EP3664965B1 - Flexible grinding means - Google Patents

Description

The invention relates to a flexible abrasive according to the preamble of claim 1.

Such abrasives, which according to the type have a flexible carrier on which abrasive grains with a very fine average particle size d ₅₀ are held, are used in particular for fine grinding work. Such an abrasive is, for example, from US 2005/276967 A1 and US 4,974,373 Known An example of this is the sanding of automobile paints, whereby the abrasives are used for both dry and wet sanding.

Typical products are given the grain size designation P1500 to P2500 according to the FEPA standard (DIN ISO 6344-3:2017-04). However, there are also products with non-standardized, finer average particle sizes d _50. Examples include the products Matador 991A ST5000 or Giant 961FOSSW ST 7000 from Starcke GmbH&Co. called KG.

The sanding process is intended to achieve a certain surface quality, e.g. a glossy effect, which depends, among other things, on the roughness created in the paint.

A series of abrasives are typically used, starting with abrasives with larger particle sizes ('coarse') and then abrasives with smaller particle sizes ('fine'). This leads to a gradual reduction in the scratch depth in the ground end product. The depth of scratches achieved in the paint depends on a number of factors. With regard to the abrasive particles, these would be, among other things, the type, the average size d ₅₀ , the shape or the distribution on the substrate.

In addition, factors such as bond types and height, type and flexibility of the substrate, the presence of other coatings and the type and condition of the surface to be processed play an important role.

It is therefore not possible to draw any conclusions about the roughness depth value ultimately achieved in the end product to be ground from the specification of the grain fineness/average particle size alone.

Paper or film with a smooth, flat and homogeneous surface are used as conventional carriers for the known flexible abrasives with fine average particle sizes d ₅₀ .

The purpose of these substrates, some of which are leveled with additional leveling coatings, is to ensure that the end product, the abrasive, has a constant thickness with a uniform, leveled surface.

The surface roughness created by the abrasive should be uniform and polishable.

In addition, a sufficient removal rate should be achieved, which means that the surface defect to be processed can be removed to a sufficient extent without the surrounding intact paint structure being deeply damaged.

In addition, a long and even grinding time is desired. With a conventional carrier, this is limited by the fact that the resulting grinding dust quickly clogs the spaces between the abrasive grains to such an extent that the grinding effect is gradually reduced to the point of total failure.

As the gaps become increasingly filled, the risk of scratches on the sanded paint surface also increases, as adhering coarse particle residues are not removed during the sanding process.

In order to remedy this, the top of the abrasive is provided with a dust-repellent layer of stearates during dry sanding, which binds the sanding dust and makes it easier to remove. However, even this cannot completely prevent clogging of the abrasive.

As a further measure to avoid clogging, wet grinding is known, in which the dust that forms during grinding is flushed out using a liquid, but this is associated with a high level of cleaning and preparation effort, which conflicts with the always desired cost optimization. In addition, this process generally creates higher roughness depths in the paint.

Dust extraction, which requires a perforated abrasive that is connected to a suction device using a sanding plate adapted to it, is also unsuitable for the specific purpose. This is particularly because appropriately sized suction openings must be present in the usually small-sized abrasive. Furthermore, extraction systems are not available everywhere

The invention is based on the object of further developing an abrasive of the generic type in such a way that its usability is improved and its service life is increased.

This task is solved by an abrasive with the features of claim 1.

The flexible abrasive according to the invention has a carrier and abrasive grains held on a surface with an average particle size d ₅₀ of the abrasive grains of 3 to 15 μm, the ratio of the average particle size d ₅₀ to the average roughness R _ZT of the surface of the carrier 1 holding the abrasive grains : 1.5 to 4, the area ratio between the elevations and depressions of the surface of the abrasive is 1: 0.1-1.5 and the surface of the abrasive has an average roughness Rzs of 10-20µm and a maximum roughness R _maxS of 40µm has, whereby the microstructure of the carrier in the form of elevations and depressions is also retained in the finished abrasive.

This described microstructure has proven to be particularly advantageous in that the grinding dust that is produced collects in the gaps formed without impairing the grinding performance.

By taking appropriate measures during the grinding process, namely wiping the grinding surface between grinding processes, this grinding dust can be repeatedly removed from the recesses, which results in a significant improvement in the service life of the abrasive. Such wiping is not effective to the desired extent if the depressions are too pronounced.

Advantageous embodiment variants of the invention are the subject of the subclaims.

According to a preferred embodiment variant, a fabric with warp and weft threads is used as the carrier, on the surface of which a finish is applied at least on one side, which closes the fabric and effects an improved bond to abrasive grains.

The finished surface is structured in the form of depressions and elevations by the mentioned warp and weft threads and the connecting knots that may arise from them.

Conventional carriers, on the other hand, strive for a surface that is as smooth and flat as possible.

Abrasive grains are adhered to this now structured surface using a binder.

The microstructure of the carrier surface in the form of elevations and depressions is retained in the finished abrasive.

The microstructure can be described in more detail using the unevenness of the surface, for which the average roughness depth R _Z is a common measure.

A stylus device is used to record surface roughness, in which a scanning needle slides along the surface of the workpiece. The parameters determined from this are defined for individual measuring sections according to DIN EN ISO 4287.

The average roughness depth R _Z is now the average of the individual roughness depths of five successive individual measuring sections in the roughness profile.

The maximum roughness depth R _max indicates the difference between the deepest groove and the highest peak within this total measuring distance.

The finished fabric carrier has a microstructure with an average roughness depth R _ZT of 10 - 20 µm, preferably 14 µm and a maximum roughness depth R _maxT of 40 µm.

The finished abrasive also has an average roughness R _ZS of 10 - 20 µm, preferably 14 µm and a maximum roughness R _maxS of 40 µm.

As has been shown, another criterion for optimal design of the abrasive is the area ratio between the grinding areas, i.e. the elevations, and the depressions, i.e. a sanding dust reservoir in which the sanding dust collects, which must be taken into account. An area ratio between the elevations and the depressions of 1:0.1 to 1.5, preferably 1:0.2 to 0.3, is particularly favorable.

The abrasive grains, which are applied to the finished surface of the fabric forming the backing, have an average particle size d ₅₀ of 3 to 15 µm. The value d ₅₀ is defined as the average particle size according to DIN 13320.

Grain size and quantity of grain must not level the structure of the associated surface of the carrier. Consequently, the size of the abrasive grains and the carrier microstructure are within a comparable range.

A preferred ratio of the average particle size d ₅₀ to the carrier roughness R _ZT is 1: 1.5-4, so that the microstructure of the carrier in the form of elevations and depressions is also retained in the finished abrasive.

The roughness depth of the abrasive surface R _ZS considered here is not identical to the roughness depth created on the workpiece to be ground. Other factors play a role here, such as contact pressure or surface quality of the object to be ground.

Due to the flexible nature of the fabric carrier, the new abrasive is able to conform to a surface to be sanded. Compared to a conventional rigid carrier according to the prior art, which is available as a film or paper, the new carrier is more elastic and reversibly deformable, so that the abrasive is used over the entire area, which ultimately leads to an increase in service life.

Thanks to the improved adaptivity, the pressure on a flat surface is distributed more evenly, resulting in homogeneous roughness depths, with the result that the paint structure surrounding the area to be sanded is significantly better preserved.

In addition, depending on the choice of material, higher tensile strengths can be achieved than is possible with paper, for example, so that the abrasive can also be used as a belt material.

According to a further idea of the invention, the usable surface of the abrasive can be provided with a stearate layer that binds the grinding dust that is produced. The structure containing abrasive grains is initially visually leveled, but during the grinding process the abrasive grains become effective in the desired manner.

The basic structure of the carrier fabric (e.g. number of threads and thread fineness, type of weave) or the resulting microstructure of the carrier can be changed according to the respective requirements, for example according to the desired grain size.

The threads from which the support is made can consist of natural fibers, such as cotton, cellulose or the like, or of artificial fibers, for example PES, PA or viscose. What is crucial is that the surface structure of the fabric with elevations and depressions results in a periodically contoured structure that is only slightly covered by the applied finish, with the elevations and depressions continuing to remain intact even after the binder loaded with the abrasive grains has been applied.

Phenolic resin, polyurethanes, acrylates, polyesters or epoxides can be used as binders between the carrier and the grain, with phenolic resin-based paints being preferred. The binder layer can be applied in various ways, for example by dipping or by roller application.

The abrasive grain consists of known materials such as SiC, aluminum oxide, zirconium corundum, ceramic grain, diamond or boron nitride and can also have a treated surface. Application can be done in various ways, e.g. by dipping or sprinkling.

Finally, a dust-repellent layer consisting, for example, of lauryl sulfates, waxes, polyfluorinated hydrocarbons, is preferred However, they can be applied from stearates. This overlays the structure, but only superficially.

During the grinding process, the essential basic structure is exposed again to such an extent that the full grinding effect is achieved. The stearate/dust/grain mixture created during sanding can be wiped off so that the full functionality of the abrasive can be restored between sanding cycles.

What is crucial for the improved properties of the new abrasive is that there is a specific surface structure with elevations and depressions in a periodically contoured structure, which is only slightly covered by subsequent overlying layers.

Such a structure can also be contained in other supports due to their structure. For example, knitted fabrics, knitted fabrics or scrims also have a three-dimensionally structured surface. In this case too, the size ratios described above offer optimization in the grinding process.

• Prägung einer Oberfläche, d.h. Umformung einer Oberfläche mit Druck zu einem Relief
• Übertragen von Material auf einen Träger wodurch eine Struktur gebildet wird, z.B. Prägedruck, Siebdruck, 3D- Druck
• Mechanische Veränderung wie z.B. Ätzen, Lasern oder Schneiden
  Zur Befestigung des Schleifmittels an einer Schleifeinrichtung kann die der Schleifseite abgewandte, sozusagen Rückseite des Schleifmittels mit entsprechend Vorkehrungen ausgerüstet sein.

Furthermore, structured surfaces can also be achieved using manufacturing processes:

• Embossing a surface, ie forming a surface into a relief using pressure
• Transferring material to a support to form a structure, e.g. embossing, screen printing, 3D printing
• Mechanical modification such as etching, lasering or cutting
  To attach the abrasive to a grinding device, the back of the abrasive, so to speak, facing away from the grinding side, can be equipped with appropriate precautions.

These include adaptive layers, e.g. made of Velcro glued to the backing or self-adhesive layers that are covered with a protective paper before attaching the abrasive to the grinding device.

Also possible are padding layers, e.g. made of foam, which are glued to the carrier, depending on the modification of the grinding device.

Claims (7)

1. Flexible abrasive having a backing and abrasive grains held thereon on a surface with an average particle size d₅₀ of the abrasive grains of 3 to 15 µm, the microstructure of the backing in the form of elevations and depressions also being retained in the finished abrasive and the surface of the abrasive having an average peak-to-valley height Rzs of 10-20 µm and a maximum peak-to-valley height R_maxS of 40 µm and a maximum peak-to-valley height R_maxs of 40 µm,
   characterized in that

   - the ratio of the mean particle size d₅₀ to the mean peak-to-valley height R_ZT of the surface of the backing holding the abrasives is 1 : 1.5 to 4,

   - the area ratio between the elevations and depressions of the surface of the abrasive is 1 : 0.1 - 1.5.
2. Flexible abrasive according to claim 1, characterized in that the surface structure is achieved by

   - Embossing of a surface, i.e. forming a surface with pressure into a relief,

   - Transferring material onto a substrate to form a structure, e.g. embossing, screen printing, 3D printing, or

   - Mechanical modification such as etching, lasering or cutting.
3. Flexible abrasive according to claim 1 or 2, characterized in that the backing consists of a woven fabric with warp and weft threads with a structured surface having depressions and elevations, whereby at least the surface carrying the abrasive grains is provided with a dressing.
4. Flexible abrasive according to one of the preceding claims, characterized in that the peak-to-valley height Rzs is 14 µm.
5. Flexible abrasive according to one of the preceding claims, characterized in that the abrasive grains held on the dressed surface by means of a binder are at least partially covered with a stearate layer.
6. Flexible abrasive according to one of the preceding claims, characterized in that the surface facing away from the abrasive layer for connection to an abrasive device comprises a Velcro velour, a self-adhesive layer or a layer of foam, the Velcro velour and the foam being glued to the backing.
7. Flexible abrasive according to any one of the preceding claims, characterized in that the area ratio between the elevations and depressions of the surface is 1 : 0.2-0.3.