You can freely build on How Microwave Crisping Sleeves Work

What this patent covers

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 is now free to use

All 1 claims of US 4267420 are in the public domain. Specifically:

The 1 dependent claim add narrowing limitations and are also free.

What is NOT covered

Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.

• 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.

Who is building on this today

General Mills originally patented this, and major packaging giants like Graphic Packaging International and Bemis (now Amcor) built heavily on this susceptor technology to create modern microwave active packaging.

Products built on expired version of this technology

How to cite this patent in your documentation

General Mills Inc. US Patent 4267420. Packaged food item and method for achieving microwave browning thereof. Granted 1981, expired 1998. Now in the public domain.

Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.

PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.