HK1117091B - Method for designing a die - Google Patents

Description

The present invention relates to a method for designing a die for production of hollow panel, according to the preamble of claim 1. An example of such a method is known from EP-A-1630241 .

There have been known dies to form hollow sections from a blank by extrusion and there have also been known methods for designing such dies.

Japanese Patent No. 3645453 discloses a die which consists of more than one entry port for flow splitting of raw material, a chamber (merging space) in which the flows of raw material passing through each entry port merge and weld together, and a forming space in which the raw material which has been welded together in the merging space is formed into a hollow section by extrusion. The hollow section extruded from the die has more than one weld mark resulting from the welding of the split raw material passing through the entry ports. In addition, the die-designing method disclosed in said patent specifies each entry port which has an adequate position of gravitational center and.an adequate area of orifice so that there exists a correspondence between the ratio of volume distribution among regions welded together in the hollow section and the ratio of flows of raw material passing through each entry port. The thus designed die permits uniform extrusion from the forming space without causing excess distortion to the raw material. This yields hollow sections with high dimensional accuracy and straightness.

The die disclosed in said patent mentioned above, however, does not yield any satisfactory thin, wide hollow panel due to unintended wavy distortion.

EP-A-1 630 241 discloses a high-strength aluminium-alloy extruded material with excellent corrosion resistance and method of producing the same in which a ratio of a flow speed of the aluminium alloy in a joining section to the flow speed in a non-joining section is set at 1.5 or less.

US-A-3 527 079 discloses a feeder hole die with improved metal flow which permits substantial increase in volume of material moved through the die while minimizing die wear.

US-B1-6 557 388 discloses a method of determining dimension of extrusion die and extrusion die produced based on the same including determining a dimension of the extrusion die based on sizes of a plurality of figures.

The present invention was completed to provide a method for designing a die for production of well-shaped thin, wide hollow panels free of the foregoing problem.

As mentioned above, unintended distortion is inevitable in thin, wide hollow panels even thought each entry port is specified such that there exists a correspondence between the ratio of volume distribution among regions welded together and the ratio of flows of raw material passing through each entry port. The present inventors' investigation revealed that this problem arises from unbalanced strain in individual parts in the hollow panel. This finding led to a method for designing a die having more than one entry port for flow splitting of raw material, a merging space in which the flows of raw material passing through each entry port merge and weld together, and a forming space in which the raw material which has been welded together in the merging space is formed into a hollow section by extrusion, wherein in said method, each of said entry ports and said merging space have characteristic design values such that said hollow panel extruded from said die has more than one weld line formed by raw material passing through said entry ports and also has a limited strain which does not differ more than ±10% between the average value of strain in all regions in the welded parts and the individual values of strain in each region in the welded parts.

According to the die-designing method mentioned above, the entry ports and the merging space are assigned characteristic design values so that no difference exceeding ±10% occurs between the average value of strain in all regions in the welded parts and the individual values of strain in each region in the welded parts. The thus designed extrusion die yields a hollow panel with a limited strain which differs only slightly from one region of the welded parts to another. The limited strain prevents the extruded hollow panel from suffering unintended wavy distortion. This is true particularly in the case of thin, wide hollow panels with low stiffness. Thus, the extrusion die according to the present invention is suitable for production of thin, wide hollow panels free of distortion.

The above-mentioned die-designing method should preferably be modified by changing the characteristic design values of said entry ports and said merging space such that said hollow panel extruded from said die has a limited strain which does not differ more than ±6% between the average value of strain in all regions in the welded parts and the individual values of strain in each region in the welded parts. The thus designed extrusion die yields a hollow panel with a limited strain which differs only slightly from one region of the welded parts to another. The limited strain prevents the extruded hollow panel from suffering unintended wavy distortion. This is true particularly in the case of thin, wide hollow panels with thick parts and an opening at the center in the widthwise direction. Thus, the extrusion die according to the present invention is suitable for production of thin, wide hollow panels free of distortion.

The die-designing method should preferably be further modified by changing the characteristic design values of said entry ports and said merging space such that said hollow panel extruded from said die has a limited strain which does not differ more than ±3% between the average value of strain in all regions in the welded parts and the individual values of strain in each region in the welded parts. The thus designed extrusion die yields a hollow panel with a limited strain which differs only slightly from one region of the welded parts to another. The limited strain prevents the extruded hollow panel from suffering unintended wavy distortion. This is true particularly in the case of thin, wide hollow panels with thick parts and uneven stiffness and an opening away from the center in the widthwise direction. Thus, the extrusion die according to the present invention is suitable for production of thin, wide hollow panels free of distortion.

The above-mentioned method for designing a die should preferably be characterized in that each of said entry ports and said merging space have characteristic design values such that there exists a correspondence between the ratio of volume distribution among regions welded together in the hollow panel and the ratio of flows of raw material passing through each entry port and said hollow panel has a limited strain which does not differ more than ±10% between the average value of strain in all regions in the welded parts and the individual values of strain in each region in the welded parts. The die designed in this manner achieves extrusion such that there exists a correspondence between the ratio of volume distribution among regions welded together in the hollow panel and the ratio of flows of raw material passing through each entry port. Therefore, it is capable of uniform extrusion of raw material through the forming space without causing excessive distortion to raw material, and the resulting hollow panel has high dimensional accuracy and good straightness. Moreover, the die designed in this manner gives a hollow panel characterized by a limited strain which does not differ more than ±10% between the average value of strain in all regions in the welded parts and the individual values of strain in each region in the welded parts. Therefore, it is capable of forming a satisfactory thin, wide hollow panel as mentioned above. The resulting hollow panel has a high dimensional accuracy and good straightness.

The die according to the present invention is produced by any one of the above-mentioned die-designing methods. It is capable of extruding hollow panels while suppressing unintended wavy distortion. Hence, it is suitable for extrusion of thin, wide hollow panels in good shape.

As mentioned above, the present invention provides a method for designing a die for extruding a thin, wide hollow panel in a good shape.

• Fig. 1 is a perspective view showing the structure of the hollow panel formed by extrusion through the die according to the present invention.
• Fig. 2 is a side view (in the direction of extrusion) of the hollow panel shown in Fig. 1.
• Fig. 3 is a schematic sectional view of important parts showing the structure of the extrusion apparatus to form the hollow panel (shown in Fig. 1) by extrusion.
• Fig. 4 is a partly cut-away perspective view showing the die designed according to the present invention.
• Fig. 5 is a perspective view showing how the billet (as a blank) changes in shape while passing (for extrusion) through a die attached to a forming apparatus.
• Fig. 6 is a diagram showing a deformed hollow panel.
• Fig. 7 is a side elevation (viewed in the extrusion direction) showing a hollow panel.
• Fig. 8 is a side elevation (viewed in the extrusion direction) showing a hollow panel.

A detailed description of the preferred embodiment of the invention will be made below with reference to the accompanying drawing.

The first embodiment

Figs. 1 and 2 show the structure of the hollow panel 1 formed by extrusion through a die 24 (mentioned later) designed according to the present invention. The hollow panel 1 consists of paired flat sheets 2 and 2. The sheets 2 and 2 are arranged parallel to each other, a certain distance away from each other in their thickness direction, and they are joined to each other with a number of ribs 4 formed between them. The sheets 2 and ribs 4 have the same thickness. There is an internal space 6 (extending in the extrusion direction or the lengthwise direction of the hollow panel 1) among the sheet 2 and the ribs 4 and 4 adjoining in the widthwise direction of the hollow panel 1.

This hollow panel 1 is produced by using a forming apparatus 10 shown in Fig. 3. The forming apparatus 10 consists of a container 12, a platen 14 (positioned away from the container 12), and a forming unit 16 (positioned between them).

The container 12 has an inner hole 12a extending in the direction of extrusion of the billet 18 (blank) of aluminum alloy. The inner hole 12a is provided with a stem 19 which is driven by a driving mechanism (not shown). The stem 19 is movable back and forth in the direction of extrusion of the billet 18. The platen 14 is fixedly positioned in the downstream of extrusion of the billet 18 from the container 12. (This position is at the right side in Fig. 3.)

The forming unit 16 consists of a die slide 20, a die ring 22, a die 24, a backer 26, and a bolster 28. The die slide 20 is movable in the direction perpendicular to the direction of extrusion of the billet 18. It is so constructed as to move back and forth between the set position (intermediate between the container 12 and the platen 14) and the retreat position away from the set position in the direction perpendicular to the direction of extrusion of the billet 18.

The die ring 22 is held by the die slide 20. The die ring 22 and the bolster 28 are juxtaposed to each other (in the lateral direction in Fig. 3), and they are held between the container 12 and the platen 14. The die ring 22 is an annulus ring, which holds therein the die 24 and the backer 26. The die 24 and the backer 26 are sequentially arranged in the direction of extrusion.

The die 24 consists of a male die 30 and a female die 31. It is constructed as shown in Fig. 4, which is a partly cut-away perspective view. The male die 30 and the female die 31 are sequentially arranged in the direction of extrusion of the billet 18, so that the male die 30 fits into the female die 31. The male die 30 has the entry ports 30a that penetrate in the direction of extrusion. The entry ports 30a communicate with the inner hole 12a of the container 12. See Fig. 3. The male die 30 has a projection 30b extending in the direction of extrusion of the billet 18. The projection 30b has one end (close to the female die 31) which conforms to the shape of the internal space 6 of the hollow panel 1.

The female die 31 has a merging space 31a (which communicates with the entry port 30a of the male die 30) and a forming hole 31b (which communicates with the merging space 31a). The merging space 31a permits communication between the entry port 30a and the forming space 31b. The forming hole 31b has a cross section (perpendicular to the direction of extrusion) which conforms to the peripheral shape of the hollow panel 1. The forming hole 31b holds therein the projection 30b of the male die 30, with a gap left between the projection 30b and the inner wall of the forming hole 31b. The gap (between the projection 30b of the male die 30 and the inner wall of the forming hole 31b of the female die 31) constitutes the forming space S which conforms to the shape of the hollow panel 1. The forming space S communicates with the merging space 31a.

Fig. 5 shows the process in which the billet 18 (as a blank) changes in shape in the forming apparatus 10 during its extrusion into the hollow panel 1 through the die 24. The following steps proceed to form the hollow panel 1. The billet 18 is forced into the die 24 by the stem 19 from the inner hole 12a of the container 12. Then, the billet is split into the flows 18a of raw material by the entry ports 30a. The flows 18a of raw material are forced out of the entry ports 30a. After passing through the entry ports 30a, the flows 18a of raw material merge (weld together) into the single flow 18b of raw material in the merging space 31a. The flow 18b of raw material is formed into the hollow panel 1 (with a hollow cross section) by extrusion through the forming space S. The resulting hollow panel 1 has the linear weld lines 1a extending in the direction of extrusion. (The weld lines 1a are formed as the flows 18a of raw material weld together after passing through the entry ports 30a.) In other words, the hollow panel 1 has the (unwelded) regions 1b between the weld lines 1a at the positions corresponding to the positions of the flows of raw material 18a passing through the entry ports 30a.

The extrusion process mentioned above causes the flows 18a (18b) of raw material to move from the entry ports 30a into the forming space S through the merging space 31a. However, they do not necessarily move in the entire region of the entry ports 30a and the merging space 31a. This is illustrated in Fig. 4, in which each of the entry port 30a and the merging space 31a is divided into two regions by the imaginary line A. In the region inside the imaginary line A, the flow of raw material 18a (18b) moves toward the forming space S, while in the region outside the imaginary line A, the flow 18a (18b) of raw material stops. The fact that the flow 18a (18b) of raw material separates into the moving portion and the stagnant portion during extrusion results in strain that remains in the hollow panel 1 even after extrusion from the forming space S. In other words, the strain remains in the region 1b between the weld lines 1a of the hollow panel 1 which has been formed from the flow 18a of raw material passing through the entry ports 30a.

The present inventors' investigation to address the above-mentioned problem revealed that the unintended wavy distortion in the thin, wide hollow panel 1 (shown in Fig. 6) is due to difference in strain existing in the regions 1b between the weld lines 1a. This finding led to a technical idea of suppressing unintended distortion in the hollow panel 1 by reducing difference in strain in the regions 1b between the weld lines 1a.

According to this technical idea, the present invention is intended to provide a method to design the die 24 by establishing the design characteristic value for each of the entry ports 30a and the design characteristic value for the merging space 31a such that there exists a correspondence between the ratio of volume distribution ratio (Vi) among the regions 1b between the weld lines 1a of the hollow panel 1 and the ratio of flow (Xi) of raw material 18a passing through each entry port 30a and the hollow panel 1 has a limited strain which is defined by a difference (Δε) not exceeding ±10% between the average value of strain (εi) in all regions 1b in the welded parts 1a and the individual values of strain (εi) in each region in the welded parts 1a.

The ratio of volume distribution (Vi) is defined as the ratio of the individual volume of the region 1b between the welded part 1a to the total volume of the unit length of extrusion of the hollow panel 1. The ratio of flow (Xi) is defined as the ratio of the individual flow of raw material 18a passing through each entry port 30a to the total flow of raw material 18a passing through all the entry ports 30a. The design characteristic values for the entry port 30a include Sei (which is the sectional area perpendicular to the direction of extrusion), Lei (which is the circumferential length), Hei (which is the depth, shown in Fig. 4), and rei (which is the center position). The design characteristic values for the merging space 31a include Hc (which is the depth, shown in Fig. 4). Incidentally, the depth (Hei) of the entry port 30a is the length (in the direction of extrusion) of the entry port 30a formed in the male die 30. The depth (Hc) of the merging space 31a is the length in the direction of extrusion of the entry port 30a between the end (adjacent to the male die 30) of the female die 31 and the bottom of the merging space 31a. The subscript "i" denotes a variable to specify individual entry ports 30a, such that i = 1 to n, where n is the number of entry ports 30a.

The die 24 is practically designed in the following manner. The first step is to temporarily establish the position of each welding part 1a of the hollow panel 1. The second step is to calculate the total volume per unit extrusion length of the hollow panel 1. The third step is to calculate individually the volume of the region 1b between the welding parts 1a. The fourth step is to calculate the ratio of the individual volume to the total volume, thereby obtaining the ratio of volume distribution (Vi) of each region 1b between the welding parts 1a.

The subsequent step is to derive the function f(rei) expressing the relation between position and flow rate, the function g(Sei,Lei,Hei) expressing the relation between shape and flow rate, and the function h(Sei,Hei) expressing the relation between dimension and flow rate, from the above-mentioned design characteristic values (Sei,Hei,rei) of each entry port 30a. These functions are combined together to give the following formula (1) that represents the ratio of flow (Xi) passing through each entry port 30a.

$\mathrm{Xi}=f\left(\mathrm{rei}\right)\cdot g\left(\mathrm{Sei},,\mathrm{Lei},,\mathrm{Hei}\right)\cdot h\left(\mathrm{Sei},,\mathrm{Hei}\right)$ The formula (1) is given arbitrary design characteristic values (Sei, Hei, and rei) to obtain the ratio of flow (Xi), and the thus obtained value of Xi is compared with the ratio (Vi) of volume distribution. If no correspondence between Xi and Vi is reached, the design characteristic values Sei, Lei, Hei, and rei are partly or entirely modified and the above-mentioned procedure is repeated until a correspondence between Vi and Xi is reached. In this way the design characteristic values (Sei, Lei, Hei, and rei) are obtained as desired.

Next, calculations are carried out for strain (εi) in each region 1b between the welding parts 1a of the hollow panel 1. This strain (εi) is obtained from the following formula (2), in which Sei represents the sectional area (perpendicular to the direction of extrusion) of each entry port 30a and Soi represents the sectional area (perpendicular to the direction of extrusion) of the corresponding region 1b of the hollow panel 1.

$\mathrm{\epsilon i}=\log \left(\mathrm{Sei},,,/,\mathrm{Soi}\right)$ The strain (εi) obtained from the formula (2) is put in the following formula (3) to give the relative error (Δε) of the strain (εi) in each region 1b between the welding parts 1a to the average value (Σεi/n) of the strain in each region 1b between the welding parts 1a.

$\mathrm{\Delta \epsilon }=\left(1,-,\mathrm{\epsilon i},/,\left(\mathrm{\Sigma \epsilon i},/,n\right)\right)\times 100$ The formula (3) is evaluated to see if it gives a relative error (Δε) within ±10%. If the result is negative, modifications are made to the design characteristic values (Sei, Lei, Hei, and rei) for the entry ports 30a and the design characteristic value (Hc) for the merging space 31a. The formula (3) is evaluated again to calculate the relative error (Δε) and judgment is made again to see if the relative error (Δε) is within ±10%. The foregoing procedure is repeated to obtain the design characteristic values (Sei, Lei, Hei, and rei) for the entry ports 30a and the design characteristic value (Hc) for the merging space so that the relative error (Δε) is within ±10%.

The foregoing steps are repeated to reconfirm the correspondence between Vi and Xi and the relative error (Δε) according to the formulas (1) and (3). The foregoing procedures are employed to design the die 24 according to the invention.

The design characteristic values (Sei, Lei, Hei, and rei) obtained as mentioned above are employed to form the entry ports 30a, and the characteristic value (Hc) obtained as mentioned above is employed to form the merging space 31a. In this way the die 24 is produced.

The resulting die 24 is attached to the forming apparatus 10. Extrusion through the die 24 yields the hollow panel 1 in which there exists a correspondence between Vi (the ratio of volume distribution in each region 1b between the welding parts 1a of the hollow panel 1) and Xi (the ratio of flow of raw material 18a passing through the die 24 and the entry ports 30a) and there is a relative error (Δε) within ±10% for the strain (εi) in each region 1b between the welding parts 1a to the average value of the stain (εi) in each region 1b between the welding parts 1a.

As mentioned above, a die designed by the method of the invention gives the hollow panel 1 in which there exists a correspondence between Vi (the ratio of volume distribution in each region 1b between the welding parts 1a) and Xi (the ratio of flow of raw material 18a passing through the die 24 and the entry ports 30a). The result is uniform extrusion from the forming space S of the die 24 without excessive distortion in the raw material 18a (18b). The hollow panel 1 extruded in this manner has high dimensional accuracy and straightness.

Moreover, the invention permits the die 24 to extrude the hollow panel 1 in which there is a relative error (Δε) within ±10% for the strain (εi) in each region 1b between the welding parts 1a to the average value of the stain (εi) in each region 1b between the welding parts 1a. The resulting hollow panel 1 has a reduced difference in strain ((εi) among the individual regions 1b between the welding parts 1a. This helps reduce unintended wavy distortion in the thin, wide hollow panel 1 with low stiffness which is extruded from the die 24. Therefore, the invention permits the designing of a die for extrusion of thin, wide hollow panels with a good shape as well as a high dimensional accuracy and straightness.

Fig. 7 shows the structure of the hollow panel 41 produced by a die designed according to a second preferred embodiment of the present invention. The second preferred embodiment is concerned with extrusion of the hollow panel 41 which has thick-walled parts 43 at its both lateral ends and an opening at the center of its width.

The hollow panel 41 according to the second preferred embodiment has at its lateral ends the thick-walled parts 43 which are thicker than the sheet 2 and the rib 4. It has its lateral ends closed by the thick-walled parts 43, and it has the upward opening 45 at the center of its width. The opening 45 extends in the lengthwise direction (or the direction of extrusion) of the hollow panel 41.

It is to be noted from Fig. 7 that the region 2a near the opening 45 and the region 2b under the opening 45 are thicker than the upper and lower sheets 2. The regions 2a and 2b are joined together by the ribs 4a, which are also thicker than other ribs 4. The hollow panel 41 with such a structure has the region near the opening 45 reinforced by the thick-walled regions surrounding the opening 45.

The hollow panel 41 mentioned above is more subject to unintended wavy distortion than the hollow panel 1 (shown in Fig. 2) according to the invention because of variation in thickness and unbalanced stiffness due to the opening 45.

This problem is addressed in the second preferred embodiment by designing the die 24 (shown in Fig. 4) with the design characteristic values for the entry ports 30a and the design characteristic values for the merging space 31a such that the hollow panel 41 has a relative error (Δε) within ±6% for the strain (εi) in each region 41b between the welding parts 41a to the average value of the stain (εi) in each region 41b between the welding parts 41a. To be concrete, this object is achieved by establishing the design characteristic values for the entry ports 30a (which include the sectional area Sei (perpendicular to the direction of extrusion), the circumferential length Lei, the depth Hei, and the central position rei of the entry port 30a) and the design characteristic value for the merging space 31a (which includes the depth Hc) by using the formulas (2) and (3) mentioned above, so that the relative error (Δε) is within ±6%.

The second preferred embodiment is also carried out in the same way as the invention. The procedure consists of steps of calculating the design characteristic values (Sei, Lei, Hei, and rei) for the entry ports 30a by using the formula (1) mentioned above, so that there exists a correspondence between the ratio of volume distribution (Vi) and the ratio of passing flow (Xi). The foregoing steps are repeated in order to reconfirm the correspondence between Vi and Xi and the relative error (Δε) within ±6%, in the same way as in the invention.

The die 24 designed as mentioned above yields the hollow panel 41 which is less liable to unintended distortion than the hollow panel in the invention on account of the reduced difference in strain (εi) in each region 41b between the welding parts 41a. The second preferred embodiment is identical with the invention except for the above-mentioned constitution.

As mentioned above, the second embodiment permits the designing of a die 24 to extrude the hollow panel 41 in which there is a relative error (Δε) within ±6% for the strain (εi) in each region 41b between the welding parts 41a to the average value of the stain (εi) in each region 41b between the welding parts 41a. The resulting hollow panel 41 has a reduced difference in strain (εi) among the individual regions 41b between the welding parts 41a, owing to the specifically established design characteristic values for the entry ports 30a and the merging space 31a. This helps reduce unintended wavy distortion in the hollow panel 41 having the thick-walled part 43 and the opening 45 at the center of the width.

Fig. 8 shows the structure of the hollow panel 51 produced by a die designed according to the third preferred embodiment of the present invention. The third preferred embodiment is concerned with extrusion of the hollow panel 51 which is similar in structure to the hollow panel 41 (shown in Fig. 7) according to the second preferred embodiment except that the opening 55 is shifted from the center of its width.

The hollow panel 51 (shown in Fig. 8) according to the third preferred embodiment has the opening 55 in its upper sheet at a position shifted rightward from the center of its width. The opening 55 extends in the lengthwise direction (or the direction of extrusion) of the hollow panel 51.

The hollow panel 51 lacks uniform stiffness, unlike the hollow panel 41 (shown in Fig. 7) according to the second preferred embodiment, because of the opening 55 formed at a position shifted from the center of its width. Therefore, it is more subject to unintended wavy distortion than the hollow panel 41 (shown in Fig. 7) according to the second preferred embodiment.

This problem is addressed in the third preferred embodiment by designing the die 24 (shown in Fig. 4) with the design characteristic values for the entry ports 30a and the design characteristic values for the merging space 31a such that the hollow panel 51 has a relative error (Δε) within ±3% for the strain (εi) in each region 51b between the welding parts 51a to the average value of the stain (εi) in each region 51b between the welding parts 51a. To be concrete, this object is achieved by establishing the design characteristic values for the entry ports 30a (which include the sectional area Sei (perpendicular to the direction of extrusion), the circumferential length Lei, the depth Hei, and the central position rei of the entry port 30a) and the design characteristic value for the merging space 31a (which includes the depth Hc) by using the formulas (2) and (3) mentioned above, so that the relative error (Δε) is within ±3%.

The third preferred embodiment is also carried out in the same way as the invention. The procedure consists of steps of calculating the design characteristic values (Sei, Lei, Hei, and rei) for the entry ports 30a by using the formula (1) mentioned above, so that there exists a correspondence between the ratio of volume distribution (Vi) and the ratio of passing flow (Xi). The foregoing steps are repeated in order to reconfirm the correspondence between Vi and Xi and the relative error (Δε) within ±3%, in the same way as in the invention.

The die 24 designed as mentioned above yields the hollow panel 51 which is less liable to unintended distortion than the hollow panel in the invention on account of the more reduced difference in strain (εi) in each region 51b between the welding parts 51a. The third preferred embodiment is identical with the invention except for the above-mentioned constitution.

As mentioned above, the third preferred embodiment permits the die 24 to extrude the hollow panel 51 in which there is a relative error (Δε) within ±3% for the strain (εi) in each region 51b between the welding parts 51a to the average value of the stain (εi) in each region 51b between the welding parts 51a. The resulting hollow panel 51 has a further reduced difference in strain (εi) among the individual regions 51b between the welding parts 51a, owing to the specifically established design characteristic values for the entry ports 30a and the merging space 31a. This helps reduce unintended wavy distortion in the hollow panel 51 having the thick-walled part 43 and the opening 55 at a position shifted from the center of the width.

The embodiments disclosed herein should be construed to be merely exemplary in all respects, and they are not limited to the disclosure. The scope of the present invention should be defined by the appended claims but not by the disclosed embodiments, and it embraces any modifications equivalent to what is defined in the claims.

For example, modifications may be made to the method for designing the die 24 according to the invention and embodiments mentioned above such that there exists a slight difference (instead of exact correspondence) between Vi (the ratio of volume distribution in each region 1b (41b, 51b) between the welding parts 1a (41a, 51a) in the hollow panel 1 (42, 51) and Xi (the ratio of flow of raw material 18a passing through the entry ports 30a). It is permissible to establish the design characteristic values for the entry ports 30a and the design characteristic values the merging space 31a such that there exists a relative error (Δi) within ±10% for the strain (εi) in each region 1b between the welding parts 1a to the average value of the stain (εi) in each region 1b between the welding parts 1a in the hollow panel 1 according to the first embodiment. It is also permissible to establish the design characteristic values for the entry ports 30a and the design characteristic values for the merging space 31a such that there exists a relative error (Δi) within ±6% in the hollow panel according to the second embodiment. It is also permissible to establish the design characteristic values for the entry ports 30a and the design characteristic values for the merging space 31a such that there exists a relative error (Δi) within ±3% in the hollow panel according to the third embodiment. Particularly, a slight difference between Vi (the ratio of volume distribution) and Xi (the ratio of flow rate) does not appreciably affect the dimensional accuracy and straightness in the case of short hollow panels. Therefore, according to the present invention, it is possible to design a die form hollow panels in good shape while suppressing any trouble resulting from the lack of correspondence between Vi (the ratio of volume distribution) and Xi (the ratio of flow rate).

Claims (4)

1. A method for designing a die (24) to be used for extruding a hollow panel (1), the die (24) having:

   more than one entry port (30a) for flow splitting of raw material (18a),

   a merging space (31 a) in which the flows of raw material (18a) passing through each entry port (30a) can merge and weld together, and

   a forming space (S) in which the raw material which has been welded together in the merging space (31 a) can be formed into a hollow section by extrusion; and

   the hollow panel (1) having:

   a sectional area (Soi) of an unwelded region (1 b), the unwelded region (1 b) being formed between weld lines (1 a) at positions corresponding to positions of flows of raw material (18a) passing through said entry ports (30a);

   wherein said method is characterized by:

   providing characteristic design values for each of said entry ports (30a) and said merging space (31 a), wherein said characteristic design values include a sectional area (Sei) perpendicular to the direction of extrusion, a circumferential length (Lei) and a depth (Hei),

   calculating a strain (εi) by the following formula (f1): $\mathrm{\epsilon i}=\log \left(\mathrm{Sei},/,\mathrm{Soi}\right);$ and

   limiting a strain to a value which does not differ more than ±10% between the average value of strain in all regions in the welded parts and the individual values of strain in each region in the welded parts by evaluating the following formula (f2): $\mathrm{\Delta \epsilon }=\left(1,-,\mathrm{\epsilon i},/,\left(\mathrm{\Sigma \epsilon i},/,n\right)\right)\times 100;$ wherein εi denotes a strain, Sei denotes a sectional area of each entry port (30a), Soi denotes a sectional area (Soi) of an unwelded region (1 b) of said hollow panel (1), Δε denotes a relative error and Σεi/n denotes an average value of strain in each unwelded region (1 b),

   to see if the formula (f2) gives a relative error Δε within ±10%, and if the result is negative, modifying the sectional area (Sei) and evaluate the formula (f2) again, and repeat the foregoing procedure until a sectional area (Sei) is obtained for which the relative error Δε is within ±10%.
2. The die-designing method as defined in Claim 1, in which change is made to the characteristic design values (Sei, Lei, Hei) of said entry ports (30a) and said merging space (31 a) such that said hollow panel (1) extruded from said die (24) has a limited strain (εi) which does not differ more than ±6°% between the average value (Σεi/n) of strain in all regions in the welded parts and the individual values of strain (εi) in each region in the welded parts.
3. The die-designing method as defined in Claim 1, in which change is made to the characteristic design values (Sei, Lei, Hei) of said entry ports (30a) and said merging space (31 a) such that said hollow panel (1) extruded from said die (24) has a limited strain (εi) which does not differ more than ±3% between the average value (Σεi/n) of strain in all regions in the welded parts and the individual values of strain (εi) in each region in the welded parts.
4. The die-designing method as defined in Claim 1, which is wherein each of said entry ports (30a) and said merging space (31 a) have characteristic design values (Sei, Lei, Hei) such that there exists a correspondence between the ratio (Vi) of volume distribution among regions welded together in the hollow panel (1) and the ratio (Xi) of flows of raw material passing through each entry port (30a).