## Facts

- Met-Coil-related entities owned U.S. Patent No. 4,466,641 covering roll-formed sheet-metal HVAC duct sections with integral flanges intended to simplify field connection compared with rivets or spot welds.
- Engel Industries took a license under the patent and paid royalties, then later challenged the patent and sought a refund.
- Engel filed a declaratory judgment action in the Eastern District of Missouri seeking declarations of invalidity and noninfringement and repayment of royalties.
- After trial, the district court held all claims invalid for failure to satisfy the best mode requirement of 35 U.S.C. § 112 and found inequitable conduct based on nondisclosure of the best mode to the PTO, rendering the license agreement void.
- The district court rejected an obviousness challenge under 35 U.S.C. § 103.
- Met-Coil appealed the best-mode invalidity and inequitable conduct rulings; Engel did not cross-appeal the no-obviousness determination.

## Issues

1. Whether the patent was invalid for failure to disclose the inventor’s best mode of practicing the claimed invention under 35 U.S.C. § 112.
2. Whether the alleged failure to disclose best mode constituted inequitable conduct before the PTO, making the patent unenforceable and the license void.

## Decision

- The Federal Circuit reversed the judgment of invalidity based on best mode.
- The Federal Circuit reversed the finding of inequitable conduct and the resulting holding that the license agreement was void.
- The court left undisturbed the unappealed determination that the patent was not shown invalid for obviousness.

## Legal Principles

- Best mode under 35 U.S.C. § 112 turns on the inventor’s subjective knowledge at filing of the best way to carry out the claimed invention and whether a better mode was concealed.
- The best mode requirement does not compel disclosure of non-claimed manufacturing details, commercial preferences, or routine engineering refinements (e.g., particular machine settings or tooling adjustments) when they are not essential to practicing the claimed invention and can be determined by a skilled artisan without undue experimentation.
- Inequitable conduct requires material misrepresentation or omission coupled with intent to deceive the PTO; absent material nondisclosure, inequitable conduct cannot be sustained.
- A finding of best-mode deficiency does not automatically establish inequitable conduct; the required materiality and intent elements must be independently supported.

## Conclusion

The Federal Circuit held that the district court applied an overly expansive view of best mode by treating preferred industrial implementation details as required disclosures, and it therefore reinstated the patent’s validity and enforceability and rejected inequitable conduct and license-voiding relief based on the asserted nondisclosures.


Engel Indus., Inc. v. Lockformer Co., 946 F.2d 1528 (Fed. Cir. 1991)