Lubrication Process

Abstract

The specification discloses processes for lubricating compaction presses, including automatic presses having double-sided rotary press heads, with solid lubricants.

Description

BACKGROUND OF THE INVENTION

Compacting presses have in the past been plagued by early wear-out attributable to the abrasive character of the solid particulate material which is fed to them. The difficulty of abrasiveness exists with many inorganic solids, and with some organic solids, but particularly with powdered metals. Because the maintenance of tight tolerances is critical to successful operation of the compaction presses now in use, accelerated wear rapidly impairs their efficiency. The problem has been sufficiently acute to force either the frequent replacement of die sockets, punches and punch sockets, or the equipping of the presses with replaceable hardened bushings in the punch sockets, tungsten carbide dies and high carbon or carbide-tipped punches.

While these expedients have circumvented or reduced wear problems, they have not eliminated them, and there is no question that there is a definite need for further improvements. This need becomes evident when one considers the demands of high speed rotary compaction presses designed to turn out 750, or even 1000 to 1500 parts per minute, high pressure impacts on the order of 50 tons p.s.i., and particular working materials, such as tungsten powders used in forming electrical contacts, ferrites used in forming electronic computer memory cores, carbides used in forming studs for insertion in pneumatic tires, ceramic materials included in compacted filter elements and other materials.

Lubricants have been applied in various ways to combat tool wear. For instance, in the compaction of ceramic powders to form pellets, a lubricant is added to the feed material prior to compaction, and during the pelletizing operation, some of the lubricant rubs off of the feed material granules onto the compacting members of the pelletizing apparatus, lubricating same. However, when the resultant pellets are subjected to after-treatment in an oven to further densify them, the decomposition or volatization of the residual lubricant in the pellets can foul the oven, the product or the atmosphere surrounding the oven.

Lubrication of the inside walls of the compacting chamber in a more direct fashion has also been attempted. Many presses are fitted with upper and lower punches, the latter being arranged to pass at one point in their operating sequence through a supply of lubricant so as to become coated with a film of the lubricant. The punches then move, bearing the dragged-out film of lubricant, into contact with the die and transfer a portion of the lubricant film thereto. This method limits the effective height of the compaction chamber and tends to deposit more lubricant in the lower portion of the die than in the upper portion. Thus, if the lubricant is supplied in sufficient quantity to properly lubricate the upper portion of the die, there may be sufficient excess in the lower portion to unite with the finer particles in the powder feed to form a thin deposit which obstructs lubricant feed and/or blocks up the lower punch.

Liquid lubricants and dry lubricants in volatile solvent have also been applied to the walls of compaction dies by spray-guns which are synchronized with the operating rhythm of the press. Nonuniform application is also a problem with this system. The problem increases in magnitude as the complexity and height of the compacting chamber increases.

Thus, the need which exists may be more particularly defined as a need for the discovery of a process for lubricating the compacting members of a compacting press which is successful in retarding wear, is effective in bringing the lubricant to the exact place where it is needed without undue waste, spillage or misplacement, and minimizes contamination of the compacted product.

BRIEF SUMMARY OF THE PRESENT INVENTION

Briefly stated, the present invention fulfills the aforementioned need by providing a process of lubrication in which solid lubricant and solid particulate working material are alternatively subjected to compression in the same die of a compacting press.

More particularly, the invention is directed to the lubrication of manual or automatic, single or multiple, rotary or nonrotary compaction presses for use in the compaction of organic or inorganic solid particulate working materials by successively compressing solid lubricant material in the die of the press with a compaction member (e.g., the punch) in the substantial absence of the working material for causing a portion of the lubricant material to adhere to the internal working surfaces of the die while the remaining portion of the lubricant material, which is in excess of that which will adhere to said internal working surfaces, does not adhere to the die, ejecting the excess lubricant material from the die, introducing abrasive solid particulate material into the die, and compacting the abrasive solid particles in the die with the adhered lubricant material in place on the internal working surfaces of the die.

In the present specification and the appended claims, "solid," as applied to lubricants, refers to lubricants which exist substantially in the solid phase under the conditions of temperature and pressure which prevail in the compaction press during their introduction thereto and removal therefrom. If there is a temporary liquification of the lubricant while the press is closed, followed by a return to solid phase when it opens, the process will still operate. However, many of the available solid lubricants, including those which are preferred, appear to remain in the solid phase throughout the press cycle.

The term "lubricant" is used herein in a special expanded sense. It refers not only to materials whose sole function is lubrication but to those which have additional functions. It refers to lubricants consisting of a single material having lubricating properties as well as to mixtures of materials, some of which may and others of which may not have lubricating properties. Thus, the term "lubricant" includes particles of a carrier material in (meaning "in" or "on" or both) which is dispersed a solid or liquid material having lubricating properties and which is available at the surfaces of the carrier particles in an amount which is effective to have a significant wear-reducing effect on the compacting press in which the present invention is practiced.

Where the material having lubricating properties is dispersed in a carrier mainly in the sense of being dispersed throughout the bodies of the carrier particles, the particles must have sufficient pores or other channels of communication between their interiors and the surfaces thereof to make the material having lubricating properties available at the surface thereof. Where the material having lubricating properties is dispersed on a carrier mainly in the sense of being distributed on the surfaces of the carrier particles, the porosity of the carrier is not of critical importance. However, the presence of porosity (including cellular formations) in the carrier at the surfaces of the carrier particles facilitates the preparation of the lubricant.

The weight of material having lubricating properties, based on the weight of the carrier in which it is dispersed, will generally range from about 0.1 percent to about 20 percent, but any minor amount (less than 50 percent by weight) which is effective in significantly reducing wear may be used, with about 1 percent to about 10 percent being preferred. Where the dispersed material having lubricating properties is a liquid, it will be necessary to restrict the quantity thereof, relative to the carrier, to an amount which will not deprive the lubricant of its substantially solid character. The upper limit of liquid which can be tolerated will vary with different liquids and carriers, but can be readily determined by gradually adding liquid to the carrier with agitation until periodically withdrawn samples of the wetted carrier begin to show signs of localized excesses of liquid manifested as droplets of the liquid filling the interstices between the carrier particles.

The materials which impart lubricating properties to the lubricant may be organic or inorganic or both. For instance, a fatty acid soap is a desireable lubricant. Inorganic lubricants may also be used; for instance, graphite may be dispersed on small carrier beads (e.g. 1 mm.) of polystyrene. Liquid materials having lubricating properties for dispersion in a carrier include for instance synthetic and natural oils such as animal, marine, vegetable and mineral oils which have been refined, if necessary, to remove any deleterious materials contained therein. Also, it will of course be understood that the carrier and any other materials included in the lubricant will be free of abrasiveness and other undesireable properties.

The size of the particles of solid lubricant may vary widely, and the size to be used may be readily chosen by those skilled in the art. Generally speaking, the smaller the particles, the better, as this results in the best flowability and the highest degree of contact between the lubricant particles and the walls of the compacting chamber. Thus, the size of the particles should be a small fraction (e.g. less than 10 percent) of the height, width or depth of the compacting chamber.

The shape of the lubricant particles may also vary widely. Thus, the particles may be spheres, powder particles, granules, spangles or any other form. Spheres are preferred because of their readily flowable character and the resultant convenience involved in feeding them to the compacting chamber of the press. Less force is required to convey the spheres if they are nonadherent in respect to one another prior to compression, and thus in some cases the force of gravity will be sufficient to move them from any suitable dispensing mechanism into the compacting chamber or zone.

The lubricant particles are "yieldable." That is, when they are compressed, they undergo a change in shape which brings them into more complete contact with the walls of the compaction chamber, while tending to conform generally to the shape of the chamber while under compression. The change in shape may persist after the compression ceases, as will be the case where the lubricant is a solid fatty acid per se. The change of shape may also not persist, as where the lubricant includes material having lubricating properties, e.g. fatty acid, dispersed on elastomeric carrier particles, which tend to return to their original shape after compression is released.

The quantity of lubricant employed will usually be sufficient to fill the compacting chamber or zone with the compacting member(s) (e.g. punch) in retracted position and the lubricant in its uncompressed form (the form which it has prior to compression). There is no objection however to using smaller amounts, so long as the amount is not reduced to the point where the effectiveness of lubrication is impaired. A quantity of carrier particles having a very high loading of material having lubricating properties may under certain circumstances have the same lubricating effect as a larger quantity of carrier with a lower loading of material having lubricating properties. In any event, the quantity of lubricant should be sufficiently large to contact all of those surfaces of the compacting members which contact the material to be compacted when the compacting members are in their fully extended (maximum compression) position.

It should be evident that the invention is applicable to virtually any solid material which is capable of being compacted in a press. A number of examples of such materials have been given in the foregoing discussion of the background of the invention and others are set forth in the text and examples which follow. Those skilled in the art will readily apply the invention to the compaction of other particulate solid working materials.

By "alternately," as applied to the sequence of compressing the solid particulate working material and the solid lubricant particles, it is meant to include a mode of operation in which lubricant and working material are compressed in turn, first lubricant, then working material, then lubricant, then working material and so on on a repetitive or continuous basis. However, the term is also intended to include a mode of operation in which several compressions of working material intervene between each compression of lubricant and vice versa, although the latter mode will not be likely to be used as often as the former.

The expression "in the substantial absence of the working material" as applied herein to the compressing of the lubricant material, means that the whole of the compacted working material from a preceding compaction step, with the exception of any residue of the working material which is normally left behind in the compaction chamber, has been removed from the compaction chamber prior to compression of the lubricant. Moreover, before the next charge of working material enters the compaction zone, the excess lubricant is removed from the zone. Thus, excepting for the presence of normal residual amounts of the working material and the lubricant which are left behind in the zone after each has been compressed and ejected, the working material and lubricant are compressed separately from one another. Thus, employed in respect to such compression steps, the term "separately" is to be construed as not excluding the presence of the aforementioned residual amounts.

The terms "compacting" and "compaction," as employed herein, are intended to refer to operations in which particles of a solid working material are subjected to mechanically applied pressure by compacting members in compacting apparatus. The shape and the degree of cohesiveness which are imparted to the resultant product do not constitute limitations on the invention. However, it will be apparent that the invention is useful in pelleting operations as well as in operations for the formation of more complex shapes, and that the superior performance of the invention will be particularly evident in the lubrication of compaction dies which are deep and/or have complex shapes.

The invention may be better appreciated upon consideration of a preferred embodiment disclosed in the drawings and in the text which follows.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic view of the rotating table, die openings, feeder mechanism, knock-off bars and ejection chutes of a double-side rotary compaction press.

DESCRIPTION OF PREFERRED EMBODIMENTS

Although the invention is, as suggested above, applicable to virtually any type of compacting press, it can be applied with particular advantage to presses of the double-sided rotary type depicted in the drawing. Such presses are well-known to those engaged in the manufacture of shaped powder metallurgy parts (e.g. carbide snow-tire studs). Such presses have a horizontally disposed rotary table which has a plurality of vertical die openings through the table disposed at angularly spaced intervals near its periphery. In one type of such press which is manufactured by Wilhelm Fette, Schwarzenbek-Hamburg, Germany, the table is caused to rotate in registry with discs which are located on the same axis as the table above and below it. These discs carry upper and lower punches associated with each die opening, which are adapted to be extended and retracted, as the table and discs rotate in coordination, by the striking of stationary cam members fixed on the frame which supports the rotating discs and table. During each rotation of the table and discs, the end of each punch strikes four different cams which cause it to extend and retract twice. The extension and retraction of the upper and lower punches are phased so that the lower punch extends and retracts while the upper punch is retracted for ejection of a compacted product and refilling of the compaction chamber or die with a fresh charge of powder. These double-sided rotary presses have two product ejection stations adjacent the two locations on opposite sides of the apparatus where the upper punches retract and the lower punches extend for ejection of the compacted shapes. A short distance further around the apparatus in the direction of rotation of the table are two filling or working material feeding locations, again on opposite sides of the apparatus, where the upper and lower punches of each die both assume a retracted position to receive the fresh powder. Still further around the table, again in the direction of rotation, are two compression stations on opposite sides of the apparatus at which the upper punch is extended (and at which in some circumstances the lower punch may be partially extended) for compaction of the powder to form a shaped article.

In normal practice, both of the two sets of ejection, filling and compression stations of double-sided rotary presses are processing solid particles of working material into shaped compacts at the same time. Any lubrication which is supplied is accomplished by one of the above-described prior methods or by other methods which form no part of the present invention. In an informative discussion of double-sided rotary presses, entitled "High Volume Production of P/M Parts," International Journal of Powder Metallurgy, 3(4) 1967, pp. 33-43, hereby incorporated herein by reference, it is pointed out that such presses may have as many as 27 punch and die sets and, depending on the powder feed, can produce as many as 250-750 compacts per minute with compression capabilities of up to 100 tons. The development of presses capable of turning out 1000-1500 parts per minute has been predicted.

In accordance with the preferred embodiment of the present invention, one side of a double-sided rotary press is fed particulate solid lubricant, while the other is fed particulate solid working material. In this way, each die and its corresponding punches is alternately lubricated and used to form a compact. The invention can be applied just as readily to presses having any plurality (e.g. three or more) of sets of ejection, feeding and compression stations, with the number of sets which are appropriated for lubrication purposes being determined by the severity of the lubrication requirements.

The schematic illustration of the application of the invention to double-sided presses which is set forth in the drawings includes a circular table which rotates in the direction indicated by the arrow and has been divided (for purposes of illustration only) into sections I and II. As shown, the dies in section I of the table are passing beneath the outlet of a hopper 1 from which lubricant is dispensed into the open dies. The dies then rotate counterclockwise to a position, identified by reference numeral 2, where the aforementioned cams (not shown) partially extend the punches for precompression of the lubricant. The table and dies rotate further in the same direction to compression station 3 for completion of compression and then to ejection station 11 where the lower dies previously mentioned (not shown) force the lubricant out of the die so that it will be deflected by a fence 12 into an ejection chute 4. If desired, the lubricant material may be recycled manually or by automatic recycle means 10, such as a conveyor, from deflection chute 4 back to hopper 1.

While the dies in section I are being fed with and caused to compress lubricant, those in section II are being fed with and used to compress working material which issues from hopper 5 into a feed frame 6 and from thence into the dies. The dies rotate to a precompression station 7, compression station 8 and ejection station 13, where they are urged by deflection fence 14 into ejection chute 9. Section II of the table then passes beneath hopper 1, picks up a charge of lubricant, compresses and ejects same as it passes stations 2, 3 and 11 and then returns to hopper 5 for a fresh charge of working material. In normal operation the table rotates continuously with each of the dies alternately and separately receiving, compressing and ejecting lubricant and working material.

The recycling of lubricant is especially advantageous where the working material is radioactive and the disposal of contaminated lubricant is a problem. For best results, recycling should be conducted with a lubricant which includes a material having lubricating properties dispersed in particles of elastomeric spheres of carrier material which are not permanently deformed during compression.

Among the polymers which may be included in the elastomeric carriers are polystyrene (preferred on account of its inexpensiveness), polyethers, polyurethanes, polysiloxanes, polybutene, polypropylene, polybutadiene, and natural rubber. Any synthetic or natural polymeric substance having plastic or elastic properties may be used as a carrier in the practice of the invention, but elastic carriers are preferred, especially where there is to be recycling of lubricant. If it is desired to have rougher carrier particles (which is helpful in holding the lubricant), they may be of foamed resin or rubber. Their diameter will generally be on the order of 1 millimeter, but diameters within as well as above and below the range of about 0.1 to about 5 millimeters may also be used.

Where no elastic carrier is employed in the lubricant, it is still possible to recycle lubricant. For instance, if solid granules of unadulterated zinc behenate are employed as lubricant, the pellet or other shaped mass of zinc behenate ejected from the press may be converted again to small particles of a size suitable for feeding to the press, so that they may then be recycled. Such conversion may be accomplished in any known manner such as on a flaking belt or with an extruder-chopper apparatus for forming small pellets.

The preferred lubricant is zinc behenate on a carrier of polystyrene, but virtually any solid organic or inorganic lubricant can be used. There are of course the soaps of fatty acids, including stearates, palmitates, laurates and arachinates of any suitable metal, such as for example aluminum, zinc, magnesium, and calcium. Also to be considered are the esters of fatty acids, such as for example glycerol mono stearate. Other illustrative examples of lubricants include paraffin wax, molybdenum disulfide, graphite and so on.

Those skilled in the art will readily select suitable lubricants and quantities of lubricant based on the efficiency of the lubricant and various other considerations. For instance, such factors as working materials of greater abrasiveness, higher punch speeds and pressures, higher operating temperatures, large dies and a high recycling factor will increase the amount of lubricant required and/or correspondingly reduce the number of work compression operations which may occur between lubrication steps. Reversal of the foregoing factors will reverse the requirements for quantity of lubricant and the number of work compression operations which can be tolerated between lubrication steps. While the invention has been illustrated with a double-sided rotary press, it is apparent that the invention can also be used with more conventional single or multiple punch reciprocating presses for forming small or large parts, and in such presses there is the possibility of performing several work-forming steps between each lubrication step.

The present invention has the advantages of being very simple and easy to carry out. It can utilize the normal movement of the compaction members in the press to force the lubricant into contact with the die and other parts of the compaction chamber both through pressing and wiping actions, and can be used effectively no matter how large or small or complex the compaction chamber may be. Excess lubricant is effectively removed from the compaction chamber before the next charge of working material, leaving behind a thin film of lubricant which is effective in reducing the formation of undesireable deposits of fines separated from the working material. The thinness of the lubricant film left in the die also minimizes interference of the compacted product. Further, the present lubricating method is independent of the granulometry of the raw material for the compaction operation. Other advantages of the invention will occur to those of ordinary skill in the art.

By way of illustration and not limitation, the following examples are presented:

EXAMPLE 1

Polystyrene balls having a diameter of 0.3 mm. are rolled in a cylindrical container with finely divided zinc behenate until the balls have picked up an average of 1-3 percent by weight of the salt.

EXAMPLE 2

Example 1 is repeated, but is continued until a pick-up (average) of 5-10 percent by weight is attained.

EXAMPLE 3

A double sided rotary compacting press of the type described above is operated continuously. The product of example 1 is fed at one feeding station and Uranium Oxide powder is fed at the other. The powder and the operating characteristics and conditions of the press are in all respects comparable to those conventionally used in pelletizing Uranium Oxide, except for the feeding of the lubricant. The lubricant is fed in sufficient quantity to fill each of the dies into which it is produced. The extension and retraction of the punches is the same during the working and lubricating steps, which are conducted in alternation. The ejected flowable spheres of elastic carrier bearing excess material having lubricating properties are recycled and periodically replaced with fresh lubricant when nearly exhausted. The nearly exhausted particles are treated to recover residual uranium oxide. The pellets recovered from the process exhibit excellent uniformity of dimensions and have a very low order of organic contamination which is confined to the surfaces and closely adjacent portions of the pellets rather than being distributed throughout the bodies of the pellets. The operation of the press is trouble-free.

EXAMPLE 4

The procedure of example 3 is repeated, substituting the product of example 2 as the lubricant and Uranium Carbide powder as the working material.

EXAMPLE 5

Ovoid natural rubber particles are coated with calcium stearate to a loading level of 10 percent and found to possess satisfactory lubricating properties.

EXAMPLE 6

The procedure of example 5 is repeated, substituting SBR for the natural rubber. Similar results are obtained.

Claims

What is claimed is:

1. A method of lubrication comprising, in sequence: providing lubricant dispersed on carriers which yield on compression and tend to return to their original shape on release of compression; introducing said lubricant dispersed on the carriers into a compacting chamber of a compacting press; lubricating the interior walls of the chamber by compressing the carriers in the chamber by the mechanical action of compaction members in the press in the substantial absence of working material, normal residues of working material excepted; releasing the compression on the carriers; removing the carriers from the chamber in the form of flowable particles, a film of lubricant being left on the interior of the chamber; introducing working material into the chamber for compaction therein; and reusing the flowable particles removed from the compaction chamber in a subsequent compression step for lubricating the interior of the chamber.

2. A method in accordance with claim 1 wherein the compressions of working material and carriers are performed alternately.

3. A method in accordance with claim 1 including periodic introduction of carriers bearing fresh lubricant into the process.

4. A method in accordance with claim 1 wherein the carriers are of elastomeric material.

5. A method in accordance with claim 1 wherein said working material is uranium oxide powder.