The opinion of the court was delivered by: Judge Rebecca R. Pallmeyer
MEMORANDUM OPINION AND ORDER
Plaintiff United States Gypsum Company ("USG") is a large manufacturer of wallboard, also known as drywall. Wallboard is made of gypsum slurry, which is mixed in a mixer, sandwiched between pieces of paper, dried in a kiln to harden, and cut into boards. In 1997, USG patented a manufacturing process that involves the injection of foam into the gypsum slurry as the slurry exits the mixer. U.S. Patent No. 5,683,635 ("the '635 patent"). In this lawsuit, USG charges Defendant Lafarge North America Inc. (Lafarge) and its parent company, Lafarge S.A., with infringing the '635 patent during the period from 2000 to 2004. The parties have presented competing interpretations for several terms in the claims of the '635 patent. The court's construction of those terms follows.
The typical wallboard manufacturing process involves mixing various ingredients-including calcined gypsum, water, and other chemicals-in a mixer to form a viscous slurry. The mixer is typically equipped with some kind of mechanical agitator, used to generate high levels of agitation for mixing up the ingredients into slurry. One of the ingredients frequently added to slurry is aqueous foam. Adding foam to the mixture induces small bubbles in the slurry, which ultimately result in voids when the crystalline gypsum in the mixture dries and hardens. The voids improve the resulting board by making it lighter and less dense. If unevenly distributed throughout the board, however, voids can result in visible imperfections in the finished wallboard.
Though the use of foam in wallboard predates the patented process, foam was historically added directly into the mixer (or into a secondary mixer) with the slurry.*fn1 This practice ensured that foam would be relatively evenly distributed throughout the slurry, but the high levels of agitation inside the mixer resulted in the wasteful destruction of large quantities of foam. The process invented by USG and described in the '635 patent addresses this problem by adding the foam as the slurry exits the mixer. By locating the foam inlet close to the discharge outlet from the mixer, the patented process subjects the foam to less agitation while maintaining a relatively even distribution of foam throughout the slurry. Disputed claim 25 of the '635 patent describes this process.
From the mixer, the slurry is sandwiched between top and bottom sheets of paper, shaped to a desired thickness, dried in a kiln to harden, and cut into boards.*fn2 In a modern wallboard manufacturing plant, all of this occurs at high rates of speed.
Because the finished boards may be subject to a great deal of handling before their ultimate use in construction, manufacturers find it desirable to produce boards with hard edges, which can survive more wear. Disputed claim 36 of the '635 patent describes one process for achieving hard edges on otherwise more porous board. The process involves separating out some slurry directly from the mixer, where the slurry is thicker and heavier because it has not been mixed with foam, and depositing it in separate streams along the edges of the paper. The remaining slurry is mixed with foam after exiting the mixer and distributed onto the paper in a different "core stream" between the streams of thick slurry, with the result that heavier slurry (without foam) hardens on the edges of the board and lighter slurry (with foam) hardens in the middle.
B. The Disputed Claims of the '635 Patent
The '635 Patent, titled "Method For Preparing Uniformly Foamed Gypsum Product With Less Foam Agitation" sets forth 47 claims. The disputed terms, emphasized below, are found in Claims 25 and 36:
25. A method of preparing a foamed gypsum board comprising, continuously and concurrently: inserting calcined gypsum and water into a mixing chamber through one or more inlets;
agitating the contents of the mixing chamber to form an aqueous dispersion of the calcined gypsum;
discharging the contents of the mixing chamber through a discharge outlet into a discharge conduit;
inserting an aqueous foam through an inlet into the discharge conduit, such that the foam is mildly agitated to thereby minimize destruction of the foam while uniformly dispersing the foam in the aqueous gypsum dispersion;
discharging the resultant dispersion from the discharge conduit and depositing the dispersion onto a moving cover sheet;
applying a second cover sheet over the deposited dispersion; and allowing the resultant assembly to set and dry such that the calcined gypsum forms set gypsum having voids uniformly dispersed therein. . . .
36. A method of preparing a foamed gypsum board having a hard edge or edges, comprising, continuously and concurrently: mixing and agitating calcined gypsum and water to form an aqueous dispersion of the calcined gypsum;
dividing the aqueous dispersion to form a core stream of the aqueous dispersion and one or more edge streams of the aqueous dispersion;
mixing an aqueous foam into the core stream, such that the foam is mildly agitated to thereby minimize destruction of the foam while uniformly dispersing the foam in the aqueous dispersion; depositing the core stream onto a moving cover sheet; depositing the edge stream or streams onto the cover sheet contiguous to one or both edges of the deposited core stream; applying a second cover sheet over the deposited streams; and allowing the resultant assembly to set and dry such that the calcined gypsum forms set gypsum and the set gypsum in the deposited core stream has voids uniformly dispersed therein.
USG initially filed its patent application, including disputed claims 25 and 36, in December 1995. (Office Action Summary, Pl.s Ex. 3, at 1.) The U.S. Patent and Trademark Office ("PTO") originally rejected claims 25 and 36 in September 1996. Claim 25 was rejected as being anticipated by U.S. Patent 4,735,755 (Bischops). (Id. at 5-6.) The Bischops patent involved inserting aqueous foam by injecting it into a discharge conduit for calcined gypsum. (Id.) Claim 36 was rejected as being anticipated by U.S. Patent 4,279,673 (White). (Id. at 4-5.) According to the PTO, the White patent involved "dividing the aqueous dispersion to form a core stream. . . and one or more edge streams. . . [and] mixing an aqueous foam into the core stream, such that the foam is mildly agitated to thereby minimize destruction of the foam while uniformly dispersing the foam. . ." (Id.) Claims 25 and 36 were also rejected as obvious based on those two patents and other examples in the prior art. (Id. at 7-12.)
In response to the PTO's rejection, USG amended its claims. (Request for Reconsideration, Pl.'s Ex.4.) The amendment to claim 25 indicates that the process is designed to be used specifically for the manufacture of wallboard, as opposed to other gypsum products. The amended language states, further, that after slurry exits the discharge conduit, the dispersion would be deposited onto a moving cover sheet, a second cover sheet would be applied, and "the resultant assembly" would be allowed "to set and dry. . . ." (Id. at 3.) USG explained that the amendment was intended "to indicate that the method is being employed to prepare a gypsum product which is a board having cover sheets on both faces thereof (the conventional wallboard configuration)," (Id. at 5.), to distinguish it from the Bischops patent, which dealt with cements and plasters. (Id. at 8.)
USG amended claim 36, as well, adding that the process was designed "such that the calcined gypsum forms set gypsum and the set gypsum in the deposited core stream has voids uniformly dispersed therein." (Id. at 4.) The amendment to claim 36 was made "simply to make it clear in the claim itself that the setting produces set gypsum from the calcined gypsum and that the uniform distribution of foam in the dispersion is such that a uniform distribution of voids is produced in the final product. This is the main goal of the invention as discussed throughout the specification." (Id. at 5.) USG disagreed with the PTO that the White patent anticipated USG's process. "[The White patent] nowhere teaches that the foam should be added to the main slurry after edge streams have been divided out. . . nor does it teach any method of subjecting the foam in the main slurry to milder agitation than the calcined gypsum." (Id. at 7.) The PTO apparently agreed and accepted USG's amended claims in March 1997. (Notice of Allowance, Pl's Ex. 5 at 1.)
In 2003, USG filed a complaint against Lafarge, a significant competitor in the wallboard manufacturing business, alleging that Lafarge infringed on claims 25 and 36 of the '635 patent. Defendant Lafarge S.A., a French corporation that owns a majority share of Lafarge North America, is also alleged to be liable for patent infringement. The parties agree that Lafarge used the accused process between 2000 and 2004. USG also alleges several state and federal claims against Lafarge, its parent company, and ten individuals who worked at USG before subsequently going to work at Lafarge.
In July 2007, the court granted in part and denied in part Defendants' motion for summary judgment. See United States Gypsum Co. v. Lafarge North America, Inc., 508 F. Supp. 2d 601 (N.D. Ill. 2007). In denying summary judgment on the patent claims now at issue, Judge Hart partially construed several terms of claims 25 and 36. Id. at 616-21. Construing claim 25, Judge Hart determined the void uniformity referred to in that claim applied only to the core of the wallboard. "A person skilled in the art would know that wallboard is manufactured with edges that have a density different than the core. Such a person would understand that claim 25 does not make any claim regarding the edges of the board." Id. at 616.
Judge Hart also found that Lafarge did not literally infringe on Claim 36 because Lafarge's accused process applied its edge streams before depositing its core stream to the paper, rather than depositing edge streams after the core stream as contemplated by the '635 patent. Id. at 617-19. Nonetheless, Judge Hart denied defendants' motion for summary judgment as to Claim 36 under the doctrine of equivalents, holding that a trier of fact could conclude that Lafarge's accused process performed "the same function in the same way to obtain the same result as distributing the core stream first." Id. at 618.
Lastly, Judge Hart partially construed the disputed term "mildly agitate," holding that "mildly" should be given "its ordinary meaning of being on the low end of an absolute scale." Id. at 620. Judge Hart made clear, however, that his construction of the term "mildly" did not foreclose all possible comparisons to agitation as it occurred elsewhere in the manufacturing process. "Viewed as points on an absolute scale, 'mildly agitated' would involve less agitation than 'agitating the contents ...