Abstract:
Method and apparatus for the continuous extrusion of rods of plastic raw material. The rods contain at least one inner channel which is at least in part spiral-shaped. The raw material may be a powder-metallurgical or ceramic mass. The material is extruded through a nozzle mouthpiece and made to rotate by an arrangement of flow guiding surfaces. At least one thread made of a flexible or elastic material is entrained and gives the rod a spiral shape with a predetermined pitch. The thread is retained upstream of the nozzle mouthpiece in an eccentric position relative to the rod axis that extends through the nozzle mouthpiece. The movement of rotation of the raw material is adjusted by an outer adjusting force which modifies the angle between the arrangement of flow guiding surfaces and the longitudinal axis of the nozzle mouthpiece to adjust the position and pitch of the spiral channel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process and an apparatus for continuous extrusion of rods of plasticized raw material provided with at least one internal channel which is helical in at least portions. 
     2. Discussion of the Background 
     Such a process as well as apparatuses, or in other words extrusion heads, for performing such a process are used, for example, when a rod preform of a plasticized powder compound, such as a powder-metallurgical compound, or in other words a hard-metal or ceramide compound, is to be shaped to a preform, or in other words a sintered-metal or a sintered-ceramic preform, from which a preform in the form of a cylindrical rod for a high-performance tool is then produced in a sintering or baking process. By virtue of the dies or powder mixtures used, these preforms are characterized by extremely high basic strength, especially as regards mechanical stresses and strains as well as abrasion, and so a trend has developed toward using such preforms especially in the manufacture of drilling or milling tools. Since these tools are frequently operated with extremely high cutting speeds, it is important that the lubricant being used be supplied selectively and in many cases under very high pressures to those regions of the cutting edges which are subject to the highest stresses and strains. This is best ensured by co-formed, internal cooling channels, which then emerge on a predetermined pitch circle on the front end of the tool, or in other words preferably on a flank of the ground surface of the tool. Because sintered hard-metal tools can be machined only with costly methods, it is desirable that the shape of the preform approach the final geometry of the tool as closely as possible. This is achieved most simply by using an extrusion process, with which the capability exists of making the preform such that it already has finish-formed internal cooling channels in a continuous process, which has the particular additional advantage that tools of very great length can be made without changing the process operation. 
     Certainly the process is economical only if it is possible to make the rod such that the geometry and in particular also the position of the at least one internal lubricant or cooling channel are kept within very narrow tolerance limits. This problem becomes more acute if the tool to be made—as is the case for a drilling tool, for example—must be provided with clamping grooves. Because fully hard metal drilling tools are now made with relatively large axial lengths, the at least one internal cooling channel must be co-formed sufficiently exactly that it is disposed at exactly the predetermined position in the drill web in each cross section of the drilling tool, since only then is it ensured that the drill stability will be constant over the entire length and that the exit point of the internal cooling channel will remain unchanged relative to the main cutting edge during finish grinding of the tool. 
     There are already known numerous attempts to make such tool preforms from plasticized powder compound in the extrusion process in such a way that the requirements of accuracy of position and form of the internal cooling channels are met. 
     In U.S. Pat. No. 2,422,994 there is already described an extrusion process in which a plasticized powder-metallurgy compound is extruded through an extrusion nozzle. The inside surface of the extrusion nozzle has projections of predetermined cross section and, in the region of the center of the nozzle, there extend axially oriented, rod-like elements fixed to a core, which is disposed upstream from the extrusion nozzle and around which the plasticized compound flows. This process operates in multiple stages, wherein the plasticized raw material is first shaped to a preform with at least one straight, external groove, whereupon the preform obtained in this way is twisted by a relative rotational movement between the extrusion nozzle and the raw material. Such a two-stage forming process is already unfavorable for most raw compounds that have come into use since then, however, because the preform emerging from the extrusion nozzle is extremely pressure-sensitive. Even very small external forces acting on the preform would cause undesired large deformations, especially of the internal co-formed channels, whereby the preform would immediately become unusable. 
     To overcome this problem, German Patent DE 3601385 describes an extrusion process in which the helical profile of the at least one internal coolant channel is produced simultaneously with the extrusion of the plasticized compound, although in this case the nozzle mouthpiece must be provided internally with a helical profile. At the center of the extrusion nozzle there are provided elastic pins, which at their upstream ends are fixed to a nozzle core and the elasticity of which is chosen to be sufficient that the pins can conform to the swirl flow induced by the internal contour of the nozzle mouthpiece. It has been found that it is difficult to form the cooling-channel helix sufficiently accurately in the preforms with this extrusion head. The projections and depression on the inside surface of the nozzle mouthpiece had to be provided in large numbers in order to induce appropriate rotation of the flow of the compound. As a result, the nozzle mouthpiece is relatively expensive and, moreover, the projections present on the sintered preform must be ground off first of all, leading to material losses. 
     In European Patent Application EP 465946 A1 there are described a process and an apparatus according to the preamble of claim  1  or of claim  5 , with which it is possible to eliminate the process step of cylindrical surface grinding of the finish-sintered cutting-part preforms. For this purpose, the inside surface of the nozzle mouthpiece is formed by the envelope surface of a regular cylinder. A swirl device disposed in the flow of the compound is mounted upstream from the nozzle mouthpiece. Corresponding to one alternative, a swirl motion acting uniformly over the cross section of the strand is imparted to the extrusion compound by means of this swirl device whereas, according to a second alternative, a spinning or revolving movement is imparted to the swirl device by the extrusion compound. To form the at least one internal channel, a filament-like material which conforms to the swirl or rotational motion penetrates into the flow of compound. Thus the pitch-circle diameter on which the cross section of the at least one internal cooling channel is eventually disposed in the extruded preform is influenced by the flow velocity and by the friction losses in the nozzle mouthpiece. According to a further variant of this known process, it is therefore proposed that the nozzle mouthpiece be designed to revolve, thus allowing the rotational motion of the flow of compound to be corrected by the revolving movement. 
     With this known process, plasticized compounds can be processed in the extrusion process to preforms which are characterized by extremely high accuracy as regards their outside dimensions and the geometry and position of the at least one internal cooling channel. Of course, in this known process and these known apparatuses for performing the process, there exists the need to keep the working accuracy largely independent of the operating parameters of the process, such as the flow conditions in the inlet region of the nozzle mouthpiece, the composition of the plasticized compound and the flow velocities through the nozzle mouthpiece, etc. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to further develop a process and an apparatus such that the interfering effects mentioned hereinabove can be suppressed with little complexity, so that the manufacturing accuracy itself is preserved when system-related parameter fluctuations occur. 
     According to the invention, there is integrated into the nozzle mouthpiece an array of flow-guiding surfaces, whose angle of orientation relative to the longitudinal axis of the nozzle mouthpiece is adjustable by a positioning device, which preferably can be actuated by an external positioning force. Besides the fact that hereby there can be manufactured a multitude of geometries of the extruded preform without complex modifications and with a simplified extrusion head, there is achieved thereby the particular advantage that the rotational motion of the plasticized raw material can be continuously corrected during the extrusion process such that the position and profile of the at least one internal cooling channel can be kept within narrow tolerance limits. In this way fluctuations of the process parameters of the extrusion process can be reliably stabilized or compensated. At the same time, the advantage is retained that the process uses material sparingly, and so subsequent machining of the sintered preform is not required. The preform is extruded with a smooth, regular cylindrical outside surface which—allowing for the percentage shrinkage which will occur—is maintained such that the least possible removal of material is needed during finish-machining of the preform. Because the angle of orientation of the array of flow-guiding surfaces can be corrected at any time during the extrusion process, the helical pitch of the at least one internal channel can be kept within narrow limits which heretofore were unattainable, specifically even if the mass flowrate of the plasticized compound and/or other physical conditions of the extrusion process were to be changed. 
     In principle there exist two options for disposing the array of flow-guiding surfaces at the extrusion head. According to this further embodiment, the plasticized compound flowing through the nozzle mouthpiece develops autorotation by virtue of the angle of orientation of the guiding surfaces relative to the longitudinal axis of the nozzle mouthpiece and because of the static friction at the inside wall of the nozzle mouthpiece. The rotational velocity depends on the one hand on the flow velocity of the plasticized feed compound and on the other hand on the preselected angle of orientation of the array of flow-guiding surfaces which exists at the time. Hereby velocity fluctuations of the flow of compound can be smoothed out, because the speed of revolution of the array of guiding surfaces or of the nozzle mouthpiece is automatically adapted to the velocity of the flow of compound. The helical pitch of the at least one internal cooling channel in the produced preforms is thus kept constant, specifically regardless of whether the plasticized compound enters the nozzle mouthpiece rapidly or slowly. 
     By virtue of the natural compensation of the interfering effects caused by existing fluctuations of parameters of the extrusion process, the process according to the invention and the apparatus according to the invention are suitable for processing a broad spectrum of powder-metallurgical, plasticized compounds. It must be emphasized, however, that other mixtures and compositions, even those with extremely different physical characteristics and thus different flow behavior, can be processed by the process according to the invention without having to relinquish the advantages cited hereinabove. 
     A particularly simple layout of the array of flow-guiding surfaces is achieved. It has been found that the flow of compound, which splits temporarily in the boundary region of the flow while passing around the guide blade, closes back to a complete circular cross section immediately downstream from the guide blade under the effect of the extremely high molding pressures prevailing in the nozzle mouthpiece. Thus the flow of compound is distorted as slightly as possible, whereby the microstructural quality of the preform manufactured by the process according to the invention can be maintained at an extremely high level. 
     When the nozzle mouthpiece is fixed to the extrusion head such that it revolves therewith, it is advantageous for the array of flow-guiding surfaces to extend over a substantial portion of the overall length of the nozzle mouthpiece. 
     In contrast, if—corresponding to further variants of the subject matter of the application—the nozzle mouthpiece is held revolvably on the extrusion head, in such a way that the axis of revolution coincides with the central axis of the nozzle mouthpiece, it is preferable for the array of flow-guiding surfaces to be laid out such that it extends only over an axially limited inlet portion of the nozzle mouthpiece. Thereby it is ensured that the revolving motion of the nozzle mouthpiece induced by the array of flow-guiding surfaces is reliably capable of maintaining or stabilizing the autorotation motion over the remaining flow length of the flow of compound in the nozzle mouthpiece of the compound by the effect of the static friction conditions at the inside wall of the nozzle mouthpiece. In this case the array of flow-guiding surfaces is advantageously laid out or adapted to the geometry of the nozzle mouthpiece such that the extruded flow of compound rotates with the same angular velocity as the nozzle mouthpiece when it emerges. In this way adjustment and automatic control of the rotational motion of the flow of compound become even more accurate, with particular advantages when the positioning device for the array of flow-guiding surfaces is integrated into a control loop of the extrusion apparatus. 
     In principle, it is possible to adjust the angle of orientation of the array of flow-guiding surfaces in stages. It is particularly advantageous, however, when the adjustment is made infinitely variably or in extremely small steps, for example by means of a stepping motor. In this way any desired swirl angle of the internal cooling channel can be generated and controlled. 
     When the at least one guide blade is braced at least over a substantial distance against the inside surface of the nozzle mouthpiece, preferably via surface contact therewith, relatively large forces can be absorbed. The advantage is then obtained that the radial extent of the guide blade can be increased, with the result that the coupling between flow velocity and speed of revolution of the nozzle mouthpiece and thus the rotational flow becomes more exact. 
     To suppress interfering vibrations of the extrusion system, it is advantageous for the positioning device for the array of flow-guiding surfaces to have a vibration-damping device. This vibration-damping device is advantageously incorporated in a positioning mechanism, preferably in the form of a damped elastic means. Such a vibration-damping device is advantageous in particular when the positioning device is incorporated in a control system for the geometry of the at least one internal cooling channel. 
     The process according to the invention works by using easily bendable or highly elastic filaments, which are fixed locally with their upstream end disposed preferably in the inlet region of the nozzle mouthpiece. It is equally possible, however, to perform the process using filaments or internal rods which have higher modulus of elasticity in order to impart greater dimensional stability, in which case these thin rods or pins are held on a support which is mounted to revolve around an axis of revolution coinciding with the axis of the nozzle mouthpiece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A practical example of the invention will be explained in more detail hereinafter with reference to a schematic drawing. 
     FIG. 1 shows a schematic cross section through the downstream region of an extrusion head for performing the process according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, reference symbol  10  denotes an extrusion head with which there can be performed a process of continuous extrusion of rods of plasticized raw material provided with at least one internal channel which is helical in at least portions. The plasticized raw material can comprise, for example, a powder-metallurgical or ceramic compound, wherein the powder is chosen preferably from the group of ceramic powders, hard-metal powders such as a mixture of tungsten carbide and cobalt, and metal powders, as well as from mixtures of these constituents, such as the cermet mixtures. The FIGURE shows the downstream end of the extrusion head, which tapers conically and forms the inlet portion  12  of a nozzle mouthpiece  14 . In inlet portion  12 , or in other words in extrusion head  10 , there is disposed a retaining device  16 , on which there are fixed upstream ends of filaments  18 , with which internal cooling channels  22  can be produced in extruded regular cylindrical preform rod  24  during extrusion of the plasticized raw material. 
     In the practical example shown in the FIGURE, filaments  18  comprise easily bendable or highly elastic material such as plastic, or a chain whose links hang movably on each other. Filaments  18  have a downstream end  18   a , which extends beyond front end  26  of nozzle mouthpiece  14 . Filaments  18  are attached to retaining device  16  on a pitch-circle diameter TKD1, and in fact are preferably adjustable, in order to permit adaptation to each particular nozzle mouthpiece  14 , or in other words to outside diameter D of preform rod  24  to be made. 
     Arrow S denotes the parallel flow of plasticized powder compound entering nozzle mouthpiece  14 , which parallel flow—as can be seen in the FIGURE—aligns highly elastic or easily bendable filaments  18  in parallel. In nozzle mouthpiece  14  there is provided an array of flow-guiding surfaces in the form of a plurality of guide blades  28  distributed uniformly over the circumference and mounted adjustably in nozzle mouthpiece  14 . For this purpose there are provided substantially radially directed bores  30 , through which there extends a positioning spindle  32  of each particular guide blade  28 . Arrow R indicates that guide blade  28  in question is adjustable by means of a positioning device, which is not illustrated in more detail, such that the angle of orientation of guide blade  28  relative to longitudinal axis AL of nozzle mouthpiece  14  is adjustable, preferably in infinitely variable manner. The FIGURE shows that the adjustment of guide blades  28  can be effected by an external positioning force, with the result that the angular orientation of the array of flow-guiding surfaces in the form of guide blades  28  can be varied at any time during the extrusion process. 
     Reference symbol  36  schematically represents a bearing by which the nozzle mouthpiece is fixed to revolve on extrusion head  10 , specifically such that the axis of revolution coincides with longitudinal axis AL of nozzle mouthpiece  14 , which has a concentric cylindrical internal bore  38 . Guide blades  28  are laid out or disposed in nozzle mouthpiece  14  such that their axial extent EA amounts to only a fraction of the total end-to-end length LB of nozzle mouthpiece  14 . Furthermore, downstream edge  40  of guide blades  28  is disposed at a minimum distance BA from the outlet end, or in other words from front end  26  of the nozzle mouthpiece, which is sufficiently large to ensure that the flow of plasticized compound split by guide blades  28  is closed back to complete a circular cross section downstream from guide blades  28 . 
     The structure shown in the FIGURE leads to the following functional principle of the extrusion apparatus: 
     The plasticized compound enters inlet portion  12  of nozzle mouthpiece  14  on the left side in the FIGURE, specifically in such a way that it has the form of a parallel flow on entering nozzle mouthpiece  14 . This parallel flow now strikes guide blades  28 , which are adjusted to an angle of orientation α and by means of which—due to the hydrodynamic forces—autorotation is imparted to nozzle mouthpiece  14 . The speed of revolution of nozzle mouthpiece  14  depends on the flow velocity of the arriving plasticized compound and on angle of orientation α. 
     Because of the static friction condition which the flow passing through nozzle mouthpiece  14  has at inside surface  38  of the nozzle mouthpiece, rotational motion around axis AL is also imparted to the plasticized compound, while length LB ultimately determines with which rotational velocity the plasticized compound will exit nozzle mouthpiece  14 , or in other words with which rotational velocity around axis AL rod preform  24  will emerge from nozzle mouthpiece  14 . By suitable provisions, such as air-cushioned bearing of emerging rod preform  24 , it is possible reliably to prevent pressure-sensitive rod preform  24  from being impermissibly deformed as it emerges in rotating condition. 
     Because of the rotation of the plasticized compound and of emerging rod preform  24 , easily bendable or highly elastic filaments  18  are also aligned with the flow of plasticized compound, or in other words they are shaped, by the flow of plasticized compound as it passes through, into helical form, the pitch of which can be adjusted as desired by angle of orientation a. Expressed otherwise, by means of the action of the external positioning device on guide blades  28 , the profile of internal cooling channels  22  as well as the position of cooling channels  22 , or in other words pitch-circle diameter TKD2 in finish-extruded preform rod  24 , can be exactly defined. 
     Positioning spindles  32  of guide blades  28  are preferably components of a central positioning mechanism, which has the form, for example, of a planetary gear, so that angle of orientation α of the guide blades can be varied synchronously and uniformly. To prevent vibrations from developing in the extrusion apparatus or in the positioning system, a suitable vibration-damping means can be provided. This vibration-damping means is formed, for example, by elastic components with self-damping behavior. 
     At the outlet of the extrusion head, or in other words in the region of emerging rod preform  24 , there is advantageously provided a device for measuring and monitoring the geometry of the at least one internal cooling channel  22  or for determining the position and size of pitch-circle diameter TKD2. This measuring and sensing device is a component of a control loop, in which the corresponding measured signal is fed back to the positioning device for guide blades  28 , so that the desired position and geometry of the at least one internal cooling channel  22  can be automatically controlled regardless of the interfering effects which occur, such as the flow velocity and the physical properties of the plasticized compound. From the foregoing description it follows that the extrusion system according to the invention already smooths out any fluctuations of velocity of the flow of compound which may occur because they are inherent to the system, by the fact that the speed of revolution of nozzle  14 , which is in autorotation condition, is continuously and automatically adapted to the velocity of the flow of compound. The helical pitch of the internal cooling channels produced in rod preforms  24  during the extrusion process is thereby always of constant size regardless of the flow-through velocity, whereby substantially narrower tolerances of position and geometry of the internal cooling channels can be achieved. 
     By means of the external positioning device according to the invention, it is further possible, with one and the same nozzle mouthpiece  14 , to make rods in which the internal cooling channels have different pitches. In the extreme case, the array of flow-guiding surfaces in the form of guide blades  28  can be adjusted such that guide blades  28  have an angle of orientation α of 0°, so that a preform rod  24  with straight internal channels can be made. 
     The concept according to the invention is equally applicable for the case that the nozzle mouthpiece is fixed to extrusion head  10  such that it revolves therewith. In this case guide blades  28 , which are adjustable by the positioning device, ensure alone that the desired swirl or rotational motion, with magnitude determined by adjustable angle of orientation α, is imparted to the plasticized compound entering mouthpiece  14  as a parallel flow. In this case also the positioning device for the array of flow-guiding surfaces can be integrated into a control system in which the positioning device is driven as a function of the measured signals. 
     Guide blades  28  are illustrated only schematically in the FIGURE. Guide blades  28  are braced against inside surface  38  of the nozzle mouthpiece, preferably such that they maintain surface contact therewith, in which case frictional locking can additionally be provided. A further advantage can be achieved by shaping guide blades  28  such that the guide surfaces continuously rest snugly on inside wall  38  of nozzle mouthpiece  14  during adjustment of angle of orientation α. This is possible, for example, when the guide blades are constructed from members which press resiliently against the inside surface. 
     Embodiments differing from the practical examples described hereinabove are obviously possible without departing from the basic idea of the invention. For example, it is possible to operate the process according to the invention using pins which have limited elasticity and which are fixed instead of filaments  18  to a retaining device mounted to revolve around the central axis of the nozzle mouthpiece in the extrusion head. The pins, or in other words the at least one pin can be pre-twisted into helical form already corresponding largely to that helical form which the at least one internal cooling channel is supposed to have after extrusion of the extruded preform. It is possible to provide, for the retainer of this core pin comprising material of high modulus of elasticity, a separate drive, by means of which fine adjustment of the helical profile is possible by incorporation into a suitable control loop. 
     It is also possible to vary the number, size and arrangement of guide blades  28 . For example, the array of flow-guiding surfaces can be provided with a plurality of arrays of guide blades axially staggered along the nozzle mouthpiece. It also is not absolutely necessary to dispose guide blades  28  with uniform circumferential spacing. For vibration-related reasons it may be practical to provide an irregular arrangement over the circumference. Furthermore, as a modification of the illustrated practical example, provisions can be made to correct the rotational motion of extruded preform rod  24  by means of a further drive device. This additional drive can be provided either on nozzle mouthpiece  14  itself or downstream from this component. 
     The invention therefore provides a process and an apparatus for continuous extrusion of rods of plasticized raw material, such as a powder-metallurgical or ceramic compound, provided with at least one internal channel which is helical in at least portions. The plasticized raw material is pressed out of a nozzle mouthpiece, an array of flow-guiding surfaces provided therein participating in imparting thereto a rotational motion, which entrains at least one filament of easily bendable or elastic material, said filament being retained upstream from the nozzle mouthpiece at a point off the rod axis and extending through the nozzle mouthpiece, and shapes it to helical form with predetermined pitch. To increase the manufacturing accuracy and manufacturing tolerances of the extruded preform rod with simultaneous simplification of the associated apparatus, the invention provides that, for adjustment of the position and/or the pitch of the at least one helical internal channel, the rotational motion of the plasticized raw material is adjusted by an external positioning force, which varies the angular orientation of the array of flow-guiding surfaces relative to the longitudinal axis of the nozzle mouthpiece.