US2378386A - Method of making abrasive articles - Google Patents

Description

June 19, 1945. R. A. BAUMGARTNER METHOD OF MAKING ABRASIVE ARTICLES Filed June 28, 1944 v INVENTOR. fi/c/mza fla /vane r/vzz Patented June 19, 1945 METHOD OF MAKING ABRASIVE ARTICLE Richard A. Baumga'rtner, Niagara Falls, N. Y., as'signor to The Carborundum Company, Niagara Falls, N. Y., a corporation of Delaware -Application dune 28, 1944, Serial No. 542,567

6 (Claims.

whereby the finished article is facilely and quickly formed and in which the resultant product is characterized by a, more uniform density and hardness grade throughout, freedom from defects, and a mild or soft abrasive action which combines an effective cutting rate with an emcient and satisfactory polishing action.

There has always been a need for abrasive articles of the bonded type which would combine an effective cutting action with satisfactory finishing or polishing whereby a. reasonable amount of materialwould be removed from the work being abraded and at the same time the article would be-left with a desirable surface finish or polish. Abrasive articles heretofore provided for such dual purposes have failed to be completely satisfactory in accomplishing both goals and have been found to be defective in one respect or another. Among the reasons for their unsatisfactoriness have been the failure to combine an appreciable degree of cutting ability with a resilience or softness of cutting action, nonhomogeneity of the abrasive article, lack of permanency ofthe abrasive content, i. e., failure of the abrasive wheel structure to retain the abrasive particles within the abrading body, nonuniformity of abrasive action, inflexibility, etc. Provision of satisfactory properties in one respect has usually been at a sacrifice of one or more of the other characteristics desirable in such products.

Ordinary bonded abrasive articles of the prior art consisting of abrasive grains and a binder therefor in which various fibers have been incorporated for reinforcing means or for other purposes, on account of their harsh, hard cutting behavior, have never been found suitable for the purposes herein set forth. They do not provide an action equivalent or comparable to the combined soft cutting and polishing action of the present articles.

Recently, abrasive articles having a combined cutting and polishing action have been made from a plurality of felted fibrous, abrasive-containing sheets superimposed one on the other and adhesively or otherwise secured in position for -grinding and polishing use. Such articles and rnethods of making them have been set forth and 7 described in 'copendin application Serial No. 408,936, filed August 30, 1941, which was issued as Patent No. 2,355,667 on August 15, 1944. According to the procedure described therein, abrasive wheels and shapes are formed by cutting pieces of the abrasive-containing fibrous sheet material of the size and shape of the desired article from a roll of the abrasive web material and assembling the pieces to form the desired article. While the resultant articles have shown a resilient cutting action of the desired type, the method of making them, and especially the operration of pressing the sheet material between and in direct contact with the press platens, has not been entirely satisfactory. The method used comprises first hot-pressing the assemblage of sheets and then curing the adhesive therein by heating the compacted articles in an oven. In hot-pressing the sheet material between the platens of the press in accordance with the procedures heretofore followed, the top and bottom layers of sheet material in direct contact with the press platens become hot and are dried out long before the interior of the sheet material has been brought to the desired temperature. The premature drying out of the surfaces of sheet material hinders the efiective drying out and fluxing of the adhesive of the inner layers of sheet material and as a result the finished articles are often either overheated on the external surfaces, if the innermost portions of the article are heated to the proper degree to develop the necessary adhesiveness or else the innermost parts of the article are underheated. Accompanying this non-uniformity of drying and fluxing is the tendency under such conditions for one or more of the fibrous sheets, and especially the surface layers, not to properly combine with the adjacent sheets of material with the result that the final product is blistered or bloated. It is impossible to overcome the unevenness resulting from the hot-pressing treatment by the subsequent oven cure to which the articles are subjected, and the oven cure does not remove the blistered or bloated condition when it occurs. The thicker the article being formed, the more pronounced these undesirable conditions become.

I have discovered improved methods, and especially the compacting step thereof, of making the aforesaid articles wherebythe above andother difliculties are obviated and by which improved abrasive articles can be formed by consolidation of a number of superimposed sheets of felted fibrous abrasive-containing web material in an efiicient and effective manner. The

resulting articles are not only uniform throughv through the line suits desired. The herein-described metbodsby reason of the improved uniformity of product obtained thereby and the rejected due to defects therein, result in economic savings as well as the accrual of other advantages, all of which will become apparent as the description proceeds. In accordance with the present invention, improved abrasive articles of the resilient or soft acting type are made by a process in which the top and bottom surface layers of the abrasiveincluded felted sheet or web material, after the sheets have been suitably assembled in superimposed relation, are maintained in a suitably humidified condition during the final consolidation and formation of the article in the hot press. One satisfactory method of maintaining the desired degree of humidity during the hot-pressing operation is by the insertion prior to the prwsing step of water-soaked cloths, such as heavy canvas or duck or similar fabrics or felted materials, between the platens of the press and the top and bottom surfaces of the sheet material being compacted. Upon the application of heat and pressure the water in the cloth penetrates the surface layers of abrasive-included fibrous material and retards the drying and heating of that portion to the extent that the innermost layers of abrasive-included fibrous sheet material are given an opportunity to dry and mature at a rate substantially equivalent to that of the outer portions. As a result, the article is uniformly matured throughout and the surface layers of the sheet material do not blister or otherwise separate from the remainder of the sheet material. Following the hot-pressing operation, the composite shape is subjected to an oven cure after which it is dressed and edged according to customary procedure; whereupon it is ready for use. In order to better understand the nature of the invention, reference is made to the accompanying drawing in which:

Figure 1 is a diagrammatic vertical cross section through a hot press set-up adapted for carrying out the present invention, and depicts an assembly of abrasive-included felted fibrous sheet material as positioned between the press platens and prepared for proper humidification during the hot-pressing operation;

Figure 2' is a top plan view of an abrasive wheel made in accordance with the present invention; and

Figure 3 is a. diametrical vertical cross-section 111-411 of Figure 2.

The abrasive-included fibrous web material used in making the products of the present invention can be manufactured in several ways. The abrasive and adhesive content of the fibrous material is incorporated and distributed throughout the fibrous web at the time of its manufacture and constitutes an integral part thereof. A very satisfactory method of making included abrasive sheet material of the herein required type is that set forth and fully described in U. S. Patents Nos. 2,284,715 and 2,284,716. Briefly, the felted fibrous web is formed by feeding a plurality of thin carded fibrous membranes from a number of carding assemblies onto a moving endless support so that each membrane is deposited or superimposed upon the preceding membranes until a web of loosely felted fibrous material of the desired thickness is built up on the traveling support. A number of abrasive grain hoppers are also disposed between the carding assemblies and above the traveling conveyor. Abrasive grain is iowernumber of articles fed from the hoppers onto the fibrous membranes at various stages in the building up of the final web, so that, as a result, the abrasive material is applied between the individual membranes makcluded abrasive webv is then wound into rolls for use as a source material for the manufacture of the products herein described.

, In practicing the invention, any of the abrasive materials in common use may be employed, such as silicon carbide, diamonds, boron carbide, fused aluminum oxide, fiint, corundum, emery, rouge and similar substances. The size of the abrasive particles may vary from the finest polishing or bufilng powders to the coarser grit sizes used in grinding.

Other methods of incorporating abrasive material internally of the fibrous sheet material during its manufacture may be employed. For example, the abrasive particles can be thoroughly admixed with the adhesive binder and the mixture applied to the uncompacted web by the usual adhesive-applying rolls. This method has been found to be particularly satisfactory for the inclusion of the finer abrasive materials of the size employed in hitting and polishing operations.

Another method is to project the abrasive material into the web or sheet after it has been built up to the desired thickness and immediately prior to consolidating the web. The projection of grain is suitably carried out by means of a blast of air or gas against one or both surfaces of the fibrous web, theair stream being laden with the abrasive material 'to be included internally of the web. The other steps in the formation and consolidation of the web are carried out in a manner similar to that used in the previous procedures.

Felted fibrous webs having abrasive material included within the web which are suitable for use in the fabrication of the herein-described products can also be made by a modification of the above processes in which the individual fibers are interwoven and interlocked by a gentle air or gaseous agitation of the thin carded membranes during their deposition. This process is termed aerodynamic weaving and is used to promote the strength and eliminate any laminations from the web. For a more complete description of such a process and product, reference ismade to U. S. Patents Nos. 2,284,738 and 2,284,-

, 739 in which further details are also given of the above procedures for including abrasive materials within the fibrous structures.

Abrasive-included sheet material of the hereinrequired type can also be made by wet-felting procedures employing various types of paperstock fibers and including abrasive therein at the time of making the sheet material. One satisfactory method of makingthe abrasive-included Briefly, the felted fibrous paper-stock material having abrasive the neighborhood of 0.5 to 5% of the suspension, collecting the fibrous suspension on a suitable support, introducing abrasive grains with or without an adhesive binder into the fibrous web while it is in a highly aqueous. or fluid, condition, extracting the water or other liquid medium from the web, and further drying and compacting the web to the desired density. Optionally, the abrasive grains can be given a preliminary coating of a suitable adhesive, which ispreferably soluble or softenable in the liquid suspending medium or a component thereof, to assist in holding the fibrous, abrasive-containing web in compacted form after it has been compressed. The process can be carried out in a number of ways.

In one such modification the fibrous suspension of paper-stock is deposited upon a foraminous support in one step and the abrasive grain is deposited in several increments while the web is still in a highly aqueous or fluid condition. That portion of abrasive applied first penetrates deepest into the web structure, and the abrasive of each succeeding deposition penetrates the web to a lesser degree by reason of the continually decreasing amount of water present and the consequent thickening of the fibrous body. The distribution of abrasive within the web can thus be controlled by variation of the amounts and places of deposition during the dewatering of the web so as to obtain any desired effect.

In a modified form of the process a fiuid fibrous suspension is collected upon a plurality of revolving foraminous supports, such as cylinders, which are partially immersed in the fibrous suspension. The thin fibrous membranes collected upon the foraminoussupports are then successively transferred to a common carrier. usually a moving endless felt blanket or belt, in superimposed relationship to form a fibrous web structure. Abrasive grain is incorporated into the fibrous web structure by deposition or projection between and into the various individual membranes as they are deposited on the carrier support. This method has the advantage that, since the fibrous suspensions can be varied, the composition or structure of various portions of the web can be varied if desired. Adhesive binders can be incorporated in the fibrous web, regardless of the procedure used, at various stages in the process, such as by inclusion in the fibrous suspension, introduction during dewatering of the web, with the abrasive grains as a coating thereon, or after the web has been substantially dewatered and prior to the final compacting and drying of the web.

A still further modification which can be used to make fibrous abrasive materials is the process of forming a wet fibrous abrasive lap by a cylinder wet-press process, in which a fibrous suspension similar to that used in the cylinder method above is fed, usually from a single cylinder mold, onto a traveling wet felt carrier belt. The thin fibrous membrane is conveyed over one or more suction boxes to remove a portion of the water and passed between a pair of pressure rolls. As the fibrous material passes between the pressure rolls it is transferred to the upper pressure roll upon which it is wound in a series of successive layers until the desired thickness of fibrous material is obtained, whereupon the material is removed by hand or by a suitable knife or doctor blade. Abrasive grain is fed onto the fibrous sheet previous to its passage between the pressure rolls and is thereby incorporated within the fibrous structure.

Another satisfactory method of making abrasive-included paper-stock web material is that set forth and fully described in copending application Serial No. 461,140, filed October 7, 1942. In accordance with the process therein set forth, fibrous paper-stock abrasive sheet material is made from anaqueous suspension of around 3% fiber content in which the fibers are distributed in the suspension as substantially individually separated fibers. This fibrous suspension is agitated, beaten, or otherwise mechanically treated, with sufficient vigor to render the fibers partially gelatinous, abrasive grain being incorporated in the suspension to the desired extent. A sumcient amount of abrasive grain is added to provide an abrasive grain content of over 40%, and preferably -70% or more, by weight of the final product after the water has been extracted. The agitation of the fibrous suspension previous to and/or during the addition of abrasive grain is very important in preparing a liquid in which the abrasive particles will remain uniformly in suspension. This stability of suspension is believed to be due, at least in part, to the adherence of the abrasive particles to the partially gelatinized fibers. The amount of agitation or beating of the fibrous suspension necessary to provide a suitable stable mixture of abrasive and fiber of the proper consistency depends upon the size of the abrasive particles. For example, a very slight agitation serves the purpose when the finest abrasive polishing powders are used whereas with the use of 60 or grit abrasive grains, it is desirable to subject the suspension to a much longer and/or more vigorous agitation. Suspension of the abrasive particles may be assisted by incorporating an adhesive in the fluid mass, either in the form of a preliminary coating of the abrasive particles or by the addition of the adhesive to the liquid. In either case, the adhesive should preferably be soluble in the liquid medium of the suspension orat least softenable or soluble in a component thereof.

After a fiber-abrasive suspension of suitable consistency has been prepared as above outlined, it is flowed or otherwise deposited on a suitable foraminous support in the form of a continuous, highly aqueous layer from which the water or other liquid suspending medium is extracted, and the resulting film is further dried and compacted to the desired density. These operations can be performed by flowing the suspension onto a moving endless wire screen or by collecting a plurality of thin membranes onto cylinder molds and transferring them to a suitable felt blanket in superimposed relation to form a web of the desired thickness. While it is usually desirable to proceed in the described manner of forming continuous fibrous webs by this particular method,

the process can be practiced in modified form by using the same thin suspension of fibers and abrasive to form individual abrasive-included sheets by a series of discontinuous steps. Adhesive binders can be incorporated in the fibrous web, regardless of the procedure used, at various stages in the process, such as by inclusion in the fibrous suspension, introduction during dewatering of the web, as a coating on the abrasive grains, or after the web has been substantially dewatered and prior to the final compacting and drying of the web. I

Modifying agents such as waterproofing compounds, antifriction agents, fiexibilizers, plasticizers and other fillers may be incorporated in the web at the time of making, irrespective of the particular procedure followed, in order to render the web resistant to water or impart other specific desirable properties to all or certain controlled portions of the web.

Abrasive-included felted fibrous web material such as that made by any one of the aforementioned methods is used as a source of raw material for the making of abrasive articles in accordance with the present invention. As a specific example of the manner in which the present process has been carried out and abrasive articles made in accordance with the teachings of the invention, the following procedure is given.

A flexible, felted fibrous sheet material made in accordance with the teachings of U. S. Patents Nos. 2,284,738 and 2,284,739, and containing approximately '75% of fused alumina particles of 80 mesh grit size, 15% cotton fibers and natural rubber derived from latex is sized with a 20% aqueous solution of casein glue, such as the casein glue made and sold by Casein Corporation of America under the trade name Casco, in a quantity sufiicient to provide about 10% casein by weight based on the fibrous sheet material.

This sized material is dried and then out to the desired size and shape for use in making the abrasive article. For example, in the making of abrasive grinding wheels the adhesively coated web material is cut in the shape of discs having a diameter equal to that of the abrasive wheel to be formed. The central arbor hole can be out out at the time of cuttin the disc or, in the case of thin wheels, it can be cut out after the sheet material has been compacted. Instead of applying the adhesive to the abrasive web material prior to cutting to size, the individual pieces can be coated with the desired adhesive after it has been cut to size, although the former method is preferred because of the greater facility for sizing and drying the material in roll form.

More recently, improved methods of handling the adhesive-coating and cutting of the sheet material to the ultimate size for use in forming abrasive bodies of the desired shape have been devised. These improved methods have been more fully set forth and described in copending application Serial No. 542,321, filed June 27, 1944; they are applicable to carrying out the present invention, and in fact, constitute the preferred method of preparing the sheet material for hotpressing. Briefly, the abrasive-included fibrous sheet material is first sized in roll form or in the shape of relatively large sheets of the material after which a sufficient number of large sheets of the adhesively-sized material are assembled in superimposed relation to provide a slab of specified thickness, as for example 4-inch thick when compressed by cold-pressing. The number of individual sheets to be used in forming one of the compacted slabs is determined by weighing. In

the case of the herein-illustrated example, these preformed slabs of sheet material are died out on a punch press or by similar mechanism, or out Or punched out by hand, to provide a number of disc-shaped segments of slab thickness. Each segment consists of a number of individual discshaped sheets of abrasive-included fibrous web material having the diameter of the abrasive wheel to be formed, and suitably compressed and ready for use in making the molded abrasive article. After the abrasive-included felted fibrous web material has been suitably coated with adhesive and reduced to the desired size for forming the abrasive article, either in the form of individually adhesively-coated sheets of the material or in the form of segments of preliminarily cold-pressed slab material in accordance with the preferred procedure set forth above, it is placed between the platens of the press for the subjecting to heat and pressure.

Referring further to the drawing, Figure 1 shows in detail the assembly of elements as they are arranged for the hot-pressing operation. The abrasive-included fibrous material consists of a numberv of abrasive-included fibrous sheets 4 which preferably have been initially coldpressed into a slab and one or more of these prepressed slabs is inserted between the upper platen 5 and lower platen 6 of the press. A sheet of fabric 1 which has been saturated with water is placed between the upper platen 5 of the press and top layer 4a of the fibrous sheet material, and a similar water-soaked sheet of fabric 8 is placed between the lower platen 6 of the press and the bottom layer 4b of fibrous material. Heavy canvas or duck which has been saturated with water, such as by dipping in a tank filled with water and running through a pair of soft rubber rolls to remove the excess water, has been found highly satisfactory for such use. However, any woven or felted fabric capable of retaining an appreciable amount of water will be suitable for the purpose.

Other means of humidifying the surface sheets 4a and 4b of fibrous material can be employed, such as by wetting the surfaces of the assembled sheet material wtih water. However, it has been found that such wetting of the surface sheets with water is more diificult to control and when the entire thickness of fibrous sheet material is Wet or becomes excessively dampened before pressing, the tendency for blisters to form or delimination to take place is greatly increased and the presence of the moisture throughout the piece becomes detrimental instead of beneficial. It is therefore essential that the humidifying action be confined to the surface sheets of material for satisfactory results.

The press platens 5 and 6 may be heated by passing hot water or steam through the ducts In which are connected by flexible hoses ii to a hot water or steam pipe 12. This assembly is subjected in the case of a 4-inch thick wheel to approximately 3000 pounds per square inch pressure at a temperature of 260 F. for 15 minutes. A- number of abrasive articles can be formed at one time by carrying out this operation on a multiple platen press or by the use of a number of rigid steel plates between individual assemblies of abrasive-included sheet material, in which case each assembly of the abrasive-included sheet material is placed between water-saturated cloths as described above. The hot-pressed wheel or Wheels are then removed from the press and cooled, after which the central arbor holes, if not already punched, are cut out and the wheels placed in an oven for 5 hours at 260 F. to complete the curing of the adhesive binder. The articles are then dressed and edged according to customary practice, whereupon they are ready for use.

Figures 2 and 3 show an abrasive wheel l4 having an arbor hole l5 and formed from a plurality of abrasive-included fibrous sheets 4 in accordance with the present invention. The provision of water-saturated cloths between the press platens and the sheets 4 of fibrous material during the pressing operation prevent the premature drying out of the top and bottom surface sheets 4a and db of fibrous material, as a result of which the wheel is press-cured more uniformly throughout its thickness and the surface sheets asvasee la and lb are strongly united to the inner sheets I of material without blistering or bloating, as is commonly the case with similar articles formedv without the use of humidifying means.

It has been found that the character of the abrasive articles produced by the herein-described process, as for example the grade of hardness, can be'altered by variations in the quantity and character of the adhesive binder em ployed in making the abrasive-containing fibrous sheet material, and particularly by the choice of auxiliary or supplemental adhesive substance which is admixed with the sheet material as a size thereto. It is desirable that the adhesives selected for making the aforesaid articles do not smear during grinding operations. This is especially true in polishing wheels and devices where smearing of the bond tends to produce a hot cutting" or burning action which is ruinous to the finish being produced.

Among those substances which can be satisfactorily used as bonding materials herein in addition to the specific adhesive sizing compositions set forth and which are non-smearing are included glue adhesives, particularly when treated with plasticing agents such as ethylene glycol, sorbitol, glycerine and the like. The plasticizer may amount to as much as 40% by weight of the total adhesive binder. smearing adhesives which can be used are casein glues, natural or synthetic rubber latices, urea resins, phenol-aldehyde resins and other natural or synthetic resins, or mixtures of two or more of such adhesives.

The herein-described invention offers numerous improvements and advantages over the prior art. It providesa practical method of fabricating abrasive articles such as grinding and polishing wheels and shapes having soft or resilient combined cutting and polishing action, and in which the abrading body is of uniform cutting characteristcs throughout and is free from de-' fects.

The articles of the present invention have a relatively high rate of stock removal and at the same time produce a surface finish equal to that obtained by' an ordinary bonded abrasive article embodying abrasive particles several it sizes smaller. Abrasive products made as herein-de scribed cut both eificiently and effectively, with an accompanying polishing action, and are capable of high operating speeds without chattering, operating smoothly to remove burrs and produce very high finishes on metal castings at surface speeds of tieorzed that the soft" cutting action of such abrasive articles is the result of the abrasive grains being cushioned by the surrounding felted fibers and binder so as to prevent gouging and scratching and to provide the abrasive grains with a yielding background by which they are caused to maintain individually more effective contact with the work and at the same time absorb and take up inequalities of surface so as not to scratch or mar the surface being finished.

Although I have set forth, in the specific example, certain specific conditions, it will be understood that my invention is not limited to the I conditions there described. The length of time that the article is subjected to heat and pressure is determined to some extent by the size or shape of the article being molded but is usually in the range of 15 minutes for the first A inch .of thickness and an additional 5 minutes for each 9500 feet per minute. It is Other nonadditional V4 inch in thickness. likewise, articles of different grades can be made by varying the fiber, abrasive and adhesive content of the abrasive-containing felted fibrous material, the amount and kind of adhesive applied thereto as a size, and by the density to which the articles are compacted. In general the sheet material contains 5-20% adhesive, 55-20% fibers, usually cotton, and 65-90% abrasive.

Having described and set forth the invention in detail, the scope 'of the invention is not to be confined other than by the appended claims.

I claim: I I

1.In the method of making abrasive articles from a plurality of superimposed layers of abra-- sive-include'd, felted fibrous sheet material, the step which comprises selectively maintaining the outer layers of the superimposed sheet material being compressed in a humidified condition while subjecting the assembly of said superimposed sheet material to heat and pressure to form the desired shape.

2. The method of consolidating ,a pluralityof superimposed layers of abrasive-included, felted fibrous sheet material toform an abrasive article therefrom, steps of which comprise humidifying the surface layers of the material to be compressed and subjecting the superimposed layers of sheet material to heat and pressure to form the desired article.

3. The method of consolidating a plurality of superimposedlayers of abrasive-included, felted fibrous sheet material to form an abrasive article therefrom, steps of which comprise placing the sheet material to be compacted between the platens of a press, interposing sheets of porous water-wet material between the press platens and the material being compacted, and subjecting the said material to heat and pressure to form an abrasive article therefrom.

4. The method of consolidating a plurality of superimposed layers of abrasive-included, felted fibrous sheet material to form an abrasive article therefrom, steps of which comprise placing the sheet material to be compacted between the platens of a cloths between the press platens and the material being compacted, and subjecting the said material to heat and pressure to form an abrasive article therefrom.

5. The method of making abrasive articles from a plurality of superimposed layers of abrasiveincluded, felted fibrous web material which comprises assembling a plurality of sheets of said web material in superimposed relation, thoroughly wetting sheets of fabric, placing the assembly of superimposed abrasive-included fibrous sheet material between the wet sheets, placing the entire assembly between platens of a press, and subjecting the same to heat and pressure to unite the sheets of abrasive-included, fibrous sheet material and form abrasive articles therefrom.

6. In the method of making abrasive articles from a plurality of superimposed layers of abrasive-included, felted fibrous web material, the

RICHARD A. BAUMGARTNER.

press, interposing water-soaked-

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