Machine for fractionating ground grain products

Abstract

The plansifter or the semolina purifier has at least one screening compartment with an arrangement of a plurality of screening elements with screens (23). The screening compartments can be set to oscillate, for example in the form of circular movements. At least one of the screens (23) is designed as a flat body, in particular a metal sheet. The screen holes are present as through-holes. According to a first option, the screen can be present as a metal screen with etched through-holes. According to a second option, the metal screen can be manufactured by an additive manufacturing method. It is shown that, by providing the screen as a flat body with through-holes, the efficiency of the fractionating process can be markedly improved by comparison with woven screens.

Description

[0001] MACHINE FOR FRACTIONING

[0002] GRAIN MILLING PRODUCTS

[0003] The invention relates to a machine for fractionating grain milling products, for example a plansifter or a semolina purifier.

[0004] Machines for fractionating grain mill products are used to separate the components of a mill product into coarser and finer grained components and, depending on the case, components of different densities, as well as to remove foreign matter from the mill product. Separating the mill product into components of different grain sizes is also called "classifying" or "fractionating." Among the machines for fractionating grain mill products, plansifters and so-called semolina purifiers are particularly well-known. Plansifters are generally used in the milling industry to fractionate grain mill products between and after passes through the roller mill of a grain mill. They can also be used for so-called control sifting, i.e., the sieving of flour that is otherwise ready for sale. Semolina purifiers are used to separate semolina components in a process downstream of the milling process or between milling processes.

[0005] The process of fractionating grain mill products should not be confused with the upstream process of cleaning, to which grain kernels are subjected in order to separate straw, stones, foreign bodies, seedlings or sand from the grain, and for which - comparatively coarse - sieves are also used.

[0006] Plansifters have screening compartments, each containing a stack of screens acting as planar screens. The screens are each mounted on primary frames, so-called "insertion frames," which in turn are inserted into screen frames (referred to here as "secondary frames"). A suitable drive mechanism sets the screening compartments in horizontal oscillating motions, particularly circular oscillations in the screening plane. Plansifters generally also employ so-called screen cleaners, i.e., movable elements driven by the movements of the screening compartments. Their movement relative to the respective screen frees it of blocked material.

[0007] In purifiers, the screens used are also arranged in multiple layers and are also subject to vibration. However, unlike flat screens used in plansifters, they do not vibrate (only) in the screening plane, which is why they are classified as throw screens. Purifiers also generate an air stream to which the material being screened is exposed during the screening process.

[0008] Within a machine for fractionating milled grain products, sieves of various mesh sizes are generally used, allowing fractionation into more than two components, which are then packaged for sale or subjected to further processing steps, such as a repeat pass through the roller mill. Common sieves for such plansifters or semolina purifiers are woven fabrics made of plastic yarn or filament, e.g., PET or polyamide. Meshes made of metallic wires are also available, specifically for pre-screening with large mesh sizes, as this process often involves significant abrasion. Since the sieves are woven, the meshes are square. The mesh size is defined as the distance between adjacent threads or wires.

[0009] For attachment, a sieve is placed on a designated wooden, plastic, or metal frame, pre-tensioned, and glued in place while tensioned, for example, using cyanoacrylate. The use of such adhesives, which may come into contact with the food being processed, is increasingly viewed as disadvantageous.

[0010] So-called "shale shakers" are known from the petroleum industry; they can be used to remove solid residues from drilling mud, such as that produced during shale oil extraction. WO 2017 / 019580 proposes using a metal perforated plate as a sieve. The document lists all common methods for creating metal perforated plates, including laser drilling, chemical etching, spark erosion, waterjet cutting, punching, and others, and claims that an extremely wide range of hole sizes between 1 μm and 5000 μm is possible. Such a "shale shaker" is completely unsuitable for processing ground grain products.

[0011] DE3041270 and FR9851710 each teach a sieve for a hammer mill and, respectively, for grain products in general. The sieves are designed as metal sheets with elongated sieve slots. Due to the elongated shape of the sieve slots, these sieves would not be suitable for machines in which a plurality of sieves form sieve compartments arranged, for example, one above the other, with the sieve compartments being set into vibration. In general, the sieve compartments in machines of the type described here are subject to circular movements, which is why sieve holes with essentially the same length and width should generally be used.

[0012] US2166367 relates to a process for producing metallic 'screens,' i.e., grid structures such as those used in sieves, but also in printing processes. According to US2166367, the process is intended to create significantly more regular holes than simply etching holes into a metal sheet. The process is multi-step and involves the galvanic deposition of material on a specially manufactured matrix with raised surfaces. The process is relatively complicated and time-consuming. For this reason alone, grid structures produced according to US2166367 would not be suitable as screens for industrial use in machines such as plansifters or purifiers.

[0013] To date, woven sieves have been the only ones used for applications in plansifters or semolina purifiers.

[0014] In the milling industry, there is a constant need to increase the efficiency of the fractionation process. The throughput per screen area for a specific mesh size depends on the number of meshes per screen area. This number can be optimized by using thinner threads or wires to increase the ratio between open screen area and total screen area, or more precisely, between open screen area and net screen area (screen area minus frame and any bars, i.e. area over which the screen hangs freely). However, optimization in this way is limited by the fact that stability must be guaranteed, which in fact sets a lower limit for the permissible thread or wire diameter for each material used. Other optimization options, e.g. the use of optimized screen cleaners, only lead to further efficiency improvements to a limited extent.

[0015] It is therefore the object of the present invention to provide a machine for fractionating ground grain products and a screening element for such a machine, which overcomes the disadvantages of the prior art and makes the screening process of ground material produced in grain mills more efficient. A further object of the present invention is to further develop machines for fractionating ground grain products so that they have additional functions and / or are suitable for new applications.

[0016] Machines of the type described here have at least one screening compartment – ​​often several, for example, arranged side by side – each with an arrangement of several screening elements with screens. Generally, the screening compartments have stacks of screening elements. The machine is designed to set the screening elements into vibration, for example in the form of circular movements triggered by the rotation of an unbalanced mass; forced movements are also conceivable. In the context of this text, circular movements are included when reference is made to "vibrations", regardless of the type of excitation. Generally, the screening compartments are set into vibration as a whole, i.e. the stacks of screening elements make joint, particularly circular, movements.

[0017] According to the invention, the screen is designed as (at least) a flat body, for example, made of metal. The screen holes are provided as through holes. Non-metallic materials are also conceivable, in particular materials with sufficient dimensional stability that are suitable for an etching process and / or an additive manufacturing process—for example, ceramic materials suitable for 3D screen printing, for example, or hard thermoplastic or curable (thermoset) plastics.

[0018] A "flat body" is a foil or plate, i.e. a continuous object extending in two dimensions with a constant thickness (extension in the third dimension) which is at least one order of magnitude smaller than the extensions in the other two dimensions (length and width). Depending on the thickness, such a flat body is perceived more as a foil or a plate. A flat body can in particular be a sheet in the broader sense of the term; in specific embodiments, the flat body in the present context is also a sheet in the narrower sense of the term, i.e. a metal product produced by rolling.

[0019] Surprisingly, it has been shown that the design as a flat body with through holes—instead of the conventional woven fabric of threads or wires—significantly increases efficiency. The time required to force a specific amount of material (flour, dust, semolina, grits, etc.) through a specific sieve area, assuming all other relevant parameters (mechanical excitation, size of the sieve holes, number of sieve holes) remain the same, can be significantly reduced.

[0020] It can be particularly advantageous if a defined edge is formed between the top and, for example, the bottom surface of the screen on the one hand and the walls of the through holes on the other, which edge extends around the respective through hole, whereby the angle can be approximately 90°. Such a pronounced edge exists when the top surface merges into the wall of the through hole without a continuous curvature, or when the average radius of curvature of a transition area that can still be determined under microscopic observation is much smaller than half the thickness of the flat body, for example, less than one-fifth, one-eighth, or one-tenth of the thickness of the flat body.

[0021] As is known from prior art sieves, through holes differ from slot-like holes in that they are essentially as wide as they are long in the sieving plane. They are typically round or approximately in the shape of a regular polygon. The average ratio between the longest dimension of the through holes in the sieving plane and the smallest dimension in the sieving plane can accordingly be comparatively small, e.g., no more than 1.5, and in particular no more than 1.3, 1.2, or 1.1. Through holes with this characteristic are suitable for machines in which the sieve elements are subjected to uniform, e.g., circular, movements in the plane. Furthermore, they enable good discrimination of sieved material particles, even if they are not spherical, which will inevitably be the case with milled grain products.

[0022] In a first group of embodiments, the screen is created from a metal sheet or a foil / plate made of another material using an abrasive process. Among the abrasive processes, parallel processes are particularly suitable, in which the removal process is carried out simultaneously for all through-holes—or for a large group of through-holes, e.g., entire contiguous areas of the screen. One such process is etching. The screen element can thus comprise an etched metal screen. Etching creates through-holes with walls that are approximately perpendicular to the screen surface.

[0023] In a second group of embodiments, the screen is manufactured by an additive manufacturing process, i.e. a manufacturing process in which the material of the screen is applied layer by layer to produce the screen

[0024] Essentially parallel processes are also particularly well-suited among additive manufacturing processes. In these essentially parallel processes, at least the curing process occurs layer by layer across the entire surface of the screen or across at least one partial surface in parallel. This contrasts with processes in which a laser beam cures locally selectively and therefore has to traverse the entire screen surface. Essentially parallel processes can particularly include processes from the broader category of extrusion processes, in which a 2D mold is used through which the material to be cured is forced.

[0025] One such essentially parallel process is 3D screen printing. In this process, a suspension containing particles of the material from which the object to be manufactured is to be made, in this case the screen, is pressed layer by layer using a squeegee through a grid provided with a stencil, the so-called screen printing screen (not to be confused with the screen being produced on the machine). The stencil thus serves as a printing form in the sense of an extrusion process and defines (in two dimensions) the shape of the object to be produced. Each layer is cured before the next layer is applied. After all layers have been applied, sintering can take place for densification. When applied to the production of the screen, a screen printing screen with a stencil that represents a negative of the screen to be produced is created and then used for the additive manufacturing process.The production of a sieve using such an additive process also produces through holes with walls that are approximately perpendicular to the sieve surface and with pronounced, well-defined edges between the sieve surface and the walls.

[0026] Sieves of the type according to the invention—this applies to both groups of embodiments—therefore have webs between the sieve holes that are rectangular in cross-section (i.e., substantially rectangular, as described in more detail below), in contrast to the round cross-sections of the threads or wires of woven sieves. Furthermore, the sieve forms a flat structure, in contrast to woven sieves, where the threads or wires overlap at intersection points.

[0027] These geometric differences may provide a starting point for explaining the greater efficiency of the inventive approach compared to the prior art. In particular, the edges that form on top of the webs in a sieve according to the invention may have an important influence. First, they can lead to a particle subject to random movement being less likely to encounter a surface where it becomes trapped and more likely to encounter an edge of the etched sieve. Second, they can lead to a lower probability of temporarily stable structures forming from the sieved material that extend beyond the sieve hole.

[0028] However, the findings of the present invention do not depend on a specific explanation for the observed beneficial effect. Another property of etched screens or those produced by an additive manufacturing process is that they are free of structures that protrude beyond the screen plane. This distinguishes them, for example, from punched structures, such as those found in coarse perforated sheets, which each have a sharp ridge on one side while tapering conically from the other. It has been shown that the absence of such structures has a positive effect on the efficiency of the etched screens.

[0029] A further advantage is that the sieve, as a flat body with through holes, is easier to clean than the state of the art, especially using ultrasound.

[0030] The through holes can be arranged in a hexagonal structure, i.e., staggered from row to row. This utilizes another advantage of the inventive approach: the arrangement of the sieve holes can be freely chosen, meaning there is no need to specify a rectangular arrangement. The hexagonal structure has the advantage that, for a given minimum web width, the highest possible number of sieve holes can be arranged per sieve surface. This can further increase the efficiency gain.

[0031] The same applies to the shape of the sieve holes: Within limits defined by the etching process or the additive manufacturing process, the shape can also be freely determined. Therefore, not only the arrangement but also the shape of the sieve holes can be approximately hexagonal. This allows the ratio between open sieve area and net sieve area to be maximized for a given minimum web width. For example, the through holes of very fine sieves in the machine, e.g., up to a mesh size of approximately 0.3 mm, can be approximately round, while larger through holes are hexagonal.

[0032] The use of etched screens or screens produced by an additive manufacturing process, i.e., screens with through holes produced by etching, is particularly advantageous for mesh sizes between 0.08 mm and approximately 1 mm. Accordingly, a machine according to the invention comprises, for example, at least one screen with through holes having a mesh size between 0.08 mm and 1 mm. In particular, it can comprise at least one screen with a mesh size of more than 0.3 mm and with hexagonal through holes.

[0033] For non-square through holes, the mesh size is defined in this text as the square root of the area of ​​the through holes in the horizontal section.

[0034] The width of the webs between the through holes can be approximately equal to the thickness of the sheet. For webs with a non-constant width, this applies to the width at the narrowest point. "Approximately equal to the sheet thickness" here means that the web width corresponds to no less than 40% or no less than 50% of the sheet thickness and no more than 160% or no more than 130% of the sheet thickness, and in particular, can be between 60% and 100% of the sheet thickness.

[0035] In the first group of embodiments, as already mentioned, the sieve is manufactured by etching (industrial etching processes are sometimes referred to by the English term "chemical milling"; in this text, "etching" generally refers to material-removing processes using substances that attack the material). Etching has the advantage that the process is parallel, unlike, for example, laser cutting, which can be crucial for production efficiency, especially with fine-mesh sieves. The sieve holes (through holes) can be etched from both sides, i.e., from above and below.

[0036] In embodiments of the first group, the screens are designed such that the etching is asymmetrical from the top and bottom, with more material removed from the bottom and the narrowest point of the through-holes being located above a center plane of the flat body. This minimizes the probability of a particle of the screened material becoming jammed in a through-hole.

[0037] In embodiments of the second group, with a screen manufactured using an additive manufacturing process, the layers from which the screen is constructed are particularly parallel to the two large surfaces (top and bottom) of the flat body. In embodiments of the second group, the screen is also particularly metallic. In embodiments of the second group, there is not necessarily a defined narrowest point between the planes of the screen defined by the large surfaces.

[0038] If the flat body from which the sieve is made is metallic, it can also be made of a magnetic (i.e., magnetizable, i.e., ferromagnetic or ferrimagnetic) material, such as magnetic steel. Ferromagnetic stainless steels (particularly martensitic or ferritic steels, or steels with corresponding proportions) are known both as hard magnets and as steels with soft magnetic properties. The use of a magnetizable material has the following advantage: In the event of sieve breakage, which can occur over time, it is possible to remove a magnetic sieve fragment from the material being sieved by exposing the area containing the fragment to a magnetic field.

[0039] The use of metals or certain plastics as the material for the flat body has another potential advantage: the screen can be attached to a primary frame by welding. This eliminates the need for adhesives. A prerequisite for weldability is the design of the screen as a flat body with through holes. Only with this design can forces acting on the webs between the through holes be sufficiently well distributed across the entire periphery of the screen and from there onto the primary frame. Welding wires to form a wire mesh screen, as is known from the prior art, would theoretically also be possible; however, it would be complex and impractical.

[0040] In addition to a primary frame, to which the sieve is directly connected and which can therefore also be considered part of the sieve, the sieve element can have a secondary frame, onto which the primary frame is placed or which holds and / or receives the primary frame in some other way. The secondary frames of a sieve compartment can be stacked on top of one another and / or they can be held by a sieve compartment housing and serve as modules thereof. However, a secondary frame additional to the primary frame is not necessary. It is also possible for the primary frames to be attached directly to the sieve compartment housing and / or stacked on top of one another and thus serve as sieve frames. In this case, the sieve elements consist only of the primary frames with the sieves attached to them.In addition to a machine for fractionating ground grain products, such as a plansifter or semolina purifier, the present invention also relates to a sieve for use in such a machine, as well as this use per se and a method for producing a sieve for such a machine. Sieves for machines for fractionating ground grain products have, in many embodiments, a relatively large surface area of ​​more than 100 cm. 2 , often at least 0.35 m 2 .

[0041] A screen according to the inventive approach can be a single piece, i.e., made from a single flat body. However, it is also possible to use more than one flat body, for example, two or more flat bodies placed side by side as strips or pieces and attached to the primary frame.

[0042] The primary frame, to which the screen is directly attached, may have an outer frame section that spans the screen surface, as well as bars spanned by the screen. In this case, the screen is attached not only to the outer frame section, but also to the bars.

[0043] In the case of seven of the first group of embodiments, the manufacturing method comprises providing the flat body and producing the through holes in a material-removing process, for example etching, for example from both flat sides.

[0044] In screens of the second group of embodiments, these steps are replaced by the additive manufacturing process. This involves providing a suspension with particles suspended in a solvent, in particular metallic particles (or particles made of another suitable, e.g., weldable, material), and possibly additional components such as binders or other additives. In a further step, the process involves applying the suspension in layers, each layer having a shape that corresponds to the shape of the flat body to be produced with the through holes (horizontal section). This can be done using a stencil (in 3D screen printing) or, in a non-parallel process, also by selective curing with a laser beam. In the first case, each layer is at least partially cured after application before the next layer is applied.This curing can be performed as an actively induced step, e.g., by irradiation with electromagnetic radiation (infrared or possibly visible and / or UV), or by applying hot air with a hot air gun, and / or another type of energy supply. Alternatively, it is also conceivable, in principle, that the writing is carried out passively, with the material curing itself, for which sufficient time is provided.

[0045] Processes in the first group and processes in the second group have in common that they produce a very flat surface with pronounced edges between the surface (i.e., the top and bottom) on the one hand, and the walls of the through-holes on the other, despite the very small size of the through-holes due to the application. In processes in the first group, the pronounced edges arise due to the properties of the abrasive etching process used. In processes in the second group, they arise because the successively applied layers will always be relatively thin compared to the thickness of the entire screen, and because, due to the peculiarities of the processes, each layer can have exactly the same dimensions.

[0046] This sharp-edged property distinguishes the screens manufactured according to the procedures described here, firstly, from braided screens known from the prior art. Secondly, it also differs from screens such as those known from US Pat. No. 2,166,367, in which the screens are formed using a casting technique and have rounded edges due to the surface tension of the still-flowing mass during the casting process and also due to the demolding process—in addition to the rounded edges already present in the mold used.

[0047] Furthermore—and this applies to both groups of embodiments—the method can include attaching the flat body to a primary frame, for example, by welding, in particular laser welding or spot welding. A particular advantage of the inventive procedure is that the flat body does not need to be clamped particularly tightly—it may be sufficient for it to be correctly dimensioned and placed on the primary frame and then attached to it. This eliminates both the disadvantageous gluing and the use of a clamping device.

[0048] A sieve according to the second group of embodiments can optionally be configured to enable automatic detection of sieve breakage. For this purpose, the sieve comprises, in addition to electrically insulating material, a conductor track which is particularly embedded in the electrically insulating material. At at least two points—particularly at the two ends of the conductor track—the conductor track forms an electrode each. The machine is then configured to determine, using a suitable sensor, whether there is electrical contact between the two electrodes. If this is not the case, the conductor track is interrupted, which is an indication of a sieve breakage. The machine can indicate the sieve breakage to the user, thus enabling the sieve to be replaced early without causing damage due to unscreened material passing through a sieve surface or due to sieve fragments in the material being screened.A further aspect of the present invention relates to the use of a sieve of the type described in this text, which is designed as a flat body with through holes, as a sieve of a machine for fractionating cereal mill products - in particular a machine of the type described in which the sieves form sieve compartments (open or closed) which are set in oscillating movements.

[0049] Yet another aspect of the invention relates to a novel application of a machine for fractionating grain mill products with sieves which are set in motion, for example circular movements, in order to promote the at least partial passage of the material to be fractionated, wherein at least one of the sieves is designed as a flat body with through holes.

[0050] This innovative application is the fractionation of spent grain. 'Spent grain' refers to the byproduct that remains as a residue after the extraction of malt components during beer production or similar processes. Spent grain is a grain milling product because the grain—for example, barley—is crushed after malting and kilning and before mashing, i.e., subjected to a (relatively coarse) milling process.

[0051] Spent grain typically contains—and is generally intentional—the husks of the grain used, in addition to insoluble protein and other ingredients. These husks make the spent grain unsuitable for subsequent fractionation, as they quickly clog the sieves of plansifters and similar machines. This is one of the main reasons why spent grain is primarily used as animal feed. However, due to its high protein content, the spent grain itself would also be suitable for the production of high-quality human food, for example, meat substitutes. An efficient approach for sifting out husks and other coarse components from the spent grain would therefore be desirable.

[0052] Tests have shown that, in contrast to similar machines (plan sifters or throw sifters and other sifters with conventional woven screens), machines with screens designed according to the invention as flat bodies with through-holes, specifically with pronounced edges between the surfaces and the walls of the through-holes, do not experience clogging when sifting spent grain. Rather, it has been shown – quite surprisingly – that machines of this type with flat screens are readily suitable for fractionating spent grain. This solves a problem that the state of the art could not address.

[0053] Embodiments of the invention are described below with reference to the drawings. In the drawings, like reference numerals denote like or similar elements. The drawings show partially corresponding elements in different sizes from figure to figure. They show:

[0054] Fig. 1 : a view of a plansifter;

[0055] Fig. 2: a sieve stack;

[0056] Fig. 3 : an exploded view of a sieve element with sieve, primary frame and secondary frame;

[0057] Fig. 4: a primary frame with sieve;

[0058] Fig. 5: schematically a section of a sieve with hexagonal structure;

[0059] Fig. 6 also schematically shows a section of a sieve with a square grid arrangement; Fig. 7: a cross-section through the sieve according to Fig. 5;

[0060] Fig. 8 shows a cross section through a sieve according to the prior art;

[0061] Fig. 9 schematically shows an alternative cross-sectional shape;

[0062] Fig. 10 shows a cross-section through a sieve produced using an additive manufacturing process;

[0063] Fig. 11 shows a measurement of the separation efficiency curve for a sieve designed as a flat body with etched through-holes, in comparison with a sieve according to the prior art;

[0064] Fig. 12 a horizontal section through a sieve with automatic sieve break detection; and

[0065] Fig. 13 shows a section of a cross section through the sieve according to Fig. 12.

[0066] Figure 1 shows, as an example of a machine for fractionating ground grain products, a plansifter 1, as used in grain mills. The plansifter comprises a plurality of sieve compartments 3, which are mounted in a space via a common suspension device 4 in such a way that common horizontal oscillating movements are possible. A drive (not visible in Figure 1) is configured to set the ensemble of sieve compartments into, for example, horizontal circular oscillations. Furthermore, the plansifter has flexible feed lines 6 as the sieve material inlet and flexible outlet lines 7 as the sieve material outlet.

[0067] Each screening compartment 3 has a stack of screening elements 11, shown schematically in Figure 2, each with a screen, whereby the screens can have different mesh sizes. Furthermore, there is a screening material guide, for example, laterally relative to the screens, through which, at each screen, non-screened screening material portions (which are, for example, discharged laterally and guided to another, lower screen element) and possibly also screened screening material portions are transported further. Overall, the screening compartments are designed such that the supplied screening material is selected into screened screening material portions of different grain sizes, so that - depending on the supplied screening material - the screening material portions guided into the various screening material discharge lines 7 have different properties.

[0068] According to the present invention, this principle, which is known per se, is implemented in such a way that at least some of the sieves used are present as, for example, metallic flat bodies with through-holes, for example as etched or additively manufactured sieves.

[0069] Like plansifters, purifiers also have multiple screen elements, which can be arranged one above the other. Purifiers are generally not referred to as "screen stacks"—the number of screens arranged one above the other is often fewer than in plansifters—but rather as layers of screens. Unlike plansifters, the vibrating motion of the screens is different, which is why the screens function as throw screens. The following statements regarding the properties of the screen elements and screens generally apply to machines for fractionating milled grain products, including both plansifters and purifiers.

[0070] Figure 3 shows a schematic exploded view of a sieve element 11. A sieve frame 21 (secondary frame) acts as a supporting structure. It may also have a structure (not shown in Figure 3) for guiding so-called sieve cleaners, which are also not shown in the drawings and can be optionally present, for example, to tap the sieve from below. The actual sieve 23 is attached to a primary frame 25, which can be placed inside the sieve frame 21 and is guided by it.

[0071] The primary frame 25 has an outer frame part 26 which forms a rectangle, as well as bars 27 spanned by the sieve 23. The frame elements of the outer frame part 26 as well as the bars 27 spanned by the sieve can each have rectangular or other cross-sections.

[0072] Figure 4 shows the primary frame 25 with the screen 23 attached to it. An advantage of the invention is that the attachment can optionally be achieved by a welded connection, for example, a spot weld. Corresponding weld points 31 are shown schematically in Figure 4.

[0073] In embodiments, the sieve is welded not only to the outer frame part 26, but also to the bars 27 spanned by the sieve 23, for example also by a spot or laser welding connection.

[0074] Metal sheets have a certain moderate elasticity in their plane. This property is particularly advantageous in the context of the present invention: The sieve can be attached to the insert frame 25 without the need for dedicated means for applying a tension force along the plane, as is the case with woven sieves. Rather, the flat body with the through holes simply needs to be placed on the frame and then connected to it, for example, by the aforementioned spot welding or by laser welding. The use of an adhesive is also unnecessary, which is considered advantageous for a component that comes into direct contact with food.

[0075] Figure 5 schematically shows a section of a sieve 23. The through-holes 42 are arranged in a hexagonal pattern. Furthermore, the shape of the through-holes 42 is also approximately hexagonal. Depending on the size of the through-holes 42, such a hexagonal shape can be approximated more closely (with larger through-holes, i.e., when the sieve has a comparatively coarse mesh) or less accurately, for example, when the sieve is manufactured by etching. For very fine-meshed sieves, depending on the manufacturing process, the through-holes are approximately round. However, the advantages of the hexagonal arrangement remain independent of the actual shape.

[0076] Even if a hexagonal arrangement and shape are particularly advantageous, the sieve can in principle have any - generally regular - arrangement and any shape of through-holes. Figure 6 shows a square grid arrangement with square through-holes, as is inevitable in woven sieves, and as can optionally also be present in sieves of the type described here made from a sheet metal or other flat body. Figure 6 also shows the definition of the mesh size w, which is defined as the width of the space between adjacent threads (and not, for example, the distance between the thread axes); in a non-planar grid, in projection onto the sieve surface.

[0077] In the case of a sieve with non-square through-holes, such as the one in Fig. 5, the mesh size can be defined as the square root of the area a of the through-holes in horizontal section, i.e. the mesh size that would result if the through-holes had area a but were square. It can also be provided that, depending on the shape of the through-holes, the area is deliberately chosen to be somewhat smaller - or possibly somewhat larger - than for square through-holes in order to achieve the same classification. For example, in the case of approximately round through-holes - such as those created by etching very small through-holes of, for example, less than 0.3 mm in diameter; additive processes also result in a slightly rounded shape for very fine structures - a diameter can be chosen which has an area of ​​only approx.95% of the area a, which then results in approximately the same selection as with square through holes of mesh size ^(a). Even with hexagonal through holes—which can be created with etched screens with a slightly larger mesh size of, for example, at least 0.3 mm—the through hole can also have a slightly smaller area a than the corresponding square through hole with the same classification effect.

[0078] The comparison between Figures 5 and 6 also illustrates that, for a given web width bst and mesh size, the ratio between open screen area (the important parameter for throughput through the screen) and net screen area (which is generally determined by the dimensions of the screen element and cannot be influenced) is better with a hexagonal arrangement and with approximately hexagonal through holes than with the conventional square arrangement. It can be shown that, for example, with hexagonal through holes, the open screen area can be increased by approximately 4% compared to steel meshes and by approximately 11% compared to plastic meshes (which require slightly thicker threads). This represents a very significant increase in efficiency for industrial processes such as those found in a grain mill.

[0079] In addition, however, tests have shown that the screening throughput can be increased much further by a screen of the type according to the invention compared to the prior art than could be explained by a larger open screening area, which will be explained below.

[0080] Figure 7 shows a vertical cross-section through a screen of the type according to the invention, which is produced by etching. Figure 7 also illustrates the feature that the web width b st, for example, corresponds approximately to the thickness d of the flat body, ie the webs have an aspect ratio of approximately 1. In Fig. 7 one can also see that the webs 41 are rectangular in cross-section, for example approximately square, whereby in the case of production by etching etching takes place in particular from both sides, i.e. from above and from below, which is why there is a very slight narrowing of the through-hole towards the central plane, which, however, has no significant negative influence on the achievable results.

[0081] A corresponding section through a woven sieve according to the state of the art is shown in Figure 8.

[0082] A comparison between Fig. 7 and Fig. 8 shows that in sieves of the type according to the invention, edges 44 arise along the webs, which is not the case with a woven sieve. The existence of such edges 44, particularly on the upper side, i.e. towards the material to be sieved, represents a possible explanation for why the inventive approach increases efficiency so significantly. A flour or grit particle or a group of such particles is much less likely to form part of a stable structure on the upper side of the sieve and is more likely to be deflected in its downward movement at the edge than is the case with the woven sieve with its round cross-sectional structures. Figure 9 firstly schematically illustrates the principle that during manufacture by etching it is not necessary to etch to the same depth on both sides, but that asymmetric configurations are also possible. In Fig.9 was etched approximately twice as deep from below as from above, so that the point of greatest constriction is above the mid-plane. Such an asymmetric configuration, with the narrowest point above the mid-plane, can be advantageous because it reduces the likelihood of particles becoming trapped in the through-hole. This potentially further increases efficiency.

[0083] Secondly, Fig. 9 illustrates that the web width b s t can also be smaller than the thickness d, which was already discussed above.

[0084] Thirdly, Fig. 9 also illustrates that the angle between the upper surface of the screen and the wall of the through-hole does not have to be exactly 90°, even in the presence of a sharp edge - which may be advantageous as mentioned - but can be somewhat smaller.

[0085] Figure 10 shows a vertical cross-section, analogous to Fig. 7, through a sieve of the type according to the invention, which was not produced by etching but by an additive manufacturing process. Dotted lines indicate that the sieve is constructed from a plurality of layers 51, with the layers running horizontally. The layered structure may, but does not have to, be still recognizable on the sieve. The considerations relating to the dimensions and shape (with edges 44) explained with reference to Fig. 7 and, with regard to the web width, Fig. 9 also apply to embodiments of the second group, an example of which is shown in Fig. 10. Example 1: Approximately 100 g of commercially available white flour was placed on a sieve produced by etching, measuring 275 mm * 175 mm (net sieve area) with a hexagonal arrangement of a total of 427,000 through holes. The through holes had an area of ​​a= 0 as measured using CAD software.05309mm. A 2, which corresponds to a mesh size of 0.23. The sieve was rotated horizontally in a circular motion (approximately 3 rotations per second; diameter of the movement circle: 5 cm). After a total of approximately 25 seconds, all the flour had passed through the sieve, except for a small residue.

[0086] Under the same conditions (same sieve dimensions and net sieve area) and using the same circular movements, an equal amount of the same flour was placed on a comparable commercially available nylon sieve (mesh size 0.236 mm; number of holes 426,000) and sieved. After 40 seconds, a significant amount of flour still hadn't passed through the sieve, so a sieve cleaner was placed on top of the sieve to assist the process. Only after another 20 seconds—i.e., after a total of 60 seconds—had the flour passed through, leaving only a small residue of a comparable amount.

[0087] In this example, efficiency was increased by significantly more than a factor of 2.

[0088] This example represents a completely realistic application. It is common practice for flour ready for sale to be sieved again in so-called control sifters before delivery to filter out any foreign matter. A sieve such as the standard nylon sieve with a mesh size of 0.236 mm can be used for such processes. - TI -

[0089] Example 2: As in Example 1, but using rye flour instead of white flour as the screening material. Rye flour is highly agglomerating and therefore difficult to screen. The efficiency increase according to Example 1 was confirmed.

[0090] Example 3: Analogous to Example 1, but a sieve produced by etching with a square arrangement of approximately square sieve holes was used (due to the etching process, the shape of the sieve holes is that of squares with clearly rounded corners). The effective area of ​​the through holes was measured using CAD. It corresponded to the area of ​​the square with the mesh size 236, i.e. 0.236 mm. After just 16 s, due to the circular movements, practically the entire amount of 100 g of white flour had passed through the sieve. A further comparison measurement with the same amount of white flour and with the nylon sieve from Example 1 and a mesh size of 0.236 mm, but without the use of a sieve cleaner, showed that even after 93 seconds the white flour had not yet completely passed through, i.e. the residue was larger than with the sieve according to the invention after just 16 s.

[0091] Example 3 confirms that the arrangement and also the shape of the sieve holes are rather secondary, and that the effect of the invention is due to the fact that the sieve has an intrinsically different structure than a woven sieve.

[0092] Figure 11 shows, based on a comparative measurement, that the inventive procedure not only improves the efficiency of the sieving process but also the sharpness of the separation. The measurement was carried out using the inventive sieve and the commercially available nylon sieve according to Example 1. This grain mill product was sieved using the parameters according to Example 1. The so-called reject, i.e. the portion of the sieved material that was not sieved through, was analyzed using a commercially available laboratory device (manufacturer: Microtrac) that uses the principle of dynamic image analysis (ISO 13322-2). Specifically, the size distribution of the grain mill product particles was determined, with the minimum diameter xc,min as the characteristic quantity. xc,min is the minimum dimension of the corresponding particles – for spherical particles, xc,min corresponds to the diameter, for ellipsoidal particles, it corresponds to twice the value of the smallest semiaxis. Fig.Figure 16 shows the so-called separation efficiency curve, i.e., the proportion of the number of grain mill product particles with an xc.min value that is smaller than the value indicated on the abscissa. It can be seen that the first curve 61, obtained with a machine according to the invention with an etched sieve, has a significantly sharper demarcation (steeper slope) than the second curve 62, which results from the machine with a conventional sieve. This immediately demonstrates that the machine according to the invention and the sieve element according to the invention not only provide improved efficiency but also improved discrimination quality.

[0093] Figures 12 and 13 illustrate the possibility of providing a screen element of the type according to the invention with an additional function, namely automatic screen break detection. This possibility exists in particular for screen elements produced using additive manufacturing processes, in particular the 3D screen printing process. The screen element contains at least one embedded conductor track 71, which runs between two contacts 74. In the illustrated embodiment, the course of the conductor track 71 is meandering. In the event of a screen break 81, the conductor track is interrupted and can thus be detected by a simple sensor or a simple measuring device, which checks whether an electrical connection exists between the two contacts 74. The conductor track 71 runs embedded, i.e., there is no contact with the upper or lower surface of the flat body. In embodiments in which these surfaces are metallic - i.e.,If the sieve element is essentially metallic, at least one electrically insulating layer 92 will be present between the conductor track 71 and the metallic layers 81 on the surfaces; Likewise, the layer containing the conductor track 71 can be filled with electrically insulating material (i.e., it can be electrically insulating at those positions in the sieve element plane where the conductor track is not). Suitable electrically insulating materials are the aforementioned ceramic materials or hard thermoplastic or thermosetting plastics.

[0094] In the illustrated embodiment, in addition to the (first) conductor track 71, a second conductor track 72 is present in another layer. This second conductor track 72 also runs in a meandering manner, but with a different, particularly orthogonal, primary direction. This allows for the detection of screen breaks such as the second screen break 82 shown, which run in such a way that they do not sever the first conductor track 71. The second electrodes 75 for the second conductor track are read separately from the (first) electrodes 74 of the first conductor track 71.

Claims

PATENT CLAIMS 1. Machine (1) for fractionating grain milling products, comprising at least one sieve compartment (3) with a plurality of sieves (23), a material inlet (6) and a material outlet (7), wherein the machine is configured to set the sieves (23) into oscillating motions in order to allow at least partial passage of material introduced through the material inlet conveying the material through the sieves, characterized in that at least one of the sieves (23) is designed as a flat body with through holes (42).

2. Machine according to claim 1, wherein the flat body is metallic.

3. Machine according to claim 2, wherein the metallic flat body is magnetic.

4. Machine according to one of the preceding claims, wherein the sieve (23) is mounted on a primary frame (25).

5. Machine according to claim 4, wherein the primary frame has an outer frame part (26) which extends along a circumference of the screen, and rods (27) spanned by the screen, and wherein the screen is supported both at the outer The frame part (26) is also attached to the rods (27).

6. Machine according to claim 4 or 5, wherein the sieve (23) is welded to the primary frame (25).

7. Machine according to one of the preceding claims, wherein the through holes (42) of at least one of the sieves form a hexagonal arrangement.

8. Machine according to one of the preceding claims, wherein the through holes (42) of at least one of the sieves have a hexagonal shape.

9. Machine according to one of the preceding claims, wherein for webs (41) which are formed between adjacent through holes (42) the following applies: O.5*d <bst<l .6*d, wobei d die Dicke des Flachkörpers und b st denotes the width of the webs (41) at their narrowest point.

10. Machine according to one of the preceding claims, wherein the flat body forms an edge (44) around the through holes (41).

11. Machine according to any one of the preceding claims, which is free of adhesive in contact with the sieve (23).

12. Machine according to any one of the preceding claims, wherein the Through holes are created by etching.

13. Machine according to one of claims 1-11, wherein the sieve is manufactured by an additive manufacturing process. A machine according to any one of the preceding claims, configured to vibrate the screens (23) in a horizontal screen plane, thereby making the machine a plansifter. A machine according to any one of claims 1-13, configured to vibrate the screens (23) such that a screen plane is subjected to vibrations, thereby making the screens act as throw screens. A machine according to any one of the preceding claims, wherein at least one of the screens (23) is equipped for automatic detection of screen breaks by having a conductive track (71, 72) formed in electrically insulating material, the conductive track forming an electrode (74, 75) at each of two locations, thereby making it possible to determine whether the conductive track (71, 72) runs continuously or intermittently between the electrodes (74, 75). A screen element for a machine according to any one of the preceding claims, comprising a flat body with an area of ​​at least 100 cm². 2with a regular arrangement of through-holes (42), and a primary frame (25) to which the flat body is attached. Use of a sieve designed as a flat body with through-holes (42) as a sieve (23) of a machine for fractionating grain milling products.

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