Abstract:
Enables the uniform, accurate application, onto microscale areas, of an in-solution oil repellent of low viscousness and in which the solvent is of extremely high volatility. A contacting piece that comes into contact with a target for application of the repellent is encased inside a sheath structure that, including the contacting piece, is lent rigidity. Therein, while the in-solution oil repellent is fed along the inside of the sheath structure, along the contacting piece itself, and onto the repellent-application target, it is coated on by the contacting piece tracing the surface of the application target. Giving at least the application start-point two coats, or a number of applications greater than that, yields a coating film of still higher uniformity.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to methods of superficially applying low-viscosity liquids onto given components. In particular, the invention relates to a method of coating on a low-viscosity, in-solution oil repellent obtained by dissolving a fluorine-based oil repellent resin in a highly volatile organic solvent. 
     2. Description of the Related Art 
     Forming coatings of oil-repellent resins, which primarily are fluoroplastic resins, on designated portions of mechanical devices to impart water-repellency or oil-repellency to those areas is conventional. Forming such coatings without restrictions has, however, proven difficult, in that various contrivances to do so have been brought about to date. 
     For example, methods of brush application, of spraying on with a sprayer, of dipping in a solution and subsequently drawing out and drying, of spin-coating, of transfer-printing, and of dripping a solution onto designated regions with a brush or like instrument are known. Furthermore, direct formation of an oil-repellent film onto the target surface by means of vacuum deposition or plasma polymerization has also been proposed. 
     Vacuum and other vapor-deposition techniques, however, necessitate large-scale equipment. Dip-coating and spin-coating are prohibitive of application to designated areas. With brush application, because the tip of the brush deforms, applying a material onto designated areas proves challenging. A particular problem with brush application is that the oil repellent solidifies due to evaporation of the solvent and clings to the brush in the vicinity of the tip, whereby the flexibility of the brush is compromised and at the same time clumps of the solidified oil repellent end up adhering to the target object, such that the applicability of this technique to precision components is especially problematic. 
     Japanese Unexamined Pat. App. Pub. No. 2004-289957 to Misu et al. discloses an ingenious method in which, using a pair of nozzles whose tips are closely adjacent, an in-solution oil repellent is on the one hand supplied from one of the nozzles while being aspirated through the other, whereby the oil repellent is applied locally with the nozzles being kept out of contact with the target object. 
     BRIEF SUMMARY OF THE INVENTION 
     A method of applying an in-solution oil repellent according to the present invention includes contacting onto a surface of a target object an applicator tip having rigidity such that it basically does not deform from the level of pressing required for applying the solution, and, via a contacting piece in the applicator tip, coating-on the in-solution oil repellent. A capillary gap through which the in-solution oil repellent is supplied opens near the contact surface where the contacting piece contacts the target object. 
     According to this method, the oil repellent is supplied from near the contact surface, and therefore the adverse effect of solidification of the oil repellent due to evaporation of the solvent is relatively small. Moreover, since the contacting piece has rigidity, microparticles of the oil repellent, which form due to solidification, pulverize by being pressed by the contacting piece and dissolve again in the in-solution oil repellent that is supplied subsequently. Therefore, the microparticles do not produce dust or contaminate the surrounding area. Moreover, since the contacting piece has rigidity, the size of the contact area does not vary throughout the application operation, so that the in-solution oil repellent can be applied at a constant width. 
     The applicator tip may be one that deforms when pressed against the repellent-coating target object. As long as the pressing force is constant, however, the amount of deformation will necessarily be constant. Likewise, if the force is removed, the tip will necessarily return to its original form quickly. 
     An application method in which the solution is applied, and after the solvent evaporates and the oil repellent loses flowability, the solution is applied once again to the same location also produces beneficial results. In some cases, broken bits of oil repellent solidified near the tip end of the contacting piece do not disappear sufficiently by a single application. Even in such cases, uniformity of the coating film can be enhanced by applying the solution a plurality of times. 
     In the double application, at least the starting point of the oil repellent application needs to be coated two times. The oil repellent solidified at the tip end of the contacting piece is very likely to remain at the repellent-application starting point, but by applying the oil repellent two times to at least that portion, it is possible to improve the portion in which the problem is most likely to occur. 
     Since the viscosity of the in-solution oil repellent is often very low, the amount of outflow may be too large unless the size of the capillary gap or the like is selected appropriately. Even when the size of the capillary gap cannot be selected freely, a large amount of outflow of the in-solution oil repellent due to hydraulic pressure is prevented by keeping the opening of the capillary gap and the liquid surface of the in-solution oil repellent substantially at the same level. 
     As another method of adjusting the amount of outflow of the in-solution oil repellent, a porous material or the like for restricting the flow of fluid may be disposed in the interior of a reservoir for the in-solution oil repellent, or in a flowpath for supplying the in-solution oil repellent to the capillary gap. This may be disposed at a portion that is immersed in the in-solution oil repellent, or may be attached near the opening of the reservoir that is not immersed in the in-solution oil repellent. The flow resistance of the in-solution oil repellent or the flow rate of the air coming from outside into the reservoir drops, allowing the outflow rate of oil repellent from the opening of the capillary gap to lower. When this method is used, it is possible to employ a configuration in which hydraulic pressure is applied intentionally. 
     The applicator tip may be configured so that the circumference of the contacting piece is covered by a sheath. Covering with the sheath prevents the in-solution oil repellent from evaporating. In addition, by imparting rigidity or resilience to the sheath, it becomes possible to select a material with smaller rigidity or resilience for the contacting piece. 
     A rolling object may be employed as the contacting piece. The contacting piece is rotated relative to the object to which the in-solution oil repellent is applied, so that the in-solution oil repellent can be applied while the contacting piece is being rolled. The application may be made even with materials that are not suitable for application by sliding the contact surface. 
     As a configuration of the applicator tip, a solid member may be used as the contacting piece and a capillary gap may be secured between the contacting piece and a sheath. Conversely, a material having micro-gaps in the interior thereof may be selected as the contacting piece, and the micro-gaps may be used as the flowpath of the in-solution oil repellent. Moreover, a porous material may be used as the contacting piece. The applicator tip may have an elongated shape. This facilitates the operation of coating repellent onto very narrow areas. Application to narrow areas between component parts is also facilitated. 
     The use of an urging mechanism for pressing the applicator tip against the object to which the oil repellent is applied is efficacious. A high-quality coating film is obtained with a simple mechanism. 
     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 
         FIG. 1  illustrates an example of an applicator for applying an in-solution oil repellent; 
         FIG. 2  is an enlarged view of the applicator; 
         FIG. 3  is another example of the applicator for applying an in-solution oil repellent; 
         FIG. 4  illustrates an example of an applicator tip; 
         FIG. 5  illustrates another example of the applicator tip; 
         FIG. 6  illustrates still another example of the applicator tip; and 
         FIG. 7  illustrates an in-solution oil repellent reservoir. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Spindle motors that are built into such devices as hard disk drives in many cases have a shaft on the surface of which an oil-repellent film composed of fluoropolymer is formed to prevent the lubricant from leaking. In the in-solution oil repellent applied to the shaft surface the concentration of the fluoropolymer is typically 1% and the solvent used is highly volatile. The application method according to the invention was carried out to apply this kind of solution using application apparatus as described in the following. 
     First Embodiment 
       FIG. 1  illustrates the overall configuration of an applicator  2  for applying the in-solution oil repellent on a circumferential surface of the shaft.  FIG. 2  illustrates a portion of the applicator near applicator tips for applying the in-solution oil-repellent, viewed in a shaft-axis direction. 
     A work holder  7  serves to hold the shaft in a position where the oil-repellent is applied to the shaft. In this example, the shaft has a diameter of 2.5 mm. The work holder  7  has an inwardly curved surface at its center, and the inner side of the curved portion is provided with two applicator tips spaced apart along the shaft axis. A shaft  1 , which is an object to which the oil-repellent is to be applied, is connected to a rotating mechanism  22  via a chuck  21  so that it can be rotated when applying the oil-repellent. The shaft  1 , the chuck  21 , and the rotating mechanism  22  are supported by a hinge joint  24  so that the front end side of the shaft can be lifted, as illustrated by dotted line in the figure. With the configuration illustrated in  FIG. 1 , because the left hand side of the hinge joint  24  in the figure is heavier, the shaft  1  is automatically pressed against the work holder  7  due to the effect of gravity. If the pressing force is so large that the tip ends of the applicator tips  3  deform, a spring  23  may be provided to tug backwards on the rotating mechanism  22  to reduce the urging force to an appropriate level. 
     Even if the shaft  1  is attached to the rotating mechanism  22  slightly tilted, the tilt may be compensated since the shaft&#39;s front end can be lifted easily; therefore, the oil-repellent is applied stably. The same applies even if the shaft surface has slight surface unevenness. 
     Referring to  FIG. 2 , two applicator tips  3  for applying the in-solution oil repellent to the shaft surface are provided along the shaft circumference. Providing two applicator tips  3 ,  3  enables them to support the shaft  1  stably when applying the solution. The two applicator tips  3 ,  3  may supply in-solution oil repellents with varying concentrations. The applicator tips  3 ,  3  are supplied with the in-solution oil repellent via flowpaths  9 ,  9  from reservoirs  5 ,  5 . Varying the concentration of the in-solution oil repellent in the reservoirs enables application of solutions with varying concentrations. 
     Because the solvent of the in-solution oil repellent vaporizes very quickly, under conditions in which the shaft is rotated about two times a second, the in-solution oil repellent loses flowability and solidifies before the shaft undergoes one rotation. The in-solution oil repellent applied by the applicator tip that is on the right in  FIG. 2  solidifies before it reaches the applicator tip that is on the left. 
     It is also acceptable that the applicator tip  3  be in a single location. Since the application is carried out while the shaft  1  is being rotated, only one applicator is sufficient to apply the in-solution oil repellent onto the whole circumference, and to apply two coats easily. While in this embodiment application was conducted at a rate of rotation of 100 rpm, the rate of rotation may be faster; but application becomes problematic at a rate higher than 300 rpm. 
     Second Embodiment 
       FIG. 3  is a schematic view illustrating an applicator  12  according to another embodiment. In the applicator  12  an applicator tip  3  is supported by a sliding mechanism  25  that can move along a radial direction of the shaft  1 . The applicator tip  3  and a reservoir  5  are pressed against the shaft surface by a spring  23 , so that the in-solution oil repellent is applied while the shaft  1  is being rotated. The spring and the sliding mechanism serves to compensate the tilt and surface unevenness of the shaft  1  to enable stable application of the solution. It should be noted that a chassis or the like for mounting the applicator  12  is not depicted in  FIGS. 1 through 3  for simplicity. 
     Third Embodiment 
       FIGS. 4A and 4B  are enlarged views illustrating examples of the applicator tip  3 , which show cross-sectional views on the left and front views on the right. 
     Referring to  FIG. 4A , the applicator tip  3  has a contacting piece  4  that is rectangularly prismatic in form, and a sheath  6  for accommodating the contractor  4  therein. Capillary gaps  11  extending along the axis form at four circumferentially separate locations between the contractor  4  and the sheath  6 . The interior of the capillary gaps  11  are filled with the in-solution oil repellent to openings  46  near the tip end of the applicator tip  3 . 
     A contact surface  45  forms adjacent to the openings  46  of the capillary gaps, on which the in-solution oil repellent spreads along the surface of the contacting piece  4 , and the in-solution oil repellent is applied onto the surface of the object to which the in-solution oil repellent is to be applied in the application work. 
     Referring to  FIG. 4B , a contacting piece  41  has a capillary gap  11  extending along the axis and having an opening in the center of a contact surface  45 . Outside the region depicted in the drawing, the capillary gap  11  is connected to a reservoir from which the in-solution oil repellent is supplied. Unlike the method illustrated in  FIG. 4A , solidified oil repellent rarely forms at the opening of the capillary gap during the application work because the opening is at the center of the contacting piece. 
     Since the sheath  6  has a sufficient rigidity in both the applicator tips shown in  FIGS. 4A and 4B , the applicator tips  3  as a whole have great rigidity so that precise application work can be carried out stably. 
     Fourth Embodiment 
       FIGS. 5A and 5B  are enlarged views illustrating other examples of the applicator tips  3 , which show cross-sectional views on the left and front views on the right. 
     Referring to  FIG. 5A , a contacting piece  42  is made of a bundle of fine fibrous material having micro-gaps through which a liquid can flow along the axis direction. The contacting piece  42  is accommodated in the interior of a sheath  6 , which ensures rigidity. The rear end of the contacting piece  42  is connected to a flowpath, which is not illustrated in the figure, through which the in-solution oil repellent is supplied. The micro-gaps themselves form the termini of the flowpath. The in-solution oil repellent  10  in this case oozes out on the tip end of the contacting piece  42  to cover the tip end. 
       FIG. 5B  illustrates an example of a simpler configuration of the applicator tip. Referring to  FIG. 5B , a contacting piece  42  is likewise formed by a bundle of fine fibrous material having micro-gaps through which a liquid can flow along the axis direction. Unlike the applicator tip shown in  FIG. 5A , that shown in  FIG. 5B  does not have a sheath  6 . The rigidity of the applicator tip is ensured by the contacting piece  42  alone. For this reason, the degree of deformation of the applicator tip by depressing is greater than the configuration shown in  FIG. 5A . Nevertheless, selecting the material appropriately allows the contacting piece to have a sufficient resilience. Therefore, the configuration shown in  FIG. 5B  is also capable of smooth application work. 
     Moreover, since the configuration shown in  FIG. 5B  does not have a sheath  6 , a large amount of solvent evaporates from the side face of the 
     Fifth Embodiment 
       FIG. 6  illustrates an example in which a contacting piece  43  has a spherical shape. The spherical contacting piece  43  is held freely rotatively in a recess  47  formed at one end of a sheath  6  so that a capillary gap is provided between the inner circumferential surface of the end portion and the surface of the contacting piece  43 . The in-solution oil repellent is supplied to the capillary gap through a flowpath  9  and is delivered by the rolling motion of the contacting piece  43  to a contact surface  45  that faces an object to which the in-solution oil repellent is to be applied. The capillary gap itself forms the terminus of the flowpath  9 . In this configuration, the contacting piece  43  rotates at all times during the application, so the contact surface  45  shifts to adjacent locations on the sphere one after another. 
     The applicator tip  3  shown in  FIG. 6  applies the in-solution oil repellent by means of rolling motion, not sliding motion, of the contact surface and is therefore suitable for such applications that the application accompanying sliding is inappropriate. 
     Sixth Embodiment 
       FIG. 7  shows an embodiment in which adsorbent fibers  50  are filled in the interior of the reservoir  5  so that the in-solution oil repellent can be held in the reservoir more stably. The adsorbent fibers may pack the entire interior of the reservoir, or may be fitted only near an air vent  51  to restrict airflow into the reservoir. Either way makes it possible to reduce the flow rate of the in-solution oil repellent, which has a very low viscosity and flows out very easily, from one end of the applicator tip, so that the outflow of an excessive amount of in-solution oil repellent can be prevented. It should be noted that instead of the adsorbent fibers  50 , the reservoir may be filled to a given extent with a particulate substance. 
     Other Embodiments 
     Although the first to six embodiments above illustrate a cylinder-shaped shaft as the object to which the in-solution oil repellent is applied, applications of the application method of the invention is not limited to cylindrical components. The application method according to the invention may even be applied to an inner circumferential surface of cylindrical bearing sleeve as long as the tip(s) of the applicator can be brought in contact therewith. The application method according to the invention may of course be applied easily to non-curved surface portions of machine components, such as flat surfaces. 
     Only selected embodiments have been chosen to illustrate the present invention. To those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for limiting the invention as defined by the appended claims and their equivalents.