Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-12-29
2003-07-22
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S272200, C156S272800, C156S304300, C156S158000
Reexamination Certificate
active
06596122
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field
This invention relates to a method for joining plastic materials. More particularly, it relates to the bonding of materials by passing electromagnetic radiation through one or more transmitting plastic materials and into a radiation absorbing material with the generation of heat and resulting fusion of components to simultaneously form both lap and butt joints.
2. Background
Although one piece items are preferred because of their strength and exclusion of assembly operations, mechanical limitations and other considerations often make it necessary to join plastic materials to each other or to other parts. As a consequence, a number of methods for joining plastic materials have been developed. The use of mechanical fasteners is labor intensive, the plastic is corrupted by fastener holes resulting in mechanical joints that often leak, and the joints are weak. Press and snap fits are also used but this is often unsatisfactory since high stress components are unacceptable when the materials are subject to thermal cycling or to harsh environments.
In thermal welding, one or more of the plastic parts to be joined are placed close to or in contact with a heat source such as a hot plate or other appropriately shaped hot element device to melt the plastic at which time the parts are then pressed together to form the welded joint. Induction welding uses a material that is susceptible to heating in an induction field that is implanted within the thermoplastic matrix. Application of an induction field causes the implanted material to heat and melt the surrounding plastic material which, if in contact with the part to be joined, forms the requisite joint.
In vibration welding, one of the parts is held stationary and the other is vibrated to produce frictional heat. Alignment is critical and bonds may not be as strong as expected. In spin or friction welding, one part is held stationary while the other is rotated under sufficient pressure to keep the parts in contact with each other. The heat melts the surface at which point pressure is applied to complete the bonding process. In both vibration and spin welding, high forces are needed to generate the frictional heat to melt polymers like polyethylene and, as such, heavy and costly equipment is needed to provide substantial clamping force.
Solvents and adhesives have also been used to join thermoplastic materials. However, some solvents can adversely effect some plastics. Also, solvents present potential hazards to assembly personnel. Plastics can be bonded with a variety of adhesives but often these are solvent based and present the solvent problems noted above. In addition, adhesives can complicate polymer recycling. Both solvent and adhesive methods are complicated by waste and chemical disposal problems in addition to surface preparation requirements. Solvent and adhesive methods are typically used with polyvinyl chloride (PVC) piping.
Infrared lamps and laser beams have been used to bond one plastic to another but such techniques have been limited to single joint applications. Laser welding has additional problems. Bond strengths can be disappointing. Vaporization and flashing (ignition) of substrate can occur when gaps occur between the interface of the two materials. Laser welding also tends to create surface pits and craters. Although lasers have been on the market for a considerable time, their cost is still relatively high.
None of these methods have proven satisfactory for the joining of tubular parts such as plastic pipe. Although the following two methods of joining plastic pipe are widely used, nether of them is entirely satisfactory. In butt welding, the ends of the pipes are joined by bringing the ends of the pipes close to or in contact with a hot plate. After sufficient melting has occurred, the hot plate is removed and the ends pressed together and cooled under pressure. A good butt weld does not leave any gaps in the interface between the two pipes that are joined. Exterior weld defects can be readily determined by visual inspection and, if necessary, the joint rejected and rejoined. However, it is to be noted that a flash or weld bead is formed on both the interior and exterior of the pipe joint. The interior weld bead is disruptive of smooth flow within the pipe and leaves an uneven surface on the exterior of the pipe. Butt welds are typically used with straight runs of pipe that can be well supported to prevent failure of the joint. Butt welds are not typically used with small pipe fittings as the bulk heating can result in considerable distortion of the fittings, especially when thin-wall fittings and pipe are used.
In lap welding, an additional fitting, referred to as a coupling or collar, is slipped over the end portions of the pipes and joined to the exterior pipe surface by melting the interior surface of the coupling and/or the exterior surface of the pipe or by using a solvent or adhesive. Heating is the common method of pipe lap joint welding with adhesives or solvents being using with only a small amount of piping such as PVC pipe. Although hot element type heating can be used, implanted resistive element heating is preferred as it allows better control of the melting process. Lap joint tends to be strong because the fitting overlaps the end portions of both pipes and considerable surface area on both pipes is involved in the joint. However, the joint cannot be visually inspected and failure to evenly apply solvent, adhesive, or heat to the coupling and/or radial surface portions of the pipe can leave undetected weak joints. The ability to visually inspect lap joints has been a long felt need in all industries using plastic pipe. Visual observation would reveal the amount of fusion of the collar and pipe in the joint and thereby provide a strong indication of the soundness of the joint as well as revealing defects such as bubbles caused by overheating and decomposition of the plastic pipe or the failure to adequately melt the plastic. Another problem with the lap joint is that it leaves an open interface, i.e., gaps, between the ends of the joined pipes. Such gaps are regions of potential accumulation of microbes and foreign debris that can be quite detrimental in industries using plastic pipe for transportation of fluids such as high purity water and other liquids including semiconductor, food, pharmaceutical and chemical solutions. The use of resistive heating elements imbedded in the plastic matrix of the joint components further accentuates the contamination problem when the element melts through the plastic during the joining process and later comes in contact with the passing fluid.
It is therefore, an object of this invention to make a doubly secure and strong joint
It is on object of this invention to provide a joint with no gaps or defects in the butt weld interface.
It is on object of this invention to provide a joint with no gaps or defects in the lap joint weld interface.
It is an object of this invention to provide a butt joint without a weld bead on the interior of a pipe.
It is another object of this invention to make simultaneously both a butt and lap joint.
It is another object of this invention to provide for visual inspection of a lap joint.
It is an object of the present invention to heat only those portions of a joint involved in bonding while leaving other portions of the materials essentially unaffected, undistorted, and in their initial (pre-bonding) configuration.
It is a further object of the present invention to avoid the use of solvents and other environmentally unfriendly bonding adhesives.
It is an object of the present invention to avoid contamination from electrically resistive heating elements.
It is an object of the present invention to provide a uniform and strong bond along the bond line.
Other objects of the invention will become apparent to those with ordinary skill in the art from consideration of the present disclosure.
SUMMARY OF THE INVENTION
To meet these objects, a joining method of the present invention features a method for the
Grimm Robert A.
Savitski Alexander
Ball Michael W.
Dawsey David J.
Edison Welding Institute, Inc.
Gallagher Michael J.
Gallagher & Dawsey LPA
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