Abstract:
An apparatus and method for ultrasonically welding workpieces that reduces sonotrode adhesion during the ultrasonic welding process. The sonotrode includes a contact surface wherein a fluid is deposited on the contact surface prior to the welding process. The fluid may be applied in different ways, including providing an aperture in the contact surface of the sonotrode. In addition, the sonotrode may be cooled below the dew point of the surrounding atmosphere thus causing moisture to form on the contact surface of the sonotrode. Cooling the sonotrode to a temperature above the dew point also reduces sonotrode adhesion during the ultrasonic welding process.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to ultrasonic metal welding and, more specifically, to an ultrasonic welding apparatus that reduces sonotrode adhesion during the ultrasonic welding process. 
     Ultrasonic welding of various materials is known. The process involves vibrating overlapping or adjacent workpieces clamped between a sonotrode and an anvil. Frictional forces occurring between the vibrating workpieces create a bond or weld that occurs at the interface between the workpieces, effectively joining them to one another. Accordingly, various sonotrode and anvil surface configurations, i.e., the surface that contacts the workpieces, are known and used to transfer energy from the sonotrode to the aforementioned interface. Such configurations attempt to reduce the energy loss at the sonotrode/workpiece interface or the anvil/workpiece interface thereby increasing the energy to the workpiece/workpiece interface and increasing the overall efficiency of the ultrasonic welding apparatus. 
     Further, it is known that when using an ultrasonic welding apparatus to weld light metals, specifically aluminum, the sonotrode or more specifically, the sonotrode adheres to the workpiece being welded. The adhesion can be so severe as to (i) damage the weld when detaching the sonotrode from the joined workpieces, (ii) cause significant and unacceptable distortion of the work piece surface, and (iii) render the sonotrode unusable for subsequent welds. Sticking or adhesion to the workpiece generally results from the sonotrode sliding on the workpiece. When the sonotrode slides, it causes galling or a buildup of material on the sonotrode. 
     With many of the current sonotrode designs and surface configurations, each time the sonotrode performs a weld, a small amount of aluminum is transferred unto the sonotrode. Continued welding operations cause the aluminum to build up on the sonotrode surface. The built up aluminum on the sonotrode bonds with the material of the workpiece. When this occurs, the sonotrode sticks to, or in short, becomes welded or bonded to the workpiece. Forces of up to 5kN may be required to detach the bonded sonotrode from the workpiece material. Additionally, as aluminum builds up on the sonotrode, it clogs the gripping surface of the sonotrode and reduces the efficiency of the ultrasonic welding apparatus because the energy transferred to the workpiece to perform the weld is reduced. 
     When the sonotrode becomes clogged, the useful life thereof is reduced. The practical consequence of this is that the sonotrode needs to be cleaned after each weld. Moreover, the surface of the welded material may be severely damaged and will require costly craftsmanship work before it will meet surface finish specifications. 
     Therefore, there is a need in the art to provide an ultrasonic welding apparatus designed such that it reduces aluminum/sonotrode adhesion during the ultrasonic welding process while improving the productivity, manufacturing speed and reducing equipment downtime by reducing the sticking phenomenon that is common when ultrasonically welding materials. 
     SUMMARY OF INVENTION 
     Accordingly, the present invention is an ultrasonic welding apparatus and method that reduces adhesion between the welding sonotrode and the workpiece during the welding process. 
     In one embodiment, a fluid is deposited on a contact surface of the sonotrode before the welding process. The fluid may be deposited in several ways all of which are within the scope of the invention. For instance, in one embodiment, the fluid travels through a passageway in the sonotrode to an aperture located on the contact surface of the sonotrode. The fluid is held within the passageway by capillary action. The contact surface may also include at least one groove thereon to aid in distributing the fluid on the contact surface. 
     In accordance with an additional embodiment, a cooling medium engages the sonotrode and cools the sonotrode below the dew point of the surrounding atmosphere, causing moisture to condense or form on the contact surface of the sonotrode. The cooling medium may be externally blown across the sonotrode or it may travel through various passageways located within the sonotrode. 
     A further embodiment utilizes the use of cooling fins connected to the sonotrode to cool the sonotrode to a lower temperature, one at which the sonotrode is less likely to stick to the workpiece. 
     Further, the present invention provides a method for reducing sonotrode adhesion. The method includes several steps operating alone or in combination, including the step of depositing a liquid on the contact surface of the sonotrode. This can be accomplished by using a passageway through the sonotrode to transfer liquid to the contact surface. In addition, the sonotrode and corresponding contact surface can be cooled via a cooling medium, to below the dew point of the surrounding atmosphere thereby causing moisture to condense on the sonotrode. 
     In addition, cooling the sonotrode before performing the welding process further prevents sonotrode adhesion. Various steps can be taken in to cool the sonotrode including the use of internal cooling passageways in the sonotrode. 
     Other features and advantages of the present invention will be readily appreciated, as the same becomes better understood after reading the subsequent description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic view of an ultrasonic welding apparatus utilizing a sonotrode in accordance with the present invention. 
     FIG. 2 is a side view of a sonotrode according to the present invention for use with an ultrasonic welding apparatus. 
     FIG. 3 is a bottom view of the sonotrode of FIG. 2 according to the present invention. 
     FIG. 4 is a side view of an alternative embodiment of a sonotrode according to the present invention for use with an ultrasonic welding apparatus. 
     FIG. 5 is a bottom view of the sonotrode of FIG. 4 according to the present invention. 
     FIG. 6 is a side view of a further alternative embodiment of a sonotrode according to the present invention for use with an ultrasonic welding apparatus. 
     FIG. 7 is a bottom view of the sonotrode of FIG. 5 according to the present invention. 
     FIG. 8 is a bottom view of a further alternative embodiment of a sonotrode according to the present invention for use with an ultrasonic welding apparatus. 
     FIG. 9 is a side view of the sonotrode of FIG. 8 according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a wedge-reed ultrasonic welding apparatus, seen generally at  10 , according to the present invention. The ultrasonic welding apparatus  10  includes a reed  12 , connected to sonotrode  14 , mounted for movement in a side-to-side or horizontal direction of vibration, shown by the arrow  16 . The reed  12  also moves in a vertical manner, shown by the arrow  18 , and in cooperation with an anvil  20  clamps the first  22  and second  24  workpieces in position. Once the workpieces  22 ,  24  are clamped, a transducer  15 , connected to the reed via the wedge  11 , vibrates the sonotrode  14  at a high frequency (typically 15 to 40 kHz) to impart energy to the first  22  and second  24  workpieces at a location between the sonotrode  14  and the anvil  20  to create a bond or weld at the interface or adjacent surfaces  26  of the workpieces  22 ,  24  in accordance with known ultrasonic welding processes. As used herein the term sonotrode generally refers to the tool attached to the reed  12 . In many cases, the sonotrode also includes a replaceable sonotrode tip. Accordingly, the sonotrode is the gripping tool attached to the end of the reed  12 . 
     As shown in FIG. 2, a sonotrode  14  is inserted into the body  29  of the reed  12 . Applying a liquid, such as water, in a small amount to the contact area located between the contact surface  28  of the sonotrode  14  and the workpiece  22  prior to welding the overlapping workpieces  22  and  24  helps to prevent the sonotrode  14  from sticking to the workpiece  22 . One method for depositing a fluid on the contact surface  28  of the sonotrode  14  is by feeding a liquid through an aperture  34  at that contact surface  28  of the sonotrodel 4 . One way of feeding the liquid is to provide the sonotrode  14  with an inner cavity  30 . The inner cavity  30  forms a reservoir that stores a liquid. The liquid passes from the inner cavity  30  or reservoir through a capillary feed tube or passageway  32  to the contact surface  28  of the sonotrode  14 . As known in the art, the contact surface  28  is the surface that contacts the workpiece  22  to impart energy to the workpieces  22 ,  24  to perform the weld. The capillary feed tube  32  terminates at an aperture  34  in the contact surface  28  of the sonotrode 14 . 
     A supply hose  36  extends through the body  29  of the reed  12  and into the inner cavity  30 . An O-ring  38  seals the supply hose  36  within the inner cavity  30 . Accordingly, as fluid exits the inner cavity or reservoir  30  through the capillary feed tube  32 , the supply is replenished via the supply hose  36 . As shown in FIG. 2, the liquid is supplied by gravity and capillary action. The capillary feed tube  32  is small enough to allow capillary forces to stop free-flowing of the liquid when the sonotrode  14  is not in contact with the workpiece  22 . In the preferred embodiment, the holes are large enough and preferably have an angular opening  34  that will not easily the clogged by small particles picked up or located on the workpiece  22 . The preferred embodiment utilizes a capillary feed tube having a diameter of about 1-1.5 mm; at about 2 mm the capillary forces are no longer active to the same extent. It should be understood that the capillary forces and correspondingly diameter of the capillary feed tube  32  will vary depending upon the type of liquid used. 
     In addition, the liquid may also be supplied by a low-pressure micro pump located either in the reed  12  or separate from it, wherein the supply hose  36  extends down through the reed  12 . Further, the sonotrode  14  may include a plurality of apertures  34  in the contact surface  28  to aid in distribution of the liquid. If necessary, to further aid in distribution of the liquid to the entire contact surface  28 , one or more grooves  40  can be formed in the sonotrode  14 . In many instances, the contact surface  28  may have a knurled pattern thereon to aid in gripping the workpiece. Preferably, the grooves  40  are made slightly deeper then the knurled or gripping pattern formed on the sonotrode  14  to allow the grooves  40  to remain open during the initial stages of the ultrasonic welding process. It should be understood that the contact pressure between the contact surface  28  and the workpiece  22  stops the liquid from flowing once the welding process has started. 
     The means for depositing a fluid may also include an apparatus that applies a cooling medium, such as nitrogen or carbon dioxide, to the sonotrode  14 . The medium would cool the contact surface  28  of the sonotrode  14  to a temperature below the dew point of the surrounding atmosphere, whereby water vapor would condense on the surface of the sonotrode  14 . The moisture would affect no other part of the ultrasonic welding apparatus. The damp or wet surface would then have the non-stick properties set forth previously. 
     Other means for depositing moisture or fluid on the contact surface are also contemplated, including using a spray head to apply moisture to either the contact surface of the sonotrode  14  or the workpiece  22 . Moisture may also be applied by dripping, brushing or pressing a wet sponge on the sonotrode  14  or workpiece  22 . 
     Shown in FIGS. 4-8 are further embodiments of a sonotrode  14  according to the present invention including structure for cooling the sonotrode  14 . FIGS. 4-5 show a sonotrode  14  having radially extending fins  50  for external airflow cooling of the sonotrode  14 . The fins  50  are formed out of material that readily conducts heat away from the sonotrode  14 . In addition, a separate or nearby supply of air, or some other suitable medium, may be forced through or by the fins  50  to further increase the cooling effect thereof. As shown in the additional embodiments, air or some other cooling medium may pass internally through the sonotrode  14  and exit in an area adjacent the fins  50  to further cool the sonotrode  14 . 
     Turning now to FIGS. 6-7, there is shown a further embodiment of the present invention utilizing internal cooling passages. As shown in FIGS. 6-7, a passage  60  extends longitudinally through the center of the sonotrode  14 . The passage  60  connects with a plurality of radially extending exhaust passageways  62  ending at exhaust ports  64 . Preferably, the cooling fluid is an air or some other gas that is supplied via a supply hose to the passageway  60 . The supply of cooling fluid may be continuous or may be supplied in short bursts that coincide with or are immediately after the welding cycle is complete. As set forth above, such cooling passages may be combined with the cooling fins  50  of the previous embodiment wherein the cooling medium flows passed the fans  50 . 
     FIGS. 8-9, show a further embodiment of a sonotrode  14  having an internal cooling circuit  70 . The internal cooling circuit  70  includes an inflow passage  72  and an outflow passage  74  connected by a transverse passage  76 . As shown, the transverse passage extends inwardly from a side surface  78  of the sonotrode  14 . This is for ease of manufacturing, as it provides a simple way to connect the inflow  72  and outflow  74  passages. A plug  80  seals the opening at the side surface  78 . In use, the cooling medium, typically a liquid cooling fluid, flows in the inflow passage  72  in the direction shown by arrow  82 , across the transverse passage  76  and out the outflow passage  74  in the direction shown by arrow  84 . In this manner, fluid flowing through the sonotrode  14  acts to cool the sonotrode  14 . Depending upon the cooling medium used, such and internal cooling circuit  70  may be used to cool a sonotrode  14  to a temperature below the dew point. While shown here with a single inflow  72  and outflow  74  passages, multiple passages may be used to further increase the flow of coolant through the sonotrode  14 . The internal cooling circuit  70  may also be used in combination with the cooling fins  50  of the previous embodiment. 
     In addition, the various cooling embodiments may be combined with the fluid application embodiment such that the combination thereof further reduces the likelihood that the sonotrode  14  will stick to the workpiece  24 . For instance, it is contemplated that the internal and extra cooling embodiments of FIGS. 4-9 may be used in connection with the means for depositing a fluid disclosed herein. While we do not seek to be held for rigorous scientific exactitude, we postulate that the dropletization and/or evaporation of the liquid (both of which are visible during the practice of this invention), trapped between sonotrode  14  and material  22 , produce enough pressure surge to cause their separation, thus preventing sticking. 
     Although the wedge-reed configuration is used to describe the various embodiments of this invention, it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation. 
     Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the present invention may be practiced other than as specifically described.