Abstract:
The present invention is directed to a method and apparatus using a light guide for directing a laser beam to a weld zone. The light guide includes an entrance end, an exit end and a flexible body therebetween. The entrance end of the light guide is operatively coupled to a laser source such as a diode and is adapted to receive and communicate the laser radiation through the light guide. The light guide is formed of a flexible material to permit the exit end of the light guide to be spaced from and aligned with complex two-dimensional and three-dimensional weld zone configurations. The internal reflection of the light guide contains the laser radiation therein as it passes from the entrance end and through the exit end of the light guide. The light guide and corresponding methods of welding parts permit laser welding of complex geometric configurations.

Description:
This application claims the benefit of United States Provisional Application No. 60/196,559 filed Apr. 11, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to laser or infrared welding of parts and, more particularly, to a guide for conveying laser energy to a weld zone. 
     2. Discussion 
     The art of welding parts incorporates a variety of techniques including ultrasonic welding, heat welding, and, most recently, Through Transmission Infrared (TTIr) welding of plastic parts. During TTIr welding, laser radiation of a suitable wavelength is passed through a first transparent plastic part and impacts an absorbent polymer whereupon the absorbent polymer is heated to a critical melting temperature. When the absorbent polymer part begins to melt, the parts are pressed together. A weld or bond joins the parts as the melt cools. 
     While the area of TTIr welding has seen considerable advancement, difficulties related to the integrity and uniformity of the weld as well as controlling the transmission of the laser energy to the weld zone remain as some of the barriers to widespread commercial application of TTIr welding. In most TTIr systems a spot laser tracks the weld line either through movement of the laser or the workpiece. An alternative approach is to illuminate the entire weld zone through a coordinated alignment of laser diodes. The simultaneous illumination approach provides numerous advantages including the speed at which the weld is created and the uniformity of the resulting bond. However, simultaneous illumination of the entire weld surface requires precise alignment of the laser diodes relative to the workpiece. In weld zones consisting of linear or simple geometric configuration, the alignment of the diodes do not present a significant impediment to use of TTIr welding technology. However, for weld zones having complex two-dimensional or three-dimensional curvatures, diode alignment is a significant concern. For welds with complex curvatures the diode mounting manifolds must permit three-dimensional rotation and three-dimensional translation of the diodes for proper alignment. The configuration of the manifold will often times be different for each application and, in some instances, may be cost prohibitive. 
     An additional difficulty related to the configuration of the diode array is that the individual diodes within an array are generally aligned to slightly overlap one another to provide uniform energy along the weld zone. In this configuration, failure of a diode or diode element called an emitter creates an area within the weld zone that is subjected to less laser energy during welding. A weakness in the weld may result. While feedback circuits may be used to detect a failed diode and prevent the manufacture of a large number of defective parts, the failed diode should be replaced prior to further production. 
     SUMMARY OF THE INVENTION 
     The present invention focuses on a method and apparatus for directing a laser beam to a weld zone for infrared/laser welding. While the present invention may be most applicable for use in TTIr welding, it may also be used in other modes of infrared/laser welding such as surface heating. The invention permits mounting of the laser diodes in a configuration that need not directly correspond to the configuration of the weld zone. The laser energy generated by each diode is transmitted through a transparent flexible sheet of material referred to herein as a light guide. The entrance and exiting surfaces of the light guide are generally smooth to minimize diffusion and the light guide is preferably formed of a flexible material that provides total internal reflection. The light guide may be contoured such that its exit surface matches the weld contour while allowing the diodes to be mounted in a dissimilar configuration such as on a flat manifold. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given here below and the accompanying drawings, wherein: 
     FIG. 1 illustrates a conventional diode configuration for TTIr welding of plastic parts; 
     FIG. 2 is a top plan view of a first embodiment of the present invention; 
     FIG. 3 is a side elevational view of the welding setup shown in FIG. 2; 
     FIG. 4 is a top elevational view of a second embodiment of the present invention; 
     FIG. 5 is a side elevational view of the welding setup shown in FIG. 4; 
     FIG. 6 is a top plan view of a laser diode array according to the present invention without directional diffusion; 
     FIG. 7 is a top elevational view similar to that shown in FIG. 6 wherein the invention includes directional diffusion; 
     FIG. 8 is a top elevational view similar to that shown in FIG. 6 with a failed diode; 
     FIG. 9 is a top elevational view similar to that shown in FIG. 7 with a failed diode; 
     FIG. 10 is a top elevational view of yet another embodiment of the present invention that includes a diffusive lens between the diode array and the entrance surface of the light guide; 
     FIG. 11 is a top elevational view of the present invention wherein the light guide is deformed in two dimensions; 
     FIG. 12 is a side elevational view of the diode and light guide shown in FIG. 11; 
     FIG. 13 is a view of a diode array using the light guide to transmit laser energy from the array toward the workpiece; and 
     FIG. 14 is a view of a representative mounting structure for a circular weld zone. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a commonly used practice in laser welding wherein laser radiation  2  from a source such as a diode  4  is used to weld parts. The light or laser radiation  2  is ideally one hundred percent transparent to a clear, transparent part  10  of the plastic being welded but absorbent by another or a black part  12 . As noted above, in order to illuminate the entire weld surface using this approach, a series of diodes are commonly positioned in side-by-side relation in a diode array to produce one line that matches the contour of the weld line. The task of aligning and mounting the diodes becomes increasingly difficult as the complexity of the weld zone geometry increases. Accordingly, some of the major problems inherent in the use of conventional diode arrays include the placement, alignment, power uniformity, and fixturing of the diodes. 
     The improved welding technique and the light guide of the present invention are illustrated in FIGS. 2-14. As is shown, a light guide  14  is placed between a laser diode  16  and a workpiece  18  so as to act as a two-dimensional fiber optic cable. The laser diode  16  produces a line of radiation  20  that is retained within the light guide  14  between an entrance surface  22  and an exit surface  24  due to the one hundred percent internal reflection of the light guide  14 . The light guide  14  is preferably made of a flexible sheet of material such as rubber, silicone rubber or any transparent thermoplastic that can be molded or deformed into the desired shape as shown in FIGS. 11 and 12 and described below. It is also possible to construct the light guide  14  from transparent fibers mounted in a manner to form a geometry similar to a sheet. The flexibility of the light guide  14  allows the laser radiation to be directed to a weld zone without complex mechanical devices for alignment or manipulation. Further, the flexibility of the sheet allows relatively complex three-dimensional curvatures within the weld contour to be relatively easily and inexpensively matched. 
     The exit surface  24  of the light guide  14  is preferably spaced from the workpiece  18  and oriented to ensure that the laser impacts the workpiece  18  in an appropriate location. Once sufficient heat is generated in the workpiece  18  to provide an adequate melt along the inner face of the transparent and absorbent parts  10  and  12 , respectively, the radiation is turned off and the melts are allowed to wet or adhere and then solidify to produce the weld. Mixing and solidification of the weld is commonly promoted by the application of pressure to the workpiece  18 . 
     As noted above, the invention overcomes many problems in the art including placement, alignment, and fixturing of the array of diodes as well as power uniformity generated by the assembly. This description will initially discuss the preferred properties and operations of the light guide  14  itself followed by the power uniformly achieved through use of the light guide  14  and an exemplary method and structure for operatively aligning the exit surface  24  of the light guide  14  relative to the workpiece  18 . The light guide  14  is preferably formed of a flexible and transparent thermoplastic which can be molded into its desired shape. The magnitude of flexibility provided by the light guide  14  may be varied for any suitable application. In fact, a rigid light guide material may be most suitable in certain applications. Moreover, the light guide  14  may consist of multiple layers or fibers of material in order to optimize the light carrying properties of the light guide  14  while providing adequate structural support. 
     Another important property of the light guide  14  is that the material provides nearly one hundred percent internal reflection so that the entirety of the laser beam  20  is maintained within the light guide  14 . To this end, the light guide  14  has a higher optical density than air. 
     The light guide  14  preferably can have diffusion properties that provide a uniform beam across the entire surface area of the exit surface  24 . The entrance and exit surfaces  22  and  24 , respectively, will normally be smoothly polished to encourage the transmission of the laser radiation  20  into and out of the light guide  14 . In general, it is preferred that the entrance and exit surfaces  22  and  24  are “optically smooth”, that is, that the image quality on each side of the entrance and exit surfaces  22  and  24  are consistent. It has been determined that when using a plastic sheet as the light guide  14 , the entrance or exit surfaces  22  and  24 , respectively, may be polished with sand paper and then brushed with a flame to produce an optically smooth edge to minimize light losses. Notwithstanding this example, a variety of techniques generally known in the art may be used to provide an optically smooth edge for other light guide materials. Another available technique for minimizing the resistance to laser radiation  20  at the entrance and exit surfaces  22  and  24 , such as when the light guide  14  is formed of silicone rubber, is to couple a glass plate or other smooth and transparent material at the entrance surface  22  using an appropriate adhesive such as silicone caulking. 
     In many applications it may be desirable to diffuse or randomize the laser energy within the light guide  14  to a greater degree than is provided by the body of the light guide  14  itself. In these instances, the entrance or exit surfaces  22  and  24 , respectively, of the light guide  14  may be prepared to provide directionally specific diffusion. For example, the diffusion provided by the light guide  14  in one direction may be increased, as shown in the area indicated by reference numeral  26 , such as by scratching the entrance surface  22  in a peripheral direction with sandpaper (FIG.  4 ). The scratching may be provided such that the diffusion illustrated in the side view shown in FIG. 5 is the same as that shown in FIG. 3. A variety of techniques generally known in the art may be used to scratch the entrance or exit surfaces  22  and  24 , respectively, including chemical etching techniques similar to those used in lithography. This directional diffusion allows the light to scatter through the thickness of the sheet thereby reducing or eliminating any power density variation across the light guide  14 . The end result is a more uniform power distribution at the weld surface. In some cases, it may be necessary to mask the “over-spray”of the laser light at the edges. Another technique that can be used to randomize the light is to use many fibers that have random mixing with respect to the input and output location. 
     Additional benefits provided by directional diffusion include reduction of the adverse effects of a failed diode or emitter and more uniformity when using multiple diodes. More particularly, the laser diode  16  and the light guide  14  configuration without directional diffusion (FIG. 6) generates multiple peaks and valleys as a result of the local effects of the diodes  16 . In addition, as shown in FIG. 8, when a diode  16  in this configuration fails, a resulting “dead-spot”  25  may be produced within the weld zone. These “dead-spots”  25  result in insufficient heat applied to the workpiece  18  and therefore non-uniform welds. As shown in FIGS. 7 and 9, the directional diffusion or randomizing provided by the light guide  14  spreads the laser radiation  20  over the entire light guide  14  and, with proper light guide  14  length and internal diffusion, there are very few if any peaks and valleys produced by the local effects of the diodes  16 . In addition, as shown in FIG. 9, when a diode  16  fails in this configuration, there is no “dead-spot” produced in the weld. 
     Directional diffusion or randomizing may be further enhanced or tailored to a specific application by placing a separate lens  31  upstream of the entrance surface  22  (FIG. 10) or downstream of the exit surface  24 . Diffusive lenses of this type are generally known in the art. 
     As noted above, a principle benefit provided by the light guide  14  is that the location and pattern of the laser energy exiting the exit surface  24  of the light guide  14  may be tailored for the specific application and, more particularly, the specific geometry of the weld zone. The flexible sheet can have a two-dimensional or three-dimensional curvature that easily matches complex weld zone geometries. An exemplary two-dimensional curvature is illustrated in FIGS. 11 and 12. More particularly, FIG. 12 represents the one hundred percent internal reflection provided by the light guide  14  so as to change the general orientation and uniformity of the laser beam exiting the exit surface  24  of the light guide  14 . This allows the laser radiation  20  to be directed to the weld zone without complex mechanical devices for alignment and manipulation of the laser diodes  16 . The diodes  16  may be mounted on virtually any structure including a flat manifold. It should be noted that there is a limitation on the amount of curvature that the light guide  14  can provide and that this limitation is dependent on the application and the relative indexes of refraction between the application and the light guide  14 . For example, for a light guide  14  made of silicone rubber, plastic or glass where a ninety percent loss of laser radiation  20  through the light guide  14  is acceptable, a radius of curvature of at least one-half inch may be used. 
     An exemplary mounting assembly is illustrated in FIGS. 13 and 14. Specifically, a plurality of laser diodes  30  are mounted on a flat manifold  32  and oriented to project the laser energy toward a central opening  34  in the manifold  32 . Light guides  36  are positioned to receive the laser energy generated by each diode  30  and convey the energy through a perpendicular curvature and into the opening  34 . As is best illustrated in FIG. 14, an end  37  of the light guide  36  defining the exit surface may be coupled to a mounting structure such as a cylinder  38  in order to secure the various light guides  36  in a configuration that matches the weld zone. In the illustrated embodiments, the exit ends  37  of each light guide  36  are coupled to the mounting cylinder  38  through the use of an adhesive to match a circular weld zone. Various other mounting devices and assemblies may be used to position the exit surfaces  37  of any number of light guides  36  to match complex geometric configurations of a weld zone. FIG. 14 also illustrates that the entrance surfaces  39  of the light guides  36  may be coupled, through silicone rubber or silicone caulking, to an additional sheet  40  formed of transparent material, preferably a sheet of glass such as from a microscope slide, in order to further tailor the diffusion characteristics of the entrance surface  39  to the specific application as discussed above. 
     The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.