MULTI-FUNCTION MOULD FOR PROCESSING A STEEL STRIP AND METHOD FOR PRODUCING A WASHING MACHINE DRUM

Description

The present invention relates to a multi-function mould for processing a steel strip, typically stainless steel, used to produce the cylindrical shell of a washing machine drum, and to the relative method of operation of the mould. In particular, such a mould is capable of performing in a single station the three initial processing phases of the drum, i.e. the forming of the strip with both an aesthetic and functional function, the blanking of the holes for water drainage and the drawing of the strip around said drainage holes. In the following, specific reference will be made to the production of a washing machine drum, but it is clear that the mould described here and its method of operation can be applied to the production of other similar products which use a metal strip as a starting material and which require the same processing steps.

As is well known, a washing machine comprises a drum into which the garments to be washed are loaded, this drum having a cylindrical shape with perforated walls to allow the entry and exit of the washing liquid. To make this cylindrical shape, one starts with a stainless steel strip wound on a reel, which is unwound using a decoiler that leads the strip to a bench on which various tools and devices are placed to hold the strip, cut a rectangular panel from it, and form on said panel the aesthetic geometry and the funnel-shaped holes for water drainage. The panel processed in this way is then joined with one short side folded over the other to form a cylinder which, with subsequent processing, will become the drum of a domestic washer/dryer. This joining of the two short sides of the rectangular panel can be either mechanical (stapling) or by fusion of the overlapping joint by resistance welding, or the butt joint by laser welding.

The problem with the traditional production method lies in the fact that it requires separate and contiguous operations, i.e. the panel is moved from at least a first to a second station, if not also to a third station, in order to perform the aforementioned forming, blanking and deep-drawing operations. This results from the fact that blanking is usually the first operation performed, since it is easier to punch the holes on a flat sheet of metal, followed at the next station by the drawing of the sheet around the holes so as to move them away from the plane of the sheet in order to protect the washed garments from contact with the edge of the holes, thus limiting the risk of any blanking burrs or irregular holes damaging the garments. Finally, the forming of the sheet metal for aesthetic/functional purposes is performed in a further station by inserting the panel in a mould formed by a mobile upper part (punch-holder) fixed to the ram of a hydraulic press, and a fixed lower part (die-holder) fixed to the base of the hydraulic press.

For example, U.S. Pat. No. 5,934,110 describes an apparatus for producing a washing machine drum comprising a stamping tool formed by an upper part and a lower part between which a sheet metal is clamped, said parts being provided with corresponding patterns of holes in which tools are housed. In particular, in the holes of the lower part, stamping and drawing dies are arranged lower than the separation plane of the stamping tool, while in the holes of the upper part, corresponding stamping and drawing punches are arranged concentrically and sliding with respect to each other and to said upper part.

The production method described in U.S. Pat. No. 5,934,110 involves a traditional first blanking phase with a blanking punch that protrudes inferiorly from the upper part of the stamping tool by sliding inside a deep-drawing punch that remains stationary inside said upper part. However, the blanking also causes an initial phase of partial deep-drawing, due to the distance between the sheet and the underlying blanking die, which results in a truncated conical portion of the sheet around the hole. This operation also causes the formation of a burr protruding along the lower edge of the inclined face of the hole, and in order to bring this burr back inside the hole, a subsequent deep-drawing phase is performed, transforming the internal part of the aforementioned truncated conical portion into a flat portion. Finally, the blanking punch is retracted to fold upwards and round off the burr so that it does not pose a hazard to the operator or the laundry.

Since the apparatus and method described in U.S. Pat. No. 5,934,110 do not envisage the forming phase, this will have to be performed in a subsequent station, and such a configuration clearly entails the drawback of a significant footprint and cost of the processing bench, which must also include mechanisms for moving the semi-finished panel from one station to the next. A further drawback is that the drum resulting from such a production method will have rather large processing tolerances, as the movement between the different stations makes it difficult to perform the subsequent processing operations always in exactly the same position on the panel.

A further drawback of the apparatus and method described in U.S. Pat. No. 5,934,110 is that the blanking punch is subjected to considerable strain with risk of breakage or deformation, as well as increased wear, since it also performs an initial deep-drawing phase without even being guided in its terminal operating portion by the deep-drawing punch, which remains inside the upper part during the blanking. This also means, given the relative freedom of the sheet metal in the space above the blanking die, that the blanking punch tends to deform the sheet metal uncontrollably even further away from the hole.

It is therefore an object of the present invention to provide a multi-function mould and a method of operation which overcome the aforementioned drawbacks. This object is achieved by means of a forming mould which also includes tools (dies, punches) for the deep-drawing and blanking stages, and a method of operation of such a mould for carrying out the processes according to a sequence of forming, deep-drawing and blanking.

The main advantage of this mould is therefore that it allows the three processing steps to be carried out in a single station, so that the processing bench is much more compact and economical. This translates into lower costs for both the set-up and maintenance of the processing bench and the production of the drum, due to the reduction in processing time and the lower amount of energy required.

Another important advantage of the above-mentioned mould and its related method of operation lies in the better quality of the finished product, compared to the traditional method with separate and contiguous processing operations, because the fact that the entire processing sequence is performed without moving the panel between one operation and the next allows for greater precision and repeatability with smaller tolerances.

Yet another advantage lies in the containment of the plastic deformation of the sheet metal around the hole in the blanking phase, because this process is carried out last when the sheet metal is still pressed in the forming die and in contact with the deep-drawing punch and die, which limit the effect of the blanking punch to the area of the hole. On the contrary, in the traditional method, in the absence of any restraining element, the blanking punch tends to deform the sheet metal even further away from the hole, with undesired and uncontrolled effects.

Further advantages and features of the mould and its method of operation according to the present invention will be apparent to those skilled in the art from the following detailed description of an embodiment thereof with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view from above of a processing bench with a single moulding station comprising a mould according to the present invention;

FIG. 2 is a perspective view from below of the forming punch of the above-mentioned mould;

FIG. 3 is a perspective view from above of the forming die of the above-mentioned mould;

FIG. 4 is a perspective view from above of the three sets of tools contained in the mould for performing deep-drawing and blanking operations, with an enlarged detail in exploded view;

FIG. 5 is a sectional view of the closed mould with the tools in their final operating position at the end of the three processing stages, with an enlarged detail;

FIG. 6 is an exploded schematic side view of the open mould before starting the panel processing;

FIG. 7 is a view similar to the previous one showing the first forming stage;

FIG. 8 is a view similar to the previous one showing the second deep-drawing stage; and

FIG. 9 is a view similar to the previous one showing the third blanking stage.

Referring to FIG. 1, it can be seen that a processing bench 1 traditionally comprises a frame structure 2 which carries a plurality of tools and devices, including an hydraulic press 3, for processing a stainless steel strip inserted through inlet 4 by unwinding it from a reel via a decoiler (not shown) which pushes the strip towards bench 1. Here, a panel is cut from the strip and sequentially subjected to the forming, drawing and blanking steps in the single station comprising press 3, which carries a mould 10 according to the present invention as described below.

Once these operations have been completed, the panel is sent through outlet 5 to the next stages of processing into a finished washing machine drum. It should be noted that the panel extends almost the entire length of bench 1 between inlet 4 and outlet 5, so that its complete processing over its entire length requires multiple processing sequences in press 3 on the various portions of the panel being advanced, typically three or four sequences.

The perspective views in FIGS. 2 and 3 show forming punch 11 and forming die 12, which are part of the movable upper part and fixed lower part of mould 10 respectively. If forming punch 11 and forming die 12 were simple traditional forming tools, they would imprint the aesthetic geometry on the sheet without allowing any further processing.

A first novel aspect of the mould according to the present invention is to comprise three sets of tools contained within it and arranged to interact with each other via two hole patterns 13, 14 formed at corresponding positions in the opposing operating surfaces of punch 11 and die 12 respectively. More specifically, as shown in FIGS. 4 and 5, mould 10 comprises a set of blanking punches 15, a set of deep-drawing punches 16 and a set of deep-drawing dies 17, which also cooperate with the blanking punches 15.

In fact, both punches 16 and dies 17 have a substantially tubular structure, and punches 15 are sized to be smoothly guided along the axial holes of said tools. In particular, punches 15 preferably have an upper portion 15a having a larger diameter that substantially corresponds to the inner diameter of punches 16 (with adequate clearance), and a lower portion 15b having a smaller diameter that substantially corresponds to the diameter of the entrance bore 17a of dies 17 (with adequate clearance). In this way, the blanking punches 15 are stronger and more durable as they are not limited in their diameter by the diameter of holes 17a but can have a larger diameter, taking advantage of the larger diameter of the deep-drawing punches 16.

In practice, the die set 17 accommodates the deep-drawing punches 16 and the blanking punches 15 within the forming die 12 in the lower half of mould 10 (note that the sheet metal panel arranged in mould 10 is not shown in FIGS. 5-9). The top of die 17 can be complementary to the shape of end 16a of the deep-drawing punch 16, or it can be flat to simply act as an end stop in the deep-drawing phase. Said end 16a preferably has a hemispherical shape, but may have other shapes such as a truncated cone, a circular paraboloid or the like.

While dies 17 are fixed inside the forming die 12, punches 15 and 16 move together with the forming punch 11, which is mounted on the upper part of mould 10, and are at least partially housed inside it. In addition, punches 15 and 16 are progressively engaged and actuated in order to make them protrude inferiorly from the forming punch 11 through holes 13, so as to perform their respective processes in a given sequence. Note that the hydraulic mechanisms used to actuate punches 15, 16 are standard and therefore need not be described.

The simple and effective operation of the mould according to the present invention is immediately apparent from the above description, with the aid of FIGS. 6-9 schematically illustrating its operation with reference only to the punches and dies.

In the initial position of FIG. 6 with the die open, punches 15 and 16 are schematically shown outside forming punch 11, while dies 17 are not visible as they are inside forming die 12. In the first forming stage of the panel (not shown), the mould is closed by bringing forming punch 11 against forming die 12, with punches 15, 16 moving together with punch 11 but not relative to it (FIG. 7).

In the second deep-drawing stage illustrated in FIG. 8, the first set of drawing punches 16 are lowered so as to protrude from the forming punch 11 and act on the panel until it is pushed against the drawing dies 17. Subsequently, in the third blanking phase of FIG. 9, the second set of punches 15 is engaged and, passing through the deep-drawing punches 16 and entering dies 17, shears a circular section concentric to the deep-drawing geometry, completing the processing with the drainage holes.

The mould-opening sequence is reversed from the processing sequence described above, i.e. first the blanking punches 15 are lifted, then the deep-drawing punches 16 are lifted, and then the forming punch 11 is lifted, so that the processed panel can be advanced to outlet 5 of the moulding station.

It should be noted that the forming punch 11 could theoretically also perform deep-drawing so as to reduce the process to just two steps, dispensing with the deep-drawing punches 16, but in reality it is more advantageous to use the mould and process according to the present invention. In fact, deep-drawing requires a stronger localised action than forming, which implies a higher wear of the punch in the deep-drawing areas, so that there would be an accelerated wear of punch 11 in the deep-drawing areas. This would involve replacing the entire punch 11 even if it was still effective for the forming operation, otherwise the entire punch 11 would have to be made of a stronger, more expensive and heavier material to avoid localised wear, with obvious cost disadvantages. Note that the same problem would occur in the case of using forming die 12 for deep-drawing as well, to dispense with drawing dies 17.

Conversely, having separate tools 11, 12 for the forming phase and 16, 17 for the deep-drawing phase allows only the tools affected by wear to be changed, and possibly even the deep-drawing profile to be changed independently without changing the forming and vice versa.

It is clear that the above-described and illustrated embodiment of the mould according to the invention is only an example susceptible to numerous variations. In particular, the shape, size and mode of coupling of punches 11, 15, 16 and dies 12, 17 may vary freely according to constructional requirements, as may the drive mechanisms of punches 15, 16. For example, punches 15 might have a single diameter along the shank, or a different clearance to punches 16 than to dies 17.

The innovative method of producing a semi-finished panel for a washing machine drum by means of a hydraulic press 3 equipped with a mould 10 according to the invention can thus be summarised in the following steps:

• • a) closing of mould 10 to perform an initial forming phase using the forming punch 11 and forming die 12;
  • b) lowering the deep-drawing punches 16 until the metal panel comes into contact with the deep-drawing dies 17, in order to perform a second deep-drawing phase;
  • c) lowering the blanking punches 15 until they enter holes 17a of the deep-drawing dies 17, in order to perform a third blanking phase;
  • d) lifting of the blanking punches 15;
  • e) lifting of the deep-drawing punches 16;
  • f) opening of mould 10.

As mentioned above, it is clear that this method can be applied to the production of a semi-finished metal panel for other types of products, other than a washing machine drum, as long as the processing of such a panel involves the same forming, deep-drawing and blanking steps.