Patent ID: 12194524

PREFERRED MODE FOR CARRYING OUT THE INVENTION

A press forming method according to an embodiment of the present invention is described below with reference to the drawings.

FIG.1Ais a plan view seen from the outside of a formed product9formed by a press forming method according to the present embodiment,FIG.1Bis a front view seen in a longitudinal direction of the formed product9,FIG.1Cis a cross-sectional view of the formed product9taken along line I-I inFIG.1A, andFIG.1Dis a partial perspective view of the formed product9.

As illustrated inFIG.1AtoFIG.1C, the formed product9has a rectangular shape as seen in a plan view and is in the form of a rectangular cylinder having a bottom. The formed product9has a hat shape as seen in cross section, and is formed from a plate-shaped workpiece by the press forming method described later. Described below is a case in which the formed product9is used as a battery case for housing a plurality of battery cells to be installed in a vehicle, but the present invention is not so limited.

The formed product9is provided with a top surface90extending along a plane that is approximately orthogonal to a pressing direction F (seeFIG.1C) in the press forming method and having a rectangular shape as seen in a plan view, a vertical wall portion92extending in the pressing direction F from the four edge portions of the top surface90, and a flange portion94extending outwardly from an edge portion of the vertical wall portion92along a plane that is approximately orthogonal to the pressing direction F (i.e. approximately parallel to the top surface portion90).

The vertical wall portion92is of a cylindrical shape connecting the top surface90and the flange portion94, and is rectangular as seen in a plan view. As illustrated inFIG.1C, the vertical wall portion92inclines slightly outwardly from the top surface portion90toward the flange portion94. The vertical wall portion92and the top surface portion90are approximately orthogonal to each other, and the vertical wall portion92and the flange portion94are approximately orthogonal to each other. In the below description, the portion bending outwardly as seen in cross section between the top surface portion90and the vertical wall portion92is referred to as an outward corner portion91, and the portion bending inwardly as seen in cross section between the vertical wall portion92and the flange portion94is referred to as an inward corner93. The outward corner91and the inward corner93each has a rectangular shape as seen in a plan view. In the below description, in the outward corner portion91, portions that curve with a predetermined radius of curvature R (seeFIG.9described below) are referred to as a first curved outward corner portion91aand a second curved outward corner portion91b. Described below is a case in which, as illustrated inFIG.1C, the radius of curvature as seen in cross section of the outward corner portion91is smaller than the radius of curvature as seen in cross section of the inward corner portion93, but the present invention is not so limited.

In the top surface portion90inward of the outward corner portion91, there is formed a step portion95having a rectangular shape as seen in a plan view. The step portion95forms a step as seen in cross section, as illustrated inFIG.1C. In the below description, the portion of the top surface portion90inward of the step portion95is referred to as a first flat portion97, and a portion outward of the step portion95is referred to as a second flat portion93. The second flat portion93is closer to the flange portion94in the pressing direction F than the first flat portion97. In other words, the second flat portion93is lower than the first flat portion97.

At an outer peripheral edge portion of the flange portion94, there is formed a bent portion96having a rectangular shape as seen in a plan view. The bent portion96forms a step, as illustrated inFIG.1C. The portion of the flange portion94inward of the bent portion96is closer to the top surface90in the pressing direction F than the portion outward of the bent portion96. In other words, the portion of the flange portion94outward of the bent portion96is lower than the inward portion.

In general, when forming a formed product9provided with an outward corner portion91, a vertical wall portion92, and an inward corner portion93as described above from a flat plate-shaped workpiece by way of only one draw forming step, cracking is likely to occur in the outward corner portion91, the vertical wall portion92, and the inward corner portion93. Among the outward corner portion91, the vertical wall portion92, and the inward corner portion93, cracking is particularly likely to occur in the first curved outward corner portion91aand the second curved outward corner portion91b.

In order to suppress the occurrence of cracking as described above, it would be conceivable to form the formed product9from the flat plate-shaped workpiece by way of two draw forming steps. However, when forming the formed product9by way of two draw forming steps, wrinkling may occur in the flange portion94. Wrinkling tends to occur particularly in the portion of the flange portion94outward in the peripheral direction relative to the first curved outward corner portion91aand the second curved outward corner portion91bas seen in a plan view. Described below is a press forming method that suppresses the occurrence of wrinkling and cracking in such a formed product9.

In the below description, a plate member of aluminum or an aluminum alloy, which are suitable materials for battery cases and are prone to cracking, was used as the workpiece, but the present invention is not so limited.

FIG.2is a flowchart illustrating the specific sequence of the press forming method according to the present embodiment. As illustrated inFIG.2, the formed product9as illustrated inFIG.1is formed from a single workpiece by way of a draw forming step (S1) and a restrike forming step (S2). The specific sequence of the draw forming step and the restrike forming step is described below.

FIG.3illustrates the configuration of a first pressing device1used in the draw forming step. The first pressing device1has a lower die mechanism20having a lower die21arranged below a plate-shaped workpiece W, and an upper die mechanism30that causes an upper die31to approach or separate from the lower die21in the pressing direction F.

The upper die mechanism30causes the upper die31to approach or separate from the lower die21by means of, for example, a servo motor. The upper die31presses the workpiece W together with the lower die21, and the lower surface of the upper die31has formed therein a die surface31afor contacting the upper surface of the workpiece W. The die surface31ahas a recessed shape facing the lower die21. An annular holder32is provided around the upper die31. The end surface of the holder32is horizontal, and protrudes slightly more toward the lower die21than the die surface31a. Therefore, when the upper die mechanism30causes the upper die31to approach the lower die21, the holder32comes into contact with the workpiece W before the die surface31adoes.

The lower die mechanism20is provided with the lower die21, a base22supporting the lower die21, an annular blank holder23supporting the portion of the workpiece W that will form the flange portion94of the formed product9, and a die cushion mechanism24that raises or lowers the blank holder23.

The lower die21is provided on top of the base22and presses the workpiece W together with the upper die31. On the upper surface of the lower die21there is formed a die surface21afor contacting the lower surface of the workpiece W. The die surface21ahas a protruding shape facing the upper die31.

The blank holder23is provided in a position facing the holder32in the pressing direction F, and clamps the outer peripheral edge portion of the workpiece W together with the holder32in order to prevent the occurrence of wrinkling and displacement, etc. when pressing the workpiece W. The die cushion mechanism24raises or lowers the blank holder23in the pressing direction F by means of an elevating mechanism not shown here.

In the draw forming step, the workpiece W is formed by using the first pressing device1in the following manner. First, after raising the blank holder23to a position higher than the lower die21, the workpiece W is placed on the blank holder23. Next, the upper die31is caused to approach the lower die21. When the upper die31is caused to approach the lower die21as described above, the holder32comes into contact with the workpiece W before the die surface31adoes, whereby the outer peripheral edge portion of the workpiece W is clamped between the holder32and the blank holder23. As the upper die31further approaches the lower die21, the die surface31aof the upper die31and the die surface21aof the lower die21come into contact with both surfaces of the workpiece W. The workpiece W is thus formed into a shape mimicking the die surface21a. In the below description, the formed product formed from the workpiece W in the draw forming step is referred to as an intermediate formed product.

FIG.4illustrates a comparison of the cross-sectional shape of the intermediate formed product8formed by way of the draw forming step and the cross-sectional shape of the formed product9. InFIG.4, the die surface21aof the lower die used in the draw forming step is illustrated by a thick line, and the die surface of a lower die used in the subsequent restrike forming step, that is to say the inside surface of the formed product9, is illustrated by a dashed line.

The die surface21aof the lower die used in the draw forming step is provided with, in order from the inside toward the outside, a top surface211, a step surface212, a protruding surface213, a vertical wall surface214, a recessed surface215, and a flange surface216. As illustrated inFIG.4, in the intermediate formed product8, a first intermediate flat portion87corresponding to the first flat portion97in the formed product9is formed by the top surface211, an intermediate step portion85corresponding to the step portion95in the formed product9is formed by the step surface212, a protruding portion81corresponding to the second flat portion98and the outward corner portion91in the formed product9is formed by the protruding surface213, an intermediate vertical wall portion82corresponding to the vertical wall portion92in the formed product9is formed by the vertical wall surface214, a recessed portion83corresponding to the inward corner portion93in the formed product9is formed by the recessed surface215, and an intermediate flange portion84corresponding to the flange portion94in the formed product9is formed by the flange surface216.

As illustrated inFIG.4, in the die surface21aof the lower die used in the draw forming step, the shapes of the top surface211, the step surface212, the vertical wall surface214, and the flange surface216are approximately the same as the inside surface of the formed product9, but the shapes of the protruding surface213and the recessed surface215differ from the inside surface of the formed product9. More specifically, the radii of curvature of the protruding surface213and the recessed surface215as seen in cross section are greater than the respective radii of curvature of the inside surface of the formed product9as seen in cross section. In addition, the protruding surface213bulges further toward the top surface211in the pressing direction F than the inside surface of the formed product9as seen in cross section.

Therefore, in the intermediate formed product8formed by way of the draw forming step as described above, the radii of curvature of the protruding portion81and the recessed portion83as seen in cross section are greater than the radii of curvature of the outward corner portion91and the inward corner portion93of the formed product9as seen in cross section, and the cross-sectional shape of the protruding portion81bulges further toward the top surface portion90in the pressing direction F than the second flat portion98and the outward corner portion91.

As illustrated inFIG.4, a height in the pressing direction F from the intermediate flange portion84to the first intermediate flat portion87in the intermediate formed product8is equal to a height in the pressing direction F from the flange portion94to the first flat portion97in the formed product9.

FIG.5illustrates the configuration of a second pressing device4used in the restrike forming step (seeFIG.2, S2).FIG.5illustrates a state wherein the intermediate formed product8formed by way of the above draw forming step is placed in the second pressing device4.

The second pressing device4is provided with a lower die41, a pad actuator (not shown) that causes a pad die42to approach or separate from the lower die41, and a movable die actuator (not shown) that causes a movable die43to approach or separate from the lower die41.

The shape of a die surface41aof the lower die41mimics the shape of the portion of the formed product9inward of the bent portion96. More specifically, the die surface41aof the lower die41is provided with, in order from the inside toward the outside, a top surface412, a protruding surface413, a vertical wall surface414, a recessed surface415, and a flange surface416. The surface of the pad die42is approximately flat, and has approximately the same shape as the first flat portion97of the formed product9as seen in a plan view in the pressing direction F. Therefore, by pressing the pad die42against the first intermediate flat, portion87of the intermediate formed product8by means of the pad actuator, outflow of material outward from the first intermediate flat portion87can be suppressed.

The shape of a die surface43aof the movable die43mimics the portion of the formed product9outward of the first flat portion97, More specifically, the die surface43aof the movable die43is provided with, in order from the inside toward the outside, a top surface432, a recessed surface433, a vertical wall surface434, a protruding surface435, and a flange surface436. The movable die43is also provided with, outward of the flange surface436, a fixing portion437protruding further toward the lower die41in the pressing direction F than the flange surface436. Therefore, when the movable die43is caused to approach the lower die41in the pressing direction F by means of the movable die actuator, the fixing portion437comes into contact with the intermediate formed product8before the die surface43adoes.

As is obvious from a comparison ofFIG.5andFIG.1D, in the formed product9, the first flat portion97is formed by the pad die42, the step portion95and the second flat portion98are formed by the top surface432, the outward corner portion91is formed by the protruding surface413, the vertical wall portion92is formed by the vertical wall surfaces414and434, the inward corner portion93is formed by the protruding surface435, the portion of the flange portion94inward of the bent portion96is formed by the flange surface436, and the portion of the flange portion94outward of the bent portion96, and the bent portion96itself, are formed by the fixing portion437.

In the restrike forming step, the formed product9is formed from the intermediate formed product8by using the second pressing device4in the following manner. First, the pad die42is pressed against the first intermediate flat portion37of the intermediate formed product3by means of the pad actuator. This suppresses outflow of material outward from the first intermediate flat portion87of the intermediate formed product8. Next, the movable die43is caused to approach the lower die41. When the movable die43is caused to approach the lower die41as described above, the fixing portion437comes into contact with the intermediate flange portion84of the intermediate formed product8before the die surface43adoes. Then, as the movable die43further approaches the lower die41, the outer peripheral edge portion of the intermediate flange portion84is bent by the fixing portion437, and is restrained between the fixing portion437and a side surface41boutward of the die surface41aof the lower die41. Thus, the bent portion96is formed while suppressing inflow of material inward from the outer peripheral edge portion of the flange portion84of the intermediate formed product B. In the restrike forming step, the formed product9is formed in this way by lowering the movable die43to a bottom dead center to press the protruding portion81of the intermediate formed product B down in the pressing direction F toward the recessed portion83, while suppressing outflow of material outward from the first intermediate flat portion87and inflow of material inward from the outer peripheral edge portion of the intermediate flange portion84.

FIG.6is a cross-sectional view of the intermediate formed product8for describing a movement of material in the restrike forming step. InFIG.6, the formed product.9formed from the intermediate formed product8by way of the restrike forming step is illustrated in dashed lines.

In the restrike forming step, when the movable die43is caused to approach the lower die41while suppressing outflow of material outward from the first intermediate flat portion87and inflow of material inward from the outer peripheral edge portion of the intermediate flange portion84by means of the pad die42and the fixing portion437, the protruding surface435comes into contact with the recessed portion83as illustrated inFIG.6, and the vertical wall surface434slides against the intermediate vertical wall portion82. Therefore, the material of the protruding portion81is pulled between the die surface41aand the vertical wall surface434to move outward in the peripheral direction. In addition, the material of the intermediate vertical wall portion82and the protrusion81is pulled between the vertical wall surface434and the recessed surface413to move toward the flange surface436in the pressing direction F. As the movable die43further approaches the lower die41in the pressing direction F, the top surface432and the protruding portion81come into contact near the bottom dead center, whereby the radii of curvature of the protruding portion81and the recessed portion83of the intermediate formed product3become smaller, and thus the outward corner portion91and the inward corner portion93are formed.

FIG.7Ais a partial perspective view of a portion of the intermediate formed product8corresponding to the first curved outward corner portion91aof the formed product9for describing extension of material in the draw forming step and the restrike forming step.FIG.7Bis a plan view of the same portion as inFIG.7Afor describing extension of material in the draw forming step and the restrike forming step. As described above, the first curved outward corner portion91aand the second curved outward corner portion91bare portions of the formed product9prone to cracking. InFIG.7A, the arrow71indicates extension of material in the draw forming step, and the arrow73indicates extension of material in the restrike forming step. It should be noted that the extension of the portion corresponding to the second curved outward corner portion91bis qualitatively the same as the extension of the portion corresponding to the first curved outward corner portion91a, and thus description thereof is omitted.

As illustrated inFIG.7A, the extension of the portion that will form the first curved outward corner portion91ain the formed product9mainly occurs in the pressing direction F in the draw forming step. The portion of the protruding portion81of the intermediate formed product8indicated by a thick solid line75moves toward the portion indicated by a thick dashed line76in the restrike forming step (seeFIGS.7A and7B). In this way, the extension of the portion that will form the first curved outward corner portion91ain the formed product9mainly occurs in a direction of a plane orthogonal to the pressing direction F in the restrike forming step.

FIG.8plots a state of strain in the draw forming step and the restrike forming step of the portion that will form the first curved outward corner portion81ain the formed product9in a forming limit diagram, wherein the horizontal axis is a maximum strain εx and the vertical axis is a minimum strain εy. InFIG.8, the thick solid line indicates a forming limit FL. That is to say, when the strain exceeds this forming limit FL, cracking or necking occurs in the formed product.

As described above, the extension of the material in the draw forming step mainly occurs in the pressing direction. Therefore, the strain state after the draw forming step changes into the state indicated by the point P1inFIG.8. The extension of the material in the restrike forming step mainly occurs in the direction of a plane orthogonal to the pressing direction. Therefore, the strain state of the formed product9after the restrike forming step changes into the state indicated by the point P2inFIG.8. As described above, the press forming method according to the present embodiment can suppress the occurrence of cracking and necking by forming the formed product9by way of the two steps of draw forming and restrike forming.

FIG.9is a plan view illustrating the vicinity of the first curved outward corner portion91aand the second curved outward corner portion91bof the formed product9. As described above, the bent portion96of the formed product9is formed by the fixing portion437of the movable die43in the restrike forming step. In the restrike forming step, the bent portion96is preferably formed such that the radii of curvature R of the first curved outward corner portion91aand the second curved outward corner portion91bbecome greater than the radii of curvature r as seen in a plan view of the portion of the bent portion96outward in the peripheral direction of the first curved outward corner portion91aand the second curved outward corner portion91b, as illustrated inFIG.9. This allows for suppressing the occurrence of wrinkling in the portion of the flange portion94inward of the bent portion96.

The press forming method according to the present embodiment exhibits the following effects.

(1) In the present press forming method, a formed product9which has a hat shape as seen in cross section, and which is provided with a top surface portion90, an outward corner portion91, a vertical wall portion92, an inward corner portion93, and a flange portion94, is formed by way of a draw forming step and a restrike forming step. In the draw forming step, an intermediate formed product8including a protruding portion81corresponding to the outward corner portion91in the formed product9and a recessed portion83corresponding to the inward corner portion93is formed from a workpiece W. In the draw forming step, the radii of curvature of the protruding portion81and the recessed portion83as seen in cross section are greater than those of the outward corner portion91and the inward corner portion93, and the cross-sectional shape of the protruding portion81is caused to bulge further toward the top surface portion90in the pressing direction F than the outward corner portion91. This suppresses the occurrence of cracking in the protruding portion81and the recessed portion83that will form the outward corner portion91and the inward corner portion93, and secures the required material for forming the outward corner portion91and the inward corner portion93in the subsequent restrike forming step. In the restrike forming step, the outward corner portion91and the inward corner portion93are formed by pressing the protruding portion81down in the pressing direction W toward the recessed portion83, while an inward portion of the intermediate formed product8inward of the protruding portion81and an outward portion thereof outward of the recessed portion83are restrained so as to suppress outflow of material outward from the inward portion of the protruding portion81and inflow of material inward from the outward portion of the recessed portion83. This allows for the material of the protruding portion81formed in the draw forming step to be moved to the vertical wall portion92, while the vertical wall portion92is extended in the pressing direction F toward the inward corner portion93, whereby the outward corner portion91and the inward corner portion93having small radii of curvature can be formed while suppressing the occurrence of wrinkling and cracking in the top surface portion90, the outward corner portion91, the vertical wall portion92, the inward corner portion93, and the flange portion94.

(2) In the restrike forming step, a bent portion96having a step shape is formed while restraining an outer peripheral edge portion outward of the recessed portion83of the intermediate formed product8to suppress inflow of material inward from the outer peripheral edge portion. In the formed product9obtained by way of the restrike forming step, radii of curvature R of a first curved outward corner portion91aand a second curved outward corner portion91bthat are portions of the outward corner portion91that curve as seen in a plan view are greater than radii of curvature r of portions of the bent portion96outward in a peripheral direction of the curved outward corner portions91a,91b. This allows for suppressing the occurrence of wrinkling in the portion of the flange portion94inward of the bent portion96of the formed product9, and therefore, when the formed product S is used as a battery case, the flange portion94can be used as a sealing surface.

(3) In the restrike forming step, a first flat portion96is formed in the top surface portion90of the formed product9by restraining the first intermediate flat portion87of the intermediate formed product8inward of the protruding portion81with a pad die42. In the formed product9obtained by way of the restrike forming step, a step portion95is formed between the first flat portion97and a second flat portion98outward of the first flat portion97of the top surface portion90, such that the second flat portion98is further toward the flange portion94in the pressing direction F than the first flat portion97. By forming such a step portion95, any wrinkling that may occur between the first flat, portion97and the outward corner portion91when pressing the protruding portion81down in the pressing direction F toward the flange portion94in the restrike forming step can be made less conspicuous.

(4) In general, when forming a plate member of aluminum or an aluminum alloy by deep-drawing, cracking is likely to occur due to localized extension. However, according to the present invention, by forming the formed product9from a workpiece W of aluminum or an aluminum alloy by way of the draw forming step and the restrike forming step described above, an outward corner portion91and an inward corner portion93having small radii of curvature can be formed while suppressing the occurrence of wrinkling and cracking.

An embodiment of the present invention has been described above, but the present invention is not limited to this embodiment, various modifications to the detailed configurations are possible without departing from the spirit and scope of the present invention.

EXPLANATION OF REFERENCE NUMERALS

W Workpiece1First pressing device21Lower die21aDie surface211Top surface212Step surface213Protruding surface214Vertical wall surface215Recessed surface216Flange surface4Second pressing device41Lower die41aDie surface412Top surface413Protruding surface414Vertical wall surface415Recessed surface416Flange surface42Pad die43Movable die43aDie surface432Top surface433Recessed surface434Vertical wall surface435Protruding surface436Flange surface437Fixing portion8Intermediate formed product81Protruding portion82Intermediate vertical wall portion83Recessed portion84Intermediate flange portion85Intermediate step portion87First intermediate flat portion9Formed product90Top surface portion91Outward corner portion91aFirst curved outward corner portion (curved outward corner portion)91bSecond curved outward corner portion (curved outward corner portion)92Vertical wall portion93Inward corner portion94Flange portion95Step portion96Bent portion97First flat portion98Second flat portion