Adhesive bonding and miscellaneous chemical manufacture – Surface bonding means and/or assembly means therefor – Means joining flexible indefinite length or endless bodies...
Reexamination Certificate
2000-03-20
2003-10-07
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Surface bonding means and/or assembly means therefor
Means joining flexible indefinite length or endless bodies...
C156S304200, C156S304600
Reexamination Certificate
active
06629551
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is also related to application Ser. No. 09/295,102 filed on the same date as this application, entitled “Insert for Use in Conjoining Tubular End Portion,” by Michael W. Johnson and Jeffrey J. McKenzie. These applications are incorporated by reference herein.
BACKGROUND OF THE INVENTION
This invention relates to an apparatus and method for welding thermoplastic tubular end portions. More particularly the invention relates to an apparatus and method particularly suitable for welding PFA (perfluoroalkoxy).
Various means have been known for welding together the ends of thermoplastic pipes or tubes. U.S. Pat. No. 3,013,925 discloses inserting a heated platen between the ends of lined pipes and causing the pipes to bear against the heated platen to fuse the plastic after which, the platen is removed and the pipes are pressed together under pressure until welding has occurred and the thermoplastic material hardens as it cools. U.S. Pat. Nos. 5,037,500; 4,792,374; and 5,484,506 all disclose exterior conduction heaters in which abutted tubular ends are peripherally heated to weld the ends together.
U.S. Pat. No. 4,929,293 to Osgar utilizes the placement of an infrared heating plate in-between and not contacting the tubular end pieces to be joined. The infrared heating plate is removed and the tube joints are then engaged together to create the weld. Although such a procedure provides a high integrity weld on PFA and other melt processable plastics, such a weld typically leaves a deformation or a bead on the inner and/or outer surfaces of the joined tubular end portions.
In many applications it is unacceptable to have any deformations in the joined tube particularly on the interior surface. In sanitary systems used in the food processing and pharmaceutical industries the standards of the U.S. Code of Regulations, 7 C.F.R. ?58.128 require a conduit be smooth, permit laminar flow of fluids and be free of discontinuities that could trap particulate matter.
Traditionally, stainless steel tubing and pipe have been used in sanitary systems. However, due to corrosion, expense and other problems, plastic pipe and tubing are now seeing more use in such systems and PFA has the particular advantages of its high chemical inertness and resistance to the high temperature cleaning and sanitizing temperatures.
Applicants are not aware of any conduction heat weld system that has peripheral contact with abutted tubular ends has been shown to be suitable for welding PFA. This is due to the difficulties associated with the higher melt temperatures of PFA (approximately 590 F.) and the characteristics of PFA. For example, melted PFA sticks to many different materials that are commonly used in welding devices. Such sticking can render the completed weld defective and can cause significant operational problems with the weld equipment. Additionally, heating and melting of PFA produces fluorine gas which is highly corrosive to conventional materials utilized in fusion welding equipment. For example standard stainless steel quickly pits when exposed to fluorine gas. Additionally, PFA when melted expands and if constrained can develop extremely high pressures causing the PFA to leach out of the weld are into crevices or other undesired locations.
Also problematic with welding higher temperature melt processable plastics, such as PFA, is the extended cycle time for a weld. The typical generic steps for a weld are: 1) position the tubular end portions to be welded in the weld head; 2) close the weld head; 3) warm up the weld head and heater portion; 4) melt and weld the end portions; and 5) allow the welded part to cool; and 6) remove the welded component from the weld head. Conventional conduction heating heads utilize a pair of integral heater portions each of which extend longitudinally down the abutted tubular end portions and totally enclose the melt portion of the end portions. Such conventional weld portions are formed of stainless steel which has a relatively high thermal conductivity. This makes it difficult to isolate and minimize the melt zone which in turn effects the length of the cycle time. Moreover, in conventional weld beads and weld apparatus essentially all of the components are made of metal. This increases the warm-up period and the cool down period. With the elevated temperatures associated with PFA, these problems are exacerbated.
Attempts have been made to reduce the cycle time of welding thermoplastic tubular end portions such as by providing heat sink arrangements and forced cooling. The high thermal conductivity of metals and particularly stainless steel minimizes the effectiveness of such measures and ancillary cooling equipment adds cost, complexity, and maintenance problems.
A welding apparatus, system, and methodology is needed for creating beadless welds in plastic tubular end portions, particularly PFA, by conduction heating. Moreover, there is a need for reducing cycle time in welding thermoplastics.
When tubular end portions of PFA are heated to beyond their melt point a minimal, although significant, amount of expansion of the molten PFA material occurs. When the tubular end portions being welded are tightly constrained this expansion causes the molten PFA to leach into any crevices or imperfections in the weld head and/or mandrel. Moreover, if the tubular end portions are secured in place after the weld is cooled there will typically be a narrowing of the material at the weld site. This can cause an hourglass shape and/or a reduced wall thickness at the juncture. Where the PFA has leached into minute crevices or imperfections, flashing may exist on the exterior or interior surfaces of the tube when the weld has cooled and is removed from the weld head. These would typically necessitate manual removal and in severe cases may make the welded components unusable in particular applications. Thus, a need exists for minimizing or reducing the currents of flashing, the reduced diameter at the weld juncture, and reduced wall thickness at the weld juncture due to the expansion and contraction of PFA during the weld operation.
SUMMARY OF THE INVENTION
A conduction welding apparatus and method for bonding abutted thermal plastic tubular end portions is particularly suitable for welding PFA. In a preferred embodiment, a folding weld head embraces the abutted the tubular end portions to be welded, each tubular end portion including a flange spaced from the surfaces to be joined. The weld head includes means for securing the flanges within the weld head and further include bias means. The bias means provide an inward axial bias on the adjoined tubular end portions when the tubular end portions are displaced axially outward from the weld head due to the expansion of the plastic during the weld process. The bias means in a preferred embodiment is substantially inoperative prior to the weld and provides bias, or a substantial increase in bias, when the flanges are displaced due to the expansion of the molten PFA during the weld process. In a preferred embodiment, the securing means comprises a pair of clamps which each attach to and encompass the respective tubular end portions at the end portion flanges. The tubing clamps fit into recesses in the weld head. In such a preferred embodiment the bias means is provided by a spring loaded plate which is deflectable in an axial direction outward from the weld head. The invention also includes a process for accomplishing a weld with the described apparatus.
In a preferred embodiment a composite heater portion provides a sharp temperature gradient from the weld juncture outward. The abutted tubular end portions are positioned at a central heated section of relatively narrow thickness in the axial direction with respect to the tubular end portions. The central heated section is preferably formed of two half sections each with a semicylindrical cavity. The two semicylindrical cavities form a cylindrical cavity for embracing the abutted end portions and may be hinged together. Sandwiched arou
Johnson Michael W.
McKenzie Jeffrey J.
Ball Michael W.
Entegris, Inc.
Kilkenny Todd J.
Patterson, Thuente Skaar & Christensen
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