Technical Field.

The present invention relates, generally, to abrasive foam grinding

structures, and more particularly to methods for manufacturing a resin-based

composite structure including microballoons for controlling friability, hardness,

and cell density.

Abrasive foam materials for grinding, honing, and buffing the surfaces of

workpieces are generally well known. For certain workpieces, e.g. computer

hard drive substrates (hard disks), the desired surface finishes are becoming

increasingly difficult to obtain by conventional techniques using known soft

grinding "stones."

Presently known honing stones may be of an open-cell structure or a

closed-cell structure. Open-cell stones are generally made from a combination

of polyvinyl alcohol resins (PVA), starch, and a silicon carbide filler functioning

as the abrasive material. Appropriate catalysts and co-polymers, for example,

sulfuric acid and formaldehyde, are added to the PVA, starch, and silicon carbide

mixture to convert the mixture to a rubber-like mass with the starch particles

randomly distributed and trained therein.

The rubber-like mass is then flushed in a hot water shower to dissolve and

flush out the starch from the composite. Upon flushing the starch from the mass,

a random distribution of holes is formed having diameters in the range of 50-150

microns. The spongy mass may then be impregnated with a stiffening agent,

e.g. meiamine. The stiffening agent penetrates through the random distribution

of holes coating the complex surfaces within the spongy mass. The excess

stiffening agent within the holes may then be removed, for example, through

centrifugal extraction or a subsequent rinsing process. The resulting stiffened

sponging mass may then be dried and used to prepare the surface of

workpieces (e.g. computer hard disks.)

An alternative stiffening technique used in open-cell structures involves

adding various rigid co-polymers to the PVA before curing to enhance the

stiffness of the finished spongy mass.

Closed-cell structures are typically produced by mixing a "Part A" and "Part

B" urethane resin together. With this process, one of the two parts is used to

disperse an abrasive, such as silicon carbide. While Part A and Part B are

stored separately prior to mixing, upon mixing together, the composite resin

mixture is quickly introduced into a mold, whereupon a spontaneous exo-thermic

cross-linking reaction occurs. The exo-thermic reaction between the two resins

generates a substantial amount of heat. The heat expands blowing agents

present within one or both of the resin components, forming foam or high density

bubbles within the compound. The bubbles become entwined within the

cross-link matrix resulting in a porous, closed-cell structure suitable for grinding,

honing, and buffing.

When applied to a workpiece, an open-cell stone intentionally flakes,

liberating particulates at the stone/workpiece interface. To remove these

particulates, as well as to the remove particulates liberated from the workpiece

and the heat generated at the interface, the workpieces are typically flushed with

water or a water-based solution during the honing operation. Because of the

open-cell structure, some of the particulates may have a tendency to penetrate

into the stone, resulting in "loading" of debris within the stone. This loading can

exasperate problems associated with the finish of the workpiece and the local

stiffness deviations across the stone surface. In addition, incomplete mixing of

Part A and Part B also results in nonhomogeneous or insufficiently homogenous

regions, creating a nonuniform honing surface on the stone. A nonuniform

mechanical surface may result in uneven material removal from the workpiece,

and can also result in scratches and other blemishes being imparted to the

workpiece. Attempts to increase the mixing rate of Part A and Part B have been

unsatisfactory, inasmuch as the rate of reaction is often faster than even the

most sophisticated mixing techniques.

Closed-cell urethane stones are also unsatisfactory in several regards. For

example, the urethane stones comprise an inherently tough composite structure.

Consequently, "flaps" are created as bubble surfaces are breached and can

scratch a workpiece if the flaps are not liberated from the stone and rinsed away.

Moreover, both the PVA-based stones and the urethane-based stones require

the use of highly toxic materials. This is particularly true of many isocyanide

urethane resins.

Yet another known stone formation technique involves the use of phenolic

resins combined with a foaming agent and a catalyst. The catalyst causes the

foam "to explode," creating bubbles in the resin. The exo-thermic nature of the

reaction cures the resin, trapping the bubbles within the mass.

Presently, known stone substances and techniques for manufacturing them

are unsatisfactory in several regards. One of the principle drawbacks to known

stone pad materials relates to the non-uniform mechanical structure of the

honing surface. Non-uniform mechanical structures result in non-uniform

removal of material from the workpiece, creating unsatisfactory workpiece

surface finishes. In addition, imperfect dispersion of abrasives in resinous

stones tend to yield agglomerates or nodules which project from the mean

surface of the honing pad. Agglomerates can also cause scratching and surface

imperfections in the workpiece. Moreover, the toxic nature of many of the

components of known stones renders their manufacture, use, and disposal

environmentally compromising and expensive to handle. Finally, the cost of

manufacturing presently known stones is getting increasingly high, exacerbating

the problem of limited stone life.

A new honing stone composition and method for making and using a new

stone composition is therefore needed which overcomes the limitations of the

prior art.

Summary of the Invention.

The present invention provides an abrasive foam grinding, honing, and

buffing material which overcomes many of the shortcomings of the prior art. In

accordance with a particularly preferred embodiment of the present invention,

a brittle, low modular weight phenolic resin mixture is combined with

microballoons to produce a foam structure with highly desirable physical

properties. In accordance with the further aspect of the present invention, an

exemplary method of manufacturing a phenolic resin composite permits various

physical and structural parameters of the stone to be tuned during manufacture.

In accordance with a further aspect of the present invention, highly

consistent material removal rates may be obtained from stone to stone and from

run to run for a particular stone.

In accordance with another aspect of the present invention, the stiffness

and hence, the useful life of stones is enhanced.

In accordance with another aspect of the present invention, more uniform

mechanical properties may be obtained across the surface and throughout the

bulk of a stone.

In accordance with a further aspect of the present invention, a novel mixing

chamber and mixing technique is employed to enhance the dispersion of the

abrasive during the manufacturing process, thereby reducing the risk of

agglomerate formation within the finished structure. In accordance with a further

aspect of the present invention, enhanced hardness may be obtained.

In accordance with another aspect of the present invention, maximum

consumption of toxic materials is achieved, resulting in minimal environmental

impact and minimal liberation of toxicity.

Brief Description of the Drawing Figures.

The present invention will hereinafter be described in conjunction with the

appended drawing figures, wherein like numerals denote like elements, and:

Figure 1 is a plan schematic view of an exemplary platen

supporting an abrasive stone table. A plurality of carriers, each

carrying a plurality of workpieces to be honed are obtainably

mounted to the platen to thereby place the workpieces in frictional

contact with the abrasive stone surfaces;

Figure 2 is a prospective view of an exemplary spin blade

configuration in accordance with the present invention;

Figure 3 is a cross-section side view of the blade structure

of Figure 2; and Figure 4 is a schematic diagram of a mixing

chamber, showing a piston urging the resin material from a chamber

and through a manifold into a plurality of near net molds.

Detailed Description of Preferred Exemplary Embodiments.

The subject invention relates to abrasive stone pads for use in processing

workpiece surfaces. Although the workpiece to be processed may comprise

virtually any device requiring a controlled finish, the present invention is

conveniently described with reference to computer hard disks which require

controlled surface finishes. It will be understood, however, that the invention is

not limited to any particular type of workpiece or any particular type of surface

finish.

Referring now to Figure 1 , a work table can suitably comprise an annular

shaped platen wheel which supports a plurality of generally pie-shaped abrasive

stone segments 14A-14N. A plurality of carriers 16A-16E each carry a plurality

of workpieces 18, for example a computer hard disk substrate, in the

embodiment shown in Figure 1 , a total of five carriers 16 each carry nine (9)

workpieces 18, for a total of forty-five (45) workpieces. In accordance with a

preferred embodiment of the invention, an oppositely disposed platen carrying

a plurality of stone segments is disposed opposite platen 12 such that carrier 16

is sandwiched between opposing stony surfaces. Each carrier 16 is suitably

configured to rotate about its axis in the direction indicated by arrow A; in

addition, lower platen 12 is suitably configured to rotate about its axis in the

direction indicated by arrow B and the upper platen is configured to rotate in a

direction opposite of the lower platen. As a result, both surfaces of each

workpiece 18 may be processed simultaneously, with a desirably uniform and

even removal rate from each side simultaneously.

It should be noted, however, that any particular surface grinding or honing

machine can be used with the grind stone of the preferred exemplary

embodiment of the present invention. For example, a grinding machine may be

configured to only grind and buff one side of the unfinished parts at a time, or the

platen may comprise one large grinding stone instead of a plurality of segments

of grinding stones as discussed below.

To optimize the surface characteristics of each workpiece 18, it is desirable

that the mechanical surface of each stone segment 14 be uniform across the

entire surface of each segment 14. Furthermore, it is desirable to maintain

uniform mechanical characteristics across the surface of each segment 14 even

as the stone surface is worn away, for example by passing a diamond dressing

plate over the surface of the moving stone plates as is known in the art.

In accordance with a preferred embodiment of the present invention, typical

removal rates on the order of 1-1.5 micro-inches every 3.5 minutes are desirably

obtained for the various workpieces 18 upon which the stone surfaces operate.

This is typically done under pressures on the order of about 1 to 2 pounds per

square inch (PSI). Although the shape of each stone segment 14 in Figure 1 is

generally pie-shaped, it is understood that virtually any stone shape may be

utilized in the context of the present invention. In accordance with the preferred

embodiment, a prepared resin mixture may be urged into a mold cavity, wherein

the resinous mixture cures within the mold cavity. Upon curing, the mold cavity

is open, revealing an abrasive stone of any desired shape corresponding to the

shape of the mold.

The manner in which abrasive stones are manufactured in accordance with

the present invention will now be described.

In accordance with the particularly preferred embodiment of the present

invention, a brittle, low modular weight phenolic resin is suitably employed. A

suitable phenolic resin solution may comprise a 70-90% by weight resin

monomer in water, for example, product number ARJ11761 available from the

Schenectady Chemical Company of Schenectady, New York. As discussed in

greater detail below, microballoons are added to the phenolic resin to create a

desirable cellular structure. In accordance with one aspect of the present

invention, it is believed that the combination of the phenolic resin mixture with a

suitable diluent will permit a greater number (higher density) of micro-balloons

to be added to the resin mixture. In this regard, Applicants have determined that

a suitable diluent may be formulated by combining 40-50% by weight deionized

water and 50-60% by weight ethanol, and preferably 43% by weight water and

57% by weight ethanol. In a further embodiment of the present invention, the

diluent may comprise a mixture of water, ethanol and furfuryl alcohol. In this

further embodiment, the diluent may be formulated by combining 30-60% by

weight water, 15-45% by weight ethanol and 15-45% by weight furfuryl alcohol.

As is also discussed in greater detail, the amount of furfuryl present in the diluent

may be manipulated to control the hardness of the finished stone structure.

Upon preparing a suitable diluent, 40-55% (preferably 47.2%) by weight of

the diluent may be mixed with 45-60% (preferably 52%) by weight of silicon

carbide and a small amount (e.g., a few drops) of a suitable dispersing agent, for

example, AMP Regular manufactured by Angus Chemical Co. of Buffalo Grove,

Illinois, or Hypermer PS2 manufactured by ICI Surfactants of Wilmington,

Delaware.

The diluent, silicon carbide, and dispersant mixture is suitably bail milled

in a tumbler with small, hard beads, for example, on the order of 15 minutes or

until all agglomerates are satisfactorily crushed. The resulting ball milled meal

mixture is referred to herein as the dispersed silicon carbide.

The dispersed silicon carbide is then suitably transferred, for example

through an appropriate filtering screen to remove agglomerate structures, milling

beads, and the like into an appropriate vessel, whereupon this may be added to

a resinous compound, including approximately 20-35% by weight and preferably

about 27.5% by weight phenolic resin monomer in the mixing vessel.

In this regard, although a preferred embodiment of the present invention

employs a phenolic resin, it will be understood that virtually any material having

similar properties may be employed, such as polymethylmethacrylates, epoxies

and the like. In addition, suitable co-polymers, hardening agents and defoamers

may be added to the phenolic resin, as desired. A vacuum is then drawn in the

vessel chamber to prevent the introduction of air into the mixing chamber during

mixing, which helps control foam uniformity. It should also be noted that the

dispersed silicon carbide and phenol resin mixture should be mixed at a fairly

high shear rate. For this purpose a suitable commercial vacuum, chilled mixer,

for example one available from the Meyers Engineering Company of Los

Angeles, California may be used.

A vacuum is suitably drawn in the mixing chamber on the order of 20-30

inches of mercury, and most preferably in the range of 25-27 inches of mercury.

In this regard, because rapid stirring typically generates heat, it may also be

desirable to maintain a fairly cool environment within the mixing chamber, for

example on the order of 40-60°F, to minimize the cross-linking and other

reactions associated with the resin.

Under vacuum, approximately 55-75% and preferably 63.5% by weight

dispersed silicon carbide, and approximately 20-35% and preferably about

27.5% by weight of the resin mixture are mixed with approximately 1.0-5.0% and

preferably about 2.02% by weight of microballoons. The mixture may further

include about 7% by weight defoam, for example, from Ultra Additives of

Patterson, New Jersey. In accordance with the preferred embodiment, suitable

microballoons are on the order of about 20-200 microns, and preferably on the

order of about 80-150 microns in size. Suitable microballoons may be made

from polyvinylacrylonitrile, for example, the microballoons manufactured by Akzo

Nobel's Expancel Corporation of Sweden under the name DE80 Expancel

Balloons. However, one skilled in the art should appreciate that any type of

microballoon may be used in accordance with the preferred embodiment, for

example, microballoons made of vinyl or acrylonitrile may be used.

In a preferred embodiment, the microballoons are added to the dispersed

silicon carbide/resin mixture while these two components are being mixed together in the mixing chamber, preferably under vacuum to prevent the

introduction of air. Moreover, the addition of the microballoons may also be

followed by the addition of a suitable catalyst. For example, in accordance with

the preferred embodiment of the present invention, about 5.0-9.0% and

preferably about 7.0% by weight of phenolsulfonic acid having 65% by weight

aqueous solution may be added to the mixture.

As briefly discussed earlier, a high cell density structure is desired,

suggesting that a relatively high density of microballoons should be added to the

mixture. However, there are physical and practical limits to which balloons may

be added to the mixture while maintaining a flowabte mixture. In this regard, the

Applicants have determined that the diluent facilitates a fluid environment,

thereby permitting the addition of a large number of microballoons. In addition,

the diluent appears to impeded the curing reaction to some extent.

Referring now to Figure 2, the manner in which mixing is implemented

within the chilled chamber in accordance with a preferred embodiment will now

be described.

It is generally known that the increased dispersion of the silicon carbide

within a resin reduces agglomerate nodules. However, presently known

techniques for enhancing dispersion are not satisfactory because many typical

rotor stator high sheer mixer blades can destroy microballoons.

In accordance with a further aspect of the present invention, a Tesla style

turban configuration is employed to enhance dispersion of the microballoons and

silicon carbide within the mixing chamber. More particularly, and as shown in

Figures 2 and 3, an exemplary mixing system 20 suitably comprises a rotating

shaft 22 having a center blade 24 attached to the shaft, for example, via a well

bead 26. An upper blade 28 and a lower blade 30 are also suitably coupled to

center blade 24, for example by suitable stand off bolts or studs 32. At high

rotation rates, for example on the order of 2,500 to 4,000 RPM, the viscous

mixture within mixing vessel 34 in contact with middle blade 24 is urged radially

outward away from shaft 22 as a result of the high centrifugal force created by

the rotation of shaft assembly 20. The accelerated flow of mixture induced by

the centrifugal forces associated with blade 24 essentially cause "needles" of the

mixture to shoot radially away from shaft 22 at high rates of speed, for example

along the direction indicated by arrows 36. These high velocity "needles" create

high shear stresses in the volume of mixture surrounding the blades, effecting

dispersion. At the same time, a corresponding vacuum is created in the region of blade 24 proximate shaft 22. This low pressure region 38 (Figure 3) induces

flow of mixture from the outer regions of blade 24 back toward shaft 22 along the

direction of arrows 40.

In accordance with the illustrated embodiment, the spacing between the

parallel blades 24, 28, and 30 is suitably on the order of %-1 inch, with the outer

diameter of the blades suitably on the order of 3-4 inches. With an

approximately 6-7 gallons charge within the chamber, the dispersed silicon

carbide and resin mixture is mixed together within the chamber for in the range

of 5-20 minutes, and most preferably around 10 minutes. When sufficient

dispersion is achieved, spinning is stopped and the mixture is allowed to cool

down to approximately 60°F from the approximately 90°F temperature the

mixture achieves due to the friction generated through mixing.

A suitable carrying catalyst, for example 5.0-10% by weight of an acid (e.g.,

phenolsulfonic acid), can then be added to the mixture, being careful to avoid

introduction of air into the mixture. The mixture is again stirred vigorously, under

vacuum, for approximately 5 minutes to disperse the catalyst. Mixing is then

terminated, and the shaft and blades are carefully extracted to minimize the

introduction of air into the mixture.

Although some degree of reaction may occur within the resin, this may be

inhibited both by the presence of the diluent and by maintaining the mixture at

a suitably low temperature to impede reaction.

It should be noted that the preferred embodiment of the invention has been

described in accordance with the Testa turbine mixer shown in Figures 2 and 3.

However, any type of mixing device that can create the proper dispersion of the

mixture can be used.

Referring now to Figure 4, the mixing vessel 60, including the partially

cured or uncured mixture 62, is then transferred to or otherwise engaged by a

suitable piston 66 or other mechanism for purging the mixture from the vessel

into suitable molds.

Piston 66, under the operation of a suitable ram 67 or other urging

mechanism, is carefully urged toward the top of fluid 62, while the air and/or

other gases within the region 64 between piston 66 and mixture 62 is drawn out

of vessel 60, for example by a suitable vacuum line 68 which communicates with

region 64. Piston 66 then urges the mixture through an appropriate valve 78 into

conduit 70 and into a manifold 72 which communicates with conduit 70. From

manifold 72, the mixture is urged into a plurality of lead lines 74 and thereafter

into a plurality of corresponding molds 76.

While resident in molds 76, excess resinous liquid and/or diluent may be

conveniently extracted from the molds, for example through extraction conduits

82, such as a frit plate or the like. In a particularly preferred embodiment, a

buchner funnel or vacuum is drawn in each of extraction tubes 82 to controllably

draw fluid from the mold cells.

With continued reference to Figure 4, present inventors have determined

that all of the resinous liquid may be left within mold 76 and ultimately cured.

Alternatively, a large amount of uncured resinous liquid may be withdrawn from

the mold; indeed, any desired amount of resinous liquid may be either left in the

mold and allowed to cure or withdrawn from the mold prior to curing. Applicants

have further determined that the amount of resinous liquid left in the mold or

withdrawn from the mold largely determines the hardness of the finished stone.

This selective resin withdrawing process from the partially cured, gelatinous

composite within the mold is referred to herein as "tuning" the hardness of the

finished stone. As briefly alluded to above, it is also possible to adjust (tune) the

hardness of the finished stone by varying the amount of furfuryl alcohol added

to the diluent earlier in the process, and by varying the ratio of resin to diluent.

As a general rule, altering the amount of furfuryl alcohol added to the diluent, will

modify the hardness of the finished stone. As a further rule of thumb, the more

resinous liquid which is left in the mold during the final extraction process, the

harder the finished the stone; conversely, the more resin or resinous liquid

withdrawn from the mold during the curing phase, the lower the hardness of the

finished stone.

After extracting a desired amount of resinous liquid from the molds, the

mold may be placed in a warm (e.g., in the range of 75-110° F and preferably

about 90°F) chamber or oven to allow the composite to cure to a firm gel. In a

preferred embodiment, this may take in the range of 5-50 hours, and preferably

around 20-30 hours, and most preferably approximately 24 hours. Once the

composite has reached the firm gel stage after approximately 24 hours, an

additional vacuum extraction of resinous liquid may be performed, for example

by drawing a vacuum through extraction tube 82 in the range of 15-30, and

preferably around 26 inches of mercury, for up to approximately 5 minutes. In

this regard, the intensity of the vacuum and the length of time the gel is subject

to vacuum should be carefully controlled to avoid cracking of the composite

within the mold. Further extraction of resinous fluid at the gel stage allows

further tuning of the hardness of the finished product.

As also briefly alluded to above, the porocity of the finished stone may also

be tuned by adding more or less microballoons at the mixing stage. To facilitate the addition of more microballoons to thereby achieve a higher density cell

structure, it may be desirable to add additional diluent at the mixing stage.

Once the final fluid extraction is performed, the stones may be removed

from the molds and placed in a chamber or oven which is slowly ramped up to

a temperature in the range of 20-50°C, and most preferably around 40°C. In

accordance with one aspect of the present invention, it is desirable to slowly

ramp the temperature up to avoid boiling of the alcohol or other liquids within the

composite to thereby avoid cracking of the composite. The stones are then

maintained at approximately 40°C for on the order of 6-30 hours, and most

preferably about 24 hours. As a final curing operation, the temperature is then

ramped up from 40°C to approximately 120°C over a period of approximately 10-

35 hours, and most preferably over a period of approximately 12 hours. The

stones are maintained at a temperature of 120°C for approximately 0.1-10 hours,

and most preferably for approximately 1 hour. This is believed to fully cure the

stones and achieve maximum hardness. It is also believed that these latter

heating stages fully desiccate (dry) any unincorporated formaldehyde which is

liberated during the cross-linking process. In this regard, the final curing

operation may also be carefully implemented to fine tune the hardness of the

finished stone. For example, total curing of all residual resin within the mold will

result in maximum hardness of the finished composite; conversely, by curing

slightly less than all the available resin, a correspondingly lower hardness level

will be achieved. Control of the latter stages of hardening may be effected by

manipulating the final temperature above or below the 120°C threshold, and also

by varying the resident time at which the stones reside in the final curing stage

for more or less than the aforementioned 1 hour threshold.

Finally, after the stones have been cured, they are mounted and glued to

pie-shaped platen segments which are preferably aluminum. A belt sander is

then used to sand the stones down to the exact shape of the segments. The

platen segments with the grinding stones are then bolted to the grinding machine

platen.

In accordance with a further aspect of the present invention, it may be

advantageous to place a fabric or paper covering over the top of the composite

if molds 76 are configured such that a portion of the stony composite is exposed

to ambient air. The use of such a film or fabric will impede the formation of a

skin, which could cause the formation of hardness gradients within the mold.

In accordance with yet a further aspect of the present invention, the stones

can be cast directly onto disposable backing plates such as phenolic boards.

In accordance with yet a further aspect of the present invention, it may be

advantageous to use a sound probe to measure the fluid density in the stone.

Because the sound will travel through fluids faster than through air bodies, the

sound probe will help determine whether the proper amount of liquid has been

with out of the stone molds, thus allowing the production of more uniform batch

hardnesses.

In accordance with yet a further embodiment of the present invention, the

microballoons and a catalyst can be added to ceramics and other materials to

control the porocity or pore density of the material.

It will be understood that the foregoing description is of preferred exemplary

embodiments of the invention, and that the invention is not limited to the specific

forms shown or described herein. Various modifications may be made in the

design, arrangement, and quantity of the elements and chemical compositions

disclosed herein, as well as the steps of making and using the invention without

departing from the scope of the invention as expressed in the appended claims.