Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1994-10-17
2001-06-05
Sells, James (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S292000
Reexamination Certificate
active
06241836
ABSTRACT:
This invention pertains to the joining of thermoplastic resin bodies by vibration welding. More specifically, this invention relates to a practice that enhances the quality and speed of the vibration welding of abutting thermoplastic resin bodies that have widely separated melting temperatures.
BACKGROUND OF THE INVENTION
It is often desirable or necessary to make an assembled product by welding together two thermoplastic resin parts. There are many families of thermoplastic resin compositions such as, e.g., polyethylene, polypropylene, nylon, polyvinyl chloride, acrylic and methacrylic resins, and polycarbonate. These materials are all readily moldable or otherwise shapable into bodies that can then be welded into an assembled part.
Linear vibration welding is a known practice that is used to rapidly rub abutting complementary surfaces of thermoplastic bodies together along a joint line such that the high speed rubbing melts plastic adjacent the joint. When the rubbing is stopped and the joint cools, a welded bond is formed. The practice is particularly useful for the bonding of two thermoplastic resin bodies having chemically compatible and structurally complementary abutting surfaces. The two bodies are pressed together along designed abutting surfaces, and a resonant system is used to produce vibratory motion, usually along a single axis, of the surfaces. The back and forth motion has an amplitude of, for example, 0.060 to 0.120 inches (1.5 to 3.0 mm) at a frequency of, for example, 120 hertz or 240 hertz.
Linear vibration welding differs from ultrasonic plastic welding, which is more widely used. In ultrasonic plastic welding, high frequency mechanical vibrations, for example 20 to 40 kilohertz, are transmitted through the ultrasonic horn or tool to produce heat in the assembled parts to be welded. Linear vibration welding uses low frequency mechanical rubbing to produce frictional heat in abutting pieces. The usual joint in vibration welding is a butt joint, and it can be considerably larger than like joints made by ultrasonic welding. Linear vibration welding is also generally considered to be a more tolerant process, especially when thermoplastic resin bodies of differing melt or softening temperatures are to be joined. However, where the thermoplastic resin bodies differ in melting point by a substantial amount, for example upwards of 80° F. to 100° F., it is often difficult to obtain a rapid and clean welded joint. The lower melting body fuses, and molten material is expressed from the joint by the high pressure of the unsoftened higher melting point body required to generate the frictional heat. The expressed flash does not contribute to the strength of the weld, and the overabundant melt from the lower melting point part does not necessarily form a strong bond to the unmelted, higher melting part.
It is an object of the present invention to provide a method of increasing the welding rate and weld strength between thermoplastic resin bodies having disparate softening or melting temperatures. It is a more specific object of the present invention to provide a linear vibration welding method for joining disparate melting point thermoplastic resins in which the welding surface of the higher melting body is configured so as to initially generate localized high pressure to induce suitable controlled rapid melting of both bodies and also to provide a strengthened mechanical interlocking bond in the welded joint.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of our invention, these and other objects are accomplished in accordance with the following practice. While our invention is of general applicability to the vibration welding of thermoplastic resin bodies, it will be described in terms of the welding of a polycarbonate resin automobile lamp housing to a polymethylmethacrylate lens. In these applications, the polycarbonate housing is employed for its high temperature resistance and its toughness, while the methacrylate lens is employed for its optical transmission properties. The polycarbonate resin has a typical melting point or softening point, for example of the order of 300° F. to 310° F., while the softening temperature of the polymethylmethacrylate lens is the order of 90° F. to 100° F. lower. In normal practice, the edge of the lens member and the abutting surface of the polycarbonate housing member are both molded so that they are flat and of smooth complementary configuration. Thus, they can be pressed together in full engagement during the vibration welding process.
In accordance with our practice, the welding surface of the polycarbonate housing is stippled, knurled or otherwise roughened over substantially all of the intended contact area. The roughening is characterized by a continuous pattern of peaks and valleys of an amplitude up to about one-half of a millimeter. The bonding surface of the methacrylate lens may be flat for its full contact with the abutting housing surface or it may be tapered for only edge contact. When the bodies are pressed together at their bonding surfaces, the stippled or ribbed surface of the polycarbonate housing initially has only peak contacts with the abutting lens surface. Even though substantial clamping force is applied, the initial limited contact facilitates the linear rubbing across the whole joint. The peaks apply intense local pressure against the lower melting methacrylate surface and rapidly induce melting of both the lens surface and a portion of the peak surface on the housing to produce a film between them comprising material from both members. The process is preferably controlled so that a portion of the small peaks of the housing surface remain unmelted and penetrate into the molten film. When the rubbing is stopped, the molten material hardens between the unmelted portions of the peaks and in the valleys of the roughened polycarbonate surface. The entire practice can suitably be accomplished in the order of one to three seconds under a suitable clamping pressure of the order of 1000 to 2500 pounds per square inch. Due to the mechanical interlock between the roughened housing surface and the penetrated lens surface, the bond strength between the welded pieces is increased.
Other objects and advantages of our process will become more apparent in view of a detailed description thereof which follows.
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Novinger Edward Ray
Nyberg William Alan
Skirha Martin Dirk
Guide Corporation
Miller Ice
Sells James
Ster Brian T.
Taylor Jay G.
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