Abrasive Jet Cutting Medium

1. Technical Field

The present invention relates to the field of jet stream cutting, and particularly to abrasive jet stream cutting, wherein a suspension of abrasive particles in a fluid medium is pumped under high pressure and at high velocity against the surface of a workpiece to effect cutting operations. Such operations are widely employed in cutting of metal sheet and plate in fabrication of useful articles.

2. Prior Art

Abrasive water jets have grown to be widely employed in cutting and machining operations, particularly with metal sheet and plates to effect rapid and economical cutting and related forming operations. Typical applications have been the cutting materials which are difficult to machine, such as stainless steels, titanium, nickel alloys, reinforced polymer composites, ceramics, glass, rock and the like. Such techniques are particularly advantageous to produce cutting action through very highly localized action at low average applied force, to effect cutting of such materials with minimal thermal stress or deformation, without the disruption of crystalline structure, and without delamination of composite materials.

To effect abrasive water jet cutting, a specialized nozzle assembly is employed to direct a coherent collimated high pressure stream through a small diameter orifice to form a jet. Typically, pressures of about 200 MPa (about 30,000 psi) and higher are applied to force the media through the nozzle orifice.

Typical nozzle assemblies are formed of abrasion resistant materials, such as tungsten carbide or Boride. The orifice itself may be formed of diamond or sapphire. Abrasive particles are added to the high speed stream of water exiting the nozzle orifice by directing the water stream through a "mixing tube" and introducing abrasive particles into the tube in the region between the exit of the stream from the orifice and its entry into the "mixing tube." The mixing tube, which is typically several inches in length, is a region of contained, extremely turbulent flow in which the relatively stationary or slow moving abrasive particles are accelerated and become entrained in the high speed water stream, which may have nozzle exit velocities as high as Mach 3. The entrainment process tends to disperse and decelerate the water stream while the abrasive particles collide with the tube wall and with each other.

Relatively wide kerfs result from the dispersed stream, energy is wasted, and the tube is rapidly worn, even when made from abrasion resistant materials, such as tungsten carbide or Boride and the like. Some studies have shown that as much as 70% of the abrasive particles are fractured before they reach the workpiece to be cut.

In recent developments, water jets without abrasives have been thickened with water soluble polymers, which aid in obtaining and maintaining coherent jet streams, in reducing the level of misting, splashing and the like. Somewhat narrower kerfs can be achieved. Operating pressures and velocities remain quite high.

It is also known to suspend particulate abrasives in water jets, ordinarily relying on the thickening effect of the water soluble polymers to act as a suspending agent in the system. The abrasive cuts with greater efficiency than the water alone or the water with a thickening agent, but introduces an entire new spectrum of difficulties.

Because of the high pressures and flow rates involved in jet stream machining, it is quite difficult to maintain coherent streams of the jet. While the use of thickening agents provides important improvements, such operations tend to be expensive, as neither the water not the soluble polymer is reusable, because the high shear inherent in such operations degrades the polymer; the degraded polymer remains dissolved in the water, providing waste disposal expense.

When abrasive is added to the system, for abrasive jet stream cutting and milling, the difficulties and expense are even greater.

Nozzles employed for abrasive water jet cutting operations are more complex and require ancillary equipment to add the abrasive to the stream, normally immediately adjacent the nozzle assembly or as a part of such a nozzle. The assembly includes a mixing chamber where the abrasive is introduced into the medium, a focusing tube where the stream is accelerated, and a small orifice where the flow is collimated into a coherent jet stream projected at the workpiece.

The mixing chamber and its associated hardware are complex, required by the necessity of injecting the abrasive particles into the relatively high speed stream. The mixing chamber is required to inject the particles into the interior of the flowing stream as much as possible to minimize the extent to which the interior surfaces of the mixing chamber and orifice are abraded. Because the components have widely different densities, it has generally not been possible to premix the components prior to the nozzle assembly because, even in thickened fluids, the abrasive particles tend to separate and settle at an appreciable rate. Additional thickening is not cost effective in such systems.

Uniform dispersion of the abrasive into the stream has proved elusive and inconsistent, largely attributable to the broad differences in density of the materials, the high velocity differences between the injected particles and the fast flowing stream, and the resulting need for the stream to accelerate the abrasive particles. The mixing of the particles into the medium is often incomplete and inconsistent, the acceleration requirements of the abrasive slows the flow of the medium, and the incomplete mixing introduces inconsistencies and inhomogeneities which cause divergent flows and differing trajectories of the stream or its components exiting the orifice, producing inconsistent and/or increased kerf widths and imprecise and non-uniform cut edges on the workpiece.

The mixing process causes the abrasive to produce high rates of wear in the interior of the nozzle elements, which have, as a result, a useful life measured in hours of operation under favorable conditions, and less favorable conditions can reduce nozzle and orifice life to a matter of minutes. For example, precise alignment of the nozzle and focusing tube are quite critical.

The entrainment of the particles also tend to make the jet stream divergent rather than coherent, resulting in wide kerfs and extra time and effort in the cutting operation.

When the jet stream into which the abrasive is introduced is adequately thickened, shear degradation precludes reuse of the medium, and the cost is substantial. Considerable amounts of the polymer are required to achieve adequate thickening to effectively suspend the abrasives commonly employed.

Water jet stream nozzle orifices are typically on the order of about 0.25 mm (about 0.010 in.). When an abrasive is introduced, the minimum practical mixing tube is about three times the orifice diameter, i.e., about 0.75 mm (about 0.030 in) or greater. Smaller nozzles have intolerably short service life from excessive erosion during operations. The wider nozzle produces a wider stream and cutting kerf, and requires more medium and abrasive consumption per unit of cutting.

Hollinger, et al., "Precision Cutting With a Low Pressure, Coherent Abrasive Suspension Jet," 5th American Water Jet Conference, Toronto, Canada, Aug. 29-31, 1989, have reported improved dispersions of abrasives in aqueous solutions of methyl cellulose and a proprietary thickening agent "Super Water" (trademark of Berkely Chemical Co.). Their work was based on attaining sufficient viscosities, based on the use of 1.5 to 2 weight percent of the thickeners to permit premixing of the abrasive into the polymer solutions, eliminating the need for injection of the abrasive at the nozzle. Hollinger, et al., reported that orifices as small as 0.254 mm (0.01 in.) could be effectively employed.

The work of Hollinger et al. has subsequently been enbodied in U.S. Pat. No. 5,184,434, issued Feb. 9, 1993, on an application filed Aug. 29, 1990. Crosslinking of the polymers employed is not contemplated.

See also Howells, "Polymerblasting with Super-Water from 1974 to 1989: a Review", Int'l. J. Water Jet Technol., Vol. 1, No. 1, March, 1990, 16 pp. Howells is particularly informative concerning the reasons why polymer jet stream media, with or without abrasives, has not been recycled and reused. See Pages 8 and 9.

In many contexts, the water or aqueous based systems employed in the prior art may not be used with some materials or particular workpieces where the presence of water or the corrosion it may produce cannot be tolerated. Jet stream cutting has not been applicable to such circumstances.

In all the polymer based thickened systems of the prior art, the degradation of the polymer chains by the high applied shear rates in the system has, to date, precluded effective techniques to recover and reuse the jet stream medium, resulting in substantial waste handling requirements and considerable expense for the polymer and abrasive consumed.

It is an object of the present invention to provide a jet stream cutting and machining medium which overcomes the problems encountered in the prior art.

In particular, it is an object of the present invention to provide reusable polymer thickened jet stream premixed media which effectively suspend abrasive particles, form coherent and stable jet streams, cut with high efficiency and narrow kerfs, and which are reusable, and thereby reduce waste handling requirements and raw material costs.

A further object of the present invention is to employ jet stream cutting at lower pressures and flow volumes required in the prior art.

Another object of the invention is to permit the employment of smaller diameter orifices for abrasive jet streamcutting and milling than have been effective in the prior art.

Another object is to permit abrasive jet stream cutting using a simplified nozzle, considerably smaller and particularly shorter than those heretofore required for conventional abrasive water jet machining and cutting.

Still another object is the provision of a low cost jet stream cutting system, based on the recirculation and reuse of the thickened jet stream medium.

In one embodiment of the present invention, it is an object of the present invention to provide non-aqueous jet stream media which permits the use of jet stream cutting and machining operations with materials and workpieces not previously usable with jet stream cutting operations.

These and still other objects, which will become apparent from the following disclosure, are attained by forming a jet stream medium of a polymer having reformable, sacrificial chemical bonds, preferentially disrupted and broken during processing and cutting under high shear conditions, and which then reform to reconstitute the medium in a form suitable for recirculation in the process and reuse.

In one embodiment of the invention, the water jet stream is thickened with an ionically cross-linked water soluble polymer, wherein the ionic cross-links are formed by salts of metals of Groups III to VIII of the Periodic Table.

In a second embodiment, the aqueous jet is formed of a hydrogel of a water soluble polymer, preferably cross-linked with a gel-promoting water soluble salt of a metal of Groups II to VIII of the Periodic Table. Cross-linking in such systems is based on intermolecular bonds, i.e., hydrogen bonding, between polymer molecules.

In a third embodiment, a non-aqueous medium is formed of an intermolecular bond cross-linked polymer which itself forms predominant constituent of the jet stream. In operation, the polymer suspends the abrasive particles. The polymer may be partially broken down under the shear forces of the operation by disruption of the intermolecular bonds which produce the cross-links of the polymer. After the jet performs its work on the workpiece, the polymer is collected, the cross-linking bonds are allowed to reform, and the medium is recycled for reuse in the process.

Smaller orifice diameters, on the order of as little as about 0.1 mm (about 0.004 in.) can be effectively employed if the particle diameter of the abrasive is sufficiently small.

FIG. 1 is a schematic cross-section view of an embodiment of this invention providing recirculated media for reuse;

FIG. 2 is a cross-section view of a preferred form of nozzle according to the present invention;

The present invention is fundamentally grounded on the observation that the shear stresses imposed in the formation and use of polymer containing jet streams employed for jet stream cutting operations are necessarily high. While a number of steps can be taken to minimize the applied shear stresses in the nozzle assembly, the impact forces of the jet stream on the surface of the workpiece are also high and also break down the polymer structure. Since high shear is an inherent feature of the cutting operation, techniques for reducing polymer breakdown are, at a certain point, incompatible with the requirements of the cutting operation itself, and are thus limited.

The inclusion of one and one-half or two weight percent of thickener or polymer material in the jet stream medium typically employed in the prior art is thus a very substantial proportion of the cost of the operation. The time and energy requirements for dissolving the polymer into the aqueous medium is also a substantial factor in operating costs and can, if not adequately planned, impose substantial delays in operations because of the significant time required to dissolve such polymers. If not consistently controlled, variations in the solution may introduce a lack of uniformity in cutting and machining operations and impair the quality of the result.

After use, the degraded polymer solution is a substantial collection and handling burden on the operation, and there are no known uses for the resulting waste material. Handling and disposal costs are typically a significant cost of operations.

In that context, the employment of more complex and more expensive polymers to afford certain specific benefits to the operation are generally offset by the added costs.

The degradation of the polymers in jet stream cutting systems is produced by the breaking of the chemical bonds which make up the polymer, and particularly the chemical bonds which form the polymer chain backbone. The result of such effects is to reduce the molecular weight of the polymer, and a consequent reduction of the viscosity and loss of the capacity of the medium to effectively suspend the abrasive particles, to form a coherent jet stream, and to limit abrasive erosion of the equipment.

In the present invention, these difficulties are overcome by the employment of polymer materials which have the capacity to reform chemical bonds broken during the jet stream cutting operation, and can thus be reconstituted in a fully effective form to permit recycling and reuse. Thus, while the chemical bonds will be broken during the cutting operations, under the influence of high shear in the nozzle and by the impact on the workpiece, such effects are no longer destructive to the usefulness of the jet stream medium.

In practice, the polymers employed in the present invention may be recycled through the operation for many operating cycles. In time, there will be a more severe degradation of the chains of the polymer backbone (normally covalent bonds) which will limit the number of cycles. Generally, the preferred polymers of the present invention may be cycled through the system for twenty to one hunder cycles or more before replacement is required.

The reformation of the broken bonds to reform the polymer thickener in useful form requires that the polymer contain bonds which are sacrificed under the high shear and high impact conditions of the cutting operation, and which will reform to reconstitute the original polymer structure. This requires that the polymer contain an adequate population of chemical bonds other than covalent bonds. When covalent bonds are broken, the fragments are so highly reactive that the broken chains are normally terminated by very nearly instantaneous chain terminating reactions, and the original bonds cannot be reformed.

There are three types of chemical bonds which have thus far been evaluated in the present invention, and which have proven effective. These are ionic bonds, intermolecular hydrogen bonds and intermolecular B:O bonds.

Ionic bonds are frequently employed in ionic cross-linking of a variety of polymers. Such polymers are often water soluble types well suited to use in the present invention. When such polymers are ionically cross-linked, they typically form water swollen gels, having effective viscosity levels to effect highly durable suspensions of the high density abrasive particles to be added in the procedure of the present invention.

In ionically cross-linked hydrogels, the ionic bonds are weaker than the covalent bonds of the polymer backbone, and it is the ionic bonds which are preferentially disrupted and broken upon exposure to high shear stresses. The ionic species produced when the bonds are broken are relatively stable, and in the context of the polymer systems employed herein will react only to reestablish the broken cross-links, and thus reestablish the high viscosity hydrogel structure once the high shear stress is removed.

In an alternate embodiment, gel-forming water soluble polymers are formed into hydrogels, with or without gelation promoters such as water soluble salts of metals of Groups III to VIII of the Periodic Table. Hydrogels are based on the formation of intermolecular bonds, i.e., hydrogen bonds, between the polymer molecules. Such bonds are weaker than ionic bonds and, in the context of the the present invention, facilitate thinning of the medium under the high shear stresses imposed in the formation of the cutting jet and providing the sacrificial bonds which protect the covalent bonds of the polymer and minimize chain scission. These hydrogels also serve to promote high viscosity at rest, whether the intermolecular bonds are formed in makeup of the gel or reformed after use, which is highly desirable in preventing settling out of the abrasive particles.

While a number of water soluble polymers have been employed in formulating jet stream cutting formulations, including some gel-forming polymers, they have been employed without gelation promoters and at concentrations at which spontaneous gelation does not occur. The addition of such polymers in the prior art has focused mainly on increasing coherence of the jet. Without the formation of a substantial population of sacrificial bonds, the polymer is significantly degraded in a single use and is not reusable. The jet formulations of the prior art are normally discarded as waste.

Non-aqueous polymer formulations are also possible where the polymer is cross-linked by other types of sacrificial intermolecular bonds. Such formulations are particularly significant to cutting and machining materials which are vulnerable to water, such as ferrous metals and the like.

A preferred non-aqueous polymer, cross-linked by intermolecular bonds, is the family of polyborosiloxanes. These polymers are cross-linked by electron pair sharing between tertiary B atoms in the polymer chain with O atoms in the chain of adjacent polymer molecules. The specific properties of significance to the present invention may be very directly and finely controlled, including molecular weight of the polyborosiloxane and the like.

The formulation of cutting media based on the use of polyborosiloxanes, as described in greater detail below, is particularly preferred in the present invention because of the non-aqueous nature of the media, the close degree of control of viscosity, and the ability to balance rest viscosity and high shear reduced viscosity to suit the requirements of the cutting and machining operations to be performed.

Intermolecular bonds, whether based on hydrogen bonding or on B:O bonds, are also weaker than covalent bonds, and polymers are employed which readily form intermolecular bonds, particularly in non-aqueous jet stream processing in the present invention. Under the high shear operations involved in the production of the jet stream and under the forces of impact on workpiece surfaces, the intermolecular bonds will be broken preferentially, absorbing a portion of the energy imposed on the polymer, and preserving the covalent bonds which make up the polymer backbone.

These intermolecular bonds will readily reform over time once the high shear stress is removed, restoring the cross-linked structure and the gel-like high viscosity required of the system.

In the context of the present invention, the cross-linking bonds, i.e., ionic or intermolecular bonds, are those first broken under the high shear and high impact conditions of the operation, and thus sacrifice themselves to absorb the energy applied. They are, in that sense sacrificial bonds which serve to protect the covalent bonds from the degradation that would otherwise disrupt the polymer chains in permanent, irreversible fashion characteristic of the polymer degradation of the prior art materials and procedures.

The disrupted bonds will reform spontaneously when the shear stresses are removed, e.g., when the medium is allowed to stand. The basis for the ionic bonds remains intact, as it is the ionic species characteristic of the formation of such bonds in the original polymer medium which is produced by the breaking of the bonds during operation of the jet stream cutting process. Such bonds are reversibly formed in the first instance, and exist in an equilibrium state in aqueous media in any case. The rate of reformation of the bonds is predominantly dictated by the mobility of the polymer chains in the used and degraded medium. At the reduced viscosity of the medium under such conditions, mobility is relatively substantial, and the gel will typically reform within a few minutes of collection. It is accordingly desirable to provide for mixing of the collected polymer solution and abrasive to assure the substantially homogeneous dispersion of the abrasive particles in the hydrogel, although it is also possible to re-disperse the abrasive into the reformed gel after the ionic bonds are fully restored.

The thinning of the polymer component in response to high applied shear is itself of benefit to the abrasive jet stream formation, as the formulation will show reduced viscosity in the jet stream so that the applied energy is imparted in higher proportion to the abrasive particles, enhancing their cutting effectiveness. The polymer acts to produce a highly coherent jet stream and serves to minimize abrasion within the equipment.

It is the specific viscosity parameters and changes which permit simplification of the equipment requirements, relative to prior art abrasive water jet stream techniques. Because the entrainment of the abrasive in the medium occurs at make-up in the usual mixing equipment employed, there is no need to provide a separate supply of the abrasive to the nozzle, to feed the abrasive particles into the stream, or to provide a mixing tube, all of which are normally required in the prior art.

Disrupted intermolecular bonds will spontaneously and rapidly reform, and re-dispersion of the abrasive is rather simple to effect, if required at all.

As the polymer systems are recycled through the jet stream cutting process and the reformation of the disrupted chemical bonds, there will be some disruption of covalent bonds on each cycle. Although the proportion of irreversibly disrupted bonds in each individual cycle will be modest, the effect is cumulative, and after a substantial number of cycles, the permanent degradation of the polymer will become significant. As the polymer is cumulatively and irreversibly degraded, the viscosity of the reformed polymer will gradually decline, and the medium will eventually begin to exhibit an undesirable degree of tackiness.

In the efforts to date, the polymer thickeners employed in the water jet stream cutting operations of the present invention may be successfully recycled for up to as many as one hundred use cycles before replacement is required. The non-aqueous media of the present invention are at least as durable, and often far more durable than the aqueous systems. The number of cycles will vary, of course, with the particular polymer, the process conditions, and the like, but it is readily apparent that the medium of the present invention has contributed a significant degree of recycling compared to the prior art which does not admit of reuse of the medium after a single pass through the orifice. It is generally desirable to periodically or even continuously add small quantities of "fresh" abrasive-polymer mix to maintain the consistency and uniformity of the material during use. Equivalent increments of material are desirably removed to maintain a relatively constant volume of the medium in the equipment.

Ionically cross-linkable polymers suitable for use in the present invention include any of the water soluble polymers which form ionically cross-linked gels with metal salts, metal oxides or metal organic gelation agents of Group II to Group VIII metals. Preferred species are those water soluble polymers having substantial proportions of hydroxyl groups. The polymers may also contain active ionizable reactive groups, such as carboxyl groups, sulfonic acid groups, amine groups and the like. The ionic cross-linking polymers and cross-linking systems are similar to the hydrogels formed by intermolecular hydrogen bonding, except that the ionic bonds are only formed under conditions which promote the ionization of the cross-linking species. Such conditions may require control of pH, the presence of reaction catalysts or promoters, such as Lewis acids or Lewis bases, and the like. The formation of such ionically cross-linked polymers is generally well known and characterized in the chemical literature, as those of ordinary skill in the art will understand.

A substantial number of hydrogelable polymers and gelling agents are known, and substantially any of those available may be successfully employed in the present invention.

Examples of the preferred hydroxyl group containing water soluble polymers include, but are not limited to, guar gum, xanthan gum, hydroxypropyl and hydroxyethyl derivatives of guar gum and/or xanthan gum, and related hydroxyl group containing or substituted gums, hydroxymethyl cellulose, hydroxyethyl cellulose, and related water soluble cellulose derivatives, hydroxyl-group containing synthetic polymers, such as hydroxyethyl methacrylate, hydroxypropyl methacrylate, and other water soluble polymers, such as polyacrylamide, and the like. Also of interest are hydroxyl group terminated, water soluble species of low molecular weigh polymers and oligomers, such as polyethylene oxide, polyoxymethylene, and the like.

Among the preferred gelling promoters of Group II to Group VIII metals that may be employed are boric acid, sodium borate, and metal organic compounds of titanium, aluminum, chromium, zinc, zirconium, and the like.

A particularly preferred species for the modest cost requirements is a sodium borate gelled solution of about 2 to 2.5 weight percent guar gum in water. This particularly inexpensive hydrogel has demonstrated a capacity to survive up to twelve cycles of jet stream cutting operations at 14 MPa followed by gel reformation with no detectable permanent degradation of the polymer gel.

A preferred non-aqueous intermolecular bond cross-linked polymer is afforded by a composition of polyborosiloxane polymer, a hydrocarbon grease or oil extender or diluent, and plasticizer such as stearic acid or the like, having an effective jet stream viscosity. The polyborosiloxane polymers as a class are strong intermolecular bonding species and, when suitably plasticized to viscosities suitable for jet formation, are an excellent jet stream medium for water sensitive applications. In addition, the polyborosiloxane formulations are generally non-tacky, non-adherent materials which are readily removed from the surface of workpieces after the cutting operation is completed.

The borosiloxane polymers for use in the present invention will generally have molecular weights from about 200,000 up to about 750,000, preferably about 350,000 to about 500,000. The atomic ratio B:Si will preferably be in the range of from about 1:3 up to about 1:100, preferably from about 1:10 up to about 1:50.

The borosiloxanes are highly compatible with a wide variety of fillers, softeners and plasticizers. It is common to employ inert fillers as diluents to reduce materials costs, and to employ suitable plasticizers and softeners to further dilute the polymer and to control viscosity.

In the present invention, the abrasive particles will ordinarily be the sole inert filler, although other fillers may be employed if the amount of abrasive is correspondingly reduced. As noted above, the abrasive (and other filler, if employed) may range from about 5 to about 60 weight percent of the formulation, while about 25 to 40 weight percent is generally preferred.

Plasticizers and softening diluents are employed to regulate the viscosity of the abrasive jet medium. Suitable plasticizers for use in silicone polymers are quite numerous and well known in the art and the selection of suitable viscosity controls is not narrowly significant to the present invention. Suitable materials include, by way of example and not by limitation, fatty acids of from about 8 to 30 carbon atoms, particularly about 12 to 20 carbon atoms, such as palmatic acid, stearic acid and oleic acid; hydrocarbon paraffin oils, particularly light oils such as "top oil" and other petroleum distillates and by-products; vegetable oils and partially or fully hydrogenated vegetable oils such as rapeseed oil, safflower oil, soybean oil and the like; hydrocarbon-based greases such as automotive lubricating greases and the like; mono-, din, and tri-esters of polyfunctional carboxyllic acids such as dioctyl phthalate (DOP) and the like. Liquid or semisolid silicone oils may also be employed, and may confer considerable benefits, despite their cost, when the medium will be subjected to high temperatures and/or oxidizing conditions which may degrade hydrocarbon based plasticizers and diluents.

As mentioned, the plasticizers and softening diluents are added to control viscosity of the formulation. A standing or rest viscosity of typically about 300,000 cps at ambient conditions, as measured by a Brookfield viscosimeter is suitable and convenient. As is well known, borosiloxane polymers exhibit a substantial apparent increase in viscosity in response to applied shear, and even exhibit plug flow through configured flow paths at high shear. While there is no available technique for direct measurement of viscosity in the nozzle of the present invention, we have found that formulations with standing viscosities of from about 200,000 cps to about 500,000 cps are generally suitable and a viscosity of about 300,000 is quite reliable. We have calculated effective viscosity as a function of the applied pressure and resulting jet stream volumes and believe the effective specific viscosity at the nozzle is on the order of about 5,000 poise to about 20,000 poise.

When the jet stream material is collected and allowed stand, the viscosity rapidly returns to substantially the original standing viscosity, typically within five minutes or less, often within one minute. We believe that the return to the original viscosity demonstrates the reformation of the intramolecular B:Si bonds and the relatively insignificant level of chain scission.

While there will be some degradation over a number of use cycles, the level does not become significant until, typically, 20 or more cycles, and may not be notable until 100 cycles or more of use have occurred. The long-term degradation is readily offset by the periodic or continuous addition of fresh, unused media and withdrawal of an equivalent amount of spent media. Such a procedure also serves to replace worn abrasive particles with new, sharp particles, and to limit the accumulation of cutting or machining debris in the medium.

In the present invention, injection of the abrasive at the nozzle is not preferred, and is generally not desired. It is preferred that the abrasive particles be mixed into the gelled polymer in a separate, prior operation, and pumped by a suitable high pressure pump to the nozzle.

In the aqueous hydrogel systems, the polymer and its gelling agent will typically be on the order of from about 1 to about 20 weight percent of the medium, most often about 2 to 5 percent, and typically, for most polymers, about 2 to 3 percent. The exact proportions can be optimized for any particular gel in relation to the particular abrasive, its particle size and density, and the proportion to be added.

The abrasive will most often have a particle size of from as low as about 2 micrometers up to about 1400-1600 micrometers (about 16 mesh). More commonly, the abrasive grain size will be in the range of from about 20 to about 200 micrometers, preferably from about 20 to about 80 micrometers.

The jet stream medium may contain from about 1 to about 75 weight percent abrasive. More often, about 5 to about 50 weight percent, and preferably about 15 to about 30 weight percent is preferred.

In operation, the formulations are employed in a fashion which differs in a number of respects from jet stream cutting as practiced in the prior art and as familiar to those of ordinary skill in the art.

In the context of the present invention, the polymer formulation is sensitive to viscosity in two distinct regimes. First and foremost, the poller must afford sufficient viscosity to effectively suspend the abrasive particles in the formulation, under low shear conditions, a parameter most closely defined by static viscosity. In addition, the formation of the jet stream, under high shear conditions, can substantially affect the coherence of the jet and the homogeneity of the abrasive particle dispersion in the jet. These parameters are defined by dynamic viscosity.

Although polymer solutions are non-Newtonian, they exhibit fluid behavior which approximates Newtonian fluids under static conditions. In addition, Newtonian fluid flow characteristics again predominate at high shear conditions.

The time for a spherical particle to settle through a given height under the force of gravity in a static fluid requires a particular time. Thus, from fluid mechanics, ##EQU1## where: t=Time

.eta.=Viscosity of the Fluid

H=Settling Height

a=Particle Diameter

D.sub.p =Density of the Particle

D.sub.L =Density of the Fluid

g=Acceleration of Gravity

We have observed that the following assumptions, on which the foregoing formula is dependent, are sufficiently valid for the purposes of the present invention:

Laminar Flow: At very low velocities, characteristic of the settling of abrasive particles, flow characteristics are laminar or very nearly so.

Newtonian Fluid: Under nearly static conditions involved in particle settling, the polymer formulations are sufficiently fluid in character that substantially Newtonian flow characteristics are exhibited.

Spherical Particle Shape: The irregular shape of abrasive particles introduces some error, but because the average particles do not vary widely in their major and minor dimensions, and because over a substantial number of particles these variations tend to average out, the variation can be safely ignored in the present context.

Formulations suitable for use in the present invention will have low shear rate viscosity (Brookfield) on the order of about 200,000 to 500,000, preferably about 300,000 centipoise (cp). A 320 mesh SiC particle with a specific gravity of 3 will give a settling rate of 6.8.times.10.sup.6 seconds per inch (approximately eleven weeks, and suitable for the present invention).

At higher shear rates, the behavior of polymer formulations becomes non-Newtonian, where viscosity is dependent on shear rate, in a Power Law relationship. This dependence holds until at a high shear rate, when viscosity again becomes substantially independent of applied shear, and substantially Newtonian flow characteristics again apply.

One of the particular virtues of the jet stream formulations of the present invention is the reduction in pressure required in the formation of the jet to produce effective cutting effects. Typically, the pressures required will be on the order of about 14 to about 80 MPa (about 2,000 to about 12,000 psi), compared to pressures of typically at least 200 MPa (30,000 psi) and higher in the prior art.

As a convention, the pressure employed is measured as the pressure drop across the jet forming nozzle. As those of ordinary skill in the art will readily recognize, pressures of up to 80 MPa do not require the complex, expensive, and attention demanding equipment employed at pressures of 200 MPa and higher typically required in the prior art. Thus practice of the present invention does not require the employment of pressure compensated hydraulic pumps, high pressure intensifiers, and even accumulators can be dispensed with or at the minimum greatly simplified. The present invention can be practiced with readily available and inexpensive conventional positive displacement pumps, such as piston pumps, which may be hydraulically driven or the like at the pressures required.

At the nozzle orifice diameters effective in the present invention, nozzle velocities will range from about 75 to about 610 meters per second (about 250 to about 2,000 ft per second), preferably from about 150 to about 460 meters per second (about 500 to about 1,500 ft per second), which has proven to be fully effective in the practice of the present invention.

Selection of the abrasive material is not critical in the present invention, and any of the commonly employed materials will be effective. Examples of suitable materials include, for illustration, alumina, silica, garnet, tungsten carbide, silicon carbide, and the like. The reuse of the cutting medium permits economic use of harder, but more expensive abrasives, with resulting enhancements in the efficiency of cutting and machining operations. For example, silicon carbide may be substituted in cutting operations where garnet has been used for cost containment reasons.

In general, the abrasive will desirably be employed at concentrations in the formulation at levels of from about 5 to about 60 weight percent, preferably about 25 to about 40 weight percent. We have found that operation at the preferred range, and higher in some cases, is quite effective, and is generally substantially higher than the concentrations conventionally employed in abrasive water jet stream cutting.

As noted above, the abrasive particles can range from 2 to 2,000 micrometers in their major dimension (diameter), preferably from about 20 to 200 micrometers for cuts where a fine surface finish is desired, particle sizes of from about 20 to about 100 micrometers are particularly advantageous. It will generally be appropriate to employ the largest particle size consistent with the diameter of the jet forming orifice to be employed, in which case it is preferred that the particle diameter or major dimension not exceed about 20% and preferably not exceed about 10% of the orifice diameter.

If the particle size is larger, there is a risk that "bridging" across the orifice will occur, plugging the flow through the nozzle, which is self-evidently undesirable. At particle sizes of less than 20%, bridging seldom occurs, and at less than 10% such effects are very rare. The nozzle diameter is generally determined by other parameters.

In particular, the diameter of the nozzle orifice is determined by the following parameters:

First and foremost, the wider the orifice, the wider the jet stream and, consequently, the kerf. The accuracy of the cut will generally vary as the inverse of the orifice diameter. In cutting thin materials generally, the smaller the orifice, the better the accuracy and detail possible, subject to other parameters. Less cutting medium is used per unit length of cut.

Second, the wider the orifice, the greater the mass flow of the jet stream, and consequently the greater the rate of cutting. Thus, the wider the orifice, the better the cutting rate, subject to other considerations. More cutting medium is used in relation to the length of the cut.

Balancing of these two conflicting considerations will ordinarily override other parameters which may influence the orifice diameter.

In the present invention, nozzle diameters of from about 0.1 to about 1 millimeter (about 0.004 to about 0.04 inches) may be effectively employed, but it is generally preferred to employ diameters from about 0.2 to about 0.5 millimeters (about 0.008 to about 0.020 inches).

The orifice may be formed from hard metal alloys, hard facing materials such as tungsten or silicon carbide, ceramic formulations, or crystalline materials such as sapphire or diamond. The selection of suitable materials will ordinarily be determined by the hardness of the selected abrasive and the cost of the nozzle material. Diamond is preferred.

The standoff distance, i.e., the distance between the nozzle and the workpiece surface, has proven to be an important factor in the quality of the cut, but is not nearly so important as in abrasive water jet cutting, Although cut quality, particularly the kerf width and shape, will be affected significantly by standoff up to about 2.5 cm (about 1 in.), the present invention is capable of cutting at standoff distances of up to about 25 to about 30 cm (about 10 to about 12 inches). Although abrasive water jet cutting can be employed with materials as much as 12 inches thick, such techniques have generally required a "free air" standoff distance of no more than about 2.5 cm (about 1 in.).

Jet stream cutting in accordance with the present invention can be employed to cut any of the materials for which such techniques have heretofore been employed. Notably, particularly materials which are difficult to machine, including many metals and alloys, such as stainless steels, nickel alloys, titanium, ceramics and glasses, rock materials, such as marble, granite and the like, and polymer composites, and particularly fiber reinforced polymer laminates are all effectively cut with considerable precision by the present techniques.

Among the benefits of the present invention, achieved using gel-thickened polymer media with abrasive material in suspension is the ability of the present invention to provide premixed suspensions of fine abrasive particle sizes not previously used. Abrasive particle sizes finer than about 200 micrometers, and particularly less than about 100 micrometers, for example, are not previously preferred. Use of such fine abrasive particles in conventional abrasive hydrodynamic jet stream cutting and machining tended to result in abrasive material clogging at angles, loops and sags in abrasive material feed lines, and such fine abrasive materials are also more difficult to introduce into jet streams in a conventional mixing chamber or mixing tube. Because of these difficulties, such small particle sizes have largely been avoided in the practice of abrasive jet stream cutting and machining.

Utilization of a premixed abrasive material suspension in the present invention eliminates the need for additional feed lines and equipment in the nozzle assembly. Fine abrasive particles improve cutting and machining quality and precision, and reduce abrasive particle damage to the workpiece surfaces adjacent the cuts. Therefore, fine abrasive particles may be particularly useful in applications where additional finishing steps can be eliminated.

Having an essentially uniform suspension of abrasive materials and having abrasive particles moving at speeds comparable to those of the carrier medium, which is a consequence of using premixed abrasive material suspensions, significantly reduces the tendency for abrasive materials to bridge or pack at the nozzle orifice. Therefore, nozzle orifice diameters can be reduced. Depending on abrasive particle size, nozzle orifice diameters can be as small as about 0.1 mm (about 0.004 in.). Such smaller orifices provide comparably smaller diameter jet streams enhancing cutting and machining precision by producing smaller kerfs and decreasing media consumption rates.

Dispersions of the abrasive into the medium is achieved by simple mixing techniques, and is not narrowly significant to the practice of the invention.

As noted previously, the design and structure of the nozzle elements for use in the system of the present invention are greatly simplified by the elimination of the mixing tube, the abrasive feed mechanism, and the abrasive transport conduit, typically a hose. The features and their bulk, complexity, expense, weight and dependence on operator judgment and skill are all eliminated to the considerable benefit of abrasive jet stream cutting and machining operations.

It is also desirable that the specific design of the nozzle to be employed be configured to minimize the application of shear to the polymer constituent of the jet stream medium. It is accordingly preferred that the rate of change of the cross-sectional area of the nozzle from the relatively large inlet to the outlet of the nozzle orifice be developed in smooth, fair continuous curves, avoiding as much as possible the presence of edges or other discontinuities. Acceleration of the flow is achieved by reducing the cross sectional area through which the medium is pumped, and high shear stresses are necessarily applied to the polymer. It is believed, however, that chain scission and polymer degradation are minimized by avoiding stress concentrations at edges and the like, where the rate of change in the stress is very high, and proportional to abrupt changes in the rate of change of the cross sectional area.

Such features in the nozzle also serve to avoid producing turbulent flow in the medium. Coherence of the jet stream is favored by laminar flow through the nozzle orifice, so that the indicated nozzle configuration serves to minimize divergence of the stream.

Minimizing induced shear stresses is helpful in the context of all aspects of the present invention. In particular, shear stress magnitudes sufficient to generate turbulent flow in passing media are to be avoided. Shear stresses of this magnitude for high velocity flow are associated with passage over discontinuities and edges. A consequence of such flow is generation of stress stresses in the media of sufficient magnitude to break polymer bonds. Breaking polymer covalent bonds with the attendant irreversable molecular weight reduction are all manifestations of polymer degradation, and are best avoided or minimized when possible.

As a further aspect of the present invention, there are improvements for media catcher designs used to capture jet streams after passing through or by workpieces. Even after cutting and machining a workpiece, portions of the stream, if not the entire stream, are still traveling at high speeds so specified media catchers are required to minimize splashback, generation of mist, and damage to media catcher hardware. Additionally, media catchers need to be designed to reduce noise caused by jet stream break-up and to minimize degradation of the polymer and fracture of the abrasive particles.

Previously, elongated tubes were used for media catchers. These elongated tubes were configured and oriented to cause jet stream break-up along surface walls before jet streams reached the bottom of media catchers. Alternatively, media catchers included replaceable bottom inserts or were filled with loose steel balls to effect jet stream break-up. When replaceable bottoms were used, it was an accepted consequence that jet streams would cut the bottom. To address this disadvantage, media catcher bottoms were supposed to be designed for easy, low-cost replacement. Irrespective of the type of current media catcher used, trapped jet streams are subjected to high shear stresses that unavoidably promote polymer degradation.

The present invention provides a new media catcher design as shown in a cross section view in FIG. 1 with the media catcher generally designated by reference numeral 48. A jet stream (50) can be injected into the media catcher (48) and gently decelerated. Here, the jet stream (50) does not impact metal surfaces, but rather is directed to penetrate a contained medium (52). Preferably, this medium (52) is the same gel-thickened polymer solution or suspension as the jet stream (50). Polymer molecules in the jet stream (50) caught by media catcher (48), therefore, are decelerated over a substantial distance as opposed to impacting a metal surface and essentially being immediately decelerated. This extended deceleration avoids generation of shear stress magnitudes that would be associated with impact at metal surfaces. Though many different materials could be used for the receiving medium (52), there are disadvantages in not using the same medium as that of the jet stream (50). These disadvantages include dilution and separation difficulties, that could even be impossibilities, hen media is to be reused for jet stream cutting and machining.

Depending on the energy of the jet stream (76), and particularly the portion of the stream which has passed the cut (50) and the depth of medium (52), the jet stream (50) could penetrate through the medium (52) to the media catcher surface (54). One approach for solving this problem would be to build a media catcher (48) with sufficient volume to preclude the possibility of the jet stream (50) penetrating to the media catcher surface (54) irrespective of the energy in the jet stream (50).

Media catcher (48), of this invention, is of simple construction and can be used whether or not jet stream (50) is to be reused. Any fluid can be used for medium (52), including water, if jet stream medium (50) is not to be reused.

Since conventional piston displacement pumps can be used to generate effective jet streams (76) with gel-thickened polymers of the present invention, and a displacement pump can also be used to recycle the media (54), it is possible, and in fact convenient, to assemble equipment for a media-returning cutting and machining system using such equipment.

To use the apparatus, medium (64) for jet stream cutting and machining is loaded into the cylinder (72) of a positive displacement pump (66). A nozzle (68), preferably having a nozzle structure design substantially as shown in FIG. 2, is fitted to the displacement pump (66) output, either by a direct connection, or via a high pressure conduit for the media (75). A hydraulic actuator (70), acting through a piston rod (72), forces the piston head (74) downward, forcing the medium (64) to exit through the orifice in nozzle (68) as a high speed jet stream (76). The jet stream (76) cuts and machines a workpiece (78). After the jet stream cuts and machines workpiece (78), the now divergent flow of the jet stream (50) passes into media catcher (48). For this particular embodiment, the medium (52) is the same as the medium (64). The momentum of jet stream (50) entering media catcher (48) is progressively dissipated and the jet stream (76) medium mixes with medium (52).

When the majority of medium (64) has passed into the media catcher (48), a portion of the medium (52) can be returned to refill medium (64) in the displacement pump (66) so cutting/machining can continue. To return medium (64) into displacement pump (66), the pump (80) on return line (82) is used. Displacement pump piston head (74) is retracted to admit the media (64) on the compression side of piston head (74). If necessary, a filter (84) can be provided in return line (82) for filtering out debris, such as results from cutting and machining. This filtering is primarily intended to protect the orifice in the nozzle (68) and prevent clogging. Magnetic separation of debris may also be employed if ferrous or other paramagnetic materials are being cut. As previously stated, the force provided by piston head (74) is sufficient to force medium (64) through the nozzle (68) to produce jet streams (76) having sufficient energy to effectively machine workpieces (78). Reduced equipment cost, increased reliability, and enhanced safety for operating personnel are benefits provided by this embodiment of the present invention.

Performance of the present invention in making cuts has been demonstrated to be at least equal and often superior to the performance of prior art techniques. The greatest advantage of the system of the present invention stems from the capacity to recycle and reuse the medium, typically for 20 to 100 cycles for many of the formulations. Another considerable advantage is the simplification of the equipment required for abrasive jet stream cutting and machining operations, operating at lower pressure. These features provide considerable cost savings, and reduce dependence on the skills and experience of operators of the equipment.

The enhanced coherence of the jet streams in the present invention generally result in narrower kerf width compared to those attained in the prior art in relation to the abrasive particle size, if all other parameters are equal. The narrower kerf permits greater precision and detail in making cuts, and is a significant advantage considered alone.

For a given abrasive particle size, we have also observed that the surface finish of the cut edges is considerably better than can be achieved in the prior art. When coupled with the ability to use smaller particle sizes than can be employed in prior art techniques, it is possible to produce cuts which require no surface finishing procedures on the cut edge, reducing the number of operations and the amount of labor and equipment required in production.

While the operating pressures employed in the present invention are materially less than those employed in the prior art abrasive jet cutting processes, we have found that the cutting rates do not suffer by comparison, and are, in many cases, higher than can be attained by prior art techniques.

An aqueous solution of guar gum, at 40% by weight, is formed by mixing the gum and water at slightly elevated temperature, of about 35.degree. C., for a period of about thirty minutes, until the gum is fully dissolved. To the solution thus formed, 0.60 weight percent of a high molecular weigh alkali deacetylated polysaccharide of mannose, glucose and potassium gluconurate acetyl-ester is added and dissolved. To that solution, an equal volume of an aqueous solution of 35 weight % boric acid and 2.0 weight % sodium borate is added and mixed until homogeneously blended, accompanied by the initiation of hydrogel formation.

To the forming hydrogel, 50 parts of SiC, having a particle size of 45 micrometers (325 mesh) is added, and the combined materials are thoroughly mixed until a homogeneous dispersion of the abrasive is achieved. The result is a friable powder hereafter referred to as a precursor concentrate.

The above precursor composition is generally utilized in a dry powder form and mixed with various percentages of water, depending upon the size of the nozzle orifice through which the medium must pass during jet stream cutting and machining, together with appropriate percentages of finely divided abrasive for cutting and machining. Preferably, but not necessarily, a minor amount of paraffin oil or hydrocarbon grease is added to the composition as a humectant to inhibit formation of crust upon the medium if it is not used immediately. The characteristics of suitable formulations by volume for different nozzle orifice sizes are listed below in Table I.

The oil component in the above-defined compositions not only delays or prevents crusting. It also controls tackiness. With little or no oil, the medium is adherent to metal as well as the hands of the operator. A suitable humectant oil is, therefore, a preferred additive.

Sometimes, shelf life of the above media is limited to attack by bacterial or fungal growth. The addition of a very small amount of a biocide, such as methyl- or parahydroxy-benzoate, typically in proportions of less than about 1%, and often less than about 0.5%, is often helpful to control such attack.

The following components were combined in a planetary mixer:

The polyborosiloxane had a molecular weight of 125,000 and a ratio of Boron to Silicon of 1:25. The grease was an automotive chassis lubricating grease obtained from Exxon.

The components were mixed under ambient conditions until a smooth homogeneous blend was achieved, and was then divided into portions. Each portion was then combined and mixed with abrasive particles, as indicated in Table II, to form a plurality of abrasive jet stream media. Each formulation was adjusted by the addition of stearic acid to produce a standing viscosity of 300,000 cp.

Each of the media formulations was employed to cut quarter inch aluminum plate under the conditions indicated in Table II, and the cuts were evaluated to show the results reported in the table.

As shown by Table II, rapid, efficient and high quality cuts are obtained.

The base formulation used in Examples 4-26 was again employed, and mixed with the abrasives set out in Table III; the viscosity was again adjusted with stearic acid to a resting viscosity of 300,000 cp, and the formulation was employed to cut 0.25 inch Aluminum plate. The cutting conditions are set out in Table III.

The characteristics of the cut edges of the plate were measured for surface roughness. The measured values are set out in columns G and H of Table III.

As those of ordinary skill in the art will readily recognize, the surface finishes measured and reported in Table III are of exceptional quality in the context of abrasive jet stream cutting.

The foregoing examples are intended to be illustrative of the present invention, and not limiting on the scope thereof. The invention is defined and limited by the following claims, which set out in particular fashion the scope of the invention.