Machine for grading ground grain products

Abstract

The plansifter or grain cleaning machine comprises at least one screen compartment having an arrangement of several screen elements with a screen (23). The screen compartment may be configured to oscillate, for example in the form of a circular motion. At least one of the screens (23) is designed as a flat body, in particular a metal sheet. And the sieve holes are used as through holes. According to a first option, the screen may be present as a metal screen having etched through-holes. According to a second option, the metal screen may be manufactured by an additive method. It has been found that providing a screen as a flat body with through-holes can significantly increase the efficiency of the grading process compared to a woven screen.

Description

Technical Field

[0001] The present invention relates to a machine for fractionating ground cereal products, such as a flat sifter or grain cleaning machine. Background Art

[0002] Machines for grading ground cereal products are used to separate the components of the ground product into components of coarser or finer particle sizes, and in some cases also into components of different densities, and to remove foreign matter from the ground product. The separation of the ground product into components of different particle sizes is also expressed as "grading" or "grading". In particular, flat sieves and so-called grain cleaning machines ("cleaners") are known in machines for grading ground cereal products. Flat sieves are commonly used in the milling industry to grade the ground cereal products between and after the roller mills of the grain grinder. They can also be applied to so-called control screening, i.e. the sieving of flour that is itself ready for sale. Grain cleaning machines are used to separate grain components in a process after the milling method or also between milling processes.

[0003] The process of grading the ground cereal product should not be confused with the preceding cleaning process, to which the cereal grains undergo in order to separate straw, stones, foreign matter, seedlings or sand from the cereal grains, and for which (relatively coarse) sieves are likewise used.

[0004] The plan screen comprises screen compartments, each of which comprises a stack of screens used as a plan screen. The screens each span over a main frame, the so-called "loom", which itself is inserted into a screen frame, here indicated as "sub-frame". The screen compartments are set into horizontal oscillating motion, in particular circular oscillation in the plane of the screen, by a suitable drive mechanism. Often, so-called screen cleaners are also used in plan screens, i.e. moving elements which are driven by the movement of the screen compartments and by their movement relative to the respective screens, thereby protecting the screens from blocked screen product portions.

[0005] With grain cleaning machines, the sieves used are likewise arranged in several layers and likewise oscillate. However, in contrast to planar sieves used with flat sieves, they do not (only) oscillate in the sieve plane, which is why they are classified as throw sieves. In addition, with grain cleaning machines, an air flow is generated, to which the sieved product is exposed during the sieving process.

[0006] Usually, sieves of various mesh sizes are applied in machines for grading ground cereal products, so that grading into more than two components is possible, which are packaged accordingly for sale or subjected to further processing steps, such as grinding again by roller mills.

[0007] Woven fabrics made of plastic yarns or plastic filaments (e.g. of PET or polyamide) are known as such flat screens or screens for grain cleaning machines. Woven fabrics of metal wires are also available, in particular for pre-screening with large mesh widths, since high wear often has to be taken into account for such procedures. Since the screen is designed as a woven fabric, the corresponding mesh openings are square. The mesh width is defined in this article as the distance between adjacent filaments or threads.

[0008] For fastening, the screen is placed on a wood, plastic or metal loom designed for this purpose, pre-tensioned and glued in the tensioned state, for example by cyanoacrylate. The use of such adhesives, which may come into contact with the food to be processed, is increasingly considered disadvantageous.

[0009] So-called "vibrating screens" are known in the crude oil industry, with which solid residues can be removed from drilling sludge, such as is produced during the extraction of shale oil. WO 2017 / 019580 proposes the use of a metal perforated plate as a screen, wherein all common methods by which the metal perforated plate can be produced are listed in the document, such as laser drilling, chemical etching, spark erosion, water jet cutting, punching and further methods, wherein it is stated that a very wide range of hole sizes between 1 μm and 5000 μm can be considered. Such a vibrating screen is by no means suitable for processing ground cereal products.

[0010] DE3041270 and FR9851710 each teach a sieve for a hammer mill or generally for cereal products. The sieve is designed as a metal sheet with an elongated sieve slot. Due to the design of the sieve slot in an elongated shape, these sieves will not be suitable for machines in which a plurality of sieves form, for example, sieve compartments arranged one above the other, wherein the sieve compartments oscillate. Usually, the sieve compartments in such machines described here undergo a circular motion, which is why sieve apertures of substantially the same length and width are usually used.

[0011] US2166367 relates to a method for making a metal screen (i.e. a screen structure for use with a screen), but also for use with a printing method. The method according to US2166367 produces significantly more regular holes rather than just etching holes into a metal sheet. The method consists of several steps and includes electrodeposition of material on a specially manufactured substrate with projections. The method is relatively complex and cumbersome. For this reason alone, the screen structure manufactured according to US2166367 is not suitable as a screen for industrial applications in machines such as flat screens or grain cleaning machines.

[0012] In contrast, until today, only woven screens have proven their worth for use in flat screens or grain cleaning machines.

[0013] In the milling industry, there is a need to improve the efficiency of the classification process. For a defined mesh size, the throughput per unit screen area depends on the number of meshes per unit screen area. This number can be optimized by using finer filaments or silk threads, further increasing the ratio between the opening screen area and the total screen area, or more accurately defined as the ratio between the opening screen area and the net screen area (the area of ​​the screen minus the loom and possible rods, i.e. the area where the screen is freely suspended via it). However, due to the fact that stability must be maintained, this optimization mode is limited, which in fact sets a minimum limit for the permissible filament or silk thread diameter of each material applied. Other optimization possibilities (such as using an optimized screen cleaner) only lead to further improvements in efficiency to a limited extent. Summary of the invention

[0014] The object of the present invention is therefore to provide a machine for grading ground cereal products and a sieve element for such a machine which overcomes the disadvantages of the prior art and makes the sieving process of the ground product taking place in a cereal grinder more efficient. Another object of the present invention is to further develop the machines for grading ground cereal products so that they have additional functions and / or are suitable for new applications.

[0015] A machine of the type described here comprises at least one screening compartment, usually several screening compartments, for example screening compartments arranged next to each other, each of which has an arrangement of several screening elements with screens. Typically, the screening compartments comprise a stack of screening elements. The machine is configured to oscillate the screening elements, for example in the form of circular oscillations activated by the rotation of an unbalanced body; actively guided movements are also conceivable. In the context of this article, when "oscillations" are mentioned, this also includes circular movements independent of the manner in which they are excited. Typically, the screening compartment begins to oscillate as a whole, i.e. the stack of screening elements undergoes a common, in particular circular, movement.

[0016] According to the invention, the screen is designed as (at least one) flat body, for example of metal. In this context, the screen holes are present as through holes. Non-metallic materials are also conceivable, in particular materials with sufficient dimensional stability suitable for etching methods and / or additive manufacturing methods, such as ceramic materials suitable for, for example, 3D screen printing methods, or hard thermoplastics or curable plastics (duroplastics).

[0017] A "flat body" is a foil or sheet, i.e. a coherent object extending in two dimensions, which has a particularly constant thickness (extension in the third dimension) which is at least one order of magnitude smaller than the extension in the other two dimensions (length and width). Depending on the thickness, such a flat body tends to be perceived more as a foil or sheet. In particular, a flat body can also be a sheet in the broad sense of the term; in a specific embodiment, a flat body in this context is also a sheet in the narrow sense of the term, that is to say a metal product produced by rolling.

[0018] Surprisingly, it has been found that the design as a flat body with through holes - rather than as a woven fabric of filaments or threads as is the case in the prior art - significantly increases the effectiveness. With all other relevant parameters (mechanical excitation, size of the mesh holes, number of mesh holes) being equal, the time required to pass a certain amount of sieved product (flour, semolina, semolina, corn meal, etc.) through a defined sieve area can be significantly reduced.

[0019] It can be particularly advantageous if a defined edge around the periphery of the respective through-hole is formed between the upper surface and, for example, also the lower surface of the screen on the one hand and the wall of the through-hole on the other hand, wherein the angle can be approximately 90°. Such a distinct edge is present if the upper surface merges into the wall of the through-hole without a continuous curvature, that is, if the mean radius of curvature of the transition region 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, although the mean radius of curvature of the transition region can nevertheless be determined when observed with a microscope.

[0020] As is known from the sieves themselves according to the prior art, through holes differ from slot-shaped holes in that their width in the plane of the sieve is substantially equal to their length. In particular, they are circular or have a shape that is approximately a regular polygon. Therefore, the average ratio between the longest extension of the through holes in the plane of the sieve and the smallest extension in the plane of the sieve can be relatively small, for example not more than 1.5, in particular not more than 1.3, 1.2 or 1.1. Through holes with this characteristic are suitable for machines in which the sieve element is subjected to a uniform, for example circular, motion in a plane. In addition, they allow a good discrimination of the sieved product particles, even if these particles are not spherical, which is almost inevitable for ground cereal products.

[0021] In a first group of embodiments, starting from metal sheets or foils / plates of different materials, a screen is made by a material removal method. In particular, among the material removal methods, a parallel method is suitable, in which the material removal process is carried out simultaneously for all through holes or for a large group of through holes (e.g. a complete coherent area of ​​the screen). Such a method is specifically etching. Therefore, the screen element can include an etched metal screen.

[0022] Etching produces through holes having walls that are generally perpendicular to the screen surface.

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

[0024] In addition, in the additive manufacturing method, substantially parallel methods are particularly suitable. With regard to these substantially parallel methods, at least the curing process is carried out in a layered manner over the entire surface of the screen or in a parallel manner over at least part of the surface. This is in contrast to methods in which the laser beam is locally cured in a selective manner and therefore has to be moved over the entire screen surface. The substantially parallel method can in particular be a method from the category of extrusion methods in the broadest sense, with regard to which a 2D printing plate is used, by which the material to be cured is printed.

[0025] Such a basically parallel method is the 3D screen printing method. With regard to this method, a suspension of particles of the material of which the object to be manufactured (thus here the screen) is composed is extruded in a layered manner by a doctor blade through a screen provided with a template, the so-called screen printing screen (not to be confused with the screen (screen) of the machine to be manufactured). The template is therefore used here as a printing plate in the context of the extrusion method and defines (in two dimensions) the shape of the body to be manufactured. Each layer is solidified before the next layer is deposited. After all layers have been deposited, sintering can also be carried out for compaction. With regard to the application of the manufacture of the screen, a screen printing screen with a template representing the negative mold of the screen to be manufactured is thus produced, and is subsequently used in the additive manufacturing method.

[0026] Fabricating the screen using such additive methods also produces through-holes having walls that are generally perpendicular to the screen surface and having sharp, well-defined edges between the screen surface and the walls.

[0027] Thus, a screen of the type according to the invention - and this applies to both groups of embodiments - has webs between the screen openings, the cross section of which is rectangular (i.e. substantially rectangular, as described in more detail below), in contrast to the circular cross section of the filaments or threads of a woven screen. Furthermore, the screen forms a planar structure, in contrast to a woven screen, in which the filaments or threads overlap one above the other at the intersection points.

[0028] These geometrical differences may represent one way of explaining why the method according to the invention has a higher efficiency compared to the prior art. In particular, the edges formed at the upper side of the web of the screen of the invention may have an important influence. On the one hand, they cause particles subjected to random motion to hit the surface to which they stick with a lower probability and to hit the edge of the etched screen with a greater probability. Secondly, they may result in a lower probability of forming a temporarily stable structure by the screened product, such a structure extending beyond the screen aperture.

[0029] However, the conclusions of the present invention do not depend on a certain interpretation of the observed beneficial effects.

[0030] Etched screens or screens that have been manufactured by additive manufacturing methods are also characterized in that they have no structures protruding out of the plane of the screen. This distinguishes them from the punched structures that are common, for example, for coarse perforated plates, which each have sharp burrs on one side and taper conically from the other side. It has been found that the absence of such structures has a positive effect on the efficiency of the etched screen.

[0031] A further advantage is that the screen as a flat body with through-holes can be cleaned more easily than in the prior art, in particular by ultrasonic cleaning.

[0032] The through holes can be arranged in a hexagonal structure, so that they are staggered in rows. This makes use of another advantage of the method according to the invention: the arrangement of the screen holes can be freely selected and is no longer limited to a rectangular arrangement. The advantage of the hexagonal structure is that, for a given minimum web width, the largest possible number of screen holes can be arranged per unit screen area. This can further increase the efficiency gain.

[0033] The same applies to the shape of the mesh openings: the shape can also be freely determined within the limits imposed by the etching method or additive manufacturing method. Thus, not only the arrangement of the mesh openings but also the shape of the mesh openings can be approximately hexagonal. Thus, for a given minimum web width, the ratio between the open mesh area and the clear mesh area can be maximized.

[0034] For example, one can imagine a machine in which the through-holes of a very fine screen, for example down to a mesh width of about 0.3 mm, are approximately circular, whereas the larger through-holes are hexagonal.

[0035] The use of etched screens or screens manufactured by additive manufacturing methods (i.e. screens with through holes manufactured by etching) is particularly advantageous for mesh sizes between 0.08 mm and about 1 mm. Therefore, the machine according to the invention comprises, for example, at least one screen with through holes with a mesh width between 0.08 mm and 1 mm. In particular, it can have at least one screen with a mesh width greater than 0.3 mm and with hexagonal through holes.

[0036] In the case of non-square vias, the mesh width is defined herein as the square root of the area of ​​the via in a horizontal cross section.

[0037] The width of the web between the through holes can correspond to just less than the thickness of the flat body. For webs with a non-constant width, this applies to the width at the narrowest point. "Approximately corresponding to the thickness of the flat body" here means that the web width is not less than 40% or not less than 50% and not more than 160% or not more than 130% of the thickness of the flat body and can in particular be between 60% and 100% of the thickness of the flat body.

[0038] As already mentioned, the production of the screen in the first group of embodiments is carried out by etching (industrial etching processes are also denoted by the term "chemical milling", and in this context, "etching" is generally understood to be a method of removing material in a process using a substance that attacks the material). The advantage of etching compared to, for example, lasers is that the process is parallel, which is decisive for the production efficiency, especially in the case of fine-mesh screens. The screen openings (through holes) can in particular be etched from both sides, i.e. from above and from below.

[0039] In a first group of embodiments, the screen is designed so that the etching from above and below is asymmetrical, wherein more material has been removed from below and the narrowest position of the through-hole is therefore located above the mid-plane of the flat body. In this way, the probability of particles of the screened product sticking in the through-hole is minimized.

[0040] In the second group of embodiments, for a screen that has been manufactured using an additive manufacturing method, the layers that form the screen are particularly parallel to the two large surfaces ("upper side and lower side") of the flat body. In the second group of embodiments, the screen is particularly also metallic. In the second group of embodiments, there is not necessarily a defined narrowest position between the planes of the screen defined by the large surfaces.

[0041] If the flat body from which the screen is made is metallic, there may also be the option of making the screen from a magnetic (i.e. magnetizable, thus ferromagnetic or ferrimagnetic) material, such as magnetic steel. Ferromagnetic stainless steels (in particular martensitic or ferritic steels or steels having the corresponding parts) are known as hard magnets as well as steels with soft magnetic properties. The use of magnetizable material has the advantage that in the event of breakage of the screen which may occur over time, it is possible to remove the magnetic screen fragments from the screened product by exposing the area with the fragments to a magnetic field.

[0042] The use of metal or some plastics as material for the flat body has another possible advantage: the screen can be fastened to the main frame by welding. The use of adhesives thus becomes superfluous. A prerequisite for weldability is to design the screen as a flat body with through holes. Only with this design can the forces acting on the web between the through holes be sufficiently distributed over the entire periphery of the screen and from there to the main frame. Welding of metal wires to form a woven wire screen, as known from the prior art, although likewise possible in theory, would be cumbersome and impractical.

[0043] In addition to the main frame to which the screen is directly connected and which can therefore be interpreted as also being a part of the screen, the screen element can include a subframe, on which the main frame is placed or which holds and / or receives the main frame in another way. The subframes of the screen compartments can be stacked on each other and / or they can be held by the screen compartment housing and used as modules of such a housing. However, subframes other than the main frame are not necessary. The main frame is also likely to be directly attached to the screen compartment housing and / or stacked on each other, and therefore used as a screen frame. Therefore, the screen element consists only of the main frame to which the screen is fastened.

[0044] In addition to machines for grading ground cereal products, such as flat sieves or grain cleaning machines, sieves for use in such machines and the use thereof as well as methods for manufacturing sieves for such machines are also the subject of the invention. In many embodiments, the sieves used in the machines for grading ground cereal products have a diameter greater than 100 cm. 2 , usually at least 0.35m 2 of a relatively large area.

[0045] The screen according to the method of the invention can be one-piece, ie manufactured from a single flat body. However, it is also possible to use more than one flat body, wherein two or more flat bodies can be placed adjacent to each other as strips or sheets and fastened to a main frame.

[0046] Except the outer frame part that frames the screen area, the main frame that the screen is directly fastened to can also comprise the bar that the screen crosses thereon.In this case, the screen is for example not only fastened to the outer frame part, but also fastened on the bar.

[0047] With regard to the screen of the first group of embodiments, the manufacturing method comprises providing a flat body and producing through-holes from two flat sides by a material removal method (eg etching).

[0048] With regard to the screens of the second group of embodiments, these steps are replaced by an additive manufacturing method. This includes providing a suspension having particles, in particular metal (or consisting of another suitable, for example weldable material) suspended in a solvent and, in some cases, having further components, such as a binder or other additives. In a further step, the method comprises depositing the suspension in layers, wherein each layer has a shape corresponding to the shape of the flat body (horizontal cross section) with through holes to be produced. This can be achieved in a process using a template (for 3D screen printing) or in the case of a non-parallel method also by selective curing with the aid of a laser beam. In the former case, in particular, each layer after deposition is at least partially cured before the next layer is deposited. This curing can also be achieved as an actively induced step, for example by irradiation with electromagnetic radiation (infrared or possibly visible light and / or UV), or by inflow of hot air with a hot air blower or and / or another energy feed method. Alternatively, it is also conceivable in principle that this step is carried out passively by the material curing itself, for which sufficient time is allowed.

[0049] The methods according to the first group and the methods according to the second group have in common that a very flat surface with a sharp edge occurs between the surface (i.e. the upper side and also the lower side) on the one hand and the wall of the through hole on the other hand, and this is the case despite the fact that the size of the through hole is very small due to the application. With regard to the methods according to the first group, the sharp edge arises due to the characteristics of the material removal etching process applied. With regard to the methods of the second group, the sharp edge arises due to the fact that the layers deposited successively are always relatively thin compared to the thickness of the entire screen and that, due to the peculiarities of the method, each layer can have exactly the same dimensions.

[0050] This characteristic of the sharp edges distinguishes the screens manufactured by the described method firstly from the woven screens known from the prior art. Secondly, they also differ from the screens known from US2166367, in which the screens are molded using a molding technique and, in addition to the rounding already present in the applied casting mold, the rounding at the edges is produced due to the surface tension of the still flowable substance during the molding process and also due to the mold removal process.

[0051] Furthermore, and this applies to both groups of embodiments, the method can include fastening the flat body to the main frame, for example by welding, in particular laser welding or spot welding. A particular advantage of the method according to the invention is that here the flat body does not need to be clamped in a particularly secure manner - it is sufficient to simply place it on the main frame in a correctly dimensioned manner and then fasten it to the main frame. In this way, disadvantageous gluing and the use of clamping devices can be avoided.

[0052] The screen according to the second group of embodiments can optionally be configured to allow automatic detection of screen breakage. To this end, the screen includes, in addition to the electrically insulating material, a strip conductor, which is particularly embedded in the electrically insulating material. The strip conductor forms electrodes at at least two locations, in particular at the two ends of the strip conductor. The machine is then configured to determine, by means of a suitable sensor, whether there is electrical contact between the two electrodes. If this is not the case, the strip conductor is interrupted, which is an indication of screen breakage. The machine can display screen breakage to the user and thus allow timely replacement of the screen without damage occurring due to unscreened screened product passing through the screen surface or due to fragments of the screen appearing in the screened product.

[0053] Another aspect of the invention relates to the use of a sieve of the type described herein, which is designed as a flat body with through holes, as a sieve for a machine (in particular a machine of the type described) for grading ground cereal products, wherein the sieve forms sieve compartments that perform an oscillating movement (open or closed).

[0054] A further aspect of the invention relates to a novel application of a machine for grading ground cereal products, the machine having a sieve which moves, for example in a circular motion, in order to assist at least part of the sieved product to be graded to pass through, wherein at least one of the sieves is designed as a flat body with through holes.

[0055] This novel application is the fractionation of grist. "Grist" here refers to the by-product that remains as a residue after dissolving the malt components in beer making or similar processes. Grist is a ground cereal product, because the cereal (e.g. barley) is crushed after malting and roasting and before mashing and is therefore subjected to a (relatively coarse) grinding process.

[0056] In addition to the insoluble protein fraction and further substances, cereal residues usually (and often actually are intended to) contain the husks of the applied cereals. Due to these husks, the cereal residues are not suitable for subsequent classification, since the husks very quickly clog the sieves of flat screens and similar machines. For this reason alone, cereal residues are mainly used as animal feed. However, due to its richness in protein, cereal residues are also suitable for the manufacture of nutritional foods for humans, for example for meat substitute products. Therefore, an efficient method for screening out the husks and other coarse components from the cereal residues would be desirable.

[0057] In tests, it has been found that, compared to machines of known type (flat screens or also tossing screens and other screening machines with conventional woven screens), no clogging is observed when the grain residue is screened for a machine with a screen designed according to the invention as a flat body with through holes, in particular with a sharp edge between the surface and the wall of the through holes. In contrast, and surprisingly in fact, it has been found that a machine of this type with a flat screen is suitable for classifying the grain residue without further trouble. Thus, a problem that could not be solved with the prior art is solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In the following, examples of embodiments of the present invention are described by means of the accompanying drawings. In the accompanying drawings, the same reference numerals denote the same or similar elements. The accompanying drawings show elements that partially correspond to each other in size, which elements differ from each other in different drawings. Among them:

[0059] Figure 1 A view of a flat screen is shown;

[0060] Figure 2 A screen stack is shown;

[0061] Figure 3 An exploded representation of a screen element having a screen, a main frame, and a subframe is shown;

[0062] Figure 4 A main frame with a screen is shown;

[0063] Figure 5 Schematically showing details of a sieve having a hexagonal structure;

[0064] Figure 6 Also schematically shown are details of a screen having a square grid arrangement;

[0065] Figure 7 Shown by Figure 5 The cross section of the sieve;

[0066] Figure 8 shows a section through a screen according to the prior art;

[0067] Fig. 9 Alternative cross-sectional shapes are schematically shown;

[0068] Fig.10 shows a cross section of a screen manufactured by an additive manufacturing method;

[0069] Fig.11 shows the measurement of the fraction efficiency curve of a screen designed as a flat body with etched through holes in comparison with a screen according to the prior art;

[0070] Fig.12shows a horizontal section through a screen with automatic screen breakage identification; and

[0071] Fig.13 Shown by Fig.12 Detail of a cross section of a sieve. DETAILED DESCRIPTION

[0072] Figure 1 As an example of a machine for grading ground cereal products, a flat screen 1 for use in a cereal mill is shown. The flat screen comprises a plurality of screen compartments 3 which are assembled in space via a common suspension device 4 so that a common horizontal oscillating movement is possible. The drive (in Figure 1 The flat screen (not visible in Fig. 1) is configured to make the components of the screening compartments perform, for example, horizontal circular oscillations. In addition, the flat screen comprises a flexible feed conduit 6 as the screening product inlet and an equally flexible outlet conduit 7 as the screening product outlet.

[0073] Each sieve compartment 3 comprises Figure 2 , and each of these sieve elements has a sieve, wherein the sieves can have different mesh widths. In addition, there is a screening product guide, for example transversely relative to the sieve, by which an unscreened portion of the screening product (which is for example guided transversely and for example to another sieve element located further below) and possibly also a screened portion of the screening product are further conveyed, which is the case for each sieve. In general, the screening compartment is designed so that a selection of the screened product fed into screening product portions of different grain sizes is achieved, so that depending on the screened product fed, the screening product portions fed into different screening product discharge ducts 7 have different characteristics.

[0074] This principle is known per se and is implemented according to the invention in that at least some of the screens used are present, for example, as metal flat bodies with through-holes, for example as etched or additively manufactured screens.

[0075] In the case of grain cleaning machines, there are also several sieve elements that can be arranged one above the other, as with flat sieves. In the case of grain cleaning machines, people do not usually talk about a "sieve stack" - the number of sieves arranged one above the other is usually less than with flat sieves - but about sieve plates. The oscillating motion experienced by the sieve is different compared to the flat sieve, which is why the sieve acts as a throwing sieve. The subsequent explanation of the properties of sieve elements and sieves usually relates to machines for grading ground cereal products, including such flat sieves and grain cleaning machines.

[0076] Figure 3 Schematically an exploded representation of the screen element 11 is shown. The screen frame 21 (subframe) serves as a supporting structure. In addition, it may include Figure 3The structure not shown in the figure is used to guide a so-called screen cleaner, which is also not shown in the drawings and can be present as an option, so as to knock the screen from below, for example. The actual screen 23 is fastened to a main frame 25, which can be placed in the screen frame 21 and guided by it.

[0077] The main frame 25 comprises an outer frame portion 26 forming a rectangle and rods 27 over which the screen 23 spans. The frame elements of the outer frame portion 26 and the rods 27 over which the screen spans may each have a rectangular or other cross-section.

[0078] exist Figure 4 A main frame 25 with a screen 23 fastened thereto is drawn in FIG. An advantage of the invention is that the fastening can optionally be achieved by a welded connection (eg a spot welded connection). The corresponding weld points 31 are Figure 4 Schematically represented in .

[0079] In the exemplary embodiment, the screen is not only welded to the outer frame part 26, but also to the rods 27, over which the screen 23 is straddled, for example likewise by a spot welded connection or a laser welded connection.

[0080] The metal sheet has a certain moderate elasticity in its plane. This feature is particularly advantageous in the context of the present invention: the screen can be fastened to the loom 25 without having to use devices for applying tension along the plane, which are especially conceivable for this purpose and are known for woven screens. Instead, the flat body with the through holes only needs to be placed on the frame and then connected to the frame, for example by the mentioned spot welding connection or by laser welding. The use of adhesives also becomes superfluous, which will be seen as an advantage for parts that come into direct contact with foodstuffs.

[0081] Figure 5 Schematically shown are details of the screen 23. Through holes 42 are present in a hexagonal arrangement. In addition, the shape of the through holes 42 is approximately hexagonal. Depending on the size of the through holes 42, such a hexagonal shape inherent in the manufacture can be realized to a greater extent (assuming larger through holes, i.e., when the screen is relatively large-mesh) or to a lesser extent, for example if the screen is manufactured by etching. For screens with very fine mesh, through holes of approximately circular shape are produced, depending on the manufacturing method. However, regardless of the actual shape, the advantages of the hexagonal arrangement remain.

[0082] Even though a hexagonal arrangement and shape is particularly advantageous, the screen can in principle also have any (usually regular) arrangement and any shape of the through-openings. Figure 6 A square grid arrangement with square through-holes is shown, as is produced automatically in the case of a woven screen and as is optionally present with a screen of sheet metal or other flat body of the type described here. Figure 6Also shown is the definition of the mesh width w, which is defined as the width of the intermediate space between adjacent filaments (rather than, for example, the distance between the axes of the filaments); if the grid is non-planar, this definition applies to the projection on the screen surface.

[0083] Regarding sieves with non-square through holes, such as Figure 5 For a screen, the mesh width can be defined as the square root of the area a of the through holes in a horizontal cross section, ie, the mesh width that would result if the through holes had area a but were square.

[0084] Depending on the shape of the through-holes, it is also conceivable to deliberately choose the area to be slightly smaller (or possibly slightly larger) than for a square through-hole in order to achieve the same grading effect. For example, assuming an approximately circular through-hole (such a result is produced when etching very small through-holes with a diameter of, for example, less than 0.3 mm; with additive methods, slightly circular shapes with very fine structures are also produced), a diameter can be chosen that results in an area of ​​only about 95% of the area a, which then results in approximately the same choice as for a square through-hole with a mesh width √(a). Likewise for hexagonal through-holes (such through-holes can be produced for etching screens with a slightly larger mesh width of, for example, at least 0.3 mm), the through-holes can also have a slightly smaller area a than the corresponding square through-holes with the same grading effect.

[0085] Figure 5 and Figure 6 The comparison also shows that for a given web width b st and mesh width, the ratio between the open screen area (a variable that is important for the throughput through the screen) and the clear screen area (which is usually given by the size of the screen element and is not affected) is better assuming a hexagonal arrangement and assuming approximately hexagonal through-holes than assuming a conventional square arrangement. It has been found, for example, that in the case of hexagonal through-holes, the open screen area can be enlarged by about 4% when compared with a steel woven fabric and by about 11% when compared with a plastic woven fabric (which requires somewhat thicker filaments), which means a very significant improvement in efficiency for industrial processes such as those occurring in grain grinders.

[0086] Furthermore, however, it has been found in tests that the screen throughput by a screen of the type according to the invention can be increased even further than can be explained by a larger open screen area compared to the prior art, as will be explained further below.

[0087] Figure 7 A vertical section through a screen of the type according to the invention and produced by etching is shown in FIG. Figure 7 The web width b is also shown. stFor example, the web has an aspect ratio of approximately 1, which corresponds approximately to the thickness d of the flat body. Figure 7 It can also be seen that the cross-section of the web 41 is rectangular, for example approximately square, wherein it is assumed that it has been manufactured by etching, in particular etching from both sides, and therefore from above and below, which is why the through-hole narrows very slightly towards the middle plane, which however does not have a significant negative impact on the result that can be achieved.

[0088] Figure 8 A corresponding section through a woven screen according to the prior art is shown in .

[0089] Figure 7 and Figure 8 The comparison shows that, for a screen of the type according to the invention, an edge 44 is produced along the web, unlike in the case of a woven screen. The presence of such an edge 44, in particular at the upper side (i.e. towards the product to be sieved), represents a possible explanation for why the efficiency of the method according to the invention is so greatly improved. Compared to a woven screen assuming its circular cross-sectional structure, a flour or grit particle or a group of such particles would have a much lower probability of forming part of a stable structure on the upper side of the screen and would be deflected downwards during its movement at the edge with a much greater probability.

[0090] first Fig. 9 The schematic diagram shows the principle that when manufacturing by etching it is not necessary to etch the same depth from both sides, but rather an asymmetric configuration is also possible. Fig. 9 In the embodiment of the present invention, the depth etched from below is approximately twice the depth etched from above, so the position with the greatest narrowing is above the mid-plane. Such an asymmetric configuration with the narrowest position above the mid-plane can be advantageous because, for this reason, there is less probability that particles will become blocked in the through-holes. This leads to a potential further increase in efficiency.

[0091] Secondly, Fig. 9 The web width b is shown st It can also be smaller than the thickness d, which has been discussed above.

[0092] third, Fig. 9 It is also shown that, even assuming the presence of a (as mentioned, advantageous in certain cases) sharp edge, the angle between the upper surface of the screen and the wall of the through hole does not have to be exactly 90°, but can also be slightly smaller.

[0093] Fig.10 Shows something like Figure 7A vertical section through a screen of the type according to the invention which is not produced by etching but by an additive manufacturing method. The dashed lines indicate that the screen consists of a plurality of layers 51, wherein the layers extend horizontally. The layered structure can, but does not necessarily have to, be discernible on the screen. Figure 7 Explanation of the size and shape (with edge 44) and by Fig. 9 The considerations explained regarding the web width also apply to the second group of embodiments, one example of which is Fig.10 Indicated in.

[0094] Example 1: About 100 g of commercially available wheat flour was provided to a sieve manufactured by etching and having a size of 275 mm*175 mm (net sieve area), the sieve having a hexagonal arrangement of a total of 427,000 through holes. The through holes had a=0.05309 mm measured by CAD software. 2 The sieve is subjected to a horizontal circular motion (approximately 3 revolutions per second; diameter of the motion circle: 5 cm). After a total of approximately 25 seconds, the flour has passed through the sieve except for a small amount of residue.

[0095] An equal amount of the same flour was provided to a comparable commercial nylon sieve (mesh width 0.236 mm; number of through holes 426000) and sieved under the same conditions (same sieve size and net sieve area). After 40 seconds, a considerable portion of the flour still did not pass through the sieve, which is why a sieve cleaner was additionally placed on the sieve to assist this process. Only after a further 20 seconds (thus a total of 60 seconds) did the flour pass, apart from a small residual portion of a considerable residual amount.

[0096] Therefore, in this example, the efficiency is significantly improved by more than a factor of 2.

[0097] This example actually corresponds to a real application situation. Flour ready for sale is usually sieved again in a so-called control sieve machine before delivery in order to filter out possible foreign matter. A sieve such as a commercially available nylon sieve with a mesh width of 0.236 mm can be used for such a process.

[0098] Example 2: As in Example 1, but rye flour was used instead of wheat flour as the sifted product. Rye flour clumps easily and is therefore difficult to sieve. The improvement in efficiency according to Example 1 was confirmed.

[0099] Example 3: Similar to Example 1, but a sieve with a square arrangement of approximately square sieve holes manufactured by etching was used (the shape of the sieve holes is that of a square with significantly rounded corners due to the etching process). The effective area of ​​the through-holes was remeasured by CAD. It corresponds to the area of ​​a square with a mesh width of 236 (i.e. 0.236 mm). Due to the circular motion, practically the entire amount of 100 g of wheat flour has passed through the sieve after 16 seconds. A new comparative measurement with the same amount of white flour and the nylon sieve of Example 1 and with a mesh width of 0.236 mm (but without the use of a sieve cleaner) still does not lead to complete passage of the wheat flour even after 93 seconds; i.e., after already 16 seconds, more residues remain than with the sieve according to the invention.

[0100] Example 3 demonstrates that the arrangement and shape of the mesh openings is often secondary and that the effect of the invention is due to the mesh having an intrinsically different structure than a woven mesh.

[0101] Fig.11 Comparative measurements show that the method according to the present invention not only improves the efficiency of the screening process, but also improves the fineness of separation. Measurements were performed using a sieve according to the present invention and a commercially available nylon sieve according to Example 1.

[0102] The ground cereal product was sieved with the parameters according to Example 1. The so-called (thus the unsieved part of the sieved product) was analyzed with the aid of a commercial laboratory device (manufacturer: Microtrac) using the principle of dynamic image analysis (ISO 13322-2). In particular, the size distribution of the particles of the ground cereal product was determined, and in particular the distribution of the smallest diameter X C,min As a feature quantity. C,min is the minimum size of the corresponding particle - for spherical particles, X C,min Corresponding to the diameter, for ellipsoidal particles it is twice the minimum semi-axis value. Figure 16 shows the so-called fractional efficiency curve, i.e. X C,min The fraction of the number of ground cereal product particles whose value is less than the value specified on the abscissa. It can be seen that for the first curve 61 obtained with the machine according to the invention with the etched sieve, the dividing line changes significantly more sharply (steeper gradient) than for the second curve 62 obtained with the machine with a conventional sieve. It is immediately apparent from this that the machine according to the invention and the sieve element according to the invention lead not only to an increased efficiency but also to an increased differentiation quality.

[0103] Fig.12 and Fig.13The possibility of providing a sieve element of the type according to the invention with additional functions, in particular automatic sieve breakage detection, is illustrated. This possibility exists in particular in sieve elements manufactured using an additive manufacturing method, in particular a 3D printing method. The sieve element comprises at least one embedded strip conductor 71 extending between two contacts 74. In the illustrated embodiment example, the course of the strip conductor 71 is tortuous. In the event of a sieve breakage 81, the strip conductor is interrupted and can thus be determined by a simple sensor or a simple measuring device, which checks whether there is an electrical connection between the two contacts 74.

[0104] The strip conductor 71 extends in an embedded manner, i.e. not in contact with the upper or lower surface of the flat body. In embodiments where these surfaces are metallic (i.e. the sieve element is substantially metallic), at least one electrically insulating layer 92 will be present between the strip conductor 71 and the metal layer 81 on the surface; likewise, the layer with the strip conductor 71 can be filled with an electrically insulating material (i.e. electrically insulating at those locations in the plane of the sieve element where there is no strip conductor). Suitable electrically insulating materials are the mentioned ceramic materials or hard thermoplastics or curable plastics.

[0105] In the embodiment example represented, in addition to the (first) strip conductor 71, a second strip conductor 72 is present in another layer and likewise extends in a meandering manner, but with a different, in particular orthogonal, main direction. Screen breaks, such as the second represented screen break 82, can thus be determined, which extend such that they do not cut through the first strip conductor 71. The second electrode 75 for the second strip conductor is read separately from the (first) electrode 74 of the first strip conductor 71.

Claims

1. A machine (1) for grading ground cereal products, comprising at least one sifting compartment (3) with a plurality of sieves (23), a sifted product inlet (6) and a sifted product outlet (7), wherein: The machine is configured to cause the sieves (23) to perform an oscillating movement in order to help the sieved product to at least partially pass through the sieve, the sieved product passing through the sieve inlet, characterized in that at least one of the sieves (23) is designed as a flat body with a through hole (42).

2. The machine according to claim 1, wherein: The flat body is metallic.

3. The machine according to claim 2, wherein: The metal flat body is magnetic.

4. A machine according to any one of the preceding claims, wherein: The screen (23) is fastened to the main frame (25).

5. The machine according to claim 4, wherein: The main frame comprises an outer frame portion (26) extending along the periphery of the screen and rods (27) over which the screen straddles and wherein the screen is secured to the outer frame portion (26) and the rods (27).

6. A machine according to claim 4 or 5, wherein: The screen (23) is welded to the main frame (25).

7. A machine according to any one of the preceding claims, wherein: The through holes (42) of at least one of the screens form a hexagonal arrangement.

8. A machine according to any one of the preceding claims, wherein: The through holes (42) of at least one of the screens have a hexagonal shape.

9. A machine according to any one of the preceding claims, wherein: For the web (41) formed between adjacent through holes (42), the situation is: 0.5*d st <1.6*d, where d is the thickness of the flat body, and b st is the width of the web (41) at its narrowest point.​ 10. A machine according to any one of the preceding claims, wherein: The flat body forms a rim (44) surrounding the through hole (41).

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

12. A machine according to any one of the preceding claims, wherein: The through-holes are produced by etching.

13. The machine according to any one of claims 1 to 11, wherein: The screen is manufactured by an additive manufacturing method.

14. Machine according to any of the preceding claims, configured so that the screen (23) oscillates in a horizontal screen plane, by which means the machine is a flat screen.

15. Machine according to any one of claims 1 to 13, configured to oscillate the screen (23) so that the screen plane is subjected to oscillations, by which means the screen acts as a throwing screen.

16. 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 breakage by virtue of it comprising a strip conductor (71, 72) formed in an electrically insulating material, wherein the strip conductor forms electrodes (74, 75) at two locations respectively, by means of which it is possible to determine whether the strip conductor (71, 72) between the electrodes (74, 75) extends in an uninterrupted or discontinuous manner.

17. A screen element for a machine according to any one of the preceding claims, comprising a screen element having a screen area of ​​at least 100 cm 2 A flat body with an area of ​​and a main frame (52), wherein the flat body has regularly arranged through holes (42), and the flat body is fastened to the main frame (52).

18. Use of a sieve designed as a flat body with through-openings (42) as a sieve (23) of a machine for classifying ground products.

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