{
  "patent_number": "US 4267420",
  "country": "US",
  "title": "How Microwave Crisping Sleeves Work",
  "original_title": "Packaged food item and method for achieving microwave browning thereof",
  "summary": "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.",
  "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."
  ],
  "filed": "1978-10-12",
  "granted": "1981-05-12",
  "expires": "1998-10-12",
  "status": "expired",
  "holder": "General Mills Inc",
  "holder_url": "https://patentbrief.org/company/general-mills-inc",
  "inventors": [
    {
      "name": "William A. Brastad",
      "url": "https://patentbrief.org/inventor/william-a-brastad"
    }
  ],
  "times_cited": 234,
  "tags": [
    "materials",
    "consumer_electronics"
  ],
  "abstract": "A food item is wrapped with plastic film or other dielectric substrate having a very thin coating thereon which controls the microwave conductivity when the package is placed within a conventional microwave oven. The plastic film or other substrate and its coating conform to a substantial surface portion of the food item. The coated plastic film or other substrate converts some of the microwave energy into heat which is transmitted directly to the surface portion so that a browning and/or crispening is achieved. The coating has the additional capability of modifying the microwave field configuration so as to further enhance the heating of the food surface immediately adjacent the film in which the food is packaged. The microwave energy not converted into heat by the coated film passes through the film to dielectrically heat the single food item as is normally done when cooking with microwaves.",
  "url": "https://patentbrief.org/patent/us/4267420/microwave-popcorn-susceptor",
  "markdown_url": "https://patentbrief.org/patent/us/4267420/microwave-popcorn-susceptor/md",
  "google_patents_url": "https://patents.google.com/patent/US4267420",
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}