Method and apparatus for forming rounded ends on plastic filaments
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and process for forming round ends on thin plastic filaments. 
     BACKGROUND OF THE INVENTION 
     Thin plastic filaments made of polypropylene or some other suitable, nontoxic material are used to support certain types of intraocular lenses within the eye after it has been implanted to replace the natural lens of the eye that has been removed to correct some vision impairment, for example cataracts. Such support filaments are used on lenses like that shown in U.S. Pat. No. 4,159,546 (the Shearing patent). The properties and use of such support filaments are thoroughly described in the Shearing patent which is incorporated by reference in this patent application. 
     It will be appreciated that all the surfaces of the intraocular lens should be smooth and rounded to avoid damaging the eye during intraocular lens implant surgery or during use of the lens after implantation. It is particularly important that the free end of the support loops shown in the Shearing patent be as round and smooth as possible to avoid damaging the interior anatomy of the eye. 
     In the past, various loop finishing processes have been used to remove sharp corners that result from cutting and forming the loops during the manufacturing process. Thermal forming and abrasive tumbling have been used to provide this smoothing. Although these methods remove sharp edges and provide an acceptable degree of rounding to the ends of the loops, further improvement to form an ideal hemispherical end would be desirable. 
     Other methods for smoothing and rounding the terminal portion of an open loop are shown in U.S. Pat. No. 4,242,760 where a knob is fixed to the end of the open loop to prevent damage to human tissue within the eye. Similar rounded ends for support loops are shown in U.S. Pat. Nos. 4,244,060 and 4,366,582. 
     It would be desirable to have an improved apparatus and method for forming round ends on small plastic filaments. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and a method for forming round ends on small plastic filaments. A die is formed of two die blocks. Each die block has an interface surface in which is formed one-half of a die cavity, so that when the die blocks are placed together in proper registration with the interface surfaces facing each other a complete die cavity is formed. The exterior surfaces of each die block are preferably rectangular, but any convenient shape may be used. The interface surface of each die block has a first channel extending into the die from one of its exterior surfaces, and each first channel has an interior end formed into a section of the hemisphere so that when the die blocks are brought together, the die forms a tubular cavity which terminates in a hemisphere. Each of the first channels has a relief recess spaced a fixed distance from its peripheral edge. Each of the blocks also has a land area between the relief recess and the peripheral edge of the first channel. 
     Each of the die blocks also has a second channel extending into the other of its opposite surface aligned coaxially with the first channels and extending into the first channels. When the die blocks are placed together, the first channels preferably form a tubular cavity with a hemispherical end, and the second channels form a tubular cavity with a conic end. The tip of the cone touches the hemisphere and provides an aperture through which the two parts of the die cavity meet. This die is preferably used for a filament whose diameter is substantially equal to the diameter of the first channel. 
     A filament is placed along the interface surface of one die block so that it rests in the first channel and its distal end extends beyond the hemispherical end of the first channel into the second channel. When the die blocks are closed together in proper registration, the land areas between the first channel and the relief recess of each die block come together to cold-form part of the fiber into a hemispherical section. The portion of the fiber which is located distally of the hemispherical section rests in the second channel, and the conic surfaces of the second channel contact the fiber to provide an axially-extending, distally-directed force which tends to pull the fiber into the cavity to help form the hemispherical section. When the die closes, part of the fiber is displaced and flows through any space in the confronting surfaces of the land area into the surrounding relief recess. When the die is open, the fiber has been cold formed to provide a conical portion whose tip is aligned with a hemispherical portion. Surrounding the tip and the hemisphere is a ribbon of flash and a quantity of material that has been displaced when the conic tip and the hemispherical portion were formed. 
     After the fiber is removed from the die, it is cut at the tip of the cone. The cone portion is discarded, and the remaining portion of the filament is subjected to abrasive tumbling to remove the flash and the displaced material. 
     The present invention provides a method and an apparatus for forming a smooth, rounded end on a small plastic filament. 
     These and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of one of the two die blocks of the present invention; 
     FIG. 2 shows a perspective view of the two die blocks closed; 
     FIG. 3 shows an axial cross-sectional view of the die of the present invention taken along lines 3--3 in FIG. 2; 
     FIG. 4 shows another view of the die of FIG. 1; 
     FIG. 5 shows an additional view of the die of FIG. 2; 
     FIG. 6 shows a filament prior to being formed by the present invention; 
     FIG. 7 shows a filament after it has been cold-formed according to the present invention; 
     FIG. 8 shows a transverse cross-sectional view of the filament of FIG. 7 taken along lines 8--8 of FIG. 7; 
     FIG. 9 shows a perspective view of the filament of FIG. 7; 
     FIG. 10 shows an additional perspective view of the filament of the present invention; and, 
     FIG. 11 shows a perspective view of the finished filament. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 2 there is shown a die 10 having a first die block 12 and a second die block 14. Each die block, 12 and 14, has opposite sides 16 and 18 and interface sides 20 and 22. When the two interface sides 20 and 22 of die blocks 12 and 14 are placed in confronting relationship, channels formed in the interface sides come together in proper registration to form a die cavity 24. 
     Tubular die cavity 24 has two parts: a hemispherical end part 23 and a conical end part 25. Hemispherical end part 23 extends into first side 16 of die 10 so that one-half of a cylindrical channel 26 is cut in each of die blocks 14 and 16. Channel 26 extends a fixed distance into die 10. Conical end part 25 extends into the opposite side 18 of die 10 and is coaxially aligned with hemispherical end part 23. In the preferred embodiment, the diameter of hemispherical part 23 and conical part 25 are the same, but they need not be the same. The tip of conical end part 25 extends through the hemispherical end 23. An aperture 32 is created to provide communication between the two parts 23 and 25 of cavity 24. Aperture 32 is made very small but, as will be explained later in the application, it is large enough to permit the filament which is formed by the die to remain in one piece without being cut during the cold-forming process. 
     The half of cylindrical end part 23 that is placed in each die block 12 and 14 will be referred to as first channel 26, and the half of each conical end part 25 that is placed in each die block 12 and 14 will be referred to as second channel 28. 
     A recess 34 is provided into interface surfaces 20 and 22 of die blocks 12 and 14 respectively. Recess 34 is spaced a fixed distance from the periphery of first channel 26. Recess 34 extends preferably completely about the periphery of first channel 26 but must extend at least about the hemispherical interior end portion of channel 26. Referring again to FIG. 4, it can be seen that recess 34 will intersect the conic surface 36 of second channel 28. 
     A land portion 38 separates the periphery of first channel 26 from relief recess 34. 
     In the process of the present invention, a filament 42 having a distal end 44 and a proximal end 46 is placed along interface surface 22 of die block 14 with the distal end 44 extending past the hemispherical end portion 50 of first channel 26, and past the conic section 52 of second channel 28 into the remainder of second channel 28. It is not important how far distal end 44 of filament 42 extend into second channel 28 as long as it extends past conic section 52. The axis of filament 42 should be aligned with the axis of first channel 26 and second channel 28. Die block 12 is then closed onto die block 14 so that interface surfaces 20 and 22 are in precise registration with one another to form die cavity 24 about filament 42 and to cold-form filament 42 according to the geometry of die cavity 24. 
     Referring now to FIG. 7, it can be seen that the cold-forming process shapes filament 42 into two sections. A hemispherical portion 60 is formed on filament 42 at a point spaced proximally from distal end 44 of filament 42. A conic portion 62 is formed in conic section 52 and second channel 28, and the tip 64 of conic section 62 is coaxially aligned and integral with the center of hemispherical portion 60. The connection at the tip 64 and hemisphere 60 is permitted during the cold-forming process by the presence of aperture 32 which provides communication between first channel 26 and second channel 28. 
     It will be appreciated that a portion of the closing force for die 10 is transmitted through conic surface 36 of second channel 28 to the conic section 62 of filament 42 to provide an axially-aligned, distally-directed force on the conic section 62 of filament 42. This force helps form a hemispherical section 60 of filament 42 by making sure that filament 42 is drawn completely into the hemispherical portion 50 of first channel 26. 
     When die blocks 12 and 14 are closed together and filament 42 is shaped as shown in FIG. 7, the material which is compressed by the die to form hemispherical section 60 and conic section 62 is directed into relief recess 34. The width of land area 38 is small, about 0.002 inches. This gives land area 38 a small area, and thus a high pressure is created when die 10 is closed. As die blocks 12 and 14 start to close on filament 42, excess material which is removed from filament 42 to form hemispherical section 60 and conic portion 62 flow across land area 38 into relief recess 34. As die blocks 12 and 14 continue to close, great pressure on land area 38 creates a very thin flash area 66. Substantially all of the material displaced from filament 42 to form conic section 62 and hemispherical section 60 is displaced into relief recess 34 and spaced apart from the periphery of filament 42 by a distance equal to the width of land 38. The excess material 68 which is displaced into recess 34 may be easily removed from filament 42 by abrasive tumbling, since it is connected to filament 42 by a thin flash area 66. 
     Filament 42 is then removed from die 10 and conic section 62 of filament 42 is cut from the hemispheric portion 60 of filament 42. The cut is made at tip 64 of conic section 62. Conic section 62 is discarded and the remaining portion of filament 42 which has hemispheric section 60 is subjected to abrasive tumbling to remove flash 66 and the excess material 68 which is displaced during the cold-forming operation. The abrasive tumbling process is well known to those skilled in this art and will not be described in detail in this patent application. 
     It will be seen that the present invention provides an apparatus and a method for forming a round end on a thin plastic filament. The present invention has been described in conjunction with certain preferred embodiments. Those skilled in this art will appreciate that many modifications and changes may be made to the preferred embodiments without departing from the present invention. It is, therefore, not intended to limit the present invention except as set forth in the appended claims.