HIGHLY EFFICIENT ABRASIVE PREFORMED BODY FOR VIBRATORY FINISHING OF METALLIC AND NONMETALLIC PARTS Technical Field The present invention relates to a highly efficient abrasive preformed body for vibratory finishing of metallic parts, such as for example metallic parts obtained by casting, pressure die-casting or pressing, or nonmetallic parts, such as for example parts made of plastics obtained by molding or extrusion or parts made of natural stone, such as marble and granite. Background Art

As is known, one of the methods used to finish metallic parts, such as for example metallic parts obtained by casting, pressure die-casting or pressing, or nonmetallic parts, such as for example parts made of plastics obtained by molding or extrusion or parts made of natural stone, such as marble and granite, is vibratory finishing.

This technique consists in arranging the parts to be subjected to finishing in a vibrating tank together with a certain quantity of abrasive preformed bodies, also known as "chips", and water. The vibration of the tank causes the friction of the abrasive preformed bodies against the parts, removing casting or pressing burr or other imperfections from the parts being treated.

The abrasive preformed bodies for this use are generally constituted by a hardened mixture of organic or ceramic bonding agent and abrasive filler, constituted by an abrasive material in granules. In abrasive preformed bodies having an organic bonding agent, the bonding agent is generally constituted by an unsaturated polyester resin or by urea formaldehyde, and the abrasive filler is constituted by granules of very hard material, such as quartz, feldspath, zirconium silicate, zirconium oxide, alumina, corundum, silicon carbide or other abrasive materials, melted electrically or not, which are known in the field of the finishing of

part surfaces.

Abrasive preformed bodies which use a polyester resin as a bonding agent are composed generally of a percentage by weight which can vary from 30 to 55% of bonding agent and a percentage by weight which can vary from 45 to 70% of abrasive filler, with reference to the total weight of the mixture. In this type of abrasive preformed bodies, the bonding agent is an unsaturated polyester which contains styrene as a reactive solvent, which is cured by reaction with an oxidizing substance assisted by an accelerating agent. Such abrasive preformed bodies have, after polymerization, high- level characteristics of compactness and a surface hardness of approximately 100 Shore D.

The abrasive preformed bodies which use urea formaldehyde as bonding agent are composed generally of a percentage by weight which can vary from 50 to 60% of bonding agent and a percentage by weight which can vary from 40 to 50% of abrasive filler, with reference to the total weight of the mixture. In this type of abrasive preformed bodies, the bonding agent is a two-part product of the reaction between a base and an appropriate crosslinking agent. Both of such kinds of abrasive preformed bodies can be used exclusively when wet, i.e., together with a certain amount of water, to finish parts which can tolerate the presence of water during the finishing process.

During the finishing process, the preformed bodies gradually wear, releasing part of the materials that compose them in the form of micronized dust, which together with the material removed from the parts being machined forms, together with the water, a sludge-like suspension, which is necessary for the process and is more or less thick depending on the amount of water that is present in the vibrating tank and on the amount of dust released by the abrasive preformed bodies. The use of such abrasive preformed bodies suffers some problems.

The presence of water, which is needed for the process, entails, as explained above, the formation of a sludge-like suspension, which in order to be disposed correctly and allow reuse of the contained water forces the use of a treatment plant and very often the use of chemical treatments to remove the chemical compounds which collect in the water during the vibratory finishing process.

The use of abrasive preformed bodies with a bonding agent constituted by polyester resin generally requires, in order to achieve the intended finishing result, the use of chemical compounds, such as passivators and/or polishing agents based on surfactants, which mix with the water used during the process and must be eliminated before reusing the water or discharging it into the sewage system.

Likewise, the use of abrasive preformed bodies with a bonding agent constituted by urea formaldehyde requires the elimination of the formaldehyde from the water before it is discharged into the sewage system. The residual and partially dried sludge originating from the treatment plant is normally classified as a non-toxic special waste, and as such can be disposed of in appropriate landfills without having to sustain high disposal costs. However, this sludge, although partially dried, generally contains a percentage of water which can range from 30 to 40% and increases the weight of the sludge, thereby increasing disposal costs significantly.

Another problem which can be observed in vibratory finishing operations which use such preformed bodies is the cost of the operation for drying the finished parts before storing them, an operation which is needed in order to avoid oxidation of the parts.

As can be noted, the use of water, which is needed in vibratory finishing processes with known types of abrasive preformed bodies, causes problems which affect the overall costs of the processes. For this reason, research has long been in progress to provide abrasive preformed bodies which can work satisfactorily when dry, i.e., in the absence of water.

In the field of the dry sanding of wood pieces, abrasive preformed bodies are known which have high elasticity, with a surface hardness ranging from 30 to 60 Shore A with a bonding agent constituted by thermosetting polyurethane resin. In such abrasive preformed bodies, the bonding agent is present in a percentage by weight from 25 to 50% and the abrasive filler, constituted generally by quartz, is present in a percentage by weight from 50 to 75% relative to the total weight of the abrasive preformed body.

However, such abrasive preformed bodies, due to their low surface hardness, which is suitable for working wood, cannot be used to finish metallic parts. The granules of the abrasive material, due to the elasticity of the bonding agent, when they come into contact with the surface of a metallic part, retract into the preformed body instead of facing the surface to be sanded, performing a practically nonexistent sanding action. Abrasive preformed bodies are also known which are produced by extrusion and are composed of a bonding agent constituted by a thermoplastic resin such as PET in a percentage by weight of approximately 27% and by an abrasive filler of quartz and corundum in a percentage by weight of approximately 73% relative to the total weight of the abrasive preformed body.

Such abrasive preformed bodies can work dry but require the addition of abrasive dust which must be poured continuously into the vibrating tank in order to dress the sharp parts of the abrasive preformed bodies. Disclosure of the Invention The aim of the present invention is to solve the problems described above by providing an abrasive preformed body for vibratory finishing of metallic and nonmetallic parts which can work dry and has high effectiveness in the finishing action without requiring the addition of abrasive dust or other additives. Within this aim, an object of the invention is to provide an abrasive

preformed body which allows to increase considerably the load of parts to be treated in the vibrating tank with respect to the load allowed by known types of abrasive preformed bodies.

Another object of the invention is to provide an abrasive preformed body which is highly aggressive, so as to reduce processing times, without damaging the surface of the parts subjected to the vibratory finishing treatment.

Another object of the invention is to provide an abrasive preformed body which is capable of spontaneously dressing its sharp parts during its progressive wear.

This aim and these and other objects, which will become better apparent hereinafter, are achieved by an abrasive preformed body for vibratory finishing of metallic and nonmetallic parts, comprising a cured mixture of a bonding agent and an abrasive filler, characterized in that said bonding agent is present in the mixture in a percentage by weight substantially ranging from 5% to 25% and in that said abrasive filler is present in the mixture in a percentage by weight substantially ranging from 75% to 95% with reference to the total weight of the mixture.

This aim and these and other objects are also achieved by a method for obtaining an abrasive preformed body for vibratory finishing of metallic and nonmetallic parts, comprising the steps of: a) preparing a mixture which comprises a bonding agent in a percentage by weight ranging substantially from 5% to 25% and an abrasive filler which is present in a percentage by weight substantially ranging from 75% to 95% with reference to the total weight of the mixture, and then b) using the mixture of step a) to prepare said preformed body; wherein step b) comprises the curing or hardening of said mixture.

This aim and these and other objects are also achieved by the use of a mixture comprising a bonding agent in a percentage by weight ranging substantially from 5% to 25% and an abrasive filler which is present in a

percentage by weight ranging substantially from 75% to 95% with reference to the total weight of the mixture, in order to produce an abrasive preformed body for vibratory finishing of metallic and nonmetallic parts.

This aim and these and other objects are also achieved by the use of an abrasive preformed body as defined above for the vibratory finishing of metallic and nonmetallic parts, wherein the vibratory finishing is preferably dry vibratory finishing.

It is understood that any characteristic which is mentioned with reference to a single aspect of the invention but can be also referenced to other aspects is to be considered equally valid with reference to said other even if it is not repeated explicitly.

Brief Description of the Drawings

Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment of the abrasive preformed body according to the invention, illustrated by way of non-limiting example in the accompanying drawings, wherein:

Figure 1 is a perspective view of an abrasive preformed body according to the invention; Figure 2 is a partially cutout perspective view of an abrasive preformed body according to the invention.

Ways of carrying out the Invention

In the invention, the mixing of abrasive filler and bonding agent must occur at any time before the production of the preformed body in which the viscosity of the bonding agent is such as to allow to incorporate the abrasive filler homogeneously. For thermosetting polymers, the person skilled in the art may therefore select any moment prior to the final curing of the bonding agent, whereas for thermoplastic polymers mixing can occur even after a first curing of the bonding agent, after liquefying it again. Accordingly, after its production, the mixture of bonding agent and abrasive filler must be

"cured" according to any curing method which is known in the field and is suitable for the specific selected bonding agent, so as to ensure the physical and dimensional stability of the preformed body.

With reference to the figures, the abrasive preformed body according to the invention, generally designated by the reference numeral 1, comprises, and preferably consists substantially of, a cured mixture of bonding agent 2 and abrasive filler 3.

Preferably, in the invention the bonding agent 2 is present in the mixture in a percentage by weight which ranges substantially from 13% to 23%, and the abrasive filler 3 is present in the mixture in a percentage by weight ranging substantially from 77 to 87% with reference to the total weight of the mixture.

In the invention, the bonding agent 2 is advantageously selected from the group that consists of one or more thermosetting resins, one or more thermoplastic resins and mixtures thereof, preferably one or more thermosetting resins.

Suitable thermoplastic resins are selected advantageously from the group that consists of polyamides, polyethylene, polyurethanes, and mixtures thereof. In particular, the thermoplastic resin can be constituted by a polyurethane elastomer with thermoplastic properties.

Examples of elastomers which can be used for the application comprise, but are not limited to, polyurethanes of the TEXIN family produced by Bayer, in particular TEXIN 5275.

Preferably, the bonding agent 2 is constituted by one or more thermosetting resins. Preferably, the thermosetting resins are selected from the group that consists of polyurethane resins, polyester resins, urea resins, epoxy resins, and mixtures thereof.

Even more preferably, the thermosetting resin is constituted by a polyurethane resin, such as for example: i) a thermosetting polyurethane elastomer produced by a reaction

between a polyisocyanate having a functionality of two or more and one or more polyols having even a variable molecular weight and having a functionality of two or more. For example, a PTMEG polyol polyether with a molecular weight of 2000, a PPG polyol with a molecular weight of 1000, 1 ,4-butanediol and similar polyols or ii) a thermosetting polyurethane elastomer produced by the reaction between an isocyanic prepolymer or "quasi prepolymer", with a functionality of two or more, and one or more polyols having even a variable molecular weight and a functionality of two or more. Specific examples of polyols for the elastomer ii) are identical to the ones provided above for the polyols for the type i) elastomer. In a "quasi prepolymer", the stoichiometric ratio between the isocyanic groups of the polyisocyanate and the hydroxyl groups of the polyol, that is equal to 2 for a prepolymer, is unbalanced or shifted in favor of the isocyanate. A prepolymer is generally more difficult to process, since it generally has a higher viscosity than a "quasi prepolymer". Both allow to obtain final polymers with physical characteristics (compressive strength, tear resistance, breaking elongation) which are generally better than those obtainable with the mixture i), with a further advantage in favor of the pure prepolymer.

The term "functionality" as used herein references the number of isocyanate functional groups for the isocyanic products or the number of hydroxyl functional groups for alcohol products.

The abrasive filler 3 comprises abrasive material in granules having dimensions ranging substantially from 0.5 μm to 2500 μm.

The abrasive material in granules can be constituted by one or more of quartz, feldspath, zirconium silicate, zirconium oxide, alumina, corundum, silicon carbide or other abrasive materials, melted electrically and not, usually used in the field of vibratory finishing. The abrasive preformed body according to the invention preferably

has a surface hardness ranging substantially from 85 to 98 Shore A or substantially ranging from 70 to 85 Shore D.

In practice, the abrasive preformed body according to the invention has a lower surface hardness than abrasive preformed bodies composed of an organic bonding agent of the known type and further has a slight elasticity. Thanks to this fact, the vibrating tank of a vibratory finishing machine which uses abrasive preformed bodies according to the invention can be filled with a load of parts which is considerably greater than the load allowed by the use of abrasive preformed bodies of the known type. The low percentage of bonding agent in the abrasive preformed body according to the invention gives high aggressiveness to the abrasive preformed body, such as to reduce by at least 30% the processing times without compromising the quality of the surfaces of the processed parts and ensuring a fully satisfactory degree of finish. Moreover, the abrasive preformed bodies according to the invention, during the vibratory finishing treatment, spontaneously dress or renew their sharp parts, maintaining a satisfactory abrasive effectiveness without requiring the use of chemical compounds or added abrasive dust.

The abrasive preformed bodies according to the invention are intended to be used dry, i.e., without the addition of water in the vibrating tank of the vibratory finishing machine. The material arising from the wear of the abrasive preformed bodies and from the finishing of the parts being processed can be removed by means of an aspirator or other means adapted to remove from the vibrating tank the dust which forms during the process. This dust, which is a process residue, is a special non-toxic waste which can be disposed of without further treatment in appropriate landfills.

Since this processing residue is perfectly dry, it has a distinctly lower weight and consequent disposal cost with respect to the residue produced in wet vibratory finishing processes. In practice it has been found that the abrasive preformed body

according to the invention fully achieves intended aim, since it can be used for vibratory finishing processes of metallic and nonmetallic parts when dry, i.e., without the addition of water. Thanks to this fact, the abrasive preformed body allows to reduce significantly the overall costs of the vibratory finishing treatment, since it eliminates the need to provide vibratory finishing machines with treatment plants and to treat the sludge produced by the process.

Another advantage which arises from the possibility to perform, with the abrasive preformed body according to the invention, dry vibratory finishing processes is that the need is avoided to dry the parts at the end of the vibratory finishing process.

Further advantages of the abrasive preformed body according to the invention as specified above are constituted by the possibility to increase significantly the load of the parts in the vibrating tank of the vibratory finishing machine and to reduce significantly the processing times without having to resort to the addition of abrasive dust in order to maintain the efficiency of the abrasive preformed bodies during the vibratory finishing process.

The abrasive preformed body thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; all the details may further be replaced with other technically equivalent elements.

Tables are provided hereafter which describe some formulations for the abrasive preformed bodies according to the present invention, as well as tables which provide the comparative results of vibratory finishing processes of different particulars made of different materials with the preformed bodies when dry and the equivalent processes with existing preformed bodies. In particular, chips manufactured by the Applicant with a polyester bonding agent such as PCA and RPC, and a chip of the background art with urea/formaldehyde (U/F) bonding agent, have been

selected.

TABLE 1

Mixture example 1

component % weight

mixture of polyvalent polyols with a total hydroxyl number (mg KOH/g) of 8.60

225

MDI diisocyanate with an NCO content of 33.5% by weight 5.15

Siliporite 0.75

Glaze with d[0,l] of 13 μ and d[0,9] of 124 μ 41.00

silicon carbide with d[0,l] of 4.0 μ and d[0,9] of 95.6 μ 44.50

TABLE 2

Mixture example 2

component % weight

mixture of polyvalent polyols with a total hydroxyl number (mg KOH/g) of 14.35 225

MDI diisocyanate with an NCO content of 33.5% by weight 8.65

Siliporite 1.00

Glaze with d[0,l] of 4.4 μ and d[0,9] of 60.9 μ 33.70

Mullite with d[0,l] of 2.2 μ and d[0,9] of 29.3 μ 42.30

TABLE 3

Mixture example 3

component % weight

mixture of polyvalent polyols with a total hydroxyl number (mg KOH/g) of 8.65

225

MDI diisocyanate with an NCO content of 33.5% by weight 5.15

Siliporite 0.75

Glaze with d[0,l] of 13 μ and d[0,9] of 124 μ 40.90

Zirconium silicate with d[0,l] of 2.0 μ and d[0,9] of 52.0 μ 44.55

TABLE 4

Mixture example 4

component % weight

mixture of polyvalent polyols with a total hydroxyl number (mg KOH/g) of 4.83

225

MDI diisocyanate with an NCO content of 33.5% by weight 2.75

Siliporite 0.5

Zirconium silicate with d[0,l j of 2.0 μ and d[0,9] of 52.0 μ 35.00

Zirconium silicate with d[0,l] of 82.9 μ and d[0,9] of 173.7 μ 56.92

TABLE 5

Mixture example 5

component % weight

mixture of polyvalent polyols with a total hydroxyl number (mg KOH/g) of 8.60

225

MDI diisocyanate with an NCO content of 33.5% by weight 5.15

Siliporite 0.75

Glaze with d[0,l] of 13 μ and d[0,9] of 124 μ 41.04

Zirconium silicate with d[0,l] of 1.8 μ and d[0,9] of 41 μ 44.46

TABLE 6

Example of mixture of chips with polyester bonding agent of the PCA type

component % weight

Unsaturated polyester resin 38.40

Styrene 1.00

Cobalt octoate 0.20

Methyl ethyl ketone peroxide 2.40

Quartz with d[0,l] of 4 μ and d[0,9] of 78 μ 58.00

TABLEJ7

Example of mixture of chips with polyester bonding agent of the RPC type

component % weight

Unsaturated polyester resin 32.50

Styrene 1.50

Cobalt octoate 0.16

Methyl ethyl ketone peroxide 1.54

Quartz with d[0,l] of 4 μ and d[0,9] of 78 μ 41.00

Silicon carbide with d[0,l] of 4.0 μ and d[0,9] of 95.6 μ 23.30

TABLE 8

Treatment of blanked handles made of stainless steel

Circular vibratory tumbler, 300 1 Chip type capacity

Processing parameters Mixture 1 : RPM, 42-mm cone

42-mm dry cone polyester bonding agent

Breadth of vibration, mm

Initial weight of chips, kg 175 200

No. of parts to be processed 2000 1000

Duration of processing cycle, hours 10 12

Chip wear, % on 24 hours 7.50 8.50

Weight of chips worn per cycle, kg 5.5 8.50

Total weight of process waste 6.3 11.1*

Surface finishing of parts, average of 3 measurements on random sample of 5 processed parts, μ

Ra 0.45-1.15 0.75-0.89

Rt 5.8-11.9 8.8-9

Rz 3.5-6.6 5.5-5.6

including process water

TABLE 9

Treatment of pressure die-cast brass faucets

Circular vibratory tumbler, 300 1 Chip type capacity

Processing parameters Mixture 3: PCA, 60-mm parabola

60-mm dry parabola polyester bonding agent

Breadth of vibration, mm 2.5

Initial weight of chips, kg 175 200

No. of parts to be processed 220 170

Duration of processing cycle, hours 8 12

Chip wear, % on 24 hours 5 7

Weight of chips worn per cycle, kg 3 7

Total weight of process waste 3.6 9.1*

Surface finishing of parts, average of 3 measurements on random sample of 5 processed parts, μ

Ra 0.41-0.51 0.46-0.59

Rt 4.6-6.9 3.8-7.2

Rz 3-4.1 3.2-3.9

* including process water

TABLE 10

Treatment of pressure die-cast Zama or Zn+Al+Mg buckles

Circular vibratory tumbler, 300 1 Chip type capacity

Processing parameters Mixture 5: U/F, 32-mm cone

30-mm dry teardrop urea bonding agent

Breadth of vibration, mm 3 3

Initial weight of chips, kg 175 200

Parts to be processed, kg 150 100

Duration of processing cycle, hours 13 13

Chip wear, % on 24 hours 4.75 12

Weight of chips worn per cycle, kg 5 13

Total weight of process waste 5.8 17*

Surface finishing of parts, average of 3 measurements on random sample of 5 processed parts, μ

Ra 0.45-1.15 0.75-0.89

Rt 5.8-11.9 8.8-9

Rz 3.5-6.6 5.5-5.6

* ; including process water

The disclosures in Italian Patent Application no. MI2006A000380 from which this application claims priority, are incorporated herein by reference.