GB2265567A

GB2265567A - Producing stepped pressed articles

Info

Publication number: GB2265567A
Application number: GB9306019A
Authority: GB
Inventors: Gerd Hinzmann; Norbert Nies; Manfred Gunther
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1992-03-23
Filing date: 1993-03-23
Publication date: 1993-10-06
Anticipated expiration: 2013-03-23
Also published as: FR2688724A1; GB2265567B; ITMI930475A1; FR2688724B1; ITMI930475A0; DE4209767C1; IT1272012B; GB9306019D0

Description

2265,56;7 METHOD AND APPARATUS FOR PRODUCING STEPPED PRESSED ARTICLES The

invention relates to a method and an apparatus by which stepped pressed articles of pulverulent material, especially metal powder, may be produced.

A press is known from German Patent 2951716 for producing pressed articles from metal powder with one step, i.e. at least one surface with two sections of different heights. To allow production of pressed articles with as constant a density and height as possible, despite variation of the bulk density of the metal powder used, a filling amount adjustment capability is provided. The amount of powder poured in is regulated by modification of the filling volume in such a way that the pressing force remains within a predetermined tolerance range until the final position of the pressing ram is reached, i.e. until a desired height for the steps of the pressed article is reached.

As soon as an excessively high pressing force is detected in a pressing cycle, the press control system generates a signal which causes a reduction in filling volume in the next pressing cycle. To achieve this, the upper edge of the press tool is lowered with respect to the press surfaces of the bottom rain. If, on the other hand, a deviation below the desired pressing force range occurs, the reverse happens.

Alteration of the distance between the press surf aces of the bottom ram and the upper edge of the die is effected only by corresponding adjustment of the height of the die for the filling process. The result of this is that, when several bottom rams are present, correction of the filling heights above the individual press surfaces thereof is carried out for each by exactly the same amount.

This known pressing process produces a substantial improvement in the uniformity of the characteristics of the pressed articles produced. Under certain circumstances, however, this process may lead to unsatisfactory results. This is especially so if parts of complicated shape have to be pressed with high form accuracy and, at the same time, 2 high density e.g. at least 7 g/cm3 for iron powder. In such cases, fissures may occur upon removal of the pressed articles from the mould as a result of differences in density and thus different stresses between the individual stages of the pressed article. Fissures make the part unusable.

It is an object of the invention, therefore, to provide a method and a press for the implementation thereof, with which a pressed article which high density and complicated shape can be produced which is consistently and reliably fissure-free.

A method for producing stepped pressed articles of pulverulent material, in accordance with the invention, comprises filling the cavity of a die with the material, compressing the material between relatively moveable bottom rams, the press surfaces of which correspond to the steps of the pressed article, and at least one top ram moveable relative the bottom rams, detecting deviations of the filled amount of material from a desired value and setting the filled amount to a desired value such that the steps have approximately the same density by altering the filling volume of the die cavity by individually adjusting the filling levels of the bottom rams, being the distance of the press surf ace of the bottom rams from the die top edge, such that the ratios of the adjusted filling levels correspond, in a first approximation, to the ratios of desired heights of the respective associated steps of the pressed article.

A press for producing stepped pressed articles, in accordance with the invention, comprises a die with a die cavity, a filling device for filling the die cavity with pulverulent material, at least one top ram and a number of relatively moveable bottom rams, detection means for detecting deviations in the filled amount from a desired value and an electronic control system operable in response to detected excessive deviations to individually adjust the filling level of the press rams thereby to correct the die 3 cavity volume such that the ratios of the adjusted filling levels, in a first approximation, correspond to the ratios of the desired heights of the respective associated steps of the pressed article.

The invention is based on the recognition that with known method and press, only the average density of the pressed article is adjusted and that under critical process conditions, inadmissibly high density differences may arise in the pressed article. The invention provides a remedy, in. that filling level correction is not effected by the same amount for all steps or regions of the stepped pressed article by bringing the die alone to a correspondingly corrected height for the filling process, but rather the filling levels above the press surfaces of the individual bottom rams are individually corrected, the individual corrected filling levels in at least a first approximation being in a relationship to each other which corresponds to the relationship of the desired heights of the stages of the pressed article. This means that the greater the desired height of the associated step, the greater the individual amount of correction.

It is necessary for this purpose, with multiple rams one of which is stationary, to adjust the die upper edge spacing with respect to the press surface of the stationary bottom ram and to bring the press surfaces of the other (moveable) bottom rams to a suitable height relative to the die upper edge for the filling process. The adjusting signals necessary therefor are generated by the electronic control system. The control system preferably automatically detects deviations of the density of the pressed article from a desired value directly or indirectly after or during the pressing cycle. Direct detection of the density may be easily effected, when the volume is known, by a linked balance. Alterations in density may, however, also be determined indirectly by reference to variations in pressing force at the bottom and/or top rams until the desired final press position is reached, i.e.

4 until the desired volume of the pressed article is reached. An improvement results even when the control deviation is only summarily detected, which is the case with density determination via weight detection and for example also when only the total press force of all the top and bottom rams is measured.

The fact alone that the individual adjustments of filling positions of the bottom rams are not of the same absolute size has the effect of a reduction in the differences between the densities of the individual steps or regions of the pressed article. In this simplest embodiment, density differences nevertheless occur. These are however reduced in comparison with those resulting from known processes and within the pressed article cannot be detected in detail.

However, this changes if separate press force detection is effected at the individual bottom rams. According to a particularly preferred embodiment., a more refined regulation is superposed on the, first approximation; regulation, in which more refined regulation the correction amounts for the filling levels of the bottom rams are fixed individually, taking- into account the individual press force detected at each of the relevant bottom rams. Depending on the amount and direction of the press force deviation from the desired press force range, adjustment of the individual correction values is effected which is proportionally greater or lesser in comparison with those of the other bottom rams. This individual regulation is also capable of compensating for the effect of material displacements between the powder columns (located above the press surfaces) during the compression process.

Correction of the filling levels is appropriately not effected in a single step but rather iteratively in small approximate steps over several press cycles. This is desirable in the interests of stable regulation and is also perfectly tolerable since the filling density of the 1M.

pulverulent material to be compressed as a rule does not alter sharply but rather gradually.

Since the strokes of the bottom rams between the filling and press positions are altered by-the filling level correction, the press speeds are suitably adjusted analogously to the stroke alterations. This ensures that the relative powder quantities, which are displaced between neighbouring powder columns during the compression process, are not affected by the filling level correction.

The die may be stationary or moveable in the pressing direction. In the latter case, with one of the bottom rams being stationary, the die is preferably hydraulically displaceable in a position-regulated manner for adjustment of the distance between the die upper edge and the press surface of the stationary bottom ram. The electronic control system may be operable to adjust the speed of travel of the die during the pressing process. This is done in conjunction with the adjustment of the press speeds of the moveable bottom rams. The result is that the bottom rams reach their respective final press positions at the same time as prior to adjustment of the filling levels.

The drives for moving the moveable bottom rams are suitably hydraulic and are constructed in accordance with position-regulated press axes.

The invention will now be further described by way of example and with reference to the accompanying Figure which shows schenatically three different stages, I, II and III, in the formation of a stepped pressed article by a method in accordance with the invention.

The Figure shows a die 1, and a final press position III.

cavity 6 in the manner of jacket cavity 6 is closed by three bottom rams 2, 3, 4 arranged coaxially in one another and displaceable relative to each other. The innermost bottom ram 4 has a bore, through which a similarly coaxially arranged centre pin 5 is guided in a filling position I The die 1 encloses a die The bottom of the die r 6 which ends f lush with the upper edge of the die 1 and creates a corresponding through-hole - in the pressed article 8.

For reasons of clarity, the press provided about this tool, which effects movement of the die 1 and of the press rams 3, 4 and optionally also 2, is not shown, nor is the associated electronic control system.

When the bottom rams 2, 3, 4 have reached their filling position, i.e. their upper end faces (press surfaces) have reached the predetermined desired distance from the die upper edge, the die cavity 6 has a predetermined filling volume. A filling shoe (not shown) may then be pushed over the die upper edge to above the die cavity 6, the filling volume being filled completely as far as the die upper edge with the pulverulent material to be comDressed.

After removal of the filling shoe the actual pressing cycle begins. A top ram 7, which includes a hole corresponding to the centre pin 5, is first of all lowered in the direction of the bottom rams 2, 3, 4 and inserted in the die 1. In the illustrated embodiment, the outer bottom ram 2 is stationary, while the two inner bottom rams 3, 4 are axially moveable by means of position-regulated hydraulic drives, which are controlled by the electronic control system. After insertion of the top ram 7, the die 1, the moveable bottom rams 3, 4 and the top ram 7 are moved as far as their final pressing position. The movements are effected at different speeds conformed to each other in such a way that, with respect to the filled volume of pulverulent material, the top and bottom rams 2, 3, 4, 7 compress the powder from both sides as uniformly as possible. As in the present case the bottom ram is stationary, the die 1 must also be moved downwards a little to minimise skin friction thereat, as is illustrated in the Figure by the reference lines drawn in III, the final press position. The top ram 7 could, f or a dif f erent f orm of pressed article, depending on the desired surface thereof, P 7 also consist of several independently moveable top rains like the bottom rams 2, 3, 4.

With a press operating according to the counter pressing process the die would remain immoveable, whilst the top and bottom rams would be moved towards each other. However, movements relative to the pressed article are the same irrespective of the pressing process.

It may be seen that the individual rams follow different paths between the filling position I and the final press position III. The relative paths (with respect to the powder mass) may be ascertained, the path followed by the die 1 being subtracted from the path followed by the respective ram.

The bottom ram 2, which has to compress the highest powder column, has the longest relative path which is equal to the die path. The middle bottom ram 3, which compresses the lowest powder column, has the longest actual path of all the bottom rams 2, 3, 4 but the shortest relative path.

The density of the pressed article 8 is detected by determining the weight thereof after a pressing cycle or by detection of the press force at the top rant 7 and/or at the bottom rams 2, 3, 4 and comparison with corresponding desired press forces. If an inadmissible deviation in.the density of the pressed article 8 from a predetermined desired value is detected, the electronic control system intervenes in a subsequent pressing cycle to produce a corresponding correction of the filling volume. If the density is too low, the filling volume is increased and if the density is too high it is reduced.

An example of an increase in the volume of the original die cavity 6 to the corrected volume of the die cavity 61 is shown as position II in the Figure. It may be seen that to achieve this, the die 1 is not simply raised by the amount a with respect to the original filling position, as in the known process with the result that all powder columns located above the press surfaces of the bottom rams are enlarged by the same amount.

W 8 Rather the filling position is adjusted individually f or each bottom rain 2, 3, 4. Since the bottom ram 2 is immoveable, the filling position thereof is determined by the die 1 alone. The bottom ram 3 is raised by an amount b and the bottom ram 4 by an amount c. This means that the - of the distance filling level correction, i.e. adjustment between the press surface and the die upper edge is least f or the bottom ram 3, being a-b, and greatest f or the bottom ram 2, being a, while the correction value for the bottom ram 4 lies therebetween, being a-c.

Thus,' the correction values are set according to the desired levels of the steps of the pressed article 8 to be produced. This regulating strategy represents a first stage of the process which, in itself, permits the desired object substantially to be achieved.

A further improvement is achieved, if the press forces of each individual bottom ram 2, 3, 4 are detected (e.g. by means of a strain gauge) and compared with predet-ermined desired values by the electronic control system. In this way, indications of possible density differences between the powder columns of neighbouring bottom rams are obtained, whereby such differences may also be minimized by a superposed regulation. Adjustment of the individual correction values is effected in proportion to the deviation of the force of a given ram from those of the other rams. In this way, effects resulting from displacement of the powder inside the die cavity during the compression process can be controlled.

It is possible, by means of the invention, to produce pressed articles of especially metal powder, with constant volume and as constant as possible a density, density differences within the pressed article being minimised. This is of great significance for the production of precise formed parts with high density and complicated shape for the prevention of waste especially as a result of fissure formation during removal of the pressed article from. the mould.

pulverulent material,

Claims

A method for producing stepped pressed articles of pulverulent material comprising filling the cavity of a die with the material, compressing the material between relatively moveable bottom rams, the press surfaces of which correspond to the steps of the pressed article, and at least one top ram moveable relative the bottom rams, detecting deviations of the filled amount of material from a desired value and setting the filled amount to a desired value such that the steps have approximately the same density by altering the filling volume of the die cavity by individually adjusting the filling levels of the bottom rams, being the distance of the press surface of the bottom rams f rom the die top edge, such that the ratios of the adjusted filling levels correspond, in a first approximation, to the ratios of desired heights of the respective associated steps of the pressed article.
2. A method as claimed in claim 1, in which deviations in the desired value are detected directly, by determination of the weight of the pressed article, or indirectly, by detection of the pressing force at the top and/or bottom rams.
3. A method as claimed in either claim 1 or Claim 2, wherein the pressing force of the individual bottom rams is detected and compared with desired pressing force values allocated thereto and wherein upon detection of a large deviation at a particular bottom ram (s), the filling level correction is set to be correspondingly proportionally greater or smaller.
A method as claimed in any preceding Claim, wherein filling level correction is effected iteratively over several pressing cycles.
5. A method as claimed in any preceding Claim, wherein the pressing speeds of the pressing rams are W -- J 0 - adapted in such a way that the rams reach their respective final press positions at the same time as before filling level correction.
6. A press f or producing stepped pressed articles comprising a die with a die cavity, a f illing device for filling the die cavity with pulverulent material, at least one top ram and a number of relatively moveable bottom rams, detection means for detecting deviations in the filled amount from a desired value and an electronic control system operable in response to detected excessive deviations to individually adjust the filling level of the press rams thereby to correct the die cavity volume such that the ratios of the adjusted filling levels, in a first approximation, correspond to the ratios of the desired heights of the respective associated steps of the pressed article.
7. A press as claimed in Claim 6, wherein the detection means comprise an apparatus for determining the pressing f orce of at least one pressing ram and/or an apparatus f or determining the weight of the pressed article produced by the press.
8. A press as claimed in either Claim 6 or Claim 7, wherein the detection means comprises an apparatus f or separately detecting the pressing force of each individual bottom rani and passing the detected force on to the electronic control system which compares it with correspondingly allocated desired values, and wherein the electronic control system is arranged such that, when the deviations of the pressing force which are large in comparison with other bottom rams are detected at an individual bottom. ram(s), filling level correction is carried out at that bottom ram(s) by a correspondingly proportionally increased or decreased value.

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11,
9. A press as claimed in any one of Claims 6 to 8, wherein at least the drives for moving the moveable bottom rams are constructed in accordance with positionregulated press axes.
10. A press as claimed in any one of Claims 6 to 9, wherein the die is hydraulically displaceable in a position-regulated manner for adjustment of the distance between the die upper edge and the press surf ace of a stationary bottom ram.
11. A press as claimed in Claim 10, wherein the electronic control system is operable to adjust the speed of travel of the die during the pressing process.
12. A press as claimed in any one of Claims 6 to 11, wherein the electronic control system is operable to adjust the respective speeds of travel of the moveable bottom rams during the pressing process in such a way that the bottom rams reach their. respective final press positions at the same time as before filling level adjustment.
13. A method substantially as hereinbefore described and illustrated in the accompanying drawings.
14. A press substantially as hereinbefore described and illustrated in the accompanying drawings.

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