US 3246060 A
Description (OCR text may contain errors)
United States Patent 3,246,060 METHOD OF MAKING MACHENABLE HIGH ENERGY PERMANENT MAGNETS Walter S. Blume, Qincinnati, Ohio, assignor to Layman Corporation, Cincinnati, Ohio, a corporation of Ulric N0 Drawing. Filed Apr. 1, 1963, Ser. No. 269,701 Claims. (Cl. 264-62) This invention relates to the manufacture of high energy permanent magnets, and in particular relates to the manufacture of high energy magnets which are of the fine particle type and which are amenable to shaping by ordinary methods of machining.
It has been an objective of this invention to provide an improved method of making permanent magnets, especially magnets of the fine particle ferrite type, which will display a substantially improved maximum energy product as compared with previous machinable permanent magnets of similar composition.
Various types of machinable magnets incorporating fine discrete particles of barium, strontium, and/ or lead ferrite as the magnetic component are known. In such magnets the magnetic particles are bonded by a nonmagnetic matrix material which imrnobilizes the particles in the form of an integral self-supporting body that can be shaped by ordinary machining techniques, e.g., by turning, sawing, drilling and similar operations. Depending upon the particular method by which they are produced and other factors, the maximum energy product of previous machinable ferrite magnets has been in the range of approximately 0.4 to 1.1 or 1.2 x 10 gaussoers-teds.
There have also previously been known fine particle ferrite magnets having substantially higher energy products, for example maximum ener y products in the range of about 1.3 to 3.5 x 10 gauss-oersteds. However, such high energy product ferrite magnets have not been machinable, which has been a serious disadvantage in some applications. In the past, ferrite magnets having energy products of this order have been subjected to sintering as one step in their production, in order to obtain the improvement in magnetic quality. The ferrites are by nature very hard, ceramic-like or refractory materials, and when a mass of such particles is sintered, the particles coalesce to form a hard, dense, highly c0- herent body. As an inevitable result of the sintering process, the aggregate is rendered un-machinable in the ordinary manner, and can be shaped precisely only by grinding. Moreover, such magnets are prone to chip or crack with rough handling. These factors have been a serious detriment in applications wherein the magnets must be manufactured in precise sizes and shapes. Thus, in many instances, the higher maximum energy products of sintered ferrite magnets have been offset in practice by the lack of machinability and durability inherent in such magnets.
I have discovered a method of producing machinable fine particle magnets, of the ferrite and other types, which are not sintered but which have energy products approaching and in some cases virtually equalling those of previous sintered magnets of the same materials and substantially higher than those of previous machinable ferrite magnets.
Broadly put, with reference to use of a ferrite as the magnetic component, a preferred embodiment of this new method comprises, preparing the ferrite in the form of magnetically anisotropic particles of substantially single domain size, aligning or orienting the particles so that their preferred magnetic axes are substantially parallel, compacting the particles under pressure without substantially disturbing their alignment to increase the into parallelness.
3,246,060 Patented Apr. 12, lgfifi density of the particle mass, firing the particle mass at time and temperature conditions sufficient to convert it to a friable body in which the particles substantially retain their discreteness under light pressure and having an improved magnetic energy product, but without subjecting the mass to sintering conditions such that it is converted to a hard, dense, highly coherent sintered body that is too hard to be machined, introducing a binder in the fired particle mass while retaining the alignment of the particles, hardening the binder to immobilize the particles, and coining the matrix bonded shape under heat and pressure thereby causing it to conform precisely to the shape of a mold cavity.
In my Patent No. 2,964,793, for Method of Making Permanent Magnets, issued December 20, 1960, there is described a method of making a machinable permanent magnet wherein, according to one embodiment, substantially domain size ferrite particles are dispersed in water or acetone and are magnetically aligned in the liquid medium by subjecting the dispersion to a magnet field. The field rotates the anisotropic particles in the low viscosity medium so that the preferred directions of magnetization of the individual particles are brought The field is maintained while the liquid is removed from the particles, for example by expelling the liquid from a die cavity past a die punch having a slight clearance with respect to the cavity wall. The particles are then cohered in the aligned state by introducing and hardening a liquid binder in the interparticle voids, so that a strong self-supporting body is formed.
The magnetic product produced by the process of Patent No. 2,964,793 is machinable, because the particles are not sintered together but rather are coheredby an edge-cuttable binder which is introduced around and between them after their alignment. A typical ferrite magnet so produced may have an energy product of about 1.2 x 10 g-auss-oersteds. Such energy products, While excellent for machinable ferrite magnets, are not as high as those I have discovered can be obtained by the present improvement over that process, which improvement at the same time preserves machinability of the magnets.
1 The present invention is predicated upon the new discovery that unsintered anisotropic fine particle magnets, for example of the type produced by the process of my previously identified patent, can be greatly improved in respect to maximum energy product without loss of'machinability if after alignment the particulate magnet is subjected to the additional set of operations comprising, first, gradually burning oif any binder or lubricant that may be present, then subjecting the unbonded particle mass to a heat treatment or firing at less than but approaching sintering conditions of time and temperature, the firing conditions being such that the mass does not increase in density more than about 1520% and further such that the particles do not coalesce to form a highly coherent body but rather remain as a mass which is friable under hand pressure, or which, at least, is not so hard as to be unmachinable, by which firing energy product is improved without formation of a dense, very hard, nonmachinable body. This step is followed by impregnation with a binder to cohere the discrete heat-treated particles in the form of a durable, machinable body, and is preferably followed by a coining operation. By these additional steps the magnetic properties of the material are surprisingly improved, to an extent such that they approach and in many cases equal those which the magnet would have if subjected to sintering, yet the particles are not converted into an unmachinable, hard refractory aggregate. Thus, not only is it unnecessary to sinter a fine particle ferrite magnet in order to obtain major improvement in energy product, it is in fact often undesirable to sinter because sintering causes loss of desirable physical qualities without a compensating major gain in magnetic quality in many instances.
For the purpose of describing this invention, sintering" is defined as causing a powdered or earthy substance to become a highly coherent solid mass without melting. In the sintering of finely divided particles such as ferrite particles, the particles weld together and the contact area between them grows. The observed magnetic alignment increases, apparently by the absorption of unoriented particles by larger aligned particles. Because of the increase in particle contact area, shrinkage of the mass occurs, accompanied by an increase in density which is of the order of 75100% or more based on original density.
Sintering conditions for any given material depend upon particle size, degree of compaction, the presence of small amounts of impurities, and other factors. In barium ferrite, for example, BaFe O substanes such as lead monoxide (PbO), lead monosilicate, and calcium oxide are employed as fluxing oxides or modifiers to lower sintering conditions.
The time and temperature conditions to which a particulate mass must be subjected for sintering to occur are interrelated: the time required increases sharply with decreasing temperature. Thus, for a given material sintering may require 8 hours at 1450 F., but may occur in i 10 minutes at 2100 F. The minimum sintering time required at a stated temperature can be determined by testing the coherency of separate masses of the material heated to that temperature for different times: when the particles are sintered the mechanical strength of the mass greatly increases. Density also increases, by about 75- 100% or more. In contrast, the unsintered mass is friable, is not too hard to cut and can often be crumbled by hand pressure. Its density is not more than about greater than that of the unfired material. When impregnated with a binder to impart greater durability, it can be machined, e.g., by edge-cutting tools. A sintered ferrite body is of much lower porosity than the fired but unsintered material, and even if impregnated would be too hard to cut except by grinding, and will usually shatter if subjected to a sharp blow.
In the practice of this invention it is preferred, although not necessary, to produce the prefired aligned particle mass by the technique described in my Patent No. 2,964,793, to which reference is hereby made. The aligning technique is set out in detail in the specification of that patent. The reason for my preference of that method of alignment is that by aligning in a low viscosity liquid of the type disclosed in the patent a better degree of particle alignment can be effected magnetically than in liquids of higher viscosity, or indeed, than by any method of which I am aware wherein a binder is not present in quantity during the aligning step. Moreover, aligning in the absence of a continuous viscous binder phase avoids a large quantity of binder in the prefired compact; as will be explained, it is preferable that binder content prior to firing be no greater than necessary to impart to the aligned particle mass a strength sufficent to withstand the fracturing forces incidental to necessary handling before the firing step. Larger quantities of binder are generally undesirable, because the binder volatilizes during firing and the released gasses tend to crack, disrupt, warp or distort the particle mass during the heat treatment. I prefer, therefore, not to impregnate the prefired particle mass heavily with binder. As compared with a technique wherein alignment is carried out with the particles in a continuous binder phase, in the technique described in my patent binder content can readily be minimized, and in fact binder in the prefired compact can be eliminated altogether if desired. Good results are obtained if the ferrite is mixed with about 0.25 to 1% of stearic acid, which acts both as a light binder and as a lubricant preventing the aligned particle mass from sticking in the die in which it is aligned or compacted. Although this additive may dissolve in an acetone aligning medium it does not unduly increase the viscosity of that medium if used in low concentration.
The details of the invention may best be further described by reference to the following specific examples:
Example 1 At the present time, lead ferrite, PbFe O is preferred as the magnetic component of the bonded magnets produced by this invention, because of its high magnetic qualities and availability. Lead ferrite also possesses the advantage that it will respond to lower firing temperatures than barium ferrite, and the improvement in magnetic quality upon firing occurs earlier in relation to the sintering point; that is, the improvement in magnetic quality is obtained well before sintering occurs, so that the point at which firing is terminated in order to prevent the formation of a hard coherent sintered body is less critical than with barium ferrite. Although lead ferrite is toxic and is volatile at high temperatures, it can be used in the practice of this invention at temperatures such that volatility is not too serious a problem.
Suitable lead ferrite powders for the practice of this invention are available commercially.
For lead, barium, and strontium ferrites domain size is of the order of about 0.5 to 1 micron, and this size range is optimal. However, particles which are in the size range of about 0.3 to 10 microns are suitable for practice of the invention without too serious a loss of quality.
The fine ferrite powder is dispersed in a low viscosity liquid, for example, in acetone, in the approximate ratio of 1 cc. acetone to each gram ferrite. Stearic acid in the amount of (ll-1.0% based on the Weight of the ferrite is added to the dispersed powder, and acts as a lubricant during the aligning operation and adds sufficient coherency to the aligned compacted particle mass so that it can be handled with less danger of crumbling prior to firing. It should be noted that the use of stearic acid or any binder at this point is not critical; it may be omitted altogether, or an equivalent heat-fugitive substance may be used, which will burn-off in the subsequent firing.
The dispersion is introduced into the cavity of a die having movable punches which serve as the pole pieces of an electromagnet. The pole pieces are slightly smaller than the cavity, so as to provide a minimum clearance, e.g., of about .0005 to .001 inch in a /2 diameter. An aligning field is applied between the punches to bring the anisotropic particles into alignment .so that their preferred axes are parallel. With the aligning field maintained, the punches are moved together and the dispersant liquid is thereby expelled from the die through the clearance between the punches and the walls of the die cavity. With an initial field strength of about 2300 oersteds and a compression ratio of 6:1, the field increases up to a raximum value of about 14,000 oerstcds; a field of at least about 10,000l2,000 oersteds is preferred to obtain good alignment. Alignment is not lost as the punches are brought together, nor is there a detrimental carryout of the magnetic particles, which largely remain in the die cavity. The powder is ultimately compacted under a pressure of about 5,000 p.s.i. The pressure is not critical in respect to mechanical properties but the resulting increase in density is advantageous. The compacting pressure may be in the general range of about 6,00070,000 p.s.i. Stratification may occur under high pressure, but is no serious problem since the binder that is subsequently introduced after firing will cohere the strata.
The stearic acid binder gives the compacted aligned particle mass sufiicient strength to withstand the handling necessary to firing, though no more; as previously suggested, a greater proportion of binder may be used to further strengthen the mass, although this may be more difiicult to remove in the burn-02f step.
The specimen is carefully removed from the die and is then subjected to hinder burn-oil and firing. In this step it is heated gradually in air, from room temperature to a peak temperature of about 1382 F. over a period of about 4 hours. The fired mass is then cooled as rapidly as possible without causing shattering by thermal shock. During the initial or low temperature phase of the firing operation the binder burns oil.
The firing conditions indicated are below minimum sintering conditions for this material. Firing at these conditions imparts what can be called green strength to the particle mass, but the mass will crumble under light pressure or in ordinary handling and upon final impregnation is not too hard to be cut. A hard, highly coherent sintered body is not formed; the friable mass could not be used in practice as it is because of its poor strength, but as will be shown, its magnetic properties are greatly improved relative to the prefired mass, and it can be made durable by the introduction of a binder.
During firing the mass shrinks only slightly, usually barely suficient to permit it to he slipped easily into the forming die. The shrinkage is of the order of a few thousandths inches in each lineal inch, as compared with the much greater shrinkage (typically about 20% lineal shrinkage) that accompanies the production of a hard dense highly coherent sintered body.
The fired but unsintered product produced in this manner is highly porous in comparison with a sintered product, and is impregnated with a binder to impart the desired final strength to it. This may be accomplished for example with a low viscosity epoxy resin, phenol formaldehyde, polyester, acrylic resin, or the like. The magnet is then preferably replaced in the original forming die and is subjected to a coining and curing operation under moderate heat and pressure, e.g., at a temperature up to about 400-500 E, depending on the binder, and at pressure up to 5000 p.s.i. A sintered ferrite product cannot be coined without cracking, whereas the product of this invention will flow somewhat under the pressure of coining and will conform precisely to the shape of the die.
The product produced in the manner of this example is readily machinable in the sense that it can be edge-cut, i.e., it can be cut with an edge-cutting tool. As an illustration of the improvement in energy product, the energy product of the prefired mass is about 1.2 X gaussoersteds, while that of the final product is about 1.5 x 10 an improvement of about 25%.
In this example, the aligned particle mass was fired at time-temperature conditions below sintering conditions, and no significant particle adherence occurred. It is because the ferrite particle adherence resulting from this firing technique is very low that the final bonded product can be machined.
Example 2 Domain size lead ferrite powder is dispersed in acetone in the approximate ratio of 1 cc. acetone to each gram ferrite. The dispersion is introduced into the cavity of a die of the type referred to in Example 1, and is aligned by an initial field of about 2300 oersteds. The powder is ultimately compacted under a pressure of about 50,000
The aligned compacted particulate specimen is impregnated with chlorinated naphthalene, for example of the type sold commercially as Halowax 1014, at a temperature above the melting point of the wax. The binder is then permitted to harden.
In the firing operation, the specimen is heated gradually in air, from room temperature to a peak temperature of 1382 F., over a period of about 4 hours.
The fired product is impregnated with a binder to im part the desired final strength to it by immersion for a period or at least about six hours in a low viscosity epoxy resin. The sample is then replaced in the original die and is subjected to a coining and curing operation under moderate heat and pressure.
A product produced in the manner of this example was readily machinable, and had a residual induction B of 2,480 gauss, a coercive force H of 2,160 oersteds, and a maximum energy product of 1.5 x 10 gauss-oersteds. Its combined apparent density was 4.14 gm./cm.
Example 3 Substantially single domain lead ferrite particles are aligned in a low viscosity liquid in a magnetic field the initial strength of which is 2300' oersteds. The dispersant liquid is driven off, and the powder is consolidated under a pressure of 36,000 p.s.i. Following this the specimen is impregnated with chlorinated naphthalene. The impregnation is conducted with the particle confined in the die to prevent loss of alignment. After impregnation, the specimen is removed from the die and heated gradually from room temperature to 1472 F., over a period of about 4 hours, thereby burning oil the binder and imparting green strength to the still potentially machinable product. The specimen is allowed to cool slowly in the furnace and is impregnated by immersion for a period of about six hours in a low viscosity epoxy resin. The impregnated product is replaced in the original die and is subjected to a coining and curing operation under moderate heat and pressure.
A magnet produced by the method of this example had a residual induction B of 2300 gauss, a coercive force H of 2000 oersteds, and a maximum energy product of 1.2 x 10 The low energy product of this magnet in comparison to that of the magnet produced in accordance with Example 2, is believed to be due to the lower density of the product resulting from the lower compaction pres- .sure. It may be noted that the maximum energy product exhibited by the final product varies as to the nature of the ferrite, the manner in which it is prepared, the nature of the matrix and the method of incorporation thereof.
Example 4 Lead ferrite powder is aligned in a low viscosity liquid in the manner described in Example 2. After the liquid is removed, the powder is consolidated under a pressure of 50,000 p.s.i. It is impregnated with a binder and is fired at gradually increasing temperatures up to 1562" F.
over a period of about 3% hours. After firing the specimen was impregnated with a low viscosity epoxy resin and was subjected to a coining and curing operation. A product produced by the method of this example had a residual induction of 2,480 gauss, a coercive force of 2,140 oersteds and a maximum energy product of 1.44 x 10 The combined apparent density of the material was 4.14 gn1./cm. and the product was readily machinable.
Example 5 Example 6 An aligned lead ferrite powder was consolidated at a pressure of 71,500 p.s.i. The specimen was impregnated with chlorinated naphthalene in the manner described in Example 2. The specimen at this point had a residual induction of 2,400 gauss, a coercive force of 1,270
asaaoeo oersteds and a maximum energy product of'1.33 x The specimen was gradually heated from room temperature to 1472 F. over a period of about 4 hours, and after cooling was impregnated with a low viscosity epoxy resin. After coining and curing the product had a residual induction of 2,680 gauss, a coercive force of 1,570 and a maximum energy product of 1.68 x 10 The combined apparent density of the mixed material was 4.34 gm./ cm. This indicates the magnitude of the improvement in energy product derived from the firing step.
Example 7 Barium ferrite is ground to substantially domain size and is introduced into a non-magnetic die with acetone in the approximate ratio of 1 cc. acetone to each gram ferrite. Stearic acid is added in the amount of 0.5% by weight of ferrite. After positioning the punches of the die, an initial aligning field of 2,300 oersteds is applied. The acetone is removed without disturbing alignment, and the powder is consolidated under pressure of 50,000 p.s.i. The sample is heated gradually from room temperature to 1950 F. over a period of about 6 hours and is held at peak temperature for 10-15 minutes, thus burning oil the binder and imparting green strength to the still potentially machinable product. The specimen is allowed to cool slowly in the furnace, and is impregnated and subjected to a coining and curing operation under moderate heat and pressure.
A product produced in this manner had a maximum energy product of 1.8 x 10 and was machinable.
Example 8 Strontium ferrite is ground to substantially domain size and is introduced into a non-magnetic die with acetone in the approximate ratio of 1 cc. acetone to each gram ferrite. After positioning the punches of the die, an initial aligning field of 2,300 oersteds is applied. The acetone is removed without disturbing alignment, and the powder is consolidated under pressure of 46,000 p.s.i. The product is impregnated in the die with chlorinated naphthalene. After impregnation, the sample is heated gradually from room temperature to 1562 F., thus burning off the binder and imparting green strength to the still potentially machinable product. The specimen is allowed to cool slowly in the furnace, and is impregnated and subjected to a coining and curing operation under moderate heat and pressure.
A product produced in this manner had a residual induction of 2,200 gauss, a coercive force of 1,700 oersteds, and a maximum energy product of 1.12 x 10 The combined apparent density of the material was 3.23 g1n./cm.
In some of the foregoing examples, the binder which was used to cohere the prefired product was stearic acid, and in others chlorinated naphthalene was used. The binder suggested for impregnation into the final product is an epoxy. Those skilled in the art will re:ognize that a wide range of binders can be used for these impregnations within the scope of this invention. The nature of the final binder is not critical, provided it can be introduced into the particle mass without adverse chemical or disaligning effect. Binders including plastics, waxes, and low melting metal alloys can be used. In adhering the prefired product, if a binder is used it should be one which is heat-fugitive, i.e., which will melt or burn ofi from the aggregate as temperature increases to firing temperature. As mentioned, if the compact is handled carefully it is possible to omit a binder in that step altogether.
From the foregoing, it will be seen that this invention enables a substantially higher energy product to be imparted to fine particle magnets, While at the same time retaining all the desirable physical qualities of the nonsintered type of ferrite magnets.
This invention has been disclosed primarily in relation to magnets incorporating ferrites as the magnetic components, but the process is also of utility with fine particle magnets of other materials, for example with particulate Alnico magnetic materials, which are also ordinarily too hard to cut except by grinding.
Having described my invention, what I claim is:
1. The method of making an edge-cuttablc fine particle permanent magnet comprising,
forming substantially domain size anisotropic particles of a permanent magnet substance into a compacted body in which the particles are in magnetic alignment,
firing the body so compacted, without disturbing said alignment of the particles at conditions of time and temperature less than but approaching sintering conditions for the body, said conditions of time and temperature being such that any organic binder present in said body is burned off and further such that the body is converted to a friable, relatively porous body of improved energy product relative to its initial value and having a density which exceeds its initial density by up to about l5-20%, and which is not too hard to be edge cut,
cooling the fired body,
and impregnating the fired body with a binder Without disturbing particle alignment, thereby cohering the particles into a durable coherent shape.
2. The method of claim 1 wherein the magnetic substance is selected from the class consisting of the ferrites of lead, barium, and strontium.
3. The method of claim 1 wherein a small proportion of a heat fugitive organic binder is employed to impart temporary coherency to the compacted body prior to said firing and wherein the binder is burned off gradually during said firing.
4. The method of claim 3 wherein said binder comprises stearic acid in the amount of about 0.1-1.0% of the Weight of the magnetic particles.
5. The method of claim 1 wherein said body is coined after firing in a die to conform it to the shape of said die.
6. The method of making an edge-cuttable fine particle permanent magnet comprising,
forming substantially domain size anisotropic particles of a permanent magnet substance selected from the class consisting of the ferr-ites of lead, barium, and strontium into a particulate mass in which the particles are in alignment,
compacting the mass in a die under pressure of the order of 6,00070,000 p.s.i., not more than about 1% by weight binder being present in said mass,
firing the mass so compacted without disturbing said alignment of said particles at conditions of time and temperature less than but approaching sintering conditions, said conditions of time and temperature being such that said mass is converted to an unbonded iriable, relatively porous body of improved energy product relative to its initial value and having a density which exceeds its initial density by up to about 15-20%, and which is not too hard to be edge cut,
cooling the tired body,
and impregnating the fired body with a hardenable, low
viscosity binder without disturbing particle alignment to bond the particles into a durable coherent shape which is edge cuttable.
7. The method of making an edge-cuttable fine particle permanent magnet comprising,
forming substantially domain size anisotropic particles of a permanent magnet substance selected from the class consisting of the ferrites of lead, barium, and strontium into a particulate mass in which the particles are in alignment,
compacting the mass in a die under pressure of the order of 6,00070,000 p.s.i., not more than about 1% by weight binder being present in said mass,
9 10 firing the compacted mass without disturbing the align- 9. The method of claim 8 wherein the firing temperament thereof at conditions of time and temperature ture is gradually increased to said peak temperature over less than but approaching sintering conditions, said a period of about 4 hours. conditions of time and temperature being such that 10. The method of making an edge-cuttable fine parsaid mass is converted to a friable, relatively porous 5 ticle permanent magnet comprising, body of improved energy product relative to its iniforming substantially domain size anisotropic particles tial value and having a density whicn exceeds its of barium ferrite into a compacted body in which initial density by up to about 15-20%, and which the particles are in magnetic alignment, is not too hard to be edge cut, firing the compacted body at a peak temperature of the cooling the fired body, order of about 1900-2200 *F. for a period sufiicient impregnating the fired body with a hardenable, low to increase the energy product of said body without viscosity binder without disturbing particle alignment increasing the density of said body by more than to bond the particles into a durable coherent shape about -20% and without converting said body to which is edge cuttable, a highly coherent sintered body which is too hard and coining the bonded shape in a die having a shape 15 to be edge cut,
approximating that of the first mentioned die, at cooling the fired body, moderate heat and pressure up to about 400 F. and and impregnating the fired body with a binder without 500 p.s.i. disturbing particle alignment, to bond the particles 8. The method of making an edge-cuttable fine parinto adurable coherent shape. ticle permanent magnet comprising,
forming substantially domain size anisotropic particles References Cited y the Examiner of lead ferrite into a compacted body in which the UNITED STATES PATENTS particles are in magnetic alignment, firing the compacted body without disturbing the aligng 264-333 i I ume 264-24 ment thereof, said firing being done at a peak tem- 25 965 953 12/1960 B I I a aermann 264-24 XR perature 1n the range of about 1350-1650 F. for a period sufficient to increase the energy product 2984971 5/1961 Venerus 264-24 3,024,392 3/1962 Baermann 317-201 of said body without increasing the density of sa1d 3 085 291 4/1963 H l 19 16 5 bod more tha about 1520% and without convertat a y n 3 115 461 12/ 1963 Dams 252-625 mg said body to a highly coherent sintered body whlch 15 too d to be edge ROBERT F. WHITE, Primary Examiner.
and impregnating the fired body with a binder Without disturbing particle alignment, to bond the particles into a durable coherent shape.
ALEXANDER H. BRODMERKEL, Examiner.
J. A. FINLAY'SON, Assistant Examiner.