FILTER ELEMENT FOR FLOW CALMING AND PURIFICATION OF MELT OBTAINED DURING POURING, AS WELL AS A METHOD FOR MANUFACTURING A FILTER ELEMENT

Description

[0001] The invention relates to a filter element for calming the flow and cleaning a melt used in casting, and to a method for manufacturing a filter element.

[0002] In the field of molten metal filtration, there are various approaches to increasing the filtration efficiency of the filters used. In most cases, the filter material is modified or substituted with other substances. For example, DE 10 2011 109 681 A1 describes an increase in the filtration efficiency of "common molten metal filter geometries" through an active surface coating. "Common molten metal filter geometries" include open-cell foam ceramic geometries, honeycomb geometries, spaghetti filter geometries, perforated filter geometries, and woven fiber filters, none of which correspond to a defined filter and flow structure for optimal laminarization of the melt. DE 10 2011 109 682 A1 and DE 10 2020 000 969 A1 describe the use of carbon-bonded materials for iron and aluminum molten filtration, respectively. The filters are defined as "foamed structures".DE 10 2011 109 684 A1 for the removal of gases from melts using filters, DE 2016 106 708 A1 for the application of filters for continuous metal melt filtration and DE 10 2018 201 577 A1 for the removal of inclusions of different chemical composition using ceramic metal melt hybrid filters also describe filters with a foamed or not clearly defined filter structure.

[0003] WO 2017 / 008092 A1 describes a process for manufacturing metal filters from molybdenum or tungsten. In this process, molybdenum or tungsten powder with varying grain sizes is sintered. The sintered bridges provide the filter's strength, while the unsintered areas form the pores for molten metal filtration. This filter manufacturing method also lacks clearly defined filter or flow structures. The technical solution described in EP 3 325 428 B1 involves a filter element composed of three-dimensional geometric cages designed for melt purification. This filter element is manufactured either by printing a thermoplastic and subsequently coating the printed framework with ceramic slip and firing it, or by printing a ceramic slip.While these filter elements may have defined filter structures, these only fulfill the task of melt purification and not laminarization. Furthermore, the filter elements consist of a sintered ceramic that does not thermally decompose after use and therefore cannot be recycled, or only to a very limited extent.

[0004] German patent application DE 10 2011 109681 A1 discloses ceramic filters for molten metal filtration and methods for their production. EP 3 325 428 B1 also discloses filter elements for filtering molten metals. DE 10 2017 216964 A1 relates to porous ceramic filter structures for molten aluminum filtration. JP 2004 025276 A also deals with ceramic filters for high-temperature applications.

[0005] The present invention is based on the objective of realizing defined filter and flow structures that enable the conversion of turbulent to largely laminar flow of liquid melt by flow stabilization and / or effective retention of impurities that may be contained in the molten metal, as well as simple and environmentally friendly disposal of used filter elements.

[0006] According to the invention, this problem is solved with a filter element having the features of claim 1. Claim 5 relates to a manufacturing method. Advantageous embodiments and further developments of the invention can be realized with features specified in dependent claims.

[0007] A filter element according to the invention is designed with a three-dimensional rib structure with openings serving as flow channels through which liquid melt is guided. Liquid melt can flow through the openings formed between the ribs in the rib structure, and the ribs can be designed such that the flow is laminarized by flow stabilization, allowing impurities, e.g., oxides, to be retained by the filter element.

[0008] The rib structure is formed with particles made of a material suitable for use as a mold material in foundry technology and a binder. The particles are bonded together with the binder, and the ribs are coated on their surface with a polymeric resin.

[0009] Particles preferably composed of predominantly SiO₂, predominantly Al₂O₃, predominantly aluminosilicate, cerium-stabilized ZrO₂, or chromite may be used. "Predominantly" should be understood to mean a proportion of at least 80 vol.%, particularly preferably at least 90 vol.%, and most preferably at least 95 vol.%.

[0010] One can use quartz sand or chromite, which consists mainly of chromium and iron oxide, or minerals also known as mullite.

[0011] The particles can be chemically bonded using a furan resin binder, a phenol resin-based binder, or an inorganic binder. A furan resin binder can be a furfuryl alcohol-based binder. Phenolic resin-based binders can be hot-curing (e.g., an acid-curing phenol-resol binder) or cold-curing. An inorganic binder can be a water-based alkali silicate binder.

[0012] Depending on the binder used, hardening can be achieved by thermal treatment in which a sufficiently high temperature is reached, irradiation with suitable electromagnetic radiation, or by adding a binder-specific hardener component to the binder.

[0013] The coating applied to the surfaces of the webs can be made of polymeric resin, such as epoxy resin. According to the invention, the coating is closed and has a layer thickness of at least 50 µm.

[0014] The proportion of binder with which the particles are chemically bonded should be in the range of 1 vol% to 5 vol% relative to the number of chemically bonded particles. Preferably, the proportion is approximately 2 vol%.

[0015] The struts of the strut structure should be formed with particles with a mean particle size d 50 in the range of 63 µm to 1000 µm.

[0016] The openings forming the flow channels should have open pockets in a predefined position and size to collect impurities contained in the melt. This can be achieved using additive manufacturing techniques, which will be discussed in more detail below, by carrying out appropriately controlled production.

[0017] In principle, the manufacturing process involves forming webs layer by layer with particles made of a material suitable for use as a mold material in foundry technology. These webs are bonded to a binder in a locally defined, material-bonded manner. At the beginning of the curing process, or after the binder has cured, the surface of the webs is coated with a polymeric resin. Curing can begin with the formation of a web structure in a single layer and continue until complete curing is achieved. Subsequent layers can be structured even if complete curing has not yet been achieved. In these cases, a hardener component can preferably be applied together with the binder, or curing can be achieved through irradiation with suitable electromagnetic radiation.Alternatively, thermal treatment can be carried out at a temperature sufficient for the binder to harden. This thermal treatment can be performed on a green body that already exhibits the three-dimensional basic web structure of a filter element.

[0018] One approach in powder bed-based additive manufacturing involves applying loose particles, or particles already containing or coated with binder, as a layer onto a build platform. Particles in the uppermost layer are then locally coated with the binder, resulting in a localized, material-bonded bond between them within the plane of the respective layer. The binder application should be metered and controlled two-dimensionally along the surface of each layer formed with loose particles.

[0019] After forming a predetermined geometric ridge structure for the plane of a respective layer by materially bonding particles with binder, the build platform is then lowered by one layer thickness and a new layer of loose particles is applied, in which particles are again materially bonded with binder to form a predetermined geometric ridge structure in this plane.

[0020] The process steps of layer application, locally defined material-bonded connection of particles with the binder and lowering of the build platform are repeated until the specified three-dimensional rib structure of the filter element has been formed.

[0021] Following this, loosely bonded particles are first removed, and then a polymeric resin coating is applied to the surfaces of the webs. The coating can be applied by dipping, flooding, or spraying.

[0022] As an alternative to powder bed fusion, an uncoated filter element can be produced by 3D printing. In this process, a paste / suspension, consisting of particles, a binder, and optionally a hardener, is applied layer by layer through at least one nozzle until the three-dimensional rib structure, with its ribs and openings acting as flow channels, is formed. A polymer resin coating is then applied to the rib surfaces. Depending on the specific binder, curing occurs either after the formation of a layer of paste / suspension or only after the complete rib structure has been built up. The paste / suspension can be applied in a metered manner, either as individual drops or as filaments.In this process, at least one nozzle is positioned accordingly by two-dimensional movement of its outlet opening, and the applied mass flow of suspension is controlled locally according to the geometry and dimensioning of the rib structure.

[0023] One approach, for example, is to scan a standard 20 ppi foam ceramic filter (50 mm x 50 mm x 20 mm) using micro-computed tomography to obtain a true-to-scale CAD model. This CAD model is then additively manufactured three-dimensionally using sand and a binder. Quartz sand (SiO₂: >99.1 vol.%; mean particle size d₅₀ 0.14 mm) mixed with an activator (p-toluenesulfonic acid) is applied layer by layer to a build platform and then printed with a binder that cures to form furan resin (binder content: 2 vol.%). To increase mechanical and thermal strength, the printed filter geometry, in the form of a three-dimensional rib structure, is subsequently impregnated with a polymeric resin on the rib surfaces.

[0024] The resulting filter element was subsequently tested for its thermal shock resistance in molten aluminum using a test method designed to determine its resistance to thermal shock, as well as its resistance to the dynamic and thermal stresses occurring during aluminum casting. The filter element withstood these stresses and did not break. Furthermore, lighter areas on the filter element already showed signs of thermal decomposition, which will cause the filter element to break down into its constituent parts after use, and also makes it possible to recover residual molten material.

[0025] In the preliminary test described here, a foam ceramic filter structure was chosen for the template to control the additive manufacturing process, as it acts as the worst case for the occurring mechanical and thermal forces due to its small web thickness of 0.5 mm.

[0026] Casting trials with molten aluminum have shown that, despite the small strut thicknesses, an additively manufactured filter element, consisting essentially of the bound particles of a molding material, withstands the expected stresses and that melt purification can also be achieved. Furthermore, additional trials have demonstrated that a filter element according to the invention thermally decomposes after use and can thus be completely separated from the solidified aluminum and any contaminants that accumulate on the filter element during use. The invention allows for the provision of filter elements with defined retention and / or flow structures. Application examples have shown that even very delicate three-dimensional strut structures are feasible and that, after impregnation with polymeric resin on the strut surfaces, these structures withstand the stresses encountered, at least during aluminum casting.

[0027] The invention enables the production of flow-optimized filter geometries using additive manufacturing with an advantageous filter element material. This allows for the design and manufacture of a filter element according to improved flow-related principles. The design freedom and reproducible manufacturing process avoid the disadvantages of a foam ceramic filter. During the processing of the filter element material, loose, pourable particles are coated or mixed layer by layer with a binder. Sufficient strength can thus be achieved through the material-bonded structure. After use and exposure to a flow of molten fluid, the filter element can at least partially disintegrate, which occurs through the thermal decomposition of the binder.The chemical bond between particles is at least largely broken, and the used particles, now in loose form, can be reintroduced into the material cycle of a foundry. No waste requiring landfill is produced.

[0028] The invention offers possibilities for an optimized design and layout of the entire filter element geometry. This particularly affects the dimensioning of webs and perforations, with the alignment of flow channels formed by these perforations. The liquid melt can flow in such a way that laminar flow is achieved after passing through the filter element, resulting in the highest possible retention capacity for contaminants. Thus, it is possible to develop and provide a filter element with defined flow channels and separation pockets. The reproducible and design-free processing of the filter material is a novel method not yet established on the market.

[0029] Research has shown that such filter element structures are not currently known to be in use or under development. The development of additively manufactured filter elements of this type is characterized by a high degree of innovation.

[0030] The invention also offers ecological and economic advantages due to the use of the new filter structures. Compared to foam ceramic filters, the additive manufacturing of the flow-optimized filter structures requires significantly less energy, as the sintering process of the ceramic slurry is completely eliminated. This makes production considerably more environmentally friendly, and foundries can reduce their CO₂ footprint by using the filter elements according to the invention. Since an additively manufactured filter element disintegrates due to the thermal effects of the melt after casting, and the particulate material can be recycled, landfill and transport costs can be saved. Furthermore, the additively manufactured filter elements are significantly cheaper to produce than foam ceramic filters, both in large and small production runs.Another significant advantage of this development is that the filter structures can be specifically designed to suit their respective area of ​​application.

[0031] The starting materials used in the production process incur only low costs, and a relatively small amount of energy is required to manufacture the filter elements.

Claims

1. A filter element for flow stabilization and cleaning of a melt which is used during casting and is passed through the filter element, wherein the filter element is in the form of three-dimensional ligament structures with openings as flow channels, through which liquid melt is guided, and wherein ligaments of the ligament structure are formed using particles of a material that can be used as a moulding material in foundry technology, and using a binding agent by which the particles are materially bonded to one another, and the ligaments, on their surface, are provided with a coating of a polymeric resin, wherein the coating is closed and is formed with a layer thickness of at least 50 µm.

2. The filter element according to claim 1, characterised in that the particles are formed of predominantly SiO2, predominantly Al2O3, predominantly aluminosilicate, cerium-stabilized ZrO2, or chromite; and / or the particles are materially bonded by a furan resin binder, a phenolic resin-based binder or an inorganic binder as binding agent.

3. The filter element according to any one of the preceding claims, characterised in that a proportion of binding agent, by which the particles are materially bonded to one another, is maintained in the range from 1 vol.% to 5 vol.% relative to the amount of particles materially bonded to one another.

4. The filter element according to any one of the preceding claims, characterised in that at the openings forming the flow channels, open pockets are formed in predefined position and dimensioning for receiving impurities contained in a melt.

5. A method for producing a filter element according to any one of the preceding claims, characterised in that ligaments are formed layer by layer using particles of a material that can be used as a moulding material in foundry technology, by material bonding of the particles with a binding agent in a locally defined manner, and the surface of the ligaments is coated with a coating of a polymeric resin at the beginning of the curing or after the curing of the binding agent.

6. The method according to the preceding claim, characterised in that the loose particles or particles that already contain binder or are coated with binder are applied as a layer onto a build platform, and the binding agent is applied to particles of the respective uppermost layer in a locally defined manner and thereby particles are materially bonded to one another in the plane of the respective layer in a locally defined manner; and after formation of a predefined geometric ligament structure for the plane of a respective layer by material bonding of particles using binding agent, the build platform is lowered by one layer thickness and a new layer of loose particles is applied, in which again particles are materially bonded by binding agent in order to form a predefined geometric ligament structure in this plane; wherein the method steps of layer application, locally defined material bonding of particles using the binding agent and lowering of the build platform are repeated as often as necessary until the predefined three-dimensional ligament structure of the filter element has been formed, and thereafter, first, any loose, non-materially-bonded particles are removed, and thereafter the coating of a polymeric resin is applied onto the surfaces of the ligaments, which cures under normal conditions and / or by thermal treatment and / or irradiation with electromagnetic radiation and / or by addition of a catalyst.

7. The method according to claim 5, characterised in that a paste formed from the particles and the binder is applied, layer by layer, through at least one nozzle until the three-dimensional ligament structure comprising the ligaments and the openings as flow channels is formed, and then the coating of a polymeric resin is applied to the surfaces of the ligaments.