CROSS REFERENCE TO RELATED APPLICATION 
     This is a Division of U.S. patent application Ser. No. 09/218,725, filed on Dec. 22, 1998, now is U.S. Pat. No. 6,195,874, which is now pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a folded fin forming method, machine and folded fin obtained therefrom. The folded fin has a corrugated configuration and is particularly used in constructing a heat sink for dissipating heat generated by Integrate Circuits (ICs). 
     2. The Prior Art 
     Due to the increased consumption of power of ICs (particularly Central Processing Units (CPUs)), heat dissipation of these electronic components is becoming increasingly important. To solve this problem, heat sinks are mounted to contact the CPUs to absorb heat generated thereby and dissipate it into the surrounding air. 
     Referring to FIG. 1, a conventional heat sink  10  is constructed by alumium extrusion to have a number of heat dissipating fins  12 . The heat sink  10  formed by this method is costly. 
     To lower the cost, a folded fin  22  having a corrugated configuration is made by stamping or roll forming a metal sheet (usually an aluminum sheet), as seen in FIG.  2 . The folded fin  22  consists of a number of inverted U-shaped heat dissipating fins  23  and is fixed to a flat base plate  24  by epoxy or riveting to form a heat sink  20 . 
     The heat sinks  10 ,  20  made in accordance with the prior art share a common disadvantage. Each of the heat dissipating fin  12 ,  23  cannot have an aspect ratio (H/W) larger than twelve, otherwise breakage of the fins  12 ,  23  will occur during manufacture of the heat sink  10  or the folded fin  22 . The limited aspect ratio of the fins  12 ,  23  limits the available heat dissipating area per length unit of the heat sink  10 ,  20 . 
     Moreover, referring to FIG. 3, in order to enhance the heat dissipating effectiveness of the folded fin  22 , when producing the folded fin  22  by roll forming, a number of louvers  252  are defined in webs  25  of the heat dissipating fins  23 . Due to the limitation of the forming direction of he roll forming, the louvers  252  can only be vertically defined in the webs  25 . Although the louvers  252  can increase the heat dissipating area of the folded fin  22 , the direction of arrangement of the louvers  252  is perpendicular to the direction of forced air flow through the heat sink. Thus, the enhanced effectiveness achievable by the provision of the louvers  252  is not as significant as horizontal louvers. 
     Hence, an improvement over the prior art heat sink is needed. 
     SUMMARY OF THE INVENTION 
     Accordingly, an objective of the present invention is to provide a method for forming a folded fin with a number of inverted U-shaped heat dissipating fins which can have an unlimited aspect ratio without breakage of webs thereof occurring during formation of the folded fin. 
     Another objective of the present invention is to provide a machine for forming a folded fin with a number of inverted U-shaped heat dissipating fins which can have an unlimited aspect ratio without breakage of the webs thereof occurring during formation of the folded fin. 
     A further objective of the present invention is to provide a folded fin with a number of inverted U-shaped heat dissipating fins each having an aspect ratio larger than twelve and preferably between twenty and forty. 
     Still another objective of the present invention is to provide a method for forming a folded fin with a number of inverted U-shaped heat dissipating fins each of which has right-angled comers so that when the folded fin is attached to a metallic base plate, a maximum contacting area exists therebetween. 
     Still a further objective of the present invention is to provide a method for forming a folded fin with a number of inverted U-shaped heat dissipating fins each defining a number of horizontal louvers in webs thereof (or wave-like structures having horizontal peaks) so that the heat dissipating effectiveness of the folded fin can be significantly enhanced. 
     To fulfill the above-mentioned objectives, according to one embodiment of the present invention, a method for forming a folded fin includes the following steps: 
     providing a horizontal metal strip; 
     forming a number of pairs of indents alternately in top and bottom faces of the strip; 
     clamping the strip with three folding tools, wherein a first tool clamps the strip between a first pair of indents in the top face of the strip, a second tool clamps the strip between a successive second pair of indents in the bottom face of the strip, and a third tool clamps the strip between a fuirther successive third pair of indents in the top face of the strip; and 
     moving the second and third tools toward the first tool to fold the strip to position where sections of the strip between the first and second tools, and the second and third tools are vertically bent, wherein the third tool moves horizontally toward the first tool and the second tool moves at an angle toward an upper side of the first tool. 
     A machine for forming the folded fin consists of a stationary frame, a folding tool carrier horizontally and reciprocally mounted on the frame, three folding tools carried by the carrier for folding a horizontal strip inserted in the machine into the folded fin, and a locating device for fixing the horizontal metal strip relative to the frame when the folding tools do not clamp the strip. 
     After he horizontal strip has been received in the machine, the three folding tools securely clamp the strip at an original position. The carrier horizontally displaces the three folding tools together with the strip a predetermined distance toward the first tool. Thereafter, the second tool moves at an angle toward an upper side of the first tool and the third tool moves horizontally toward the first tool to reach a position where sections of the strip between the first and second tools, and the second and third tools are vertically bent, thereby forming an inverted U-shaped heat dissipating fin. The locating device is driven to fix the strip relative to the frame. The three folding tools are released from the strip and the carrier displaces the folding tools a predetermined distance toward the third folding tool. The three folding tools return to their respective original positions and securely clamp the strip. Afterwards, the above operation is repeated to form successive inverted U-shaped heat dissipating fins on the strip thereby obtaining the folded fin. 
     In the machine according to the present invention, the three folding tools are spaced from each other a distance D when they are at the original position which is substantially equal to a height H of the formed inverted U-shaped heat dissipating fin (H=D+2t, t: thickness of the strip). Furthermore, each folding tool has a width d substantially equal to a width w of the formed heat dissipating fin (W=d+2t). Since during formation of the forded fin the machine in accordance with the present invention does not exert any stretching force on webs of the inverted U-shaped heat dissipating fins, theoretically, the folded fin in accordance with the present invention can have an unlimited aspect ratio without breakage occurring to the webs. In a preferred embodiment of the present invention, the folded fin has an aspect ratio between twenty and forty. 
     To facilitate the folding operation of the machine, a number of pairs of indents can be alternately defined in top and bottom faces of the strip. When the strip is received in the machine and the folding tools clamp the strip at their original position, the first tool clamps the strip between a fist pair of indents in the top face of the strip, the second tool clamps the strip between a successive second pair of indents in the bottom face of the strip, and the third tool clamps the strip between a further successive third pair of indents in the top face of the strip. 
     To enhance the heat dissipating effectiveness of the folded fin, when defining the indents in the strip, a number of slits can be defined between adjacent pairs of indents. The slits are defined parallel to the indents, whereby when the folded fin is formed a number of horizontal louvers are defined in the webs of the heat dissipating fins. 
     Alternatively, a wave-like structure can be formed between adjacent pairs of indents having peaks which are parallel to the indents whereby the folded fin is formed with the wave-like structure on the webs of the heat dissipating fins. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a first conventional heat sink; 
     FIG. 2 is a side elevational view of a second conventional heat sink; 
     FIG. 3 is a partial perspective view of a heat dissipating fin of a conventional folded fin made by roll forming; 
     FIG. 4 is a partial perspective view of an aluminum strip for forming a folded fin in accordance with a first embodiment of the present invention; 
     FIG. 5 is a cross-sectional view taken alone line  5 — 5  of FIG. 4; 
     FIG. 6 is a cross-sectional view taken along line  6 — 6  of FIG. 4; 
     FIG. 7 is a diagrammatic view of a folded fin forming machine in accordance with the present invention with the aluminum strip of FIG. 4 received therein at an original position to be folded by the machine; 
     FIG. 8 is a view similar to FIG. 7 with the machine at a second position; 
     FIG. 9 is a view similar to FIG. 7 with the machine between the second position and a third position; 
     FIG. 10 is a view similar to FIG. 7 with the machine at the third position; 
     FIG. 11 is a view similar to FIG. 7 with the machine between the third position and a fourth position; 
     FIG. 12 is a view similar to FIG. 7 with the machine at a fourth position; 
     FIG. 13 is a view similar to FIG. 7 with the machine at a fifth position; 
     FIG. 14 is a view similar to FIG. 7 with the machine between the fifth position and the first position; 
     FIG. 15 is a view similar to FIG. 7 with the machine at the first position; 
     FIG. 16 is a partially enlarged view of circular  16  as shown in FIG. 15; 
     FIG. 17 is a partially enlarged view of circle  17  as shown in FIG. 15; 
     FIG. 18 is a view similar to FIG. 4, showing an aluminum strip for forming a folded fin in accordance with a second embodiment of the present invention; 
     FIG. 19 is cross-sectional view taken along line  19 — 19  of FIG. 18; 
     FIG. 20 is a partial perspective view of a heat dissipating fin of a folded fin formed in accordance with the second embodiment of the present invention; 
     FIG. 21 is a view similar to FIG. 4, showing an aluminum strip for forming a folded fin in accordance with a third embodiment of the present invention; 
     FIG. 22 is a cross-sectional view taken along line  22 — 22  of FIG. 21; and 
     FIG. 23 is a partially enlarged view of a part of a web of a heat dissipating fin of a folded fin formed in accordance with the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention. 
     Referring to FIGS. 4 to  6 , an aluminum strip  30  to be folded by a machine in accordance with the present invention alternatively defines a number of pairs of indents  322 ,  342  in top and bottom faces  32 ,  34  thereof. The indents  322 ,  324  are devised to ensure that when the aluminum strip  30  is bent, each corner between a horizontal wall and a corresponding bent vertical wall is right-angled. A distance d between two indents of each pair of indents substantially determines a width w of an inverted U-shaped heat dissipating fin to be formed. The width w is actually equal to the distance d plus two times a thickness t of the strip  30 . The distance d is substantially equal to a width of a folding tool used with the machine. A distance D between two adjacent pairs of the indents substantially determines a height H of a folded fin to be formed. The height H of the folded fin is equal to the distance D plus two times the thickness t of the strip  30 . The distance D is equal to a distance between two neighboring folding tools when the machine is at an original position. Detailed explanations concerning this are given below. 
     Referring to FIGS. 7 to  15 , a folding machine  40  in accordance with the present invention includes a stationary frame  42 , a locating device  43 , a folding tool carrier  44  horizontally and reciprocally mounted on the frame  42 , and three folding tools  47 ,  48 ,  49  drivably mounted on the carrier  44 . The first tool  47  is located the farthest away from the locating device  43 , the third tool  49  is located closest to the locating device  43 , and a second tool  48  is located between the first and third tools  47 ,  49 . Each tool  47 ,  48 ,  49  includes an upper clamping block  472 ,  482 ,  492  and a lower clampingblock  474 ,  484 ,  494 , respectively. 
     To form the folded fin in accordance with the present invention, firstly, the aluminum strip  30  is inserted in the machine  40  a predetermined length by a feeding machine (not shown) which is well known by those skilled in the art and is irrelevant to the inventive features of the present invention, hence, a detailed description thereof is omitted herein. The strip  30  is fed into the machine  40  to a position where the first and third folding tools  47 ,  49  align with two adjacent pairs of indents  322  in the top face  32  of the strip  30  and the second folding tool  48  aligns with a pair of the indents  342  in the bottom face  34  of the strip  30 . Thereafter, the three tools  47 ,  48 ,  49  are driven to tightly clamp the strip  30 . The locating device  43  is at a released state and does not fix the strip  30  to the frame  42  (FIG.  7 ). Hereafter, this position is referred to as an original position of the machine  40 . 
     Thereafter, the carrier  44  displaces the three folding tools  47 ,  48 ,  49  a distance S to the right. In the preferred embodiment, the distance S is equal to a sum of the width W of the heat dissipating fin to be formed plus the width d of the tool (S=W+d). Then, the second tool  48  moves at an angle toward an upper side of the first folding tool  47 , and the third tool  49  moves horizontally toward the first tool  47  to begin folding of the strip  30  (FIG.  8 ). 
     As shown in FIG. 9, during movement of the second and third folding tools  48 ,  49 , sections (not labeled) of the strip  30  between the first and second tools  47 ,  48  and the second and third tools  48 ,  49  are folded to be webs of the heat dissipating fin. The distance between two neighboring tools (which is substantially equal to the distance D between adjacent pairs of indents) substantially determines the height H of the heat dissipating fin. 
     The second tool  48  is moved to a position just to the left of the first tool  47  and spaced therefrom a distance equal to t plus d. The third tool  49  is moved to a position just to the left of the second tool  48  and spaced from the first tool a distance equal to d plus W (FIG.  10 ). 
     By the movement of the second and third tools  48 ,  49 , a heat dissipation fin  36  is formed having a height H and a width W wherein as aspect ratio (H/W) can be larger than twelve and is preferably between twenty and forty. In the machine  40  of the present invention, the movement of the second and third folding tools  48 ,  49  during formation of the heat dissipating fin  36  does not stretch the strip  30  so that the thickness t of the strip  30  remains constant and the webs (not labeled) of the heat dissipating fins  36  will not break. Moreover, by the provision of the indents  322 ,  342  in the top and bottom faces  32 ,  34  of the strip  30 , the strip  30  can be easily deformed and maintained at the formed shape without rebounding which often occurs when applying a bending operation to a metal plate. Thus, each corner o the folded fin formed by the present machine  40  is right angled and top and bottom faces of the folded fin lie along the same plane to enable the folded fin to be readily and precisely attached to a base plate (not shown) and have a maximum contacting area therewith. 
     After the formation of one inverted U-shaped heat dissipating fin  36 , as shown in FIGS. 11 and 12, the locating device  43  is driven to fix the strip  30  to the frame  42 . The upper blocks  472 ,  492  of the first and third folding tools  47 ,  49  move vertically upward above the formed heat dissipating fin  36 . The lower block  484  of the second folding tool  48  moves vertically downward below the formed heat dissipating fin  36 . 
     Thereafter, as shown in FIGS. 13 to  15 , the carrier  44  displaces the three tools  47 ,  48 ,  49  toward the left a distance S. The upper block  472  of the first tool  47  moves vertically downward to cooperate with the lower block  474  thereof to fixedly clamp the strip  30 . The lower block  484  of the second tool  48  moves horizontally to return to its original position, and the upper block  482  thereof moves downwardly to the left at an angle “a” to cooperate with the lower block  484  of fixedly clamp the strip  30 . The lower block  494  of the second tool  49  moves horizontally to return to its original position, and the upper block  492  thereof moves downwardly to the left at an angle “b” to cooperate with the lower block  494  to fixedly clamp the strip  30 . The angle “b” is larger than the angle “a”. Finally, the locating device  43  releases its grip on the strip  30  and the machine  40  repeats its operation as depicted from FIGS. 7 to  15  to form successive inverted U-shaped heat dissipating fins  36  on the strip  30  thereby forming the folded fin. 
     In the present invention, since each block of the folding tools  47 ,  48 ,  49  has only a small contact area with the strip  30 , the separation of the folding tools from the heat dissipating fin  36  will not exert a stretching force on the webs of the fin  36 . Therefore, the machine  40  in accordance with the present invention will not cause the fin  36  to break even if it has a relatively large aspect ratio. 
     From the above the descriptions it can be seen that the folding method and machine of the present invention is totally different from the teaching of the prior at. A folded fin with a high aspect ratio can be obtained without breaking the webs of the heat dissipating fins  36 . Thus, the present invention qualifies to be granted a patent. 
     Theoretically, if the machine  40  has a sufficiently large size, a folded fin can be produced without limitation of the aspect ratio. Nevertheless, the folded fin manufactured by the present machine  40  preferably has an aspect ratio between twenty and forty. 
     FIGS. 16 and 17 show that the provision of the indents  322 ,  343  in the top and bottom faces  32 ,  34  of the strip  30  provides as inside of each comer of the folded fin with sufficient clearance for proper folding. Such a design not only guarantees the corresponding comer to be formed at a perfect right angle, but also eliminates an internal stress within the corner due to the bending operation, whereby the folded fin can maintain its formed configuration without subsequent deformation. 
     FIGS. 18 to  20  show an aluminum strip  50  used for obtaining a folded fin in accordance with a second embodiment of the present invention. A number of slits  52  are defined in the aluminum strip  50  between adjacent upper and lower indents  522 ,  542 . The slits  52  are defined to be parallel to the indents  522 ,  524 . When the strip  50  is folded by the machine  40  following the operation of FIGS. 7 to  15  to obtain the folded fin, a number of horizontal louvers  544  are formed in upright webs  540  of the heat dissipating fins  54 , whereby the heat dissipating effectiveness of the folded fin can be significantly enhanced. 
     FIGS. 21 to  23  shows an aluminum strip  60  for obtaining a folded fin in accordance with a third embodiment of the present invention. A wave-like structure  62  is formed on the strip  60  between adjacent upper and lower indents  622 ,  642 . Peaks (not labeled) of the wave-like structure  62  are parallel to the indents  622 ,  642 . When the aluminum strip  60  is subject to a folding operation of the machine  40  to become a folded fin, upright webs  640  of the heat dissipating fins  64  are formed with wavelike structures  644  each having horizontal peaks, whereby the heat dissipating effectiveness of the folded fin can be significantly enhanced. 
     While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.
A method for forming a folded fin includes at least the following steps: providing a horizontal metallic strip; forming number of pairs of indents alternately in a top face and a bottom face of the strip; clamping the strip with three folding tools, wherein a first tool is located between a first pair of indents in the top face of the strip, a second tool is located between a successive second pair of indents in the bottom face of the strip and a third tool is located between a fuirther successive third pair of indents in the top face of the strip; and moving the second and third tools toward the first tool to a position where parts of the strip between the first and second tools and between the second and third tools are bent to be vertical, wherein the third tool moves horizontally toward the first tool and the second tool moves at an angle toward an upper side of the first tool.