Patent Issued for Selective Cure of Adhesive in Modular Assemblies

By a News Reporter-Staff News Editor at Journal of Engineering -- Tesla Motors, Inc. (Palo Alto, CA) has been issued patent number 8322393, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventors are Hermann, Weston Arthur (Palo Alto, CA); Kohn, Scott Ira (Redwood City, CA).

This patent was filed on September 25, 2011 and was cleared and issued on December 4, 2012.

From the background information supplied by the inventors, news correspondents obtained the following quote: "The present invention relates generally to module construction for assemblies having several-to-many elements, and more specifically to systems and methods for use of selectively curable low viscosity/low surface tension adhesives for assembly construction, most particularly for battery module assembly construction.

"It is common to produce module assemblies having elements secured together into an integrated monolithic structure. One method for securing the elements together uses an adhesive that bonds the elements to one or more fixtures. These fixtures typically include many openings that may be used to provide access to the elements, the inside of the assembly, or for other use. To inhibit adhesive from escaping from these openings during manufacture, conventional solutions use adhesives having great enough viscosity/surface tension to inhibit adhesive from exiting the apertures.

"In many applications such an adhesive provides an acceptable solution. In other applications, an adhesive like this is problematic. One problem is that the viscosity/surface tension requires a fairly large hydrostatic head in order to direct a suitable quantity of adhesive into small margin bonding areas to adequately wet the bonding surfaces and provide sufficient bond strength. This hydrostatic head represents a large quantity of adhesive that is 'wasted' in the sense that it does not contribute to the security of the bonding between the elements and the fixture.

"It is not only the case that this adhesive is expensive and therefore any wasted adhesive adds to the ultimate cost. In some applications, like electric vehicles, a further drawback is that excessive adhesive adds 'unnecessary' mass to the final assembly while also being unnecessarily expensive. As the number of elements in the assembly increases, and as a packing density of the elements increases, the costs in terms of expense and mass become quite significant because of the multiplicative accumulation of individual excess across all the elements in all of the modules. Any savings in reducing the quantity of adhesive per element/module is extremely effective in these cases as it reduces both expense and mass.

"Adhesives have an associated curing profile that further influences the use and suitability of adhesives in module assemblies. There are two broad categories of adhesives--one-part adhesives and two-part adhesives. Two-part adhesives are adhesives that include a base and a hardener. In contrast, a one-part adhesive includes the functionality of both the base and the hardener, but the activation or release of the hardener depends upon some externality for curing (e.g., temperature, ultraviolet radiation, water vapor in the environment, and the like). Epoxy is an example of a two-part adhesive having a resin and a hardener, with the hardener accelerating a polymerization (i.e., curing) of the adhesive, the specifics of the curing can be influenced by temperature and choice of resin(s) and hardener(s). Whether one-part or two-part, each adhesive has a curing modality that produces a rigid and strong bond in response to one or more curing agents. A cure time, particularly a minimum cure time to reach sufficient mechanical integrity for further processing, is an additional cost of use of adhesives. The cure time is sometimes related to the total quantity of adhesive used, using less adhesive can sometimes improve cure time cost.

"Further, in order for each battery module to remain mechanically robust in harsh environments such as within an automobile, a structural connection (physical and electrical) between each battery cell and module fixture should be stiff and robust. Many commodity cells do not include mechanical features that easily allow for such connections. For those that do, it is often difficult to make a stiff and robust mechanical connection while maintaining electrical connectivity requirements. Adhesives are able to establish high shear strength between smooth cell surfaces and fixtures of the module system while also maintaining any desired electrical isolation among the cells. However as noted above, adhesives can be expensive when used in this context and often lead to addition of an undesirable amount of mass to each battery module of a multiple-module battery pack. There is an additional cost to be considered, particularly as the number and density of elements in a module increases and still further as the number of these modules increases. This additional cost relates directly to long cure times often required of adhesives, and these long cure times increase process cost.

"As noted above, in a closely packed battery module, the quantity of adhesive applied depends in part on the hydrostatic head required to drive wetting of the adhesive to the required bonding surfaces. Some methodologies apply 1-2 mL of adhesive (viscosity approximately 7000 cps) to bond each 18650 battery cell into its own shallow plastic counterbore provided in a fixture. This quantity of adhesive is considered a requirement to achieve coverage of all bonding areas. However, the actual quantity of adhesive required to fill each bonding area alone is approximately 0.050-0.100 mL. Consequently, when considering what is needed for bonding the battery cell to the module fixture alone, a large amount of the adhesive is wasted.

"The standard adhesives used in conventional solutions that are outside the bonding areas serves little purpose once the adhesive has sufficiently wetted the bonding surfaces, and this 'excess' adhesive has virtually no purpose in the finished product. Reducing a dependency on the hydrostatic head to wet the bonding surfaces in modularized assemblies has a potential to produce significant cost savings by eliminating the 'wasted' adhesive.

"What is needed is a method and apparatus for decreasing costs (expense, mass, and/or cure time) associated with use of adhesives when assembling modularized components."

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventors' summary information for this patent: "Disclosed are methods and systems for decreasing costs (expense, mass, and/or cure time) associated with use of adhesives when assembling modularized components, particularly for assemblies having many elements such as for example battery modules used in electric vehicles. The methods and systems enable use of high-wettability adhesives (defined generally in this application as low viscosity and/or low surface tension adhesives) for assembling such modularized components. A first method including (a) dispensing a high-wettability adhesive into a first module fixture populated with a plurality of elements wherein the first module fixture provides a plurality of bonding wells with each bonding well accepting a first portion of one or more of the elements with the module fixture including one more apertures communicated with one or more of the bonding wells, the adhesive being selectively curable upon application of a curing modality; (b) applying the curing modality selectively to a first portion of the dispensed adhesive in a seal zone, the seal zone including one or more regions surrounding the apertures wherein the dispensed adhesive in the seal zone is sufficiently cured to inhibit significant quantities of the adhesive from emerging from the apertures while the adhesive continues to be dispensed into the module fixture wherein the curing modality is not applied to a second portion of the adhesive outside of the seal zone; and applying the curing modality to the second portion of the dispensed adhesive.

"A high-wettability adhesive, for purposes of the present application, includes one-part and two-part adhesives having one or more wettability parameters, when considering a particular adhesive and the material of the substrate, selected from the group consisting of a viscosity in a range of about 100 to about 1000 centipoise, a surface tension with the substrate of the bonding wells producing a contact angle with the adhesive less than about ten degrees in air, and combinations thereof.

"Another bonding method includes (a) populating a module fixture with a plurality of elements, the module fixture transparent to a curing modality; (b) masking selected areas of a first portion of the module fixture using a mask opaque to the curing modality producing an unmasked first portion of the module fixture and a masked first portion of the module fixture; and thereafter dispensing a high-wettability adhesive into the populated module fixture, the adhesive being selectively curable upon application of the curing modality and wherein the first module fixture provides a plurality of bonding wells with each bonding well accepting a first portion of one or more of the plurality of elements with the module fixture including one more apertures communicated with one or more of the bonding wells, the adhesive being selectively curable upon application of a curing modality; (d) curing, during the dispensing step , a first portion of the dispensed adhesive in the unmasked first portion upon application of the curing modality to the first portion of the dispensed adhesive in the first portion without application of the curing modality to a second portion of the dispensed adhesive in the masked first portion; and thereafter (e) removing the mask; and thereafter (f) curing uncured dispensed adhesive in the module fixture, including the second portion of the dispensed adhesive, upon application of the curing modality to the module fixture.

"A system includes a module fixture supporting a plurality of elements, the module fixture defining a plurality of bonding wells with each bonding well accepting a first portion of one or more of the elements with the module fixture including one or more apertures communicated with one or more of the bonding wells with the bonding wells having a nominal depth; and a dispensing system, coupled to the module fixture, for dispensing a high-wettability adhesive into each the bonding well and surrounding each the element substantially filling the bonding well up to the nominal depth without significant overfill, the adhesive being selectively curable upon application of a curing modality; and a curing structure for selectively exposing the adhesive to the curing modality as the adhesive emerges from the apertures during dispensation of the adhesive.

"Assemblies, such as for example battery packs, composed of a large number of elements (e.g., cells) on the order of tens to hundreds to thousands or more elements, implementing preferred embodiments of the present invention preferably have a method of mechanical integration that is low cost in terms of expense, mass, and process time. The use of appropriate high-wettability adhesives permits a stiff, robust, electrically insulating mechanical connection to the battery module fixture. The high packing density of cells within a battery pack limits the room available for dispensing adhesive evenly throughout a battery module, however the high-wettability adhesive is better suited for even distribution, particularly when the module fixture is adapted with ramps, wicking channels, and guiding surfaces and the like to direct dispensing adhesive into all populated bonding wells. Features in the module fixture that allow electrical interconnects on both ends of each cell may provide potential leak pathways for the adhesive during dispensation. Curing modalities are applied to adhesive exiting from the apertures to seal the module fixture and allow the adhesive to continue to fill the bonding wells and wet the bonding surfaces around the elements populating the bonding wells. The high wettability adhesive efficiently fills the bonding wells around the elements to be bonded, permitting the bonding wells to be substantially filled without significant overfilling (that is, the bonding wells are filled sufficiently to wet all the bonding surfaces without excessive (significant non-structural use). Ports between the bonding wells, a use of ramped guiding surfaces, and other features of the fixtures promotes even distribution of the high-wettability adhesive. A curing modality is applied to all of the adhesive in the bonding wells after the bonding surfaces are properly wetted.

"In some of the preferred embodiments, the curing modality includes application of light (e.g., ultraviolet radiation) which means that at least selected structures of the module fixture that define the bonding wells where the adhesive is dispensed (and where curing and bonding occurs) is transparent, and may therefore be visually inspected. A use of a cure indicator associated with the adhesive, such as color or some other visual cue, is advantageously used in certain embodiments to provide a quick assessment of the quality and/or status of the curing process in the bonding wells.

"The embodiments of the present invention includes designs and methodologies that promote the use of a limited amount of high-wettability adhesive by selectively curing (e.g., UV-curing) the adhesive as it attempts to exit out of the apertures. As noted above, due to the high wettability, the application area may remain limited without disadvantage. The high-wettability adhesive means that a time until the bonding wells are filled and that wetting of all bonding surfaces has occurred is relatively short (in relative terms compared to conventional systems and absolute terms). In addition to the savings in process time, a total quantity of adhesive used is greatly reduced, removing mass and expense from the assembled module.

"In some embodiments, after adhesive application and during wetting, the elements populating the bonding wells may be suitably positioned/aligned by use of a curing modality-transparent fixture applied to portions of the elements outside of the bonding wells of the fixture. For assemblies having multiple fixtures, this positioning/aligning fixture may be one of the other fixtures of the final assembly. (For multiple fixture assemblies, and because the preferred embodiments use high-wettability adhesives that respond strongly to gravity, any particular fixture being processed (e.g., populated, filled, and some amount of curing) is often described as a bottom fixture. It has this orientation during processing, even in cases that the fixtures may be oriented differently during operation. This is not to say that other orientations and arrangements are excluded, for example some application may be found for dispensing adhesive in a centrifugal system in which a 'bottom' fixture would be one having a greater angular velocity than any other fixtures, with the high wettability adhesive dispensed from closer to a center of rotation of the system and flowing outward into the bonding wells and potentially leaking from the apertures, with the selective curing occurring on the periphery of the system.)

"Curing modality (e.g., UV radiation) is applied to the adhesive by directing the UV radiation to the top side of the positioning/aligning assembly, curing the adhesive in the bonding areas of the bottom fixture. In embodiments having some areas shadowed by internal structures that limit exposure of some portions of the adhesive to the curing modality, the adhesive may include a multi-stage curing process. A first stage designed to cure the adhesive in seconds in response to one curing modality (e.g., UV radiation) and a second stage to cure the adhesive over a longer period in response to another curing modality (e.g., days at room temperature). This creates a two stage curing process for two sets of mechanical requirements, one for completing the remaining manufacturing steps and the other for durability in the field. While the adhesive joints are strengthened sufficiently by the UV radiation to continue on the manufacturing line, full strength as required for field durability may not be developed for days or weeks. When the module fixture itself is UV transparent, the entire cure operation can occur within seconds.

"There may be many different implementations of embodiments of the present invention including permutations of one-part and two-part adhesives with single-stage or multi-stage curing or sealing formulations, with the staging being during dispensing and/or curing. For example, one embodiment includes use of a two-part high-wettability adhesive that 'gels' or 'skins' upon application of a sealing modality while curing in a more conventional manner. The gelling creates a barrier and seals the apertures preventing leaks and permitting the adhesive to fill the bonding wells and wet the bonding surfaces. In other applications, it is possible to vary the adhesive properties during dispensing such that a first phase adhesive is particularly formulated to enhance the sealing properties by interaction with a curing/sealing modality while a second phase of adhesive being dispensed is particularly formulated to bond the fixture to the elements."

For the URL and additional information on this patent, see: Hermann, Weston Arthur; Kohn, Scott Ira. Selective Cure of Adhesive in Modular Assemblies. U.S. Patent Number 8322393, filed September 25, 2011, and issued December 4, 2012. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=110&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=5473&f=G&l=50&co1=AND&d=PTXT&s1=20121204.PD.&OS=ISD/20121204&RS=ISD/20121204

Keywords for this news article include: Tesla Motors Inc.

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