Electric heating – Inductive heating – With power supply system
Reexamination Certificate
2002-11-22
2003-10-28
Hoang, Tu Ba (Department: 3742)
Electric heating
Inductive heating
With power supply system
C219S250000, C219S663000
Reexamination Certificate
active
06639198
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to induction heating equipment and is particularly directed to a hand held induction tool of the type which produces a magnetic field in a predetermined direction. The invention is specifically disclosed as an induction heating tool that induces eddy currents in thin foil susceptors to raise the temperature of the susceptors to melt or soften an adhesive coating on the susceptor's surface, for bonding trim to wallboard.
BACKGROUND OF THE INVENTION
Historically, bonding materials quickly without use of mechanical fasteners has been quite difficult. Some of the problem areas which exist include: assembly time, cost of materials and labor, quality of the bond between component structures being assembled, reliability of the process of assembly, the typical requirement of fairly difficult finishing steps, convenience (or lack thereof) of use for the end-users, worker safety issues, and the difficulty in maintaining a good quality of appearance of the finished goods.
In the 1940's, mechanical fasteners dominated the assembly industry, and adhesives were not as important to industry during this period. From the 1930's through World War II, the United States and Germany began to develop plastics and adhesives technology in response to the growing scarcity of natural products. In many cases, particularly early on, adhesives have been used either in combination with mechanical fasteners or where no mechanical fastener could be effectively employed. Beginning in the 1950's, the modern adhesives industry began to develop. Some of today's more common adhesive systems, developed at that time, included heat-curable thermosets (epoxies), thermoplastic hot melts, pressure-sensitive adhesives (PSA's), contact cements, water-based wood glues, and the super glues (cyanoacrylates). These were major disruptive technologies that have evolved over the last 45 years and which have slowly grown the fastening market and have significantly replaced traditional mechanical fasteners in many markets.
Adhesive bonding is generally superior to mechanical fastening, but present technology doesn't allow for cost-effective pre-positioning and rapid development of a strong bond on demand with one step. Pre-positioning of components, prior to fastening, is very important, particularly in non-automated assembly systems. Millwork is an excellent example of such an assembly system. No current adhesive system allows for pre-positioning coupled with instantaneous bonding. Most of today's adhesives are slow to cure, requiring minutes to hours, thus requiring clamping and other direct personal attention by the installer.
It should be noted that the ideal adhesive system is one where the adhesive cures on demand, is reversible on demand, has unlimited shelf life, has no VOC's (volatile organic compounds), and is safe and easy to handle. Currently, the only such systems that exist are the light-curable systems, such as those employing UV (ultraviolet) and visible light. UV and visible light systems are unique in today's adhesives world. They are liquid systems that cure only upon exposure to light. Optically transparent parts can be bonded in seconds or less to virtually any substrate. Such systems, where useful, have virtually replaced all other adhesive or mechanical fastening systems. An example would be automotive headlamp assemblies that do not need to be disassembled. It should be noted that UV-curable adhesives involve chemical handling and are not currently reversible.
There are two basic types of adhesive systems: one group of systems allows for pre-positioning of the parts to be bonded, yet by default, requires long cure times; the other group of systems provides very short, almost instantaneous cure times, but yet prevents pre-positioning of the parts.
Before describing some of the major adhesive systems available, one should be aware of the following general application notes that affect adhesive utility.
(1) Many product assembly sites are often dirty and difficult to keep clean. Certain adhesive systems cannot handle such situations.
(2) Temperature fluctuations at many assembly sites could be extreme, whether for an interior or exterior application. For example, a new home being built in the middle of the winter could see interior temperatures below 0° F. Exterior applications could easily see temperatures even lower. Another typical example could involve automotive body repair, if done inside a non-heated building.
(3) Where humidity may be important, it is clear that the humidity around a manufacturing facility in Arizona would be far lower than that in and around a facility in Florida.
(4) The ability to directly heat many product components to cure adhesives is extremely limited, particularly as many plastic components can melt, and wood-based or cellulose-based millwork can burn.
One family of fast-curing adhesives is called “super glues” (cyanoacrylates). These adhesives allow for an extremely rapid adhesive setting, but cannot in any way be pre-positioned before placement as the adhesive cures during positioning. Thus, there is no room for error. These adhesives are generally the most expensive adhesives. Furthermore, they are difficult to handle, and have a limited shelf life. Finally, there is no way to easily reverse cyanoacrylate, or super glue, bonds. Companies in this industry include Loctite Corporation, Henkel A. G., and National Starch.
Another instant adhesive technology, not often employed in structural applications, is pressure-sensitive adhesive (PSA) tapes. Like super glues, such products allow for extremely rapid adhesive bonding, but again, are extremely limited with regard to pre-positioning and, as with the “superglues,” again, there is no room for error. Furthermore, pressure-sensitive adhesives are limited in their ultimate strengths unless they are thermosetting. In the case of a thermosetting PSA, some form of heat- or moisture-activation is required which is generally impractical for non-heat-resistant products, or where humidity controls are unavailable.
The latter two above thermosetting processes are time intensive. Even more importantly, pressure sensitive adhesives can be applied only in very narrow temperature ranges, typically from 55° F. to 90° F. Furthermore, above 90° F., many common PSA's weaken dramatically. As a further note on PSA's, they are incapable of flow without heat to accommodate uneven surfaces, and if exposed to dust or other particulates, they immediately lose much of their potential adhesive strength. Finally, it is extremely difficult, if not impossible in most cases, to disassemble parts that use PSA's. Examples of companies that manufacture PSA's are 3M and Avery-Dennison, which are the two largest of the group. The cost of PSA's can range from being some of the most inexpensive to some of the most expensive adhesives available today.
Hot melt adhesives are another example of an instantaneous or fast-cure system that significantly limits the ability to pre-position parts. Such adhesives are melted either in a large tank or in a small glue gun and are then dispensed as a molten material onto the parts. The parts are then quickly mated, and the bond forms as the adhesive cools. The cooling process can be as short as a few seconds to possibly as long as ten or twenty seconds. As with the other instantaneous adhesives, there is little room for error, particularly where a clean and thin bond line is desired. Such limitations are the reasons that hot melt adhesives are used most extensively in the packaging industry and also for bonding small parts or surface areas. They are particularly useful in highly automated production systems, such as for sealing cereal boxes. Furthermore, such adhesives cannot be reheated after product assembly without significantly or entirely heating the product assembly.
On the positive side, hot melt adhesives are one-component, solid-state, zero VOC systems that have indefinite shelf life and, for
Barber John P.
Claypoole Gary L.
Cravens, II Robert Carl
Malofsky Adam G.
Riess Edward A.
Gribbell Frederick H.
Hoang Tu Ba
Nexicor LLC
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