Bonding machines are commonly used for curing adhesives that have been deposited between opposing surfaces of two objects, such as two sheets that are to be glued together. These machines have many applications, including the production of automotive body panels and components. This description is written in terms of bonding two sheets together, but it will be appreciated that Applicant's invention is not limited to such use.
Many kinds of bonding machine are currently in use, including heated platen presses, microwave and radio frequency (RF) bonders, hot-air-impingement bonders, ovens, and infrared and other radiative bonders. For example, a heated platen press forces the two sheets and interposed adhesive together between two opposing appropriately shaped platens, and the adhesive is cured by heat conducted from the platens, which may be heated by steam, electricity, hot oil, or hot water. An RF bonder heats the sheets and adhesive, which are disposed between opposing electrodes, usually by some combination of electric current and atomic-scale motion induced by RF energy applied to the electrodes. Such heating by induced motion is analogous to the heating that occurs in a conventional microwave oven. Heating devices are described in many publications, including U.S. Pat. No. 5,223,684 and No. 5,277,737, both to Li et al. and U.S. Pat. No. 5,554,252 to Foran.
The problem with both heated presses and RF bonders is that today's highly engineered adhesives often can be properly cured only by carefully controlling their temperature. Over-heating some adhesives causes degradation and reduced bond strength. Under-heating leaves some adhesives uncured and can preclude the bonded part's compliance with required bond strength and dimensional tolerances. In addition, economical mass production requires each bonded component to be heated quickly for the minimal amount of time. RF bonders are currently more able than are heated presses to meet these requirements. For example, an RF bonder using a frequency of twenty-seven megahertz (27 MHz) at a power on the order of 1-100 kilowatts can need only thirty seconds for curing a large component at 280.degree. F. (138.degree. C.) while a press heated to the same temperature can require several times as long. It will be appreciated that these parameters vary greatly depending on the bonding method and materials used.
Despite their heating speed, current RF bonders have problems in controlling the spatial temperature distribution of large components, such as automotive body panels. The sources of these problems are many. The amount of heat generated is strongly dependent on many process parameters, such as the gap between the electrodes and bonded part as described in U.S. Pat. No. 4,941,937 to Iseler et al. for example. Also, an RF bonder large enough to handle large components includes as many as 16-24 electrodes, each of which may require tuning by adjustment of a respective capacitor. Further, it is desirable to minimize the time needed to complete the production cycle for each component, i.e., the steps of moving the component into the bonder, heating the component, and moving the component out of the bonder in preparation for the next component, but doing so reduces one's control over the bonding process.
One known approach to monitoring an RF bonder involves the use of a thermal imaging system. A thermographic camera captures a continuously updated "picture" of each bonded part after it is shuttled out of the bonder. Different colors in the "picture" indicate different surface temperatures on the bonded part, and these surface temperatures are used as rough indications of the temperature at the actual bondline, which is usually at some depth beneath the surface. One problem with this system is the camera's view of the part is almost always partially obstructed, preventing measurement of all of the important portions of the part. Another problem with this system is that the indicated temperatures become more and more inaccurate, both absolutely and relatively, as one moves toward the edges of the part. Since adhesives are applied near the edges of many kinds of parts, this kind of thermal imager is most inaccurate in the areas of most interest.
Another known system employs a number of individual infrared thermometers, one aimed at each corner of the bonded part, to determine surface temperatures of parts that have been shuttled out of the bonder. The several surface temperatures determined for each part are displayed on a computer process control screen. This system has problems that are similar to the problems of the system described above. The system gives information on the heating ability of only a few out of many (e.g., four out of sixteen) bonder electrodes.
Besides their other problems, neither of these known systems is accurate enough or suitable for tuning an RF bonder. In this application, the word "tuning" means adjusting so that a desired amount of energy is deposited into an adhesive layer. As mentioned above, an RF bonder large enough to handle large components includes as many as 16-24 electrodes, each of which is tuned by adjusting a respective capacitor. This is depicted in FIG. 1, which shows one view of bonder having sixteen electrodes 1-16 disposed around the edges of a part to be bonded. In a typical bonder, the electrodes receive RF energy distributed through a grid 18 from a single RF source, and an adjustment of one electrode unpredictably changes the tuning of all of the other electrodes. As a result, tuning is currently a tedious process of trial and error, which produces a large number of scrapped parts and long bonder down times, and results are qualitative, requiring interpretation based on experience.