Method for producing a foam abrasive

Abstract

The invention relates to a method for producing a foam abrasive (10) for grinding a workpiece, in which method abrasive particles (16) are scattered over a foam (12), the surface (14) of which foam is coated with binder (18). According to the invention, prior to the foam being coated with binder, pores and / or bubbles (20) in the foam (12) which are in a region (22) of the foam (12) that is close to the surface are opened using a fluid (28) directed at the surface (14) of the foam (12). The invention further relates to a foam abrasive (10) produced in accordance with the method according to the invention.

Description

[0001] The invention relates to a method for producing a foam abrasive for grinding a workpiece. State of the art

[0002] Foam abrasives are known in principle from the state of the art. These abrasive products have the advantage of adapting their shape to the contour of the object being sanded during the sanding process by elastically deforming the soft foam base. For this reason, foam abrasives are the first choice for sanding non-flat objects.

[0003] For example, US 4887396 A discloses foam abrasives comprising a base body made of foam, wherein the base body has a higher density of the foam in areas of increased wear.

[0004] Aspects of the present subject matter emerge from US 2018 / 281148 A1, JP 2013 136146 A, US 6 709 738 B2, US 8 858 664 B2, WO 2005 / 028157 A1, US 2006 / 026904 A1 and US 5 595 578 A. Disclosure of the invention

[0005] The invention is based on a method for producing a foam abrasive for grinding a workpiece, in which a foam coated on its surface with a binder is sprinkled with abrasive grains.

[0006] The foam abrasive is an abrasive article for grinding an object. The grinding can be performed by hand and / or by machine. The term "grinding" includes polishing. The foam abrasive, in particular the foam base body that gives the foam abrasive its essential shape, can in principle be present in various forms, for example, as a block, a disc, a roll, a belt, a strip, or the like. It is also conceivable to integrate the foam abrasive into a glove in such a way that the foam abrasive forms the surface of the glove on the palm and fingertips. Furthermore, the foam abrasive can also be manufactured for use with grinding machines such as eccentric sanders.

[0007] The base body of the foam abrasive comprises at least one foam. The foam is flexible and, in particular, elastically deformable. The foam as the base body gives the foam abrasive its essential shape and specific properties with regard to flexibility and stability, in particular with regard to elasticity, extensibility, compressibility, shearability, tear strength, and tensile strength. These specific properties significantly characterize the handling and properties of the foam abrasive during a grinding process. In one embodiment of the foam abrasive, the foam can be made of, or in particular consist of, a polyurethane foam. Polyurethane exhibits advantageous hardness and tear resistance.Alternatively, the foam can also be made of ethylene-vinyl acetate copolymer (EVA), polyethylene (PE), polypropylene (PP), acrylonitrile-butadiene rubber (nitrile rubber, AB or NBR), polystyrene (PS), polyurethane (PE) or the like.

[0008] The foam abrasive has abrasive grains on at least one surface of the foam. The at least one surface can in particular be selected as the entire surface of the foam forming the main body of the foam abrasive. Alternatively, the at least one surface can also be selected as part of the entire surface of the foam. An abrasive grain is to be understood as an element that has a deforming and / or abrasive effect on an object to be machined, i.e. on a workpiece, during the performance of a grinding process. An abrasive grain can in particular be formed from a mineral and / or ceramic material, for example from diamond, corundum, silicon carbide, boron nitride or the like. In one exemplary embodiment, the abrasive grains are realized as aluminum oxide particles with a particle size between 7 µm and 300 µm.In particular, the abrasive grain can have any geometric configuration deemed appropriate by a person skilled in the art. The abrasive grain can be a so-called shaped abrasive grain or a broken abrasive grain. An abrasive grain causes friction and temperature development on the object being machined, which exerts a deforming and / or abrasive effect on or in the object being machined.

[0009] In the foam abrasive, the abrasive grains adhere to the foam surface by means of the binder. The binder adheres the abrasive grains to the foam, particularly in a desired position and distribution, and thus fixes them. The binder also serves to impart specific properties to the foam abrasive on at least one surface with regard to adhesion, elongation, tear and tensile strength, flexibility, and stability. In one embodiment, the binder can be made of thermoplastic polyurethane (TPU). The term "thermoplastic polyurethane" encompasses all urethane-based thermoplastic elastomers and refers to a solvent-free adhesive. A binder made of thermoplastic polyurethane is advantageously a hot-melt adhesive that can be processed and applied without solvents.A "hot melt adhesive" is understood in particular to mean a solvent-free hot melt adhesive that is essentially solid at room temperature, liquefied when heated, and can be applied, in particular poured or smeared, and forms a solid bond upon cooling. Alternative binders, for example, based on resins, are also conceivable and known to those skilled in the art.

[0010] Typically, the surfaces of foam abrasives are subjected to particularly heavy wear during a grinding process, so that the abrasive grains located there and the foam underneath are exposed to high stresses. As a result of this stress, abrasive grains - sometimes along with the adjacent foam material of the base body - are quickly detached (torn out) from the foam abrasive and are therefore no longer available for further grinding. Furthermore, it happens that detached abrasive grains cause unwanted scratches or grooves in the surface of an object to be ground because they are moved uncontrollably between the object and the foam abrasive during the grinding process. The described degradation of the foam abrasive typically leads to a premature end of the use of the foam abrasive.

[0011] According to the invention, it is proposed that pores and / or bubbles located in the foam are pierced and opened in this way by means of a fluid directed onto the surface of the foam, at least in a region of the foam close to the surface, before coating with binder.

[0012] "Pores and / or bubbles" refer to the cavities that generally form in the foam during production. These cavities become increasingly larger during the foaming process and eventually partially burst, creating an open-pore foam structure. This process of bubble formation and bursting is difficult to control, so that the ratio of open pores or bubbles to closed pores or bubbles varies greatly from batch to batch and even within a batch across a foam body. Using the method according to the invention, particularly uniform production conditions can be achieved by using the fluid to pierce and thus artificially open the pores and / or bubbles located in the area near the surface of the foam. Consequently, uniform ratios of open pores and / or bubbles to closed pores or bubbles can be achieved.Bubbles are realized at least in the area near the surface of the foam and thus ensured.

[0013] As a result, an open cell structure can be achieved in the area of ​​the foam closest to the surface, with the open pores or bubbles forming permeable channels in the foam that serve to absorb the binder. When applied (e.g., by doctor blade), the binder can penetrate sufficiently deeply into the foam - i.e., into the open pores and / or bubbles closest to the surface. In particular, the binder can saturate the area of ​​the foam closest to the surface. In this way, the layer formed from the applied binder is anchored to the foam with particularly stable mechanical strength. Furthermore, the binder can penetrate the open pore structure and, once cured, act as a type of material reinforcement, thereby increasing the stiffness of the foam at its surface. This side effect has a positive effect on the grip of the foam abrasive.

[0014] The closed pores and / or bubbles of the foam are sealed only by a comparatively thin film. The invention is based on the finding that this thin film is mechanically constructed in such a way that it can be penetrated by the fluid directed onto the surface of the foam, thus opening the pores and / or bubbles. Surprisingly, this opening (under constant process conditions) occurs almost constantly over a certain depth into the foam. This region, which extends to the certain depth, is referred to in this document as the near-surface region. The near-surface region therefore represents the section of the foam that is, on average, saturated or wetted by the applied binder as a result of the opened pores and / or bubbles.In one embodiment of the method, the near-surface region is created to a depth of at least 3 layers of medium pore size, in particular at least 5 layers of medium pore size, very particularly at least 10 layers of medium pore size. In particular, this near-surface region is created to a depth of at least 3 mm, in particular at least 5 mm, very particularly at least 10 mm into the foam. In this way, a particularly stable anchoring of the binder and consequently a particularly durable foam abrasive can be achieved.

[0015] Furthermore, the open pores and / or bubbles also allow air contained in the foam to escape more easily during the application of the binder. This makes it possible to create a foam abrasive article in which little to no air remains beneath the applied binder in the pores and / or bubbles near the surface, which can lead to hole formation and / or blistering on the surface of the foam abrasive (especially at the binder layer) due to thermal expansion during a grinding process.

[0016] The pores and / or bubbles are penetrated and opened by the fluid directed toward the surface of the foam. "Directed toward the surface" means that the fluid propagates in a substantially linear direction toward the foam, such as that encountered in an air jet or compressed air blast (alternatively, a gas jet or gas pressure blast).

[0017] In one embodiment of the method, the fluid is directed onto the surface of the foam at a pressure of at least 2 bar, in particular at least 7 bar, and most particularly at least 10 bar (relative to ambient pressure). In this way, reliable opening of the pores and / or bubbles can be achieved by means of the fluid.

[0018] In one embodiment of the method, compressed air and / or compressed gas (such as nitrogen) and / or pulses of compressed air and / or compressed gas and / or a liquid (such as water) are used as the fluid. The use of compressed air or compressed gas represents a particularly simple implementation of the method, which is particularly suitable for foam that is continuously moving relative to the compressed air flow or compressed gas flow. A "pulse" of compressed air or compressed gas is understood to mean a dynamic pressure change resulting from an accelerated flow of gas, in particular a pressure wave or the like, caused by the acceleration of the gas. Such a pulse can be generated, for example, using a compressor by first generating a gas under high pressure, which is then suddenly released in a specific direction.In particular, a gas pressure pulse can be generated, for example, using a gas pressure nozzle in conjunction with a valve. The valve, in particular, enables precise and rapid metering of the gas pressure pulse, while the gas pressure nozzle allows the gas pressure pulse to be concentrated in a specific direction. Alternatively or additionally, it is conceivable to use a liquid as the fluid, for example, a water jet or similar. The same statements apply to liquids as to gases.

[0019] In one embodiment of the method, the fluid is directed onto the surface of the foam by means of at least one nozzle arrangement, wherein the nozzle arrangement comprises a plurality of nozzles with nozzle diameters of less than 5 mm, in particular less than 1.0 mm, and most particularly less than 0.5 mm. In a simplest embodiment, the nozzle arrangement can be realized as a "nozzle bar," for example, an elongated tube in which the nozzles in the form of holes are provided, spaced apart from one another in the longitudinal direction of the tube. The nozzles form a row. Of course, a nozzle arrangement is also conceivable in which several rows of nozzles are arranged next to one another, with the rows being offset from one another by half a nozzle spacing in the direction of the rows. In this way, particularly good surface coverage with the nozzles can be achieved.

[0020] In one embodiment of the method, the nozzle arrangement comprises at least two nozzles, in particular at least five nozzles, and most particularly at least ten nozzles per centimeter of length of the nozzle arrangement (in the direction of a side-by-side arrangement of the nozzles). A correspondingly designed nozzle arrangement can be manufactured without great technical effort, while simultaneously achieving particularly good results in opening the pores and / or bubbles of the foam.

[0021] In one embodiment of the method, the nozzle arrangement and the surface of the foam are guided or moved relative to each other in direct contact. This enables particularly efficient opening of the pores and / or bubbles and avoids vortices and turbulence, which can counteract the back pressure of the fluid.

[0022] In one embodiment of the method, the nozzles generate a fluid flow in the form of a free jet with an opening angle (defined as the half-width of a Gaussian profile describing the free jet) of less than 70°, in particular less than 50°, and most particularly less than 35°. In this way, a fluid can be directed particularly well onto the surface of the foam, since, in particular, turbulent flow in the vicinity of the nozzles is avoided.

[0023] In one embodiment of the method using fluid pulses, in particular pulses of compressed gas or compressed air, a pulse has an average duration (pulse duration) of between 0.5 and 30 milliseconds, in particular between 1 and 10 milliseconds, most particularly between 1 and 5 milliseconds. In this way, particularly efficient pulses or pulses can be generated. The fluid pulses can be generated at a frequency (pulse frequency) of 1 Hz to 500 Hz, in particular at a frequency of 5 Hz to 100 Hz, most particularly at a frequency of 10 Hz to 40 Hz. Opening and closing of a gas pressure nozzle operated in this way or a valve used can be realized, for example, electromagnetically and / or piezoelectrically. In particular, a fluid pulse can be realized using compressed air or compressed gas (e.g., carbon dioxide, nitrogen, or the like).In particular, short, strong gas pressure pulses enable a high penetration depth into the foam.

[0024] In one embodiment of the method, the fluid is released against the foam at an angle of between ±45°, in particular between ±30°, and most particularly between ±15° to the surface normal of the foam. In particular, this allows for efficient opening of the pores and / or bubbles, as a particularly high penetration depth of the fluid into the foam is enabled.

[0025] In one embodiment of the method, the method is applied to 50% of the surface of the foam, in particular to 75% of the surface of the foam, most particularly to 100% of the surface (i.e., all sides) of the foam. This means that the foam is first treated on appropriate portions of the surface to open the pores and / or bubbles using the fluid directed onto the surface of the foam, and then coated with a binder and sprinkled with abrasive grains. Drawings

[0026] The invention is explained in more detail in the following description using exemplary embodiments illustrated in the drawings. Like reference numerals in the figures denote like elements.

[0027] They show: Figure 1 shows an enlarged schematic sectional view of an embodiment of a foam abrasive according to the invention; Figure 2 shows a schematic sectional view of an embodiment of a foam abrasive according to the invention; Figure 3 shows an embodiment of a method for producing a foam abrasive according to the invention; Figure 4 shows a schematic sectional view of a production setup for carrying out the method according to the invention; Figure 5 shows a schematic sectional view of an embodiment of a nozzle arrangement.

[0028] Figure 1shows a schematic sectional view of an embodiment of a foam abrasive 10 according to the present invention. In the embodiment shown, the foam abrasive 10 according to the invention has a base body made of foam 12 with a rectangular cross-section (cuboid), on one surface 14 of which abrasive grains 16 are arranged. The abrasive grains 16 are fixed to the surface 14 of the foam 12 by means of a binding agent 18. The foam 12 has cross-sectional dimensions of 70 mm x 30 mm (shown here with a shortened height, see the ellipses on the left and right of the foam abrasive 10). The foam 12 as the base body consists of a polyurethane foam with a closed-cell structure. Inside the foam 12, in a region 22 of the foam 12 close to the surface, there are abrasive grains produced according to the inventive method (see Figure 3) open pores and bubbles 20. The near-surface region 22 extends to a depth of at least 4 layers of average pore size (i.e. a depth corresponding to four times the average pore diameter on the surface 14) into the foam 12 (although only two pore layers are shown here for clarity). The binder 18 has completely penetrated and cured into the open cell structure in the near-surface region 22 of the foam 12, in which the open pores and bubbles 20 form permeable channels 24 in the foam. The layer of binder 18 and abrasive grains 12 is additionally coated with a cover binder 26, in particular made of phenolic resin.

[0029] Figure 2shows a schematic sectional view of a further embodiment of a foam abrasive 10 according to the invention. The foam abrasive 10 has abrasive grains 16 on all surfaces 14 shown here. The method according to the invention (cf. Figure 3 ) was applied to all visible surfaces 14, so that in the entire visible near-surface area 22 of the foam 12, opened pores and bubbles 20 (here represented by a branched bubble structure) are located according to the method according to the invention.

[0030] Figure 3 shows an embodiment of a method 100 according to the invention for producing a foam abrasive 10 according to the invention, comprising Method step 102: Providing a base body made of foam 12; Method step 104: Opening pores and bubbles 20 located in the foam 12 in a near-surface region 22 of the foam 12 by means of a fluid 28 directed onto the surface 14 of the foam 12, wherein the fluid 28 is directed onto the surface 14 by means of a nozzle arrangement 54; Method step 106: Coating the foam 12 on its surface 14 with binder 18; Method step 108: Applying abrasive grains 16 to the surface 14 coated with binder 18 and fixing the abrasive grains 16 by means of the binder 18.

[0031] Figure 4shows an exemplary production setup 50 for carrying out the method 100 according to the invention in a schematic sectional view. A foam block strip is conveyed from left to right through the production setup 50. The foam 12 initially has a closed-cell structure 52, i.e., the pores and bubbles are present as individual, self-contained pores or bubbles in the foam 12 - represented here by individual closed circles (pores). As it moves through the production setup 50, the foam 12 first passes a nozzle arrangement 54 - realized here as a nozzle bar 54a (cf. Figure 5 ) - by means of which compressed air is directed as a fluid 28 onto the surface 14 of the foam 12. The nozzle arrangement 54 has a plurality of nozzles 56 with nozzle diameters of less than 0.5 mm (cf. Figure 5). The compressed air directed onto the surface 14 (not shown in detail here) causes the pores and bubbles 20 to open, so that in the area 22 of the foam 12 near the surface, the closed-cell structure 52 transforms into an open-cell structure 58 - shown here as a branched, open bubble structure. The foam 12 (as a foam block strip) then passes through a slotted nozzle 66, by means of which the binder 18 is applied. The binder 18 flows at the application point of the slotted nozzle 66 into the area 22 of the foam 12 near the surface. The pores and bubbles 20 henceforth represent pores and bubbles 22 filled with binder 18, identified here by reference numeral 72. The binder 18 applied to the surface 14 and the binder 18 that has penetrated into the pores and bubbles 20 are shown hatched here.Finally, abrasive grain 16 is sprinkled onto the binding agent 18 by means of a spreading device 70 (shown here only on the top side) and the foam abrasive is dried out (not shown in detail here).

[0032] Figure 5 shows a schematic sectional view of an embodiment of a nozzle arrangement 54, by means of which a fluid 28 - in the examples here compressed air or compressed gas - is directed onto the surface 14 of the foam 12. The nozzle arrangement is realized as a nozzle bar 54a, which has a substantially tubular shape, wherein the tube comprises on its underside a plurality of nozzles 56 with nozzle diameters of less than 0.5 mm. As already in Figure 4As shown, the nozzle bar 54a is guided in direct contact with the surface 14 of the foam 12 relative to the surface 14 of the foam 12. The nozzle bar 54a has nine nozzles, which are arranged here over a length of 2 cm. The nozzle bar 54a is supplied with compressed air of at least 7 bar.

Claims

1. Method for producing a foam abrasive (10) for abrasive working of a workpiece, in which a foam (12) coated on its surface (14) with binder (18) is sprinkled with abrasive grains (16), characterized in that pores and / or bubbles (20) located in the foam (12) in a near-surface region (22) of the foam (12) are opened, prior to the coating with binder (18), by means of a fluid (28) directed onto the surface (14) of the foam (12).

2. Method according to Claim 1, wherein compressed air or compressed gas or pulses of compressed air or compressed gas or a liquid is used as fluid (28).

3. Method according to either of the preceding claims, wherein the fluid (28) is directed onto the surface (14) of the foam (12) by means of at least one nozzle arrangement (54, 54a), in particular a nozzle bar (54a), wherein the nozzle arrangement (54, 54a) comprises a multiplicity of nozzles (56) with nozzle diameters of less than 5 mm, in particular less than 1.0 mm, very particularly of less than 0.5 mm.

4. Method according to Claim 3, wherein the nozzle arrangement (54, 54a) and the surface (14) of the foam (12) are guided relative to one another in direct contact with one another.

5. Method according to either of Claims 3-4, wherein the nozzle arrangement (54, 54a) comprises at least 2 nozzles (56), in particular at least 5 nozzles (56), very particularly at least 10 nozzles (56), per centimetre length of the nozzle arrangement (54, 54a).

6. Method according to one of the preceding claims, wherein the fluid (28) is directed onto the surface (14) of the foam (12) at a pressure of at least 2 bar, in particular of at least 7 bar, very particularly of at least 10 bar.

7. Method according to one of the preceding claims, wherein the method is applied on all sides of the foam (12).

8. Method according to one of the preceding claims, wherein the near-surface region (22) is generated down to a depth of at least 3 layers of average pore size, in particular of at least 5 layers of average pore size, very particularly of at least 10 layers of average pore size.

9. Method according to one of the preceding claims, wherein the near-surface region (22) is generated down to a depth of at least 3 mm, in particular of at least 5 mm, very particularly of at least 10 mm, into the foam (12).

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