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.
Original patent title: “Packaged food item and method for achieving microwave browning thereof”
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. Granted to General Mills Inc in 1981 with 14 claims and 234 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
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.
The gap
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.
- 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
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.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Hot Pockets crisping sleeves
Microwave pot pie crisping trays
Microwave pizza crisping discs
Why it matters
The bigger picture
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.
Filed
October 12, 1978
Granted
May 12, 1981
Market context
Who's building on this
Companies in this space
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.
Market impact
This patent laid the technical foundation for the microwave susceptor industry. It directly enabled the commercialization of crispy microwaveable foods, transforming the frozen food aisle and spawning iconic products like Hot Pockets.
Claim 1 — Plain English
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.
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.
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.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Strong
Citation count
40/40
Highly cited
Claim breadth
9/20
Moderate scope
Recency
0/20
Older than 20 years
Assignee scale
20/20
Major company or institution
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$41K – $130K
Midpoint $81K · expired or expiring · industry ×0.9
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
The original legal language
Original claims
14 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Brastad, W. A. (1981). How Microwave Crisping Sleeves Work (U.S. Patent No. 4,267,420). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/4267420/microwave-popcorn-susceptor
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
Embed
Add this patent to your site
Drop this plain-English patent card into any blog post or article — free, no signup. It always links back to the full breakdown here.
<div data-patentlens-widget data-patent-number="US4267420"></div> <script src="https://patentbrief.org/embed.js" async></script>
Stay in the loop
Get a weekly digest of new patents.
One email per week. No spam. Unsubscribe anytime.
Keep exploring
Related patents you should know
US 4683195 · 1987
How to Make Billions of Copies of a DNA Segment
This patent describes the Polymerase Chain Reaction (PCR), a method to rapidly create many copies of a specific piece of DNA or RNA, enabling its detection and analysis.
Cetus Corp
US 8697359 · 2014
How to Edit Genes in Human Cells Using an Engineered CRISPR System
This patent describes an engineered CRISPR-Cas9 system for precisely cutting DNA in eukaryotic cells to change how genes work, opening the door for gene editing in complex organisms.
Massachusetts Institute of Technology
US 7657849 · 2010
How the iPhone's Slide-to-Unlock Gesture Works
Apple's 2010 patent describes unlocking a device by dragging a specific graphical image across the touchscreen along a predefined path, a gesture that became iconic with the original iPhone.
Apple Inc
US 4733665 · 1988
How Doctors Implant a Permanent Stent Using a Balloon
This patent describes the method for placing a permanent, expandable wire mesh tube inside a blood vessel or other body tube using a balloon-tipped catheter to widen it and keep it open.
Expandable Grafts Partnership
US 4405829 · 1983
How RSA Public-Key Encryption Keeps Digital Messages Secret
This patent describes the foundational RSA algorithm, a method for securely sending messages where anyone can encrypt a message using a public key, but only the intended recipient can decrypt it using a secret private key.
Massachusetts Institute of Technology
US 4575330 · 1986
How 3D Printers Build Objects Layer by Layer from Liquid
This patent describes the foundational method for 3D printing, where a machine builds a three-dimensional object layer by layer by hardening a liquid material with light or other energy.
UVP Inc
Semantically similar
You might also find these interesting
US 2495429 · 1950 · Raytheon Manufacturing Co
How Percy Spencer Invented the Microwave Oven
US 1773079 · 1930 · Frosted Foods Co Inc
How Clarence Birdseye Invented Modern Frozen Food
US 3997676 · 1976 · Nissin Shokuhin KK
How Cup Noodles Are Designed to Cook Perfectly
US 5961804 · 1999 · Massachusetts Institute of Technology
Tiny Capsules for Electronic Paper Displays
More to explore
More in Materials & Manufacturing
US 4575330 · 1986 · UVP Inc
How 3D Printers Build Objects Layer by Layer from Liquid
US 3953566 · 1976 · WL Gore and Associates Inc
Making Strong, Porous PTFE: The Gore-Tex Process
US 5121329 · 1992 · Stratasys Inc
How Machines Build 3D Objects Layer by Layer from Melting Plastic
US 3691140 · 1972
Sticky, Tiny Plastic Balls Made from Acrylates
New to patents?
Common Questions
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.
Patent monitoring