EP1287975B1 - Process for producing molded parts in a powder press - Google Patents

Abstract

Production of pressed parts comprises pressing metal powder and sintering the blank formed in a press. The value for the energy introduced into the upper stamp (14) of the press is stored for a blank of prescribed geometry and the energy introduced to both the upper and lower stamps of the press is stored as a second value. Sliding of the upper stamp is terminated when its energy input has reached a prescribed value and the sliding of the lower stamp (16) is carried out according to a measure of the residual energy when the total energy has reached the prescribed second value. Preferably the course of the energy input over the pressing path of the upper stamp is stored and the sliding of the upper stamp is controlled according to the course of the energy input.

Description

The invention relates to a method for the production of pressed parts, in particular cutting plates made of hard metal by pressing metal powder and subsequent sintering of the compacts according to claim 1.

It is known to produce molded parts made of hard metal, ceramic, sintered metal or the like by means of presses. The powder or granular material is to be provided in such a way that, given an applied pressure, the compact gets a homogeneous structure and can be sintered. A common shape is the so-called direct pressing in correspondingly executed molds or dies, which are associated with an upper and lower punch. Depending on the particular pressing pressure results in a different density during pressing. However, lower density compacts shrink more strongly during sintering than higher density compacts. By differently adjustable pressing paths of upper and lower punch is trying to minimize density deviations. On the other hand, different densities can arise in practice by different pressing forces, which in turn at the same height of the compacts, z. B. caused by filling fluctuations that go up to a few percent. To make matters worse in the production of compacts, z. B. for carbide inserts, is the maintenance of a predetermined total height between the insert seat and at least one cutting edge having a predetermined distance from the insert seat.

Out DE 42 09 787 It is known to measure the pressing force in order to achieve the most uniform possible density within a batch. Depending on the measured pressing force, a correction is subsequently made via the filling for the subsequent compacts.

The document DE 197 17 217 discloses a method for producing pressed parts according to the preamble of claim 1.

Out DE 197 17 217 It is also known, depending on the geometry of the compact and the starting material during the compression for a punch to determine a desired force-displacement diagram (target curve) and store. In a computer, the measured values for the path and the force of the ram are compared with the setpoint curve during compression. By means of at least one separately actuated section of the ram or a separate ram, the pressure on the pressing material during the compression phase is increased or decreased as soon as a deviation from the setpoint curve is determined in order to obtain a same density for each compact at the end of the compression phase. In this method, at least two position sensors must be provided, namely for the at least one ram and the at least one further ram or a portion of the ram, which give their measured values in each case to a control computer. In addition, a force sensor is associated with the ram or a portion of the ram. Also its values are given in the control computer. The position sensors are intended to ensure that, for example, in the case of an upper punch and a lower punch, these move to a predetermined position in the die in order to produce the predetermined geometry of the compact and to maintain its dimensions. Due to different filling, however, density fluctuations can arise, which must be avoided. Therefore, the further press die is provided, which is actuated by the control computer, if a deviation from the desired density value is determined during the respective pressing operation. This method assumes that a more or less pronounced deformation can be tolerated on a surface of the compact required to achieve the desired density. This is the case for indexable inserts on the seat, for example.

It is understood that such a method is not applicable if a predetermined outer contour of the compact must be complied with.

As already mentioned, the adherence to a given density of a compact is important because the density subsequently determines the shrinkage during sintering. Naturally, the density can only be determined indirectly via the pressing force introduced into the material, which is known per se. It should be noted that the pressing force or the maximum pressing force is not completely equal to a given density, because the material of the compact springs depending on its nature after the relief springs, so that the density changes. This change may have undesirable effects. If a specific geometry of the compact is adhered to, different pressing force values result depending on the filling quantity, but also depending on the homogeneity of the powder in the compact.

For compacts with an irregular outer contour, e.g. for indexable inserts, which have on the top grooves or Spanleitflächen or the like, results in a different density distribution even during the pressing process. However, since the location and course of the cutting edge and the associated topography of the insert are important, unequal density distribution leads to undesirable dimensional variations in sintering.

Out DE-A-199 58 999 a process for the production of tablets by means of a tablet press has become known, in which the tablets are determined during the production of a pressing energy actual value and compared with a predetermined Preßenergie-target value. The tableting machine is readjusted according to the deviation between actual and setpoint. In the known method, not only the maximum pressing force is detected, but also the time in which the tabletting die has contact with the tablet, so that pressure curve curves can be created, in which the pressing force is applied over time. With the known method, variations in the breaking strength of tablets should be minimized.

Out US-A-5,024,811 a method for the production of pressed parts made of powder material has become known in which the pressing course of at least one punch is determined as a function of the stroke during the pressing operation. The Preßverlauf is compared with a predetermined path-dependent tolerance range of the pressing force. If the tolerance range is exceeded, the end position of the punch is changed so that the height of Kompaktats is reduced in the end position by an amount corresponding to the increased restoring force of Kompaktats in its removal. If the tolerance range is not exceeded, the end position of the punch is changed in such a way that the height of Kompaktats is increased in the final position of the punch by an amount corresponding to the reduced restoring force of Kompaktats in its removal.

The invention has for its object to provide a method for producing pressed parts, in particular of cutting inserts made of hard metal, by pressing metal powder in powder presses, in which the production of the compacts lead to reproducible accurate products in the sintering process.

This object is solved by the features of patent claim 1.

The invention is based on the recognition that, for example, in the case of an indexable insert having a predetermined topography in the area of the cutting edge and the top, the upper area of the compact is sensitive and therefore special care must be taken in achieving the desired density in this area. This is done in the invention in that the energy input of the upper punch is set in the upper part of the compact. If this energy input takes place essentially in a regulated manner, the desired density and the uniform density distribution in this area are guaranteed. It is then not so important whether the remaining area of the compact is compressed exactly with the same density. However, in order to obtain a density close to the density in the upper region, the total energy input for the compact is determined. During the pressing process, therefore, care is first taken to ensure that the energy entered with the top punch is reproducible. Since a control over the pressing force does not take place at different filling amounts over the pressing path, the energy input is taken as the basis and assumed that the desired density in the upper region of the compact has been reached when the energy input has a predetermined value. The lower punch is "nachgefahren" in this method the upper punch by being advanced in accordance with the residual energy input. Residual energy refers to the energy that remains when the energy entered by the upper stamp is deducted from the given total energy. This can happen that the predetermined thickness of the compact is not achieved. In this case, reworking is required on the sintered molding. Under certain circumstances, however, it may not be harmful if the energy input of the lower punch is slightly higher in order to reach the set thickness, since the influence on the compression of the upper portion is negligible.

According to one embodiment of the invention it is provided that the course of the energy input stored on a press travel of the upper punch and the feed of the upper punch is regulated in accordance with this course. Here, it can be thought of dividing the total energy in a number of increments, after which then a control of the feed of the upper punch is possible.

For the compression of powder material, in which the grains of the powder into one another and "claw" with each other, is crucial to achieve a desired density, especially the phase in which already a greater pressing pressure is applied. Therefore, an embodiment of the invention provides that the first and second value for the energy input are stored only for the second half of the press path of the upper punch or only during the second half of the pressing operation of the upper punch the energy input of upper and lower punch with the first and second values be compared. It is understood that the control path in relation to the pressing path can also be placed close to the end of the pressing path.

The energy that is entered into a compact during pressing is the product of force pressing. In reality, however, the energy actually used is not equal to this product, but lower because the compact has rebounded, and consequently part of the press energy has not been used for compaction. According to one embodiment of the invention, the Auffederweg of the compact is measured and calculated from the unused energy. This calculation can be used to correct the compression of the next compact if the actually used input energy does not correspond to a predetermined value. By changing parameters during the pressing process, the energy input can then be changed. This can be done, for example, by changing the filling, by different travels of the filling shoe to be achieved by vibration of the die, lower and / or upper punch or the like.

Fig. 1

zeigt eine Presse zum Verpressen von Metallpulver nach dem erfindungsgemäßen Verfahren vor dem eigentlichen Pressvorgang.

Fig. 2

zeigt die Presse nach Fig. 1 während des Pressvorgangs.

Fig. 3

zeigt eine abgewandelte Presse zur Durchführung des erfindungsgemäßen Verfahrens.

Fig. 4

zeigt die Presse nach Fig. 3 während des Pressvorgangs.

Fig. 5

zeigt einen Schnitt durch eine im Sinterverfahren hergestellte Schneidplatte in schematischer Darstellung.

Fig. 6

zeigt ein Kraft-Weg-Diagramm zur Illustration des erfindungsgemäßen Verfahrens.

The invention will be explained in more detail with reference to drawings.

Fig. 1

shows a press for pressing metal powder according to the inventive method before the actual pressing process.

Fig. 2

shows the press Fig. 1 during the pressing process.

Fig. 3

shows a modified press for carrying out the method according to the invention.

Fig. 4

shows the press Fig. 3 during the pressing process.

Fig. 5

shows a section through a cutting plate produced in the sintering process in a schematic representation.

Fig. 6

shows a force-displacement diagram for illustrating the method according to the invention.

In the Figures 1 and 2 a die 10 is shown, whose bore has a mold cavity 12, which is conical in cross-section. With the help of such a mold cavity 12, a compact can be produced, for example for a cutting plate, as in Fig. 5 is shown. The cutting plate after Fig. 5 has a clearance angle. The upper edge of the mold cavity 12 (die bore) from the top edge of the die 12 is spaced x. Above the die 12, an upper punch 14 and below the die 10, a lower punch 16 is indicated. The stamps 14, 16 are actuated in a suitable manner, preferably with hydraulic press cylinders. These are controllable (not shown) to apply a desired force. In addition, they can be speed controlled to produce a desired force-time curve. Suitable force measuring devices, which are associated with the punches 14, 16, determine the respectively applied pressing forces. Displacers, not shown, determine the positions of the upper and lower punches 14, 16 during the pressing process.

When filling the die bore, the lower punch 16 has a predetermined filling position. Its position determines the filling quantity. Preferably, the position is initially slightly lower than the theoretical filling position for the predetermined amount, so that after filling the lower punch a certain distance can go up the column, so that the filling shoe, not shown, wipes excess material from the Matrizenoberseite. Subsequently, upper punch 14 and lower punch 16 are moved into the die bore, wherein the upper punch 14 moves in so far that it comes to rest on the upper side of the mold cavity 12. The entry depth into the die bore thus corresponds to the dimension x. The lower punch 16 is also moved to a predetermined position, as in Fig. 2 is shown. Subsequently, the pressing process takes place. The embodiment according to FIG. 3 and FIG. 4 is different from the one after the Figures 1 and 2 in that the bore of the die is cylindrical. The cutting plate produced by means of the method has no clearance angle. Incidentally, the embodiment according to the FIGS. 3 and 4 with the same reference numerals as Fig. 1 and 2 Mistake.

From the illustration to Fig. 5 It can be seen that the cutting plate 20, the cutting edge 22 adjacent two grooves 24, 26 has. The grooves are indicated here for illustration purposes only. Cutting plates of the type shown can be different Topographies on the top, which should improve the cutting behavior of the plate. The cutting edges 22 often have no linear extension, but are curved in space in a predetermined manner.

According to the invention, with the upper punch 14 in the upper region of the cutting plate 20, which is bounded below by the dashed line 28, a predetermined density can be generated. For this purpose, it is necessary that the energy input with the upper punch 14 corresponds to a predetermined value when the punch has reached its end position. The energy or applied work corresponds to the measured pressing force times the pressing path. Thus, this method does not proceed according to a predetermined setpoint curve for the pressing force, but is seen as achieving a constant energy input for all compacts. This includes that the pressing force of the lower punch can be quite different. The pressing force of the lower punch is "nachgefahren" in accordance with the energy input from the upper punch. It can thereby happen that in the lower region of the sintered cutting plate 20 there is a density that deviates somewhat from the upper region. However, this is to be accepted if it is ensured that 20 precise dimensions are achieved for the upper portion of the cutting plate.

In Fig. 6 a force-displacement diagram for the upper punch 14 is shown. The press force curve increases up to a maximum Presskraftwerg P _max . The integral below this curve corresponds to the energy input E during the pressing process. The energy input E is composed of individual increments ΔE, and it is readily possible to control the upper punch 14 so that predetermined energy increments are entered. However, the energy ultimately used is less than the initially registered energy, because the compact shows a Auffederungsverhalten. It forms a kind of hysteresis, as in Fig. 6 shown. The area between the branches of the curve after. Fig. 6 forms the unused energy for the compression of the compact. For the desired predetermined density in the upper region of the cutting plate 20 therefore that energy is decisive, which results from the surface below the lower curve. The springing behavior can be measured and the unused energy can be determined from the suspension spring. When pressing can be used for the respective compact springing no longer corrective, but only for the next compact. By changing parameters of the pressing process, the energy entered by the punch can be influenced and thus changed until the desired value is reached.

Claims (4)

1. A process for the manufacture of compacts, particularly cemented-carbide cutting blades, by compressing metallic powder and subsequently sintering the compacts wherein the compacts are formed in a powder press having a die-plate (10), an upper ram (14) and at least one lower ram (16) which are associated with a die-bore and are adapted to be actuated by a hydraulic press cylinder with the rams (10, 14) associated thereto force-measuring devices and path-measuring devices to measure the compression forces during the ram feed motion up to the final positions, characterized in that the value of the energy to be applied by the upper ram (10) is stored for a compact of predetermined geometry and dimensions and a predetermined material, that the overall energy (E) to be applied by the upper and lower rams(10, 14) is further stored as a second value, that the feed motion of the upper ram (10) is completed when the energy applied by the ram has reached the predetermined first value and the feed motion of the lower ram (14) is effected depending on the application of the residual energy and is completed when the overall energy (E) has reached the predetermined second value.
2. The process as claimed in claim 1, characterized in that the course of energy application is stored via a compacting path of the upper ram (10) and the feed rate of the upper ram (10) is regulated depending on this course.
3. The process as claimed in claim 2, characterized in that the first and second values are stored only for the second half of the compacting path of the upper ram (10) or that the energy applied by the upper ram (10) is compared to the energy applied by the upper and lower ram (10, 14) only during the second half of the compressing operation.
4. The process as claimed in any one of claims 1 to 3, characterized in that the spring-back path is measured for the compact upon completion of the compressing operation and the energy not utilized for compaction is calculated from the spring-back and is deducted from the overall energy (E) applied to deter-mine the real energy applied to the compact and at least one parameter influencing the compressing operation is varied during the next compressing operation if the real energy applied deviates from the predetermined second value.

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