EP0114280B1 - Method of manufacturing abrasive bodies - Google Patents

Abstract

In a process for producing hard grinding elements, a synthetic resin having a dynamic viscosity of less than 10 mPa.s is used as the binder. By adding this binder, the mixture of abrasive grains, binders, and fillers is highly thixotropic and can easily be liquefied by vibration and can be filled into a casting mold having almost any desired shape where, due to the addition of a radical forming starter system to the mixture, the synthetic resin is polymerized. Methacrylic acid esters or vinyl acetates are particularly applicable as monomers for the binder.

Description

The invention relates to a method for producing grinding wheels in a casting mold using a mixture of abrasive grains, a cold-curing, thermoplastic synthetic resin as a binder, as well as additives and a starter system, such as. B. an organic peroxide and a tertiary amine, and with hardening of the mixture to form the grinding wheel in the mold.

Such methods are used for the production of hard abrasive particles, i. H. Grinding wheels, slip rings, cups, honing tools etc.

In the abrasives manufacturing industry, the view is taken that only inorganic or thermosetting plastics with high heat resistance can be used as binders for such hard abrasive bodies ("hard here in contrast to elastomeric") because of the temperatures that occur during grinding, even under cooling.

The known hard abrasives are made using ceramic, phenolic, magnesite or epoxy binders (the less commonly used binders such as metal, silicate, polyester and others are disregarded).

Ceramic-bonded grinding wheels are mainly used in precision grinding. Because of the fact that they are burned for several days, they are expensive to produce at temperatures above 900 ° C. In addition, because of the warping that occurs during firing, these bodies must be provided with a considerable excess, which necessitates appropriate reworking.

The phenolic resin bond is based on the simultaneous use of phenolic resoles and novolaks. In addition to the necessary work hygiene measures during production, curing for up to two days at temperatures around 175 ° C is also an environmental burden in that considerable amounts of free phenol, formaldehyde and ammonia are released in addition to water. When cleaning the mixer, such substances also get into the wastewater together with the solvents, which makes complex cleaning necessary.

It is a feature of both types of bond described above that the structure of the abrasive body is porous, to be precise porous in the coarse grain area and less porous in the fine area. It is difficult, if not impossible, to adjust the abrasive density to the respective use in the case of such abrasives over the entire grit spectrum and to provide a sufficient chip gap volume, particularly in the fine and fine grain range.

In addition, the phenolic resin bond has the disadvantage that it is not very resistant to the alkaline coolants, which is why this bond is mainly used only in dry grinding.

In contrast to those in ceramic or phenolic resin bond, abrasive particles in magnesite bond are cast and therefore leak-proof. H. practically non-porous. Nevertheless, they offer an extremely smooth finish, especially for hardened steels with high stock removal rates. Therefore, such grinding tools are mainly used for grinding knives, scissors, pliers and other tools, the ends of coil springs and the like. a.

The disadvantages of magnesite bonded abrasives are manifold. So they are only approved for working speeds up to 20 m / sec, furthermore they change their hardness over time, so that they can only be used optimally in the period from 1 month to 4 months after production. A significant disadvantage is the magnesium chloride liberated during grinding, which leads to severe corrosion, in particular to the protective hoods of the machines, and represents a considerable pollution of the wastewater.

Because of these disadvantages of magnesite bonding, epoxy resin formation has become increasingly common in recent years, particularly in the cutlery industry with lower stock removal rates and in the fine grain area. A handicap of epoxy resins is their high viscosity. Base resins with a high proportion of reactive thinners are available with viscosities of approx. 1 000 mPa.s, but they show insufficient heat resistance due to the thinning. That is why such abrasives always compromise between sufficient castability, resin content (which is inevitably 40% by weight and higher), heat resistance and performance. Because of these necessary compromises with regard to heat resistance, such grinding tools can only be used in wet grinding.

Added to this are the occupational hygiene risks associated with the mostly cold-curing resin systems due to the organic amines, epichlorohydrin residues and reactive thinners in the epoxy resin.

The use of raw materials and the associated costs are very high for grinding wheels with epoxy resin bond.

A generic method, in which, in contrast to the possibilities described above, a thermoplastic synthetic resin is used as a binder, is known from DE-A-20 28 891

Depending on the composition of such a mixture, it has a different consistency in the dry or pulpy area, the flowability should, owing to the low viscosity of the mixture, be sufficient to fill the mixture without further ado in the casting mold, but this is followed by compression, for example by shaking must to prevent the formation of too large voids in the mold.

In the manufacture of abrasive grits using these known methods, they have a high binder content and also a very high grit content, the latter being only useful for very hard abrasive grits. On the other hand, such grinding wheels, in addition to their limited area of application (low grinding speeds), have the known disadvantage that special coolants have to be used to cool the object to be ground, which in turn leads to increased production expenditure and also environmental pollution from the lubricant to be removed (for example grinding oil, Sulfur).

Especially in the case of non-ferrous metals, these circumstances force reduced grinding performance, although unsatisfactory «side effects» such as. B. burrs, welds, changes in shape and / or chemical reactions with the material surface can occur individually or in combination.

. It is therefore an object of the invention to further develop the generic method in such a way that a universal grinding wheel with better grinding performance can be produced.

The invention solves this problem in that metal or amine soaps are added to the binder grinding mixture, and the granulometric composition of this mixture corresponds to the respective Fuller curve, as a result of which a thixotropic mixture with the densest possible packing of its inorganic constituents is created by using the binder Synthetic resin or the monomer or mixture of monomers used to form the synthetic resin has a dynamic viscosity of less than 10 mPa.s, and that this mixture is liquefied, for example by vibration, and poured into the casting mold.

These features essentially have the effect that, due to the defined granulometric composition of all mixture components, the abrasive grains are firmly held together, since they can be supported on «neighboring components, the compliance with the respective Fuller curve thereby minimizing the need for binding agents.

The Schleifkom-filler-binder mixtures are strongly thixotropic due to their composition according to the invention and can be easily liquefied by vibration. With appropriate vibration, their consistency is such that they easily pass the outlet of a funnel and fill even complicated molds cleanly. It is possible, for example, to manufacture cup wheels with a wall thickness of only 3 mm and a cup diameter of 250 mm, which is not possible with the binders known to date.

The addition of the starter system can be metered in such a way that a sufficient pot life is achieved, but the hardened grinding wheel can be removed from the mold no later than two hours after the start of filling and the hardening takes place essentially at room temperature. Only in the case of small grinding wheels, where the large relative mold volume inevitably absorbs a lot of reaction heat compared to the small relative grinding wheel volume, it is advisable to warm the molds to 60 ° C briefly before or after filling.

The starter system expediently consists of an organic peroxide and an aromatic tertiary amine. It has proven advantageous to add a powdered organic peroxide to polymeric methacrylate and to dissolve the aromatic tertiary amine in the binder.

The hardening shrinkage of the abrasive articles produced according to the invention is extremely low and averages 0.02%. This fact makes it possible to design the respective shape in such a way that the parts of the grinding wheel which are in contact with the shape no longer have to be machined. This even applies to the bore, provided that the bore dome has a correspondingly small excess. Compared to the known methods, considerable manufacturing costs can be saved.

Due to the low hardening shrinkage, it is also very possible to integrate metal parts directly. These can be reinforcements as well as threaded bushings, shafts for small grinding bodies, etc.

On account of the production according to the invention, the abrasive bodies show practically no imbalance and no differences in density or hardness within the finished body, provided that the casting mold is geometrically perfect.

Surprisingly, it has further been found that thermoplastics can be used for the production of hard abrasive articles, the polymerization being carried out from the monomer in the abrasive article mixture in the casting mold, the monomers optionally being stabilized with 3 to 20 ppm hydroquinone or other. As such monomers are mainly methacrylic acid esters and / or vinyl acetate in question, other monomers are conceivable alone or as an additive, but it must be taken into account that some of the other monomers in question are unsafe in terms of work hygiene.

The binder according to the invention expediently contains bifunctional or trifunctional methacrylates as crosslinking agent (“crosslinking agent”).

Fillers are required to create the necessary chip gap volume and to enable polymerization. On the one hand, these fillers must be softer than the materials to be machined, but on the other hand they must be so pressure-resistant that they fix the grinding bodies rigidly and give the grinding body as a whole the necessary hardness.

Fillers that offer a Mohs hardness of less than 6 or Knoop of less than 500 are available here, are available in different grain sizes and are environmentally friendly, and possibly due to the abrasive grain Refine the micrograph created as a polishing agent. Water-insoluble calcium compounds, such as calcite, dolomite, aragonite, gypsum, selenite and / or screed gypsum, are preferred here.

In order to minimize the need for binding agents and at the same time to optimally support and rigidly fix the grinding compound, the granulometric composition of the Schleifkom-filler combination, based on the volume, should essentially correspond to the respective fuller curve. This ensures that the inorganic constituents are packed as densely as possible.

It may prove useful here that the fuller curve begins, for example, in the range of 150 ... 60 µm with a grinding-neutral filler, the range 60 ... 20 µm the Schieifkom 280 F, and subsequently of 20 ... 2 µm again a neutral filler. It is expedient that the fillers are surface-treated, primarily in the fine range 20 ... 2 µm.

The extensive setting of the fuller curve corresponding to the respective grinding grain size makes it possible to keep the binder content relatively low, in extreme cases 8% or 20% by volume. In the medium «hardness» range, the binder content is 16% by weight or 40% by volume. This is to say that the "hardness can be adjusted to the respective use here in a similar way to the abrasive grits of hard bonds, where one speaks of" hard and "soft" ceramic or Bakelite discs. Here is with the «hardness» more or less. the strength of the comein bond meant.

As is customary with the known abrasive products, for example in phenolic resin bond, it is also possible with the mixture according to the invention to modify it by means of additives. This subheading includes Grinding aids such as cryolite, pyrite or similar, provided that they are taken into account when calculating the Fuller curve.

With dry sanding, the clogging of the sanding surface can be suppressed by adding appropriate metal or amine soaps. The fuller curve must also be observed here.

The same applies if short glass or carbon fibers are added to increase the permissible scope of work. Of course, the use of glass fabrics or rovings as reinforcing materials is also possible. Since atmospheric oxygen inhibits the polymerization, the abrasive articles according to the invention must be essentially leakproof. When grinding heat-sensitive steels, however, it can be useful to create a more open surface of the grinding wheel to take away more water-based coolant. This is done very well by adding water-soluble powder substances. Here, ground water glass is ideal, which not only has a corrosion-inhibiting effect, but is also environmentally friendly and does not impair the coolant.

All known types can be considered as Schleifkom, i.e. glass, flint, garnet, the various corundums, silicon carbide, etc., alone or as a mixture. As already mentioned, all grain sizes are possible, for example from 8 - 1200 according to FEPA.

Claims (7)

1. Method of producing grinding bodies in a casting mould by utilising a mixture of abrasive grains, a cold-hardening, thermoplastic synthetic resin as a binding agent, as well as additives and a starter system, such as, for example, an organic peroxide and a tertiary amine, and by hardening the mixture in order to form the grinding body in the casting mouid, characterised in that metal or amine soaps are added to the mixture of binding agent and abrasive grains, in that the granulomet- ric composition of this mixture corresponds to the respective Fuller curve, whereby a thixotropic mixture is produced with the densest possible packing of its inorganic ingredients, in that the synthetic resin, which is used as a binding agent, or respectively the monomer or mixture of monomers used for the formation of the synthetic resin has a dynamic viscosity of less than 10 mPa.s, and in that this mixture is liquefied, by vibration for example, and poured into the casting mould.

2. Method according to Claim 1, characterised in that substantially polymerizable liquids, such as vinyl acetate and/or methacrylate, are used as binding agents.

3. Method according to Claim 1, characterised in that the hardening is effected at ambient temperature up to a maximum of 60 °C.

4. Method according to Claim 1, characterised in that a methacrylate resin, which contains bifunctional or tri-functional methacrylates, is used as a crosslinking agent.

5. Method according to Claim 1, characterised in that filler substances with a Knoop hardness of less than 500 are used as additives.

6. Method according to Claim 5, characterised in that the filler substances are surface-treated, at least partally.

7. Method according to Claim 5, characterised in that calcite, dolomite, aragonite, gypsum, selenite and/or estrich gypsum are used as filler substances.