies from 5 to 50% of this internal surface C1, advantageously 7 to 30% and preferably from 9 to 20%. As for the support zone Zs, it occupies from 50 to 95% of the internal surface C1, advantageously from 70 to 93% and preferably from 80 to 91% of the internal surface C1. The embodiment which has been used to illustrate the invention comprises two filtration zones Zf, but more can be provided, or on the contrary only one without departing from the scope of the invention. Only the proportions given above are to be respected.The use of a partial filtration zone (which extends only over a part of the internal surface of the internal cavity) makes it easier to fill the mold, balance the flow of the material, improve the homogeneity of the transformed material, facilitate the injection of the material on all the molding surfaces, facilitate the ejection of the part, detach the molded part by blowing air into the channels thus created, clean the filtration elements, limit the fouling of the draining elements, limit the deterioration of the surface condition of the molded parts, limit the marking to finer, reduced and discreet areas.

[0038] According to the invention, the shell M1 or M2 is preferably made as a single piece of material, such as stainless steel. This means that all zones Zs, Zd and Zf are made as a single piece of stainless steel. Of course, other suitable materials can be used.

[0039] Preferably, all these zones Zs, Zd and Zf are formed from layers of cords superimposed with an angular offset, which can be of the order of 67% in order to avoid any repetition of structures. Each layer is formed by parallel strips or cords more or less spaced, as can be seen in Figure 4. For the support zone Zs, the cords B are very close together, i.e. less than 0.2 mm, so that the layered structure obtained is very slightly porous, or even impermeable. For the filtration zone Zf, the cords B are moderately spaced, of the order of 0.2 to 5 mm and preferably about 0.7 mm, in order to create a moderately porous structure, which allows the passage of water, but not that of the natural fibers which must remain in the internal cavity C.As for the drainage zone Zd, the cords B are even more spaced, i.e. beyond 1.5 mm, so as to obtain a highly porous structure which allows the passage of water without great loss of load.

[0040] Thus, the greater or lesser spacing of the cords B is used to vary the porosity in the three types of zones Zs, Zf and Zd. Preferably, the cords or bands B are all identical, regardless of the type of zone. For example, cords B can be provided with a half-cylinder shape, as can be seen in Figure 4. In this case, the cords B can have a width of 0.08 mm to 0.15 mm and preferably of the order of 0.1 mm. As for their thickness, it can be 0.02 mm to 0.08 mm and preferably of the order of 0.04 mm. With this half-cylinder configuration, a perfectly flat face and an opposite curved face, preferably in an arc of a circle, can be identified. Of course, other shapes can be envisaged for the cords B. For example, perfectly cylindrical cords with a circular, square or rectangular section can be provided.

[0041] In Figure 5, we see schematically several layers L1, L2, L3, L4 and L5 formed by parallel cords B, the layers being superimposed with an angular offset of 1° to 89°, and advantageously of the order of 67°.

[0042] To produce such a layer structure of more or less spaced parallel cords, it is possible, for example, to use or implement a selective laser melting process for a bed of metal powder, advantageously stainless steel, which uses a laser beam to melt metal powder particles in the form of a cord and successively layer by layer. Figure 6 illustrates this type of selective laser melting process very schematically. First, a laser beam L is movable along a guide rail G, so that it can move in a scanning motion. Then, a powder reserve P is used to feed, by means of an equalizing roller R, an adjacent powder tank into which the laser beam L is directed. Thus, by successive scanning, it is possible to produce a cord or strip B by melting the metal powder P.Finally, the bead B rests on a non-fused powder bed P, which is movable in height by means of a plate T. When a layer of parallel beads B has been produced, the plate T descends and the roller R brings a new layer of powder from the powder reserve P. Thus, a second layer of beads can be produced on the layer previously produced, with an angular offset, preferably of 67°. In the end, the structure shown in Figure 5 is obtained, which makes it possible to produce all the zones of the mold M from the least porous, namely the support zone Zs to the most porous, namely the drainage zone Zd, passing through the filtration zone Zf of medium porosity.

[0043] The present invention thus defines a method for manufacturing a single-material monoblock mold with a structure of angularly offset superimposed cord layers using the particular method of selective laser melting of a metal powder bed.

[0044] The use of the mold M just described is very different from a conventional mold for injection molding of plastic material. While in a conventional mold, it is sufficient to be able to evacuate the air present in the mold cavity through vents, in the present invention, it is necessary to evacuate the water in which the natural fibers were suspended. Indeed, the natural fibers in aqueous suspension are injected under pressure through the inlet E to fill the internal cavity C. The natural fibers are retained in the cavity C, while the water is evacuated from the cavity C through the filtration zone(s) Zf, the drainage zone Zd and finally the water outlet S.

[0045] A priori, one might have hoped that the injection pressure of the natural fibers in aqueous suspension in the cavity C would be sufficient to generate the evacuation of water through the filtration zones Zf. However, the injection pressure alone quickly proved insufficient in terms of molded object quality and molding time. This is why the present invention provides, in addition to the injection pressure, heating the mold and sucking the water through the water outlet S. Three actions are therefore carried out simultaneously, namely injection under pressure, heating and evacuation under vacuum. These three simultaneous actions made it possible to obtain a quality molded object and a molding time reduced to a few seconds: between 3 and 5 seconds.

[0046] As a guide, the injection pressure may be in the order of 500 to 1500 bars, preferably around 800 bars. The water suction pressure may be in the order of 0.1 to 0.8 bars and preferably around -0.3 bars. As for the drying temperature, it may be in the order of 120 to 180° and preferably around 150°.

[0047] Thanks to the single-piece mold of the invention, with these three zones of distinct porosity and its method of use with three simultaneous actions, a molded object made of good quality natural fibers is obtained. The manufacturing process by selective laser fusion of metal powder bed is particularly advantageous, since it allows the production of layers with beads of different spacing.

Claims

Claims 1. Mold (M) for injecting natural fibers, in particular cellulose, in aqueous suspension, the mold (M) defining an external surface (Se) and an internal cavity (C) having an internal surface (C1) intended to come into contact with the natural fibers, the mold comprising at least three distinct zones (Zs, Zf, Zd) of different porosities, namely: a) a support zone (Zs) of low or zero porosity forming the external surface (Se) and a part of the internal surface (C1), this support zone (Zs) defining at least one inlet for natural fibers in aqueous suspension (E) and at least one water outlet (S) connected to the internal cavity (C), b) at least one filtration zone (Zf) of medium porosity forming another part of the internal surface (C1), allowing the evacuation of water but retaining the natural fibers, and c) at least one drainage zone (Zd) of high porosity in contact with the filtration zone (Zf) and of the support zone (Zs),but away from the internal cavity (C1), this drainage zone (Zd) being directly connected to the water outlet (S), so that the water present in the internal cavity (C) can be evacuated by the water outlet (S) through the filtration zone (Zf) and the drainage zone (Zd), characterized in that the filtration zone (Zf) occupies 5 to 50% of the internal surface (C1) of the internal cavity (C), advantageously 7 to 30% and preferably 9 to 20% of the internal surface (C1) of the internal cavity (C), and the support zone (Zs) occupies 50 to 95% of the internal surface (C1) of the internal cavity (C), advantageously 70 to 93% and preferably 80 to 91% of the internal surface (C1) of the internal cavity (C)., 2. Mold (M) according to claim 1, in which the filtration zone (Zf), at the level of the internal cavity (C), forms several zones separated by the support zone (Zs), so that the support zone (Zs) comprises inclusions of filtration zones (Zf).

3. Mold (M) according to claim 1 or 2, in which the filtration zone (Zf) comprises several superimposed layers (L1, L2, L3, L4, L5, etc.) with an angular offset, advantageously of 67°, each layer (L1, L2, L3, L4, L5, etc.) being formed by spaced parallel cords (B).

4. Mold (M) according to claim 3, in which the cords (B) have a spacing of 0.2 mm to 1.5 mm, preferably approximately 0.7 mm.

5. Mold (M) according to claim 3 or 4, in which the cords (B) have a width of 0.08 mm to 0.15 mm, preferably approximately 0.1 mm, and a thickness of 0.02 mm to 0.08 mm, preferably approximately 0.04 mm.

6. Mold (M) according to claim 3, 4 or 5, in which the cords (B) each have a half-cylinder shape, with a flat face and an opposite curved face.

7. Mold (M) according to any one of the preceding claims, comprising at least two mold shells (M1, M2) assembled along a joining plane (Pj) passing through the internal cavity (C), each mold shell (M1, M2) being produced in a single piece by a selective laser melting process of a metal powder bed, advantageously of stainless steel, using a laser to melt particles of metal powder, layer by layer.

8. Method for manufacturing a mold (M) according to any one of the preceding claims, implementing a method of selective laser melting of a bed of metal powder, advantageously stainless steel, using a laser to melt particles of metal powder, layer by layer.

9. A molding method using a mold (M) according to any one of claims 1 to 8, comprising the following steps: i. injecting under pressure the natural fibers in aqueous suspension through the natural fiber inlet (E) of the mold (M) so as to fill its internal cavity (C), ii. while maintaining the injection pressure, simultaneously sucking the water through the water outlet (S) and heating the mold to a drying temperature.

10. Molding method according to claim 9, wherein the injection pressure is 500 to 1500 bar, preferably about 800 bar, the suction depression is -0.1 to -0.8 bar, preferably about -0.3 bar and the drying temperature is 100 to 180 degrees, preferably about 150 degrees.

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