EP1964664B1 - Method for controlling a powder moulding press manufacturing moulded articles - Google Patents

Abstract

The method involves measuring a ram stroke of an upper arm (3) and a lower arm (4). Nominal values together with tolerance ranges and action ranges for dimensions of a molding (7) are stored in a storage device (10). The dimensions of the molding to be tested in a test station (8) immediately after a pressing process are measured. The measured dimensions are associated based on a deviation from the nominal values. An automatic correction of the stroke is initiated based on a valence of the action ranges with which the dimensions in question are associated either immediately or with a delay.

Description

The invention relates to a method for controlling a press in the manufacture of pressed moldings of powdered or granular materials, with a pressing tool, an electronic control, as well as with a following on the pressing tool test station for the automatic detection of dimensions and for sorting of moldings from vibration-decoupling of the press, wherein the material is filled into a mold and compressed by moving at least one punch which is movable relative to the mold.

It is known to produce molded parts made of hard metal, ceramic, sintered metals or the like. With the help of presses. The powdery or granular material is formed by means of a pressing tool from the presses into a molding, which is then subjected to a sintering process. One of the most important shaping processes is axial compression. A press tool provided for this purpose consists in the simplest case of an upper and a lower punch and a forming die made of hardened steel or carbide. After filling the mold with, for example, powdered material, the powder is compacted by the punches of the pressing tool. One distinguishes between one-sided and two-sided pressing, that is, either only the upper or the lower punch or both moved at the same time. The at least one moving punch is driven by a suitable power drive, either mechanically or hydraulically.

For a hydraulic press, for example DE 197 17 217 C2 , An upper and a lower press cylinder is connected to the upper and the lower punch, which are associated with a die bore. The powder is filled with retracted into the die bore lower punch with the help of a filling shoe. An accurate dosage is achieved by then raising the lower punch by a predetermined amount and removing excess powder from the die plate surface by stripping. Thereafter, with the upper punch, which enters the die from above, the powder is compressed, whereby the lower punch can be adjusted accordingly.

In the production of pressed moldings, it is crucial that the individual moldings of a series have an approximately equal density at substantially constant dimensions. For this purpose, it is necessary to keep the maximum pressing pressure, which is achieved in the final position, for example, the upper punch, as constant as possible during each pressing operation. Naturally, the maximum pressing force in the production of the shaped articles results in a variation, depending on the stamp stroke, which is determined by the filling quantity and the distribution of the powder in the mold. Due to changes in the powder due to environmental influences, a small difference in filling quantity or an inhomogeneous distribution of the powder in the mold, molded articles produced in succession can have a different weight or dimension after pressing. Even minor deviations from one another or given reference values can result in an increased scrap content after sintering.

It is for example off DE 101 42 623 C2 From time to time it is known to determine the frequency distribution of the pressing pressure detected by means of a pressure measuring cell during each pressing operation and to determine the standard deviation therefrom. The standard deviation is known to lie between the inflection points of Gaussian distribution curve and is a measure of the dispersion of the pressing pressure and thus an indicator of the variation of weight and dimensions of the molding. If the determined standard deviation of the pressing pressure deviates from a predetermined minimum deviation, at least one parameter of the pressing process is changed from time to time according to the described method.

This type of statistical process control serves to continuously monitor and, if necessary, correct the pressing process. The production monitoring with the help of Statistical Process Control (SPC) is based on a random check of pressed moldings after the production of a part lot of a production lot. A process control coupled to such a check of the moldings essentially serves to eliminate systematic influences and has a delayed effect on the pressing process. This frequently practiced method has the particular disadvantage that any critical deviations that may occur are recognized too late and the correction of the punch stroke and / or the pressing pressure is initiated too late. There is a risk that the production lot has a correspondingly large number of molded articles with critical dimensions, which could justify a rejection of the production lot in a subsequent random sample inspection, for example in the case of a goods issue or goods receipt control with statistical evaluation. Moldings with tolerances in the range of a few hundredths can not exist in the usual evaluation methods. This would mean that, although they could be produced in principle, they would not fulfill the conditions in the case of random testing and would therefore have to be considered as unproducible.

Similar powder presses for the production of moldings and method for press control of such a press are further from the patent US 4,354,811 , the publication DE 2 264 247 and the patent US3,946,212 known.

The document US4,354,811 discloses a multi-well hydraulic press apparatus and press dies for producing compacts. The pressing device is equipped with an automatic test station, which detects the height of all compacts after the pressing process and rejects out-of-tolerance compacts by sorting. The test station also automatically corrects the filling quantity of the cavities, depending on the height determined, by adjusting the position of the counter punches for the press rams. The testing station is separated from the pressing station and thus vibration-decoupled. You are fed to the pressed compacts by means of a conveyor belt.

The publication DE 2 264 247 A describes a method for the production of moldings with tight height tolerances by means of a conventional press. After pressing, the height of a pressed blank is constantly detected with a scanner, which immediately triggers a stroke adjustment of the press ram when set tolerance values are exceeded. For this purpose, the measuring station is coupled to the press control and cooperates with a pressure measuring device for the press dies, which also automatically changes the fill quantities of the cavities.

The object of the invention is to propose a method for controlling a powder press for producing pressed moldings, with which moldings having constant dimensions can be produced within a predetermined tolerance, whereby systematic and random influences on the pressing process of moldings pressed in succession are counteracted at an early stage, so that the production lot has a reduced scattering of the dimensions of the moldings.

The document US 3,946,212 discloses a quality assurance system for the control of a grinding machine. In this procedure for ensuring the dimensional accuracy of machined workpieces, which starts from a setpoint value and an associated tolerance band with a maximum and a minimum basic value, a mean value and the associated statistical spread of measured values are determined from a number of measured measured workpieces. In this method, the tool is corrected only when the mass of the workpiece in the uncritical tolerance range approaches the limit to the critical tolerance range. In this case, a trend calculation takes place before the next machine cycle, which carries out a complex calculation of the spread of the mass of the workpieces.

This object is achieved according to the invention by a method having the features of claim 1. Further advantageous embodiments can be found in the dependent claims.

For applying the control method according to the invention for a powder press for producing pressed moldings from powdered or granular materials, a press is used with a pressing tool having a mold for filling the material, and with at least one relative to the mold movable punch, wherein the pressing tool following a test station is provided for the automatic detection of dimensions of and for sorting of moldings, which is arranged vibration-decoupled from the press. The press also has an electronic control and means for determining and automatically correcting the punch stroke. The control and / or the test station are equipped with a memory in which measures determined by the test station can be stored together with set dimensions and tolerances assigned to the setpoint dimensions. For the control of the press, the controller can automatically compare the respectively measured (actual) dimensions with the desired dimensions and with the tolerances determined by tolerances limit and acts depending on the result on parameters of the pressing process, in particular the stroke of Press stamp.

Depending on the location of a respective determined measure to the corresponding desired value, the initiation of an automatic correction of the punch stroke by the electronic control takes place. In this case, the detection of the dimensions of the molding can be made without contact or with contact to the molding. It is important, however, that the detection of one or more tolerance-critical dimensions of the molding in the test station is not affected by vibrations from the press. This is ensured by the vibration isolation of the test station from the press. To control the punch stroke is a central control, with a memory for storing press operating parameters, press tool parameters and set and actual state variables of the moldings is equipped. The central control and the memory may be part of the press, the pressing tool, the test station or one of the coupled control device.

Particularly suitable is a test station, which is arranged externally of the press and has a weighing device and one or more measuring devices for tolerance-critical dimensions. The press external testing station is not mechanically connected to the press and thus protected against interference by the press. The pressed moldings are automatically fed directly after pressing. This can be done for example with a conveyor belt or preferably with a pick-place handling system or the like. A handling system is particularly useful to perform a positionally accurate positioning for the measurement of tolerances critical dimensions. With the weighing device, the weight of the molding to determine its density can be determined. It is advantageous to equip the test station for the acquisition of several dimensions with more than one measuring device and for an accurate measurement with an automatic cleaning device for the weighing device and / or the measuring device. So it is possible to determine several dimensions at the same time, which has a positive effect on the cycle time of the test station. The cleaning device can remove particles lost from moldings in the measuring station by blowing off, sucking off or mechanically wiping them off. In addition to accurate weighing of the molded article, an accurate determination of its tolerance-critical dimensions is thus also possible, since an inclined position of the molded article is excluded in determining the dimensions.

In addition, the inspection station has a sorting device for the moldings. With the sorting device, after the determination of weight and / or tolerance-critical dimension and assignment to one of the tolerance ranges, the shaped articles can fall, depending on the valency of the tolerance range in which they fall, be sorted out automatically. For example, moldings which can not be assigned any of the specified tolerance ranges can be sorted out of the production lot simply and reliably.

• Bestimmung und automatische Korrektur des Stempelhubes mittels der elektronischen Steuerung, wobei wie folgt vorgegangen wird:
• Festlegen eines von Grenzmaßen bestimmten Gesamt-Toleranzbandes für ein oder mehrere zu prüfende Maße des Formlings;
• Aufteilen des Gesamt-Toleranzbandes für ein solches Maß in einen unkritischen Toleranzbereich und in einen darauf direkt folgenden kritischen Toleranzbereich, die um einen Sollwert angeordnet sind;
• Unterteilen des kritischen Toleranzbereiches in einen ersten und in einen daran anschließenden zweiten Aktionsbereich mit unterschiedlicher Wertigkeit, wobei dem zweiten Aktionsbereich gegenüber dem ersten Aktionsbereich die höhere Wertigkeit zugeordnet ist;
• Speichern der Sollwerte zusammen mit den Toleranzbereichen und den Aktionsbereichen für die zu prüfenden Maße in einem Speicher der Prüfstation;
• Erfassen zu prüfender Maße des Formlings unmittelbar im Anschluss an den jeweiligen Pressvorgang in einer automatischen Prüfstation;
• Speichern der jeweiligen erfassten Maße in dem Speicher der Prüfstation;
• Zuordnen der jeweiligen zuletzt erfassten Maß, in Abhängigkeit ihrer Abweichung vom entsprechenden Sollwert, zu dem unkritischen Toleranzbereich oder einem der Aktionsbereiche des kritischen Toleranzbereiches; und
• Einleiten einer sofortigen automatischen Korrektur des Stempelhubes für die Fälle, dass das zuletzt erfasste Maß nicht dem unkritischen oder kritischen Toleranzbereich zuordenbar ist, das zuletzt erfasste Maß in den zweiten höherwertigen Aktionsbereich des kritischen Toleranzbereiches fällt oder dass in direkter Folge mehrere Maße dem ersten Aktionsbereich des kritischen Toleranzbereiches zugewiesen werden, wobei der Formling nur als Ausschuss aussortiert wird, wenn sein Maß außerhalb der Toleranzbereiche liegt.

According to the method of the invention, the following steps are provided for controlling such a powder press:

• Determination and automatic correction of the punch stroke by means of the electronic control, proceeding as follows:
• Determining a total tolerance band determined by limits for one or more dimensions of the molding to be tested;
• Dividing the total tolerance band for such a measure into an uncritical tolerance range and a critical tolerance range directly following it, which are arranged around a nominal value;
• Dividing the critical tolerance range into a first and an adjoining second action range with different significance, wherein the second action range is assigned the higher priority compared to the first action range;
• Storing the setpoints together with the tolerance ranges and ranges of action for the measurements to be tested in a memory of the test station;
• Detecting dimensions of the blank to be tested immediately after the respective pressing operation in an automatic testing station;
• Storing the respective detected measurements in the memory of the test station;
• Assigning the respective last detected measure, as a function of its deviation from the corresponding setpoint, to the uncritical tolerance range or one of the action ranges of the critical tolerance range; and
• Initiate an immediate automatic correction of the punch stroke for cases in which the measure last acquired is not attributable to the uncritical or critical tolerance range, the measure last detected falls within the second higher-order range of the critical tolerance range, or several measurements directly follow the first action range of the critical range Tolerance range are assigned, the molding is sorted out only as a committee, if its size is outside the tolerance ranges.

In this method, therefore, a control or regulation of the manufacturing process takes place such that both the monitoring and the control are fully automatic, the dimensions of each pressed molding are checked and used for control. The test of the dimensional stability of the specimens is one hundred percent immediately after demolding of the molding from the shape of the pressing tool, while already the next molding is pressed. Thus, in the case of dimensional deviations of the measured molding, depending on the valence of the corresponding tolerance range, influence can already be exerted in the pressing of the next but one molding. Consequently, systematic influences are recognized early and it is counteracted, for example, by expansion of the ram through Temperature change, so that produces only a few moldings that are out of order. For changing the axial dimension of the next-but-molded article, the punch stroke is changed such that the dimensions of the next but one molding are closer to the nominal value for the molding, preferably a tolerance range can be assigned, which has a lower valency compared to the currently measured molding.

Thus, the scheme does not start only when a number of moldings is not in order. The control is used even in the case of deviations which are still within the limits, in particular in a tolerance range with noncritical significance. The dimensions of the molding are permanently changed step by step by changing the Stempelhubes from a tolerance range with critical valence to a tolerance range with less critical valence, in particular with the desired uncritical significance, so that the dispersion of the established dimensions of the moldings in the production lot is significantly reduced.

It is advantageous to use for the determination of the tolerance ranges for the dimensions of the molding maximum and minimum limit dimensions, which are limited to predetermined maximum and minimum, absolute Eckmaßen. Moldings of dimensions between the boundary and corner dimensions shall be graded as scrap and shall not be approved. They are recognized by the 100% control and eliminated from the production lot.

Preferably, in the method no correction of the punch stroke in the assignment of the respective Maßes to a first tolerance range, a correction of the punch stroke in the assignment of the respective dimensions to a second, adjacent to the first tolerance range tolerance range, and stopping the Stempelhubes at any assignment of respective dimensions introduced to the first or second tolerance range. The first tolerance range is the narrower tolerance range around the nominal value for the molding.

The first tolerance range is understood to be the target tolerance range in molding of the blank, with the aim of allowing the random spread to be one to two times the standard deviation of the Gaussian curve within this tolerance range. A correction of the punch stroke is not necessary for this case and therefore not provided.

The first tolerance range is followed by a second tolerance range with a higher priority, which reaches up to the specified permissible limit value. Dimensions which can not be assigned to the first or second tolerance range are not permitted, so that the punch stroke is stopped in this case, since a number of process parameters usually have to be changed to correct them. In the second tolerance range, the dimensional change can be effected by adapting a single process parameter, namely by correcting the punch stroke.

It is furthermore advantageous that, in particular, three dimensions assigned as a result of the first action range bring about a correction of the punch stroke. If the correction of the punch stroke does not improve the dimensional accuracy of the blanks, further measures are required.

If a measure falls within the first, less critical range of action, first several, preferably three, subsequent moldings are measured to determine whether it is a systematic or random effect. If this is a systematic influence that causes the first action area to be assigned to several pieces in succession, then the punch stroke is corrected to affect the dimensions of subsequent moldings in the direction of the first tolerance range.

It is expedient to issue a warning message and / or stop the punch stroke when a plurality of, preferably two measures assigned to the second action area occur in succession. The warning can be signaled optically and / or acoustically. Thus, further measures to improve the dimensional stability can be initiated to reduce the dispersion of the dimensions of the moldings in the production lot.

Preferably, molded articles with dimensions which extend beyond the second action range, that is to say can not be assigned to the first or second tolerance range, are automatically sorted out after the test procedure. Thus, critical moldings are removed from the manufacturing process in order to save unnecessary costs for a subsequent sintering process with or without subsequent sizing presses.

The proposed procedure will perform an automatic 100% automatic correction check on the lot, eliminating the need for sampling and eliminating rejects.

Figur 1

eine erfindungsgemäße Pulverpresse in einer prinzipiellen schematischen Darstellung;

Figur 2

ein Ablaufdiagramm zum Überwachen der Maßhaltigkeit von Formlingen und zur Steuerung des Stempelhubs einer entsprechenden Presse; und

Figur 3

die Darstellung von einem Sollwert zugeordneter Toleranzbereichen unterschiedlicher Wertigkeit.

The invention will be explained in more detail with reference to an embodiment shown in the drawing. Further advantageous features of the invention are disclosed in the drawings and in their detailed description. Show it:

FIG. 1

a powder press according to the invention in a schematic schematic representation;

FIG. 2

a flow chart for monitoring the dimensional accuracy of blanks and for controlling the punch stroke of a corresponding press; and

FIG. 3

the representation of a setpoint assigned tolerance ranges of different significance.

In the FIG. 1 A powder press 1 according to the invention, shown symbolically, with a pressing tool 2, has an upper punch 3 and a lower punch 4, which can be moved into a die 5. The die 5 contains a mold 6 for a molding 7, which is already pressed and demolded in the figure. An immediately before pressed molding 7 is inserted in a decoupled from the press 1 test station 8. With a symbolically illustrated measuring device 9, in particular for the height, the molding 7 is measured in the axial direction. Before or after, an automatic weighing process can be carried out there. The detected tolerance-critical measured values 22 are stored in a memory 10 of a controller 11 of an operating device 12 of the press 1. With the controller 11, the punch stroke of the upper punch 3 and the lower punch 4 can be influenced. For automatic displacement of the molding 7 from the press tool 2 arranged in the press 1 to the test station 8, a gripper 13 is provided. The transport to an intended electronic weighing device, for the determination of the weight of the molding 7, which is not shown in the drawing, also takes place by the gripper 13, which can also sort out moldings 7 lying outside the tolerance ranges 16, 17. In addition, the test station 8 is still equipped with a cleaning device 14 in the form of a blowing device, which cleans a measuring receptacle 15 for the molding to be measured 7 before supplying a molding 7 of powder residues. The same or a further cleaning device 14 can also clean the weighing device.

FIG. 2 shows a flow diagram of the method according to the invention for controlling a powder press 1. Based on predetermined nominal dimensions 20 and predetermined Eckmaßen 23 limit dimensions 21 are determined for the molding 7, which are limited to the Eckmaßen 23. Between the nominal dimension 20 and the limit 21 are also in the FIG. 3 shown, first and second tolerance ranges 16, 17 and action areas 18, 19 set for the second tolerance range 17, and the punch stroke of the punch 3, 4 of the pressing tool 2 determined. After pressing a molded article 7, it is transferred to the test station 8 and another molding 7 is pressed, while the dimensions of the previously pressed molding 7, parallel to the pressing process of the next molding 7, are detected in the test station 8. The dimensions are assigned to the first tolerance range 16 or the second tolerance range 17, in particular a first action range 18 or a second action range 19 of the second tolerance range 17, depending on their deviation from the desired value 20.

Depending on the valency of the respective tolerance range 16, 17, or of the respective action area 18, 19, there is no, an immediate or a delayed correction of the punch stroke. When assigning the respective dimensions to the first tolerance range 16, there is no correction of the punch stroke before pressing a further (after next) molded part 7. When the respective detected dimension is assigned to the second tolerance range 17, the punch stroke is corrected immediately or delayed, depending on this whether the measured value can be assigned to the second action range 19 removed from the first tolerance range 16 or to the first first action range 18 near the first tolerance range 16. Each dimension 22 associated with the second action area 19 effects an immediate correction of the punch stroke, while a plurality, preferably three, measures 22 assigned one after the other to the first action area 18 bring about a correction of the punch stroke.

Two consecutive measures assigned to the second action area 19 cause a warning, since obviously no improvement took place after an adjustment. In everything, as in the FIG. 2 indicated that the correction of Stempelhubs always affects only the next but one molding. A borderline 21 bordering molding 7 causes an immediate shutdown of the press. This "emergency stop" is common practice.

Claims (8)

1. Method for controlling a press (1) during the production of pressed moldings (7) made of powdery or granulate materials, with a pressing tool (2), an electronic control system (11) and with a test station (8) for automatic measuring dimensions (22) of moldings (7) as well as for sorting out moldings (7), that is following the pressing tool (2) and is vibration-decoupled from the press (1), wherein the material is filled into a die (6) and is compressed by the movement of at least one ram (3, 4) that is movable relative to the die (6), comprising the following steps:

   • determination and automatic correction of the ram stroke by means of the electronic control system (11), wherein it is proceeded as follows:

   • setting a total tolerance range defined by limit values (21) for one or several dimensions of the molding that is to be tested;

   • splitting the total tolerance range for such a dimension into a non-critical tolerance range (16) and into a critical tolerance range (17), both arranged around a nominal value (20);

   • partitioning the critical tolerance range (17) into a first (18) and an adjacent second (19) action range of different significance, wherein a higher significance is assigned to the second action range (19) with respect to the first action range (18);

   • storing the nominal values (20) together with the tolerance ranges (16, 17) and the action ranges (18, 19) for the dimensions to be tested in a storage device (10) of the test station (8);

   • measuring the dimensions of the molding (7) that are to be tested immediately after the pressing process in an automatic test station (8);

   • storing the measured dimensions (22) in the storage device (10);

   • associating the most recently measured dimensions (22), depending on their deviation from the corresponding nominal values, with the non-critical tolerance range (16) or one of the action ranges (18, 19) of the critical tolerance range (17); and

   • initiating an immediate automatic correction of the ram stroke in the case, that the most recently measured dimension (22), is not associated with the uncritical or critical tolerance range (16, 17), that the most recently measured dimension (22) does not fall into the second action range (19) with higher significance of the critical tolerance range (17), or that in sequence multiple dimensions (22) are associated with the first action range (18) of the critical tolerance range (17), wherein the molding (7) is sorted out only, when its dimension (22) lays outside of the tolerance range (16, 17).
2. Method as claimed in claim 1, characterized in that for determining the tolerance ranges (16, 17) for the dimensions (22) limit values (21) for the molding (7) are used, that are reduced in relation to given absolute maximum and minimum dimensions (23) of the molding (7).
3. Method as claimed in claim 1 or 2, characterized in that

   • no correction of the ram stroke is initiated when the respective dimensions (22) are associated with a first tolerance range (16);

   • a correction of the ram stroke is initiated when the respective dimensions (22) are associated with a second tolerance range (17) that is bordering the first tolerance range (16);

   • a stop of the ram stroke is initiated when the respective dimensions (22) are not associated with the first or the second tolerance range (16, 17).
4. Method as claimed in one of the preceding claims 1 to 3, characterized in that three dimensions (22) being in sequence associated with the first action range (18) are effecting an immediate correction of the ram stroke.
5. Method as claimed in claim 4, characterized in that multiple, preferably two dimensions (22) being in sequence associated with the second action range (19) are initiating the execution of a warning signal and/or a stop of the ram stroke.
6. Method as claimed in one of the preceding claims 1 to 5, characterized in that the moldings (7) with dimensions that cannot be associated with the first or second tolerance range (16, 17) are automatically sorted out after the test station (8).
7. Method as claimed in one of the preceding claims 1 to 6, characterized in that the dimensions (22) of the moldings (7) are measured in a test station (8) that is remote from the pressing tool (2), preferably external to the presses (1).
8. Method as claimed in one of the preceding claims 1 to 7, characterized in that the pressed moldings (7) are transported to the test station (8) automatically and the dimensions (22) are automatically measured by a measuring device (9) of the test station (8).

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