An existing patent uses step A, B, C in the independent claim.
My invention inserts a non-obvious step X before step C.
So my invention looks like this. A, B, X, C
Can the existing patent owner sue me for that?
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Sign up to join this communityCan the existing patent owner sue me for that?
In a word, yes.
If there is a patent which has a claim specifying the steps A, B and C and your product implements steps A, B and C you infringe. Adding a step X doesn't change that. Now, if X is novel and non-obvious, you might be able to obtain a patent on your improvement, but that doesn't change the infringement. You would have to obtain a license from the owner of the infringed upon patent.
I am not a lawyer and this should not be considered legal advice. As I always recommend, you should consult with an actual patent attorney when determining freedom to operate questions. Relying on internet opinions on hypotheticals is risky.
Technically speaking when a claim term includes the comprising term then any new step included in the method would infringe the patent, But recently in Amgen Inc. v. Sandoz Inc. (Fed. Cir. 2019) Federal circuit affirmed the District court claim construction "washing" and "eluting" steps of the method claimed in the '878 patent (specifically, subparts (f) and (g)) as being separate steps that required the washing step to be performed before the eluting step. Again, under this construction Amgen conceded it could not prevail on infringement because Sandoz performed these steps concurrently with step (e) (regarding application of the refolding solution), and the panel credited Sandoz's argument that the claim "logically requires a series of steps," citing (as did Sandoz) Mformation Technologies, Inc. v. Research in Motion Ltd., 764 F.3d 1392, 1398–1400 (Fed. Cir. 2014) ("a process claim is properly limited to a certain order of steps 'when the claim language, as a matter of logic or grammar, requires that the steps be performed in the order written, or the specification directly or implicitly requires' an order of steps").
In view of the above judgments in order to evaluate infringement of a method claim it is essential to understand what is actually claimed i.e., does the patent claim a method of steps ABC or does the patent claim a method of steps A followed by B followed by C.