Extrusion die device and die insert therefor

Abstract: An extrusion die device includes a die holder and at least one extrusion die insert (20) detachably held by the die holder. The extrusion die insert (20) includes a female die (20b) of a generally cylindrical shape having a die cavity (26) and a discharge passage-way (26), and a male die (20a) having a protrusion part (22). The female die (20b) includes a fitting recess (25) having a generally channel-shaped cross-section formed in one end surface thereof. The male die (20a) is of a generally plate-like shape having portions defining a fitting portion (23). The male die (20a) is associated with the female die (20b) such that the protrusion part (22) is fitted in the die cavity (26) and that the fitting portion (23) is fitted in the fitting recess (25). The die device is durable and retains the original mechanical precision of a new die during its long service life, and can be easily fabricated at high precision.

Description: The present invention relates to an extrusion die device according to the preamble of claim 1 and to a die insert according to the preamble of claim 13 as for example known from US-A-2 366 344. These devices are suitable for the production of small objects for use in electrical, automotive and related manufacturing industries. Such objects are multi-cavity flat tubes which are used in aluminum heat exchangers such as evaporators for automotive air-conditioners, condensers, radiators or the like. The features of the invented extrusion die device and die insert are that they are easier to manufacture compared with the conventional extrusion dies and that they provide a long service life, by maintaining the dimensional accuracy even after a prolonged use. Extrusion process is explained first with reference to Figure 1. In general, a process of forming an object by extrusion includes the following steps: placing a billet 51 in a container 50; pressing the billet 51 with a stem 53 toward an exit opening 54 or a depressed section; flowing the material constituting the billet 51 through a space defined by the opening 54 (die cavity), and in some case also by a mandrel which is inserted into the opening, in which the space formed in between the opening and the mandrel is shaped in a shape of the profile of the object desired. Through such a process, an object having the desired cross sectional shape is obtained. One important feature of the extrusion process is that a product of a very complex cross sectional shape can be obtained through one processing step of exerting a compressive pressure around the billet placed in the container, and squeezing the material out of the shaped die cavity. For this reason, extrusion process is applied also to forming of aluminum alloys to produce multi-cavity flat tubes for use in heat exchangers such as evaporators for automotive air-conditioners, condensers and radiators. In the following, the features of the present invention will be explained with particular reference to the production of multi-cavity flat tubes, but it will be understood that the present invention is by no means limited to this particular application. Figure 2 illustrates the shape of a multi-cavity flat tube produced by the above-mentioned conventional extrusion process, which is disclosed for example in Japanese Patent Application, First Publication No. 64-3171 and Japanese Utility Model Application, Second Publication No. 3-295. Extrusion dies suitable for the production of such shapes are known to be an integral bridge die or an insert die device. Figure 3 illustrates an example of the integral bridge type die in which a die 60 is formed of a cylindrical body having a bridge part for supporting female and male die sections which are formed integrally with the rest of the die body. There is a die cavity 61 which runs through the die 60 parallel to the die axis from one surface to the opposite surface of the die 60. In the die of this type, if one section is damaged, the entire die body becomes defective, because it is not possible to replace one section of the die body, and in some cases, the die 60 itself may have to be replaced. To overcome such problems associated with the bridge dies, an insert type die device was developed. Referring to Figure 4, the insert die device comprises a die holder 80, and a plurality of die assemblies 70 which can be inserted into or taken out of the die holder 80 freely. The die holder 80 usually has a plurality of openings 81 for receiving the die assemblies 70, respectively. Each die assembly is comprised of two engaging cylindrical dies. The first die (female die) has a certain cavity shape, and the second die (male die) has a protrusion of another shape which is inserted into the first die. Therefore, if the cavity of one of the die assemblies 70 becomes defective, it is necessary to replace only the die damaged or only the die assembly 70 concerned. The construction of each die assembly 70 of the insert die device will be explained in more detail, with reference to Figure 5 to 7. In all the descriptions which follow, the surfaces and directions are referenced with respect to the direction of travel of the material being extruded. In the case of Figure 5, the billet is placed against the second die 70b (referred to as the male die 70b), and is extruded toward the first die 70a (referred to as the female die 70a). The entry-side is defined as the side from which the material enters the die assembly, and the exit-side is defined as the side from which the material leaves the die. Generally the die assembly 70 is of a roughly cylindrical shape as shown in Figure 5, and is comprised of two parallel dies 70a and 70b whose flat surfaces are disposed transverse to the axis of the die assembly 70. The first die 70a has two concentric parts: an outer depressed part 71 of a large circular shape (female mating part whose internal wall surface 71a fits with the wall surface of the male die which will be described later); and an inner depressed part 72 having a four leaf shape, which is made by machining out the central portion of the outer depressed part 71. An elongated opening 73 is formed along a diametrical axis of the die 70a. With reference to Figure 6, the female die opening 73 comprises an extrusion cavity 73a at the exit-side of the four leaf part 72, and an exit region 73b which has a larger opening than the die cavity and which communicates with the entry-side surface of the die 70a. In this particular example, the cross sectional shape of the die cavity 73a transverse to the die axis is shown, in Figure 5, to be a wide slit with the corners rounded. It is also shown in the same figure that there is a pair of locating holes 75 disposed diametrically opposite to each other, and a pair of threaded holes 74 which are disposed similarly. The second die 70b is provided with a male mating part 76, on the exit-side surface, to fit with the female mating part 71a described above, along all its periphery. There is an integrally formed comb-shaped part 77 (Figure 5), which extends along parallel to the female die opening 73, and comprises a plurality of protrusions. The comb-shaped part 77 functions as a mandrel when inserted into the die cavity 73a of the die 70a. The male die opening 78 is formed along the extrusion direction following the contours of the comb-shaped part 77. The male die opening 78 communicates with both the entry-side surface and the exit-side surface of the second die 70b. When the dies 70a and 70b are joined together, the male die opening 78 forms a container and acts as the billet chamber in conjunction with the four leaf shaped depressed part 72 of the second die 70a. To prevent misalignment of the two dies 70a and 70b of the die assembly 70, two locating pins 80 are made to align with the two locating holes 75, and the threaded holes 79 of the second die 70b are aligned, respectively with the threaded holes 74 of the first die 70a. Manufacturing of the male dies is performed using the methods which are routine to those skill in the field of extrusion. The processing includes the following steps: 1. Machining such as lathe cutting and drilling which requires the use of cutting bits; 2. Heat treatments, including hardening; 3. Polishing; and 4. Electric discharge machining (EDM): after the hardening heat treatment process above, the dies cannot be machined by the cutting bits, so the dies are fabricated by means of electric arc discharge from electrodes such as Cu electrode while washing off the debris formed by the discharge with oil. 5. Wire discharge cutting which is a type of EDM. There are serious problems associated with such processing steps mentioned above, in particular, the lathe and milling operations require a large number of processing steps and are time consuming. Approximately twenty steps, over a period of about ten hours, are required from the start to the completion of making a male die. Female dies also require about the same number of steps over a period of about six hours. Practical steps necessary would be evident to those skilled in the art from the complex shape of the dies illustrated in Figure 5. There are additional problems in the case of the insert die device as described below. (i) The size of the entry port for aluminum extrusion is set by experience, on the basis of the die strength. However, the required cross sectional area is relatively small for most aluminum extrusions, and the required extrusion pressure is high in relation to the strength of the die material. High stresses are imposed on the die, and consequently, the die suffers slight permanent distortions. Such distortions affect the precise fitting of the male and female dies, resulting in the loss of dimensional accuracy of the product. The accuracy of alignment due to pins and screws is also affected. Even if one die assembly is replaced with a new die assembly, the combination of new and old die assemblies cannot reproduce the original dimensional accuracy. When the distortion is allowed to continue, the die must eventually be discarded. (ii) Because the die assembly is made of two separate dies, alignment means such as pins and screws are required. It is necessary to fabricate such parts, but it is difficult to attain the precision required for the pin holes and threaded holes by lathe machining. Wear is introduced during the operation, because every time a die assembly is disassembled or assembled the pins are removed or driven into the dies, thereby accelerating the loss of service life of the die assembly. (iii) Heat treatment processes are required which introduce thermal distortions in the dies, making it difficult to maintain the required precision, and because of the complexity of the die shape, it is difficult to completely correct such distortions. (iv) Many machine shops making extrusion dies lack the ability to accurately measure the internal diameter of the female die, thus making it difficult to manufacture a high precision joint part by lathe machining. (v) To improve wear resistance, it is desirable to coat the surfaces of the die with known abrasion resistant coating, but it is difficult to coat the die structure, including the pin holes, uniformly with the applicable coating techniques. If the coating thickness in the interior surfaces of pins holes becomes non-uniform, the alignment accuracy of the male and female dies become poor. (vi) The suitable die materials include such hard materials as tool steels for use in hot working, high speed steels, cemented carbides. However, because of the large size of the most insert die assembly, it is not preferable to make insert die assembly with such hard materials which are susceptible to cracking. The forces responsible for causing such cracking in the insert die assembly arises from the impact of initial loading as well as from the extrusion process. Therefore, there has long been an outstanding need for the development of a durable extrusion die device which provides a long service life without causing fracture, wear and distortions due to extrusion processes, which accepts coating processing uniformly and easily thereon and which provides a long service life by maintaining the initial machining precision of the die components. It is therefore the object of the present invention to provide a novel extrusion die device and an insert die therefor based on a design concept of an independent male die rather than an integrated male die. The invented die device provides extruded products of high dimensional precision and durable service life. The male insert die is easily replaceable and is economical to produce with the use of such precision fabrication techniques as electric discharge machining. According to the present invention, there is provided an extrusion die device comprising a die-holding means; and at least one extrusion die insert detachably held by the die-holding means, the extrusion die insert including: a female die of a generally cylindrical shape having opposite end surfaces and including a die cavity formed therein so as to open to one of the end surfaces and a discharge passageway communicated with the die cavity and opening to the other of the end surfaces; and a male die having a protrusion part formed thereon, the male die being associated with the female die with the protrusion part being fitted in the die cavity, characterized in that the female die includes a fitting recess having a generally channel-shaped cross-section formed in the one end surface thereof, that the male die is of a generally plate-like shape having opposite faces and an end face joining the opposite faces, the opposite faces and the end face having portions defining a fitting portion, and that the male die is associated with the female die with the fitting portion being fitted in the fitting recess. Another object of the invention is to provide an extrusion die insert which can be suitably used for the aforesaid extrusion die device. According to this aspect of the invention, there is provided an extrusion die insert for being detachably held by a die holder, the extrusion die insert comprising: a female die of a generally cylindrical shape having opposite end surfaces and including a die cavity formed therein so as to open to one of the end surfaces and a discharge passageway communicated with the die cavity and opening to the other of the end surfaces; and a male die having a protrusion part formed thereon, the male die being associated with the female die with the protrusion part being fitted in the die cavity, characterized in that the female die includes a fitting recess having a generally channel-shaped cross-section formed in the one end surface thereof, that the male die is of a generally plate-like shape having opposite faces and an end face joining the opposite faces, the opposite faces and the end face having portions defining a fitting portion, and that the male die is associated with the female die with the fitting portion being fitted in the fitting recess. In the foregoing, it is preferable that at least the male die be made of a hard material selected from the group consisting of tool steel for use in hot working, high speed steel and cemented carbide. Each of the dies may have a hard coating such as nitride coating formed thereon. The protrusion part of the male die may be provided between the opposite faces so as to protrude from the end face, and the opposite faces of the male die may be removed towards the protrusion part to provide inclined surfaces defining regions for flowing billets. The protrusion part may be formed into a comb-shape having a plurality of protruding pieces. The female die, which has an axis, may include a cross-shaped recess formed in the one end surface thereof and having first and second grooves intersecting each other at the axis, the first groove defining a billet chamber of a generally rectangular-shaped cross-section while the second groove serves as the fitting recess. The first groove may be formed so as to open to an outer peripheral surface of the female die. The die cavity may be disposed at the intersection of the first and second grooves so as to extend along the axis of the female die, and the die cavity may have a cross-section of an elongated shape extending longitudinally of the second groove whereas the discharge passageway may be disposed in alignment with the die cavity so as to have a topless pyramid-shape with a width increasing in a direction away from the die cavity. The male die may include a stepped portion formed on the end face so as to define an engaging portion, and the female die may include a shoulder portion formed at the intersection of the first and second grooves and held in engagement with the engaging portion, whereby the male die is prevented from being shifted longitudinally of the second groove with respect to the female die. The opposite faces of the male die may have contact portions held in contacting engagement with the second groove of the female die. The female die may include a notch formed therein for preventing twisting thereof when the female die is held by the holder. Figure 1 is an illustration of the conventional extrusion processing. Figure 2 is an example of a product which is made by an extrusion processing. Figure 3 is a schematic view to illustrate a conventional bridge type die. Figure 4 is a schematic view showing a conventional insert type die device. Figure 5 is a perspective view for showing the details of a die insert of the conventional insert die device. Figure 6 is a cross sectional view of the die taken at a plane VI-VI in Figure 5. Figure 7 is a cross-sectional view of the die taken at a plane VII-VII in Figure 5. Figure 8 is a perspective view of a die insert according to the present invention. Figure 9 is a cross sectional view of the die taken along a plane VIII-VIII in Figure 8. Figure 10 is a cross sectional view of the die taken along a plane X-X in Figure 8. An extrusion die device in accordance with a preferred embodiment of the present invention will now be described in detail with reference to Figures 8 to 10. The die device is used to produce multi-cavity flat tubes for aluminum heat exchanger applications, and comprises a die holder or any holding member and at least one extrusion die insert detachably held by the die holder. The extrusion die insert, generally designated by 20, comprises a generally rectangular male die 20a and a generally cylindrical female die 20b. The male die 20a, which is fabricated from a plate of a generally rectangular shape, includes a generally plate-like shaped main body 21 having a pair of opposite longer side faces 21a and an elongated end face 21b joining the opposite faces 21a. The opposite side faces 21a are adapted to be brought into engaging contact with the female die to prevent twisting of the male die, thereby serving as a twist prevention region. A stepped portion is formed integrally on the longitudinally-intermediate portion of the end face 21b so as to protrude therefrom, thereby defining a pair of engaging walls or portions 23 which prevent sideways shift of the male die 20a. Thus, the twist prevention region and the engaging portions 23 serve as a coupling region or fitting portion when coupling the male die 20a with the female die 20b. A comb-shaped protrusion part 22, which comprises a plurality of protruding pieces arranged in line and is thinner than the thickness of the main body 21, is formed on the top of the stepped portion so as to protrude therefrom. A pair of recesses are formed in the middle portions of the opposite faces 21a, respectively, so as to define a pair of inclined surfaces smoothly sloping inwardly of the main body in a direction towards the protrusion part 22. Thus, each of the recesses defines a flowing region 22a sloping smoothly in the metal flow direction for permitting the billets to flow smoothly. The female die 20b, which is fabricated from a generally cylindrical body having an axis and having opposite end surfaces, includes a cross-shaped recess formed in its one end surface for serving the purpose of interlocking with the male die whose twist prevention region is placed therein, as well as serving the purpose of forming a billet chamber. More specifically, the cross-shaped recess comprises first and second grooves perpendicularly intersecting each other at the axis, the first groove cooperating with the male die 20a to define a generally rectangular-shaped billet chamber 24, while the second groove serves as a fitting recess 25 into which the twist prevention region of the male die 20a is fitted. The female die 20b includes a cavity opening 26 which is shaped like a wide slit, and which is formed transversely to the extrusion direction and extending from the surface of the billet chamber 24 to the exit direction. As shown in Figure 9, the cavity opening includes a die cavity 26a opening to the billet chamber 24 and a discharge passageway 26b communicated with the die cavity 26a and opening to the other end surface. The comb-shaped part 22 is inserted into the die cavity 26a and in cooperation therewith defines the cross sectional shape of the multi-cavity flat tube extrusion. The discharge passageway 26b is formed coaxially with the die cavity 26a, and enlarges toward the exit-side of the female die 20b to form a topless pyramid-shape at the exit-side surface of the female die 20b. The shape of the discharge passageway is not restricted to this particular shape and other shapes such as rectangular shape can be used. When the die insert is used to extrude a product, the twist prevention regions of the male die 20a are placed in the fitting recess 25 of the female die 20b. The engaging portions 23 of the male die 20a are coupled to the shoulder portion 24' formed on the wall surface of the chamber region immediately adjacent to the fitting recess 25. By such a coupling of the male and female dies of the die insert, the shift of the male die 20a in both X- and Y-directions (reference to Figure 8) is prevented. The female die 20b further includes notches 27 formed on an outer peripheral surface thereof at the exit-side end, as illustrated in Figure 8. With these notches 27, the female die 20b is further prevented from twisting during extrusion. In the conventional insert type extrusion die device, it is not desirable to make the large component parts from hard materials, because of the high probability of forming cracks therein. However, since the male die of the die device of the present invention is small, it is possible to utilize tool steels for use in hot working, high speed steels and cemented carbides. The processing method includes grinding and polishing with a surface grinder to remove any distortion of the starting material, followed by EDM and wire EDM to fabricate it into a specific shape. It is preferable to treat the surfaces of the fabricated part with a known surface treatment process such as carburizing or nitriding, because such process will prolong the service life of the dies. Further, it is preferable that the surfaces of the die be coated with a hard coating of about 3 to 4 micrometers thick. The hard coating may be a nitride coating composed of a single TiN layer, or may be of a multilayered structure of different components. The material for the female die can be any conventional materials, such as tool steels for use in hot working. This is because in the conventional die designs, the extrusion pressure tended to concentrate near the center portion and its surrounding region, making the die susceptible to deformation. However, as will be explained later, the die design of the present invention provides for a larger area in which the extrusion force is spread over, thereby avoiding the stress concentration in the central region of the die. Further, because the dies are firmly housed in the die holder with precision alignment, the sections which interlock the male and female dies are not subject to significant transverse forces, thereby avoiding the exposure of such interlocking sections to the undesirable distortion forces. This can be demonstrated readily from the fact that the invented die device exhibits service life of about 30 to 40 tons of extrusions compared with the service life of the conventional die design of 10 to 15 tons of extrusions. The advantageous features of the present invention will be summarized below. Additional advantages not listed will be apparent to those skilled in the art of extrusion. (1) The simple shapes of the components in the present invention are mostly fabricable with EDM. The EDM processes are efficient and economical processing methods, promoting high precision at low processing costs. In particular in the dies of the conventional designs, it was very difficult to fabricate the critical parts by EDM, such as the engaging region between the male and female dies which require the most precision. In contrast, the male die of the present invention is a plate-shaped body having comb-shaped parts protruding from one edge. Therefore, fabrication of the male die can be performed on a surface grinder which will provide a dimensional precision of about ±0.01mm, enabling to utilize an EDM process for making parts requiring high precision, such as the comb-shaped part, engaging portions and fitting portion. (2) The simple extrusion die components utilized in the present invention are small size relative to the conventional components, thus enabling hard materials to be used without the fear of introducing cracks in the dies. For the male die in particular, since the size can be restricted to the smallest possible size that contains the comb-shaped part and its support part, a suitably hard material can be chosen so as to improve the die performance such as strength and wear resistance. Further, the simple shapes enable the required surface treatment processes to be applied efficiently and uniformly. (3) The die device designed according to the present invention enables precision alignment of the male and female dies without resorting to the conventional alignment means such as pins and screws. If one die is damaged, it can be changed independently of the other die, because the dies are hardened by heat treatment processes and are fabricated with precision by means of EDM and other precision fabrication processes. (4) The extrusion die device according to the present invention enables the coupling part to be fabricated by EDM, and further because the male and female dies are coupled in the directions of the X- and Y-axes, shown in Figure 8, the alignment of the dies is retained precisely during the extrusion process (i.e. a relative motion of the two dies is prevented). (5) The mandrels (Figure 5) in the conventional dies are made as an integral part of the opening of a male die spanning across the diameter of the opening of the cylinder, and therefore, the billet chamber opening is restricted by the area of the cylindrical stock body. If the opening of the male die is made larger, the wall thickness of the male die becomes thin, making it susceptible to deformation. In contrast, the entry opening of the insert die device of the present invention is located on the female die, and it can be enlarged (i.e. the port area can be made to be large), without the fear of deformation of the die, thus enabling a lower extrusion pressure to be used. As described above, the extrusion die device according to the present invention is shaped uniquely and differently from the conventional die device, therefore, it can be manufactured easily, and essential treatments such as coating processes on the dies can be carried out uniformly and efficiently, thereby providing durable dies which can maintain high dimensional precision. Harder materials can be chosen for making the male dies compared with the conventional design of male dies. Therefore, the design method enables the production of extrusion dies of long service life, having abrasion resistance and without being affected by deformation due to extrusion pressure, thereby maintaining the original precision of newly commissioned dies during its long service life. Theoretically, the dies can have a nearly permanent life when the coating is reapplied before it is worn off or peeled off from the dies. The above preferred embodiment presents only an application of the design concept to a case of production of multi-cavity flat tubes. The basic concept can be applied to numerous other cases of production of extruded products of similar cross sectional shapes within the limitations expressed in the appended claims. The design concept disclosed in the present invention is equally applicable to other complex shapes for which insert or mandrel type dies are necessary and when the dimensional accuracy must be maintained over a prolonged production period. Reference signs in the claims are included for better understanding and shall not limit the scope.

Claims: An extrusion die device comprising: a die-holding means; and at least one extrusion die insert detachably held by said die-holding means, said extrusion die insert including: a female die (20b) of a generally cylindrical shape having opposite end surfaces and including a die cavity (26a) formed therein so as to open to one of said end surfaces and a discharge passageway (26b) communicated with said die cavity and opening to the other of said end surfaces; and a male die (20a) having a protrusion part (22) formed thereon, said male die being associated with said female die with said protrusion part being fitted in said die cavity, characterized in that said female die includes a fitting recess (25) having a generally channel-shaped cross-section formed in said one end surface thereof, that said male die is of a generally plate-like shape having opposite faces (21) and an end face joining said opposite faces, said opposite faces and said end face having portions defining a fitting portion, and that said male die is associated with said female die with said fitting portion being fitted in said fitting recess. An extrusion die device as recited in claim 1, wherein at least said male die is made of a hard material selected from the group consisting of tool steel for use in hot working, high speed steel and cemented carbide. An extrusion die device as recited in any one of claims 1 and 2, wherein each of said dies has a hard coating formed thereon. An extrusion die device as recited in claim 3, wherein said hard coating comprises a nitride coating. An extrusion die device as recited in any one of claims 1 to 4, wherein said protrusion part of said male die is provided between said opposite faces so as to protrude from said end face, and wherein said opposite faces of said male die are removed towards said protrusion part to provide inclined surfaces defining regions for flowing billets. An extrusion die device as recited in claim 5, wherein said protrusion part is of a comb-shape having a plurality of protruding pieces. An extrusion die device as recited in any one of claims 1 to 6, wherein said female die has an axis and includes a cross-shaped recess formed in said one end surface thereof and having first and second grooves intersecting each other at said axis, said first groove defining a billet chamber (24) of a generally rectangular-shaped cross-section while said second groove serves as said fitting recess (25). An extrusion die device as recited in claim 7, wherein said first groove is formed so as to open to an outer peripheral surface of said female die. An extrusion die device as recited in claim 7, wherein said die cavity is disposed at the intersection of said first and second grooves and extends along said axis of said female die, said die cavity having a cross-section of an elongated shape extending longitudinally of said second groove, said discharge passageway being disposed in alignment with said die cavity and being of a topless pyramid-shape having a width increasing in a direction away from said die cavity. An extrusion die device as recited in any one of claims 7 and 8, wherein said male die includes a stepped portion formed on said end face so as to define an engaging portion (23), said female die including a shoulder portion (24') formed at the intersection of said first and second grooves and held in engagement with said engaging portion, whereby said male die is prevented from being shifted longitudinally of said second groove with respect to said female die. An extrusion die device as recited in any one of claims 7 to 10, wherein said opposite faces of said male die have contact portions (21) held in contacting engagement with said second groove of said female die. An extrusion die device as recited in any one of claims 1 to 11, wherein said female die includes a notch (27) formed therein for preventing twisting thereof when said female die is held by the holder. An extrusion die insert for being detachably held by a die holder, said extrusion die insert comprising: a female die (20b) of a generally cylindrical shape having opposite end surfaces and including a die cavity (26a) formed therein so as to open to one of said end surfaces and a discharge passageway (26b) communicated with said die cavity and opening to the other of said end surfaces; and a male die (20a) having a protrusion part (22) formed thereon, said male die being associated with said female die with said protrusion part being fitted in said die cavity, characterized in that said female die includes a fitting recess (25) having a generally channel-shaped cross-section formed in said one end surface thereof, that said male die is of a generally plate-like shape having opposite faces (21) and an end face joining said opposite faces, said opposite faces and said end face having portions defining a fitting portion, and that said male die is associated with said female die with said fitting portion being fitted in said fitting recess.

Inventors: HASEGAWA NOBUYUKI, INAMURA AKIRA, KANEKO MASAFUMI, HASEGAWA, NOBUYUKI,, INAMURA, AKIRA, KANEKO, MASAFUMI, Inamura, Akira, c/o Mitsubishi Aluminum Co., Ltd., Kaneko, Masafumi, c/o Mitsubishi Aluminum Co., Ltd
Applicants: MITSUBISHI ALUMINIUM, THERMALEX INC, MITSUBISHI ALUMINUM CO.,LTD., THERMALEX, INC.
Application year: 1993
Main classification: B21C25
IPC codes: B21C25, B21C23