Computer Simulation Technology has released a whitepaper discussing electromagnetic simulation: "Efficient Electromagnetic Simulation of Shielding Mechanisms in Microwave Ovens". This paper discusses how electromagnetic simulation can be used to design effective shielding mechanisms in order to adhere to legal and electromagnetic compatibility (EMC) requirements.
The door to a microwave oven forms part of the heating cavity, allowing access to the oven interior. The seal around the door is never perfect, so electromagnetic fields will leak out of the oven, potentially interfering with other electrical equipment or exceeding safety limits for power levels in human operators. Oven designs have to meet legal requirements for the amount of power which can leak beyond a certain distance from the oven (due to human exposure concerns), and also have to meet EMC requirements.
Microwave heating is caused by the interaction of a high frequency electromagnetic field with a dielectric. Common uses of microwave heating include cooking, drying and sterilisation - all effects that can be dangerous or destructive outside the controlled environment of the heating cavity. To prevent injury to operators, microwave ovens need to be well-shielded to prevent leakage of RF fields.
Another reason to improve microwave oven shielding is EMC. Electromagnetic fields produced by the oven can couple into other electronic devices, causing spurious signals and potentially damaging the device. To complicate matters, most domestic microwave ovens operate on the industrial, scientific and medical (ISM) band at around 2.45GHz. Because this band is unregulated, it has become widely used for short range wireless communication systems such as WiFi and Bluetooth. Noise from microwave ovens can cause significant interference to networks using these technologies.
If a microwave oven consisted solely of a sealed heating cavity, shielding it would be trivial. However, practical considerations mean that a real microwave oven includes several potential leakage points. Because objects need to be loaded into and removed from the oven, there needs to be a door. This introduces a seam along the edge of the cavity through which electromagnetic fields can escape.
Another potential source of major leakage is the window. While the window is necessary to allow the cooking process to be monitored, it needs to be carefully designed to make sure that EM radiation in the visible portion of spectrum can pass through, but radiation in the microwave spectrum cannot.