# How Microwave Crisping Sleeves Work > General Mills' 1978 patent on using a thin, metal-coated plastic wrap that converts microwave energy into intense surface heat to crisp and brown food like Hot Pockets. - **Patent:** US 4267420 - **Original title:** Packaged food item and method for achieving microwave browning thereof - **Owner:** General Mills Inc - **Granted:** 1981 - **Status:** Public domain (expired) - **Times cited:** 234 - **Field:** materials, consumer_electronics ## What it does This patent describes a packaging material that browns and crisps the surface of microwaved food. It uses a flexible plastic sheet (the dielectric substrate, such as polyester) coated with an extremely thin layer of metal (like evaporated aluminum). When placed in a microwave, this metallic layer acts as a semiconductor with an electrical resistance between 1 and 300 ohms per square. This specific resistance causes the metal to absorb a portion of the microwave radiation and convert it directly into thermal heat, reaching temperatures high enough to sear or crisp the food's surface. Meanwhile, the remaining microwave energy passes straight through the film to cook the inside of the food. For example, this is the technology behind the grey, metallic-looking sleeves used to crisp microwave pies or pocket sandwiches. ## What it does NOT cover - Does not cover rigid microwave browning dishes or trays that do not conform to the shape of the food. - Does not cover susceptor materials with a surface resistance outside the 1 to 300 ohms per square range. - Does not cover packaging that completely blocks microwave energy from reaching the inside of the food. - Does not cover non-flexible, non-wrapping heating elements that cannot be wrapped around or draped over a food item. ## The clever bit Instead of completely blocking microwaves with thick metal foil, this design uses an incredibly thin, evaporated metal layer. This thinness creates just enough electrical resistance to turn some of the microwave's electromagnetic fields into heat, while letting the rest of the waves pass through to cook the food's interior. ## Real-world examples 1. Hot Pockets crisping sleeves 2. Microwave pot pie crisping trays 3. Microwave pizza crisping discs ## Why it matters Before this invention, microwave ovens could only heat water molecules inside food, leaving crusts soggy and pale. This patent solved the 'soggy crust' problem, enabling the entire modern industry of microwaveable pocket sandwiches, pot pies, and pizzas. It allowed food companies to sell convenient, crispy-crust frozen meals that cook in minutes. ## Frequently asked questions ### What does How Microwave Crisping Sleeves Work cover? General Mills' 1978 patent on using a thin, metal-coated plastic wrap that converts microwave energy into intense surface heat to crisp and brown food like Hot Pockets. ### Who owns patent US 4267420? General Mills Inc owns this patent, granted in 1981. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 4267420 cited by? This patent has been cited by 234 later patents that build on its ideas. ### What problem does this patent solve? Before this invention, microwave ovens could only heat water molecules inside food, leaving crusts soggy and pale. This patent solved the 'soggy crust' problem, enabling the entire modern industry of microwaveable pocket sandwiches, pot pies, and pizzas. It allowed food companies to sell convenient, crispy-crust frozen meals that cook in minutes. ### What does this patent NOT cover? Does not cover rigid microwave browning dishes or trays that do not conform to the shape of the food. **Full plain-English explainer:** https://patentbrief.org/patent/us/4267420/microwave-popcorn-susceptor **Original patent:** https://patents.google.com/patent/US4267420 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Percy Spencer Invented the Microwave Oven](https://patentbrief.org/patent/us/2495429/microwave-oven-cooking) — This 1945 patent describes the process of using concentrated microwave energy to cook food, the fundamental technology behind the modern microwave oven. - [How Clarence Birdseye Invented Modern Frozen Food](https://patentbrief.org/patent/us/1773079/frozen-food-birdseye) — This 1930 patent describes the process of rapidly freezing food in small packages to prevent the formation of large ice crystals that ruin texture and flavor. - [How Cup Noodles Are Designed to Cook Perfectly](https://patentbrief.org/patent/us/3997676/cup-noodles-instant-ramen-ando) — A 1976 patent describing the specific shape, density, and placement of dehydrated noodles inside a cup to ensure they cook evenly and quickly when hot water is added. - [Tiny Capsules for Electronic Paper Displays](https://patentbrief.org/patent/us/5961804/e-ink-electronic-paper-display) — MIT's 1999 patent on a special ink made of tiny capsules that can change color when an electric field is applied, forming the basis for early e-readers. - [How Pringles Potato Chips Are Stacked and Packaged](https://patentbrief.org/patent/us/3498798/pringles-stackable-chips) — A 1970 patent by Procter and Gamble describing the precise method for stacking uniform, saddle-shaped potato chips into a cylindrical container to prevent breakage.