Abstract:
A tiny-diameter, lengthwise extensive impeller utilized in an ultra-small centrifugal fan is molded by an injection molding operation. In order to avert difficulties attendant on injection-molding ultra-miniature parts, the thickness and length of a reinforcing ring on the tip of the impeller are set to within predetermined ranges. Further, the thickness of each of the vanes that constitute the impeller is made maximum where they join to the impeller ring section.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to methods of manufacturing impellers for centrifugal fans, and to centrifugal fans as well.  
         [0003]     2. Description of the Related Art  
         [0004]     Device downsizing and performance upgrading of electronic equipment in recent years have entailed demands for the scaling down of cooling fans installed in such electronic devices. As one among such attempts, a centrifugal fan in which the impeller has been reduced in diameter, and the individual vanes constituting the impeller have been thinned and arranged at a denser spacing has been proposed.  
         [0005]     Meanwhile, inasmuch as centrifugal-fan impellers have traditionally been manufactured by injection molding, various techniques for enhancing the quality of the manufactured product have been developed. Examples of such techniques include a method in which in advance of infusing a mold with thermoplastic resin, the mold is evacuated, as well as a method in which excessive exhausting of gases during the molding operation is prevented by sufficiently drying the thermoplastic material beforehand and then melting it. Another example utilizes highly fluid liquid crystal polymers as base materials to make it possible to mold impellers having longer vanes.  
         [0006]     Nevertheless, to proceed to make the vanes thinner is to make it impossible to mold an impeller stably by traditional methods. In particular, designing the individual vanes of a centrifugal fan to be both thinned and elongated in order to improve the fan&#39;s performance would make it impossible to charge the inside of the mold sufficiently with thermoplastic resin.  
         [0007]     Centrifugal-fan impellers are sometimes furnished with a ring section that links the tips of the vanes. The objective in such configurations is to enhance the impeller rigidity by tying the vane tips together. The ring section is vital to implementations in which an impeller is axially extensive and its vanes are thin. For ultra-miniature centrifugal fans (e.g., centrifugal fans whose outer diameter is 25 mm or less), however, if an impeller having a ring section is to be injection molded, the flow of thermoplastic resin inside the mold would be restrained such that the ring-forming portion of the mold could not be charged sufficiently with the resin. Or, even if it could be thus charged, then meld lines would form in the ring area, deteriorating the strength of the ring section. Such phenomena are detrimental to throughput during production, and invite increases in post-manufacturing breakage.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     An object of the present invention, brought about in order to resolve the problems discussed above, is to make available a method of manufacturing, by injection molding and at high throughput, impellers for micro-diameter centrifugal fans—in particular, impellers whose axial length has been extended in order to improve the impeller&#39;s characteristics.  
         [0009]     In the present invention, in order to heighten throughput in the injection-molding manufacture of ultra-miniature impellers for centrifugal fans, the thickness of the ring section is secured, and at the same time a fixed or greater axial length for the ring section is secured. In this way securing the dimensions of the ring facilitates the flow of the thermoplastic resin in the area of the mold interior that corresponds to the ring.  
         [0010]     The causative factor behind deterioration in the strength of the ring section in ultra-miniature impellers originates in insufficiency in the flow of thermoplastic resin into the ring-forming portion of the mold, which makes it likely that meld lines will form. In the present invention, the thickness and length of the ring section are rendered fixed dimensions or greater in order to avert this problem. Doing so keeps meld lines from forming within the ring-forming portion of the mold to enhance the strength and durability of the ring section, even in impeller molding implementations in which the gate is positioned in the end of the mold opposite the ring section. In a further aspect of the present invention, the formation of meld lines is also held in check by increasing the vane thickness in the area in which the vanes connect to the ring section.  
         [0011]     Such improvement is particularly pronounced in implementations in which thermotropic liquid-crystal polymers are employed as the base material-implementations that are especially vulnerable to strength deterioration where the polymer melds.  
         [0012]     When an ultra-miniature impeller as described above is to be molded in an injection mold, in addition to sufficiently drying the thermoplastic resin base material beforehand, the inside of the mold must be evacuated during the molding operation. The evacuation port is advantageously provided along the rim of the vanes, in the end of the mold opposite its gate. For example, the port can be provided in the lateral surface of the cavity that corresponds to the ring section, or in the vicinity of the borderline between the ring section and the vane tips.  
         [0013]     In order to make the flow of thermoplastic resin inside the ring-forming portion of the mold more definite and reliable, the resin may be forced out through the evacuation port and then cut off.  
         [0014]     As another means of enhancing the strength of the ring section, a ring-shaped element formed from metal or other suitable material may be placed into a position inside in the mold equivalent to the ring section and then the thermoplastic resin infused into the mold. Exploiting such an insert-molding technique also contributes to enhancing the strength of the ring section of an ultra-miniature impeller.  
         [0015]     From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]      FIG. 1  is a vertical section view illustrating a centrifugal fan involving a first embodiment of the present invention;  
         [0017]      FIG. 2  is an elevational view representing the centrifugal fan;  
         [0018]      FIG. 3  is a transverse sectional view depicting the centrifugal fan;  
         [0019]      FIG. 4  is a chart setting forth process flow in the manufacture of an impeller by injection molding;  
         [0020]      FIG. 5  is a sectional view of a mold;  
         [0021]      FIG. 6  is a view depicting a portion of the mold in section;  
         [0022]      FIG. 7  is a view showing the mold with its core having been drawn out;  
         [0023]      FIG. 8  is a sectional view illustrating a mold in an implementation in which a ring element is used to form a reinforcing ring;  
         [0024]      FIG. 9  is a sectional view illustrating another example of a mold;  
         [0025]      FIG. 10  is a sectional view illustrating yet another example of a mold;  
         [0026]      FIGS. 11A-11C  are diagrams representing arrangements of the reinforcing ring and the vanes;  
         [0027]      FIG. 12  is a vertical section view illustrating a centrifugal-fan impeller involving a second embodiment of the present invention;  
         [0028]      FIG. 13  is view illustrating the impeller of  FIG. 12  from a lateral aspect; and  
         [0029]      FIG. 14  is an enlarged fragmentary view showing details of the impeller as shown in  FIG. 13 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Reference is made to  FIG. 1 , which is a diagram illustrating the configuration of a centrifugal fan  1  involving a first mode of embodying the present invention and represents a vertical section sliced along a plane containing the fan&#39;s center axis  10 . Reference is also made to  FIG. 2 , which is an elevational view of the centrifugal fan  1 , and to  FIG. 3 , which is a transverse view of the centrifugal fan  1  in section along the arrow-indexed locus A-A.  
         [0031]     The centrifugal fan  1  is an electromotive fan utilized in order to air-cool electronic parts in the interior of electrical products and electronic devices (portable articles in particular). The centrifugal fan  1  is equipped with: an impeller  2  that by rotating generates a flow of air; a motor  3  for rotating the impeller  2 ; and a housing  4  for housing the impeller  2  and the motor  3 , and that controls the flow of air generated by the rotation of the impeller  2 , sending the air outside the fan.  
         [0032]     The impeller  2  is approximately round-cylindrical in external form, and is furnished with: a plurality of vanes  21  for generating a flow of air; a connector section  22  for linking together and anchoring the motor-ward ends of the plurality of vanes  21 , and being the impeller end that connects to the motor  3 ; and an approximately round cylindrical reinforcing ring  23 , fixed to the vane ends on the side of the plurality of vanes  21  that is opposite the connector section  22 , that reinforces the linkage of the vanes  21 . The plural vanes  21 , the connector section  22 , and the reinforcing ring  23  are molded unitarily from a thermoplastic resin.  
         [0033]     As shown in  FIG. 3 , the plurality of vanes  21 , at a fixed distance from the impeller center axis  10 , is arrayed encompassing the center axis  10 , with the vanes spaced apart at a set pitch fp; and as indicated in  FIG. 1 , the vanes each extend parallel to the center axis  10 . When the motor  3  spins, air flows through the reinforcing-ring  23  end of the impeller, into an interior space  90  that is enveloped by the plurality of vanes  21 . This means that in the impeller  2 , the reinforcing ring  23  constitutes the rim of an opening through which air is led into the space  90 . The connector-section  22  end of the space  90  is closed off by the connector section  22  being connected to the motor  3 .  
         [0034]     The housing  4  is, as shown in  FIGS. 1 and 2 , composed of a housing main unit  45  that houses the impeller  2  and the principal components of the motor  3  (as far as the environs of the motor&#39;s stator  38 ), and a cap  46  that fits snugly into the housing main unit  45 . An air inlet  41  and a venting port  42  are provided in the housing main unit  45 .  
         [0035]     In a centrifugal fan  1  having the configuration just described, when the impeller  2  spins, air flows into the space  90  through the air inlet  41  and flows out from between the plurality of vanes  21 , traveling along the inner surface  49  of the housing  4 , and is sent out through the venting port  42 .  
         [0036]     Herein, the outer diameter  2   r  of the impeller  2  (r being the radius) illustrated in  FIG. 1  is no more than 25 mm, with the length fL of the plurality of vanes  21  in terms of their extent along the center axis  10  satisfying the relation 2≦fL/r≦20. In this embodiment, the outer diameter  2   r  is 12 mm, and the length fL is 27 mm (wherein the reinforcing ring length rL is 4 mm). It should be understood that although the working length of the vanes  21 , being fL−rL, is shortened owing to the extent taken up by the axial length of the ring section, in the present invention, because fL is large, performance degradation from the deficit in working vane length owing to the presence of the ring section is negligible. It should also be understood that the outer diameter  2   r  of the impeller  2  is defined as not including the thickness rt, as indicated in  FIG. 3 , of the reinforcing ring  23 .  
         [0037]     In the impeller  2 , by the relation 2≦fL/r being satisfied the point of maximum flow speed of the air flowing out from between the plurality of vanes  21  is put in the vicinity of midway between the two ends of the vanes  21 . The flow volume of air is increased as a result, enabling the generation of a highly efficient flow of air. At the same time, by fL/r≦20 being satisfied, vibration is held down even at rotating speeds of more than 10,000 rpm, (for example, 20,000 rpm). The configuration is thus favorable to revving the fan at high rpm, whereby the flow volume and static pressure of the air can be heightened all the more.  
         [0038]     Reference is now made to  FIG. 4 , which is a chart setting forth process steps to manufacture for the centrifugal fan  1  an impeller  2  having fine, long vanes  21  by injection molding. In manufacturing the impeller  2 , at first preparations are made by setting a mold having a cavity, which is an interior space made to match the shape of the impeller  2 , into an injection-molding machine (step S 1 ). Reference is further made to  FIG. 5 , which is a sectional view illustrating the structure of the mold  6 , and to  FIG. 6 , which is a diagram illustrating a portion of a sectional plane through the mold  6 , along the arrow-indexed locus B-B in  FIG. 5 . The orientation of the impeller that would be molded in  FIG. 5  is right-left reversed from the orientation of the impeller  2  illustrated in  FIG. 1 .  
         [0039]     The mold  6  comprises: a first plate  61 , to which a nozzle  91  of the injection-molding machine connects; a second plate  62  in contact with the left side of the first plate  61 ; a third plate  63  that is located on the leftmost side of the mold; two side blocks  64  in between the second plate  62  and the third plate  63 , located above and below to enclose the cylindrical side of the impeller  2  being molded; and a core  65  inserted into the approximately round cylindrical space flanked by the two side blocks  64 .  
         [0040]     A flowpath  611  through which thermoplastic resin ejected through the nozzle  91  passes is formed in the first plate  61 ; the gate  612  in the end of the flowpath  611  corresponds to the center of the connector section  22  of the impeller  2 . (The center of the impeller connector section  22  is actually where a hole is formed, through which the motor  3  is connected after molding—c.f.  FIG. 1 .) The second plate  62  has an inner-side surface that corresponds to the outer-side surface of the connector section  22 , and forms a space  621  that corresponds to the connector section  22 . As shown in  FIG. 6 , the core  65  is inserted into the space flanked by the two side blocks  64 , wherein the core  65  creates a conformation corresponding to the space  90  inside the impeller  2  and to the spacings between the plurality of vanes  21  (c.f.  FIG. 3 ). In  FIGS. 5 and 6 , the flutes in the core  65  that correspond to the vanes  21  are labeled with reference mark  651 . It will be appreciated that in  FIG. 5 , on the upper side of the center line  60 , depicted is a situation in which one of the flutes  651  is present, while on the lower side, depicted is a situation in which one of gill-like regions  652  (see  FIG. 6 ) of the core  65 , which are present between the plurality of flutes  651 , is present. Furthermore, a recess that extends lengthwise with respect to the center line  60 , and which corresponds to the reinforcing ring  23 , is labeled with reference mark  641  in  FIGS. 5 and 6 .  
         [0041]     The third plate  63  has an opening through which the core  65  is inserted/removed, and the right-side surface of the plate corresponds to the end face of the reinforcing ring  23 , which is the rim of the opening in the impeller  2 . In a position corresponding to the corner between the end face and lateral surface (a position pointing to the cylindrical surface) of the reinforcing ring  23 —in particular, in a position that is between the third plate  63  and one of the side blocks  64  and is in one of the flutes  651 —an evacuation port  631  is formed as a slight breach. The evacuation port  631  is connected to an evacuation passage  632  formed between the third plate  63  and the side block  64 . The evacuation passage  632  is connected to an evacuating pump in the injection-molding machine. Along the opening for the core  65  in the third plate  63 , grooves corresponding to the core&#39;s gill-like regions  652  are formed so that the core  65  can be extracted following an injection molding operation. Thus in this configuration, the flutes  651  in the core  65 , which correspond to the vanes  21 , are tangent to the inner-side surface of the side blocks  64 ; and twin walls of the grooves formed in the third-plate  63  opening through which the core  65  is introduced define projections that (where they correspond to the end faces of the vanes  21 ) close off the flutes  651 .  
         [0042]     Once the mold  6  has been set into the injection-molding machine, the evacuating pump is run to evacuate the mold  6  interior space—that is, the mold cavity—through the evacuation passage  632  to put the cavity into a vacuum state (step S 2 ). Meanwhile, a pellet of thermoplastic source material, having been dried beforehand by heating the material 2.5 to 3 hours at 140-165° C. inside a drier under a reduced-pressure environment or under a predetermined gas environment, is fed from a hopper into the injection-molding machine, without prolonged contact with external air. Within a screw cylinder in the molding machine the thermoplastic resin is melted by heating it up to 250-330° C. using a heater. The mold  6  is maintained at 70-90° C. by means of a separate heater. It should be understood that an injection-molding machine in which pre-drying of the pellet is unnecessary may be employed.  
         [0043]     Once the above-described preparations have been finished, the molten resin is ejected through the nozzle  91 , directed into the flowpath  611 , and the resin flows heading from the first plate  61  to the third plate  63 —in particular, heading from a location corresponding to the connector section  22  of the impeller  2 , to a location corresponding to the reinforcing ring  23 —whereby the cavity interior is filled with resin (step S 3 ). Gas evolving from the resin at the same time that the resin is flowing into the cavity is forced through the evacuation port  631  and exhausted from the cavity via the evacuation passage  632 . It will be appreciated that because the infused resin swiftly fills the cavity interior and thereafter hardens rapidly, the mold temperature is adjusted in advance to be 70-90° C. when the resin is being injected.  
         [0044]     Utilized as the source material are thermoplastic resins whose principal component is a thermotropic liquid-crystal polymer (here indicating that half or more of the weight is a thermotropic liquid-crystal polymer, and including instances in which the resin is exclusively a thermotropic liquid-crystal polymer), which are resins that excel in fluidity, and have high post-setting strength and outstanding mechanical properties. Specifically, a fully aromatic polyester liquid-crystal polymer to which on the order of 20 weight % fibrous matter such as glass or carbon fiber has been added—a material typified by polyphenylene sulfide (PPS) or Vectra® into which fiberglass has been mixed—is utilized. Furthermore, materials in which PPS and Vectra® are intermixed, or in which other resin(s) are mixed into a thermotropic liquid-crystal polymer, may be utilized.  
         [0045]     Notwithstanding that each of the vanes  21  is of slender form, by the exhausting of gases in the cavity interior through the evacuation port  631  formed in a region that corresponds to one end of the plural vanes  21 , and by the infusing of molten resin through the gate  612  formed in a region that corresponds to where the other end of the plural vanes  21  is (that is, a region that is associated with the other end), the cavity is appropriately filled with resin to form the vanes  21  in their entirety. Moreover, the reinforcing ring  23 , which is molded in parallel with the vanes  21 , is formed by the corresponding space inside the mold becoming appropriately filled with resin. It should be understood that, as long as the resin flows for the most part unidirectionally inside the space  651  for the vanes  21 , the gate  612  may be formed in another region of the mold  6  that corresponds to where the other end of the plurality of the vanes  21  is—for example, in a region that corresponds to the outer-side surface of the connector section  22  of the impeller  2 .  
         [0046]     After the resin has cooled and set, the molded impeller  2  is taken out of the mold  6  (step S 4 ). Initially, the core  65  is extracted from the third plate  63  and the side blocks  64 .  FIG. 7  is a sectional view depicting the core  65  having been extracted partway from the mold  6 . As described previously, grooves corresponding to the gill-like regions  652  in the core  65  are formed in the third plate  63 , wherein twin walls of the grooves define projections that oppose the end face of the vanes  21 . Thus the projections block the vanes  21  from being drawn out together with the core  65  when it is being extracted, whereby the vanes  21  remain inside the cavity, sandwiched between the two side blocks  64 .  
         [0047]     After the core  65  has been extracted the two side blocks  64  are parted slightly, and then by pushing out the connector section  22  of the impeller  2  with a shoving member  613  provided in the vicinity of the flowpath  611  in the first plate  61 , the impeller  2  is completely separated from and taken out of the mold  6 . In the impeller  2  after having been withdrawn, in a place corresponding to the gate  612 , a hole into which a rotor yoke  31  component of the motor  3  fits is formed (c.f.  FIG. 1 ).  
         [0048]     Reference is now made to  FIG. 8 , which is a sectional view depicting the recess  641  and vicinity, formed by the side blocks  64  and third plate  63  of the mold  6 . In this case, with the mold  6  having been set into the injection-molding machine, an approximately round cylindrical metal ring element  23   a , as illustrated in  FIG. 8 , is inserted ahead of time into the recess  641 , and in that state the cavity interior is evacuated and the resin injected. By having the reinforcing ring  23  be a metal element in insert-molding instances, the strength of the reinforcing ring  23  is enhanced to improve the reliability of the impeller  2 .  
         [0049]     The description turns now to  FIG. 9 , which illustrates another example by which the strength of the reinforcing ring  23  is enhanced. In the mold  6  in  FIG. 9 , apertures  633  are formed in a region that corresponds to the end face of the reinforcing ring  23 . Evacuation of the cavity interior is carried out through the apertures  633 . The apertures  633  are provided matching the depth of the recess  641 , within the third plate  63 , or else in between the third plate  63  and the core  65 , in a plurality of places running along the annular recess  641 . Furnishing the apertures  633  means that when the injection molding operation is carried out, some of the resin that fills the reinforcing ring  23  portion of the mold  6  will overflow through the apertures  633 .  
         [0050]     In utilizing the mold  6  depicted in  FIG. 9  to manufacture an impeller  2 , a step of removing the resin that has overflowed through the apertures  633  is added to the last of the manufacturing steps set forth in  FIG. 4 , that is, after the impeller  2  has been taken out of the mold  6 . Resin that has overflowed through the apertures  633  may be removed in the course of taking the impeller  2  out of the mold  6 . In that case, before the core  65  is extracted from the impeller vanes, it is advantageous to undo the side blocks  64 , and in that state trim the vane tips and the resin portions that are sticking out.  
         [0051]     In an implementation in which an impeller is molded in this manner, when the thermoplastic resin melds in the reinforcing ring  23  portion of the cavity, the resin in the vicinity of the meld lines flows fully, improving the joint strength along the meld lines.  
         [0052]      FIG. 10  shows yet another example of a configuration for enhancing the strength of the reinforcing ring  23 . In this case, in the mold  6  depicted in  FIG. 10 , the region in the third plate  63  that opposes the end face of the vanes  21  constitutes a projection  634  that juts out toward the side blocks  64 . Put differently, the recess  641  corresponding to the reinforcing ring  23  is elongated in the direction toward the third plate  63 . This configuration causes the reinforcing ring  23 , molded by evacuating and infusing with resin the interior of the mold cavity, to have a projecting portion that juts out from the ends of the plurality of vanes  21 . (C.f. projecting portion  23   b  in later-described  FIG. 11B .)  
         [0053]     In an implementation of a mold  6  configured as shown in  FIG. 10 , similarly to the implementation represented in  FIG. 9 , when the thermoplastic resin melds in the reinforcing ring  23  portion of the cavity, the resin in the vicinity of the meld lines flows fully, by the amount that the recess  641  is elongated, further improving the joint strength along the meld lines.  
         [0054]     Next, the results of actually molding impellers  2  as explained in the foregoing and testing the strength of their reinforcing rings  23  will be described. Table 1 is a tabulation setting forth three types (Characterizations 1 to 3) of injection-molded impeller  2  conformations. The units of length in Table 1 are millimeters. In the test, Vectra® was utilized as the thermoplastic resin, and samples in which, as depicted in  FIG. 11A , the end face of the vanes  21  and the end face of the reinforcing ring  23  coincide were fabricated.  
                                                               TABLE 1                                       Characterization No.                1   2   3                            Impeller o.d.   12   12   12           Number of Vanes   30   34   38           Vane max. thickness ft   0.30   0.29   0.28           Vane length fL   23   23   23           Length/max. thickness   77   79   82           Ring thickness rt   0.50   0.50   0.50           Vane spacing fp   1.26   1.11   0.99           Vane spacing × 2   2.52   2.22   1.98           Ring length rL   2.0   4.0   4.0           Ring strength   X   ◯   ◯                      
 
         [0055]     In the “Ring strength” column in Table 1, “x” indicates that in taking the impellers  2  out of the mold  6  following the injection-molding operation, there was a 70% or greater likelihood that fracturing in the reinforcing rings  23  would occur, while “∘” indicates that there was a less than 10% likelihood. It may be ascertained from the table that with Characterizations 2 and 3, in which the reinforcing rings  23  were made longer, although the thicknesses of the rings were not increased, the reinforcing ring  23  strength was sufficient.  
         [0056]     In addition, impellers as shown in FIGS.  11 B and  11 C—of a form in which part of the reinforcing ring  23  jutted out from the vanes  21 , and of a form in which the reinforcing ring  23  was connected to the end face of the vanes  21 —were fabricated under Characterization 3 in Table 1. In these implementations as well, the incidence of fracturing in the reinforcing ring in taking the impeller out of the mold was less than 10%, and thus strength in the reinforcing rings was secured.  
         [0057]     Here, by having the length of the projecting portion  23   b , which from the ends of the vanes  21  juts out paralleling the center axis  10 , of reinforcing rings  23  in the  FIG. 11B  implementation be 1.5 times the pitch fp of the vanes  21 , the resin flowing out from the flutes  651  that correspond to the vanes  21  flows sufficiently into the extension portion of the reinforcing ring  23 , whereby sufficient strength along the meld lines is secured. (C.f.  FIG. 10 .)  
         [0058]     In molding applications in which articles of extremely slender conformation are injection-molded, as is the case with the vanes of impellers  2  of the present invention, thermotropic liquid-crystal polymers of long flow length are often employed as the molded material. Thermotropic liquid-crystal polymers during molding exhibit strong anisotropy in terms of the resin flow direction, such that degradation in strength along meld lines is serious. Utilizing the present invention, however, averts compromised strength along meld lines that form in the reinforcing ring, to enable high-strength impellers to be produced.  
         [0059]     Next, referring to  FIGS. 12-14 , an explanation of a centrifugal fan involving a second mode of embodying the present invention will be made.  FIG. 12  is a vertical section view illustrating a centrifugal fan impeller  2   a , sliced through a plane containing the fan&#39;s center axis  10 , involving a second embodiment of the present invention.  FIG. 13  is lateral-aspect diagram of the impeller  2   a  seen from the right side in  FIG. 12 , looking toward the left; and  FIG. 14  is diagram in which a portion of the impeller  2   a  as depicted in  FIG. 13  is shown enlarged. As illustrated in  FIG. 13 , in a centrifugal fan involving the second embodiment, a plurality of vanes  21   a  having a transverse cross-sectional form that differs from that of the plurality of vanes  21  depicted in  FIG. 3  is provided in the impeller  2   a . Apart from this feature, the configuration is similar to that of  FIG. 1  through  FIG. 3 , and thus in the following illustration, the same reference marks will be appended.  
         [0060]     With the exception of being furnished with the impeller  2   a  depicted in  FIGS. 12-14 , a centrifugal fan involving the second embodiment is similar to that of  FIG. 1 , and thus the structure and form of the motor  3  and housing  4  are the same as that shown in  FIG. 1  through  FIG. 3 . The plural vanes  21   a , the connector section  22 , and the reinforcing ring  23  are molded unitarily from a thermoplastic resin whose principal component is a thermotropic liquid-crystal polymer. In  FIGS. 13 and 14  also, likewise as in  FIG. 3 , the pitch of the plural vanes  21   a  is labeled with reference mark fp, and the impeller  2   a  outer diameter is labeled with reference mark  2   r.    
         [0061]     In the impeller  2   a , as indicated in  14 , along each of the plural vanes  21   a  the thickness ft 2  of the region (called “ring joint” hereinafter)  211  connected to the reinforcing ring  23  is thicker than the thickness dimension of the rest of the vane  21   a , wherein each vane  21   a  gradually diminishes in thickness as the dimension parts away from the reinforcing ring  23 . Thus the minimum thickness ft 1  is in the verges  212  at the inner-peripheral side of the vanes  21   a , (with the roundness attendant on rounding off the vane edges not being deemed thickness).  
         [0062]     The process flow in manufacturing the impeller  2   a  by injection molding is the same as the flow, set forth in  FIG. 4 , for manufacturing the impeller  2  involving the first embodiment, and the configuration of the mold employed in manufacturing the impeller  2   a , except for the conformation of the cavity corresponding to the vanes  21   a , is also the same as that of the mold  6  depicted in  FIG. 5 .  
         [0063]     Next, the results of molding impellers  2   a  and testing the strength of their reinforcing rings  23  will be described. Table 2 is a tabulation setting forth two types (Characterizations 4 and 5) of injection-molded impeller  2   a  conformations, and as a comparative example, entered together with these characterizations is the impeller  2  conformation of Characterization 1 set forth in Table 1. In the test, Vectra® was utilized as the thermoplastic resin, and samples in which, in the same way as is the case with the vanes  21  and reinforcing ring  23  depicted in  FIG. 11A , the end face of the vanes  21   a  and the end face of the reinforcing ring  23  coincide were fabricated.  
                                                               TABLE 2                                       Characterization No.                1   4   5                            Impeller o.d.   12   12   5.4           Number of Vanes   30   34   24           Vane thickness ft1   0.30   0.29   0.17           Vane thickness ft2   0.30   0.35   0.20           Vane length fL   23   23   9.5           Length/max. thickness   77   66   48           Ring thickness rt   0.50   0.50   0.25           Vane spacing fp   1.26   1.11   0.7           Vane spacing × 2   2.52   2.22   1.4           Ring length rL   2.0   4.0   1.5           Ring strength   X   ◯   ◯                      
 
         [0064]     In the “Ring strength” column in Table 2, like in Table 1, “x” indicates that in taking the impellers  2   a  out of the mold  6  following the injection-molding operation, there was a 70% or greater likelihood that fracturing in the reinforcing ring would occur, while “∘” indicates that there was a less than 10% likelihood. The units of length in Table 2 are also millimeters.  
         [0065]     From the results of the test it may be ascertained that with the impellers  2   a  of Characterizations 4 and 5, in which the thickness of the vanes  21   a  gradually diminishes the further away from the reinforcing ring  23  the measurement is (that is, the characterizations in which ft 1  is smaller than ft 2 ), the reinforcing rings  23  had sufficient strength.  
         [0066]     Although methods of manufacturing centrifugal fans and impellers involving modes of embodying the present invention have been explained in the foregoing, in that various modifications of the present invention are possible, the invention is not limited to the embodiments described above.  
         [0067]     For example, in the foregoing embodiments, examples were set forth in which prior to the injection molding operation the cavity in the mold  6  was evacuated to bring it into a vacuum state, but the evacuation may be carried out in parallel, for the most part, with the molding operation. Additional examples are that in the third side plate  63  a minute evacuation port may be formed to carry the evacuation out through a position corresponding to the end face of the reinforcing ring  23 , and that the minute evacuation port may be formed in the base of the recess  641  corresponding to the reinforcing ring  23 .  
         [0068]     In any of the examples of  FIG. 5  and  FIG. 8  through  FIG. 10 , the reinforcing ring  23  may join the plurality of vanes  21  along the inner side of the vanes  21  (the same being true of the vanes  21   a  and reinforcing ring  23  of the second embodiment). Also, in the  FIG. 9  implementation, in which a portion of the resin for the reinforcing ring  23  overflows, the direction in which the resin overflows does not have to be parallel to the center axis, but may be perpendicular to the center axis. And the opening through which the resin overflows may be formed in a position corresponding to the lateral (cylindrical) surface of the reinforcing ring  23 .  
         [0069]     In the implementation illustrated in  FIG. 10 , from the perspective of facilitating reduction of the outer diameter of the reinforcing ring  23 , it is preferable that the projecting portion  23   b  (c.f.  FIG. 11 ) be formed parallel to the center axis, but the projecting portion may be rendered in a form in which it expands outward or projects inward from the reinforcing ring  23 .