Graver Tank v. Linde Air Products Co.
United States Supreme Court
339 U.S. 605 (1950)
Linde (plaintiff) patented Unionmelt, a welding flux containing calcium and magnesium silicates; Graver's (defendant) competing flux Lincolnweld substituted manganese (not an alkaline earth metal like magnesium) but achieved essentially identical welding results. Chemists testified manganese and magnesium behave similarly in relevant chemical reactions, prior art already taught substituting manganese silicate in welding fluxes, and Graver produced no evidence Lincolnweld was independently developed rather than modeled on Unionmelt; the trial court found infringement under the doctrine of equivalents.
Whether a device that performs substantially the same function in substantially the same manner to achieve the same result as a patented invention may be found to infringe under the doctrine of equivalents.