This video documents the construction and testing of a large, homemade microwave burnout kiln designed for casting metal. The creator upgrades his setup by building larger silicon carbide rings and using a safer soluble fiber blanket instead of traditional ceramic fiber. The project is driven by the challenge of casting a large, mysterious iron bracket for a viewer, which necessitates a larger microwave oven. The video details the entire process, from creating the kiln's components and sand molds to overcoming technical hurdles, such as adapting a digital microwave for use with an external cycle timer. After an initial test with a thinly insulated kiln reveals design flaws, a second, more robust kiln with double-layer insulation is built. The creator successfully casts the iron bracket twice, demonstrating the viability of the improved kiln design and sharing valuable techniques for controlled mold burnout and home foundry work.
Building a Large Microwave Kiln for Iron Casting
0:00:03 Project Goal & New Material: The creator sets out to build his largest microwave burnout kiln to date, specifically using a soluble fiber blanket, a safer alternative to ceramic fiber, to cast a complex iron part.
0:02:16 Creating the Kiln Rings: The core of the kiln is made from rings cast using silicon carbide grit and a sodium silicate binder. The process involves mixing the materials, packing them into 3D-printed molds, freezing them for easy removal, and then baking them at 130°C.
0:04:16 Overcoming Size Limitations: Realizing his new 42-liter microwave is too small for a thick-walled kiln, the creator first builds a test version with only a single, 25mm layer of insulation to see if it's viable for larger projects.
0:05:17 The "Mysterious Bracket" Project: The motivation for the project is a request from a viewer, Anthony, to cast a bracket for a 130-year-old object. This collaboration led to the acquisition of the larger microwave, funded by the viewer and channel patrons.
0:11:13 Controlling a Digital Microwave: A key innovation is shared for controlling a digital microwave with a cycle timer. Since digital units typically reset when power is cut, a 3D-printed jig is used to continuously hold down the "start" button, allowing the cycle timer to effectively turn the microwave on and off.
0:12:24 First Burnout & Lessons Learned: An incorrectly set timer leads to a very aggressive first burnout. While it successfully burned out the PLA pattern, it cracked the mold and confirmed that the single layer of insulation was insufficient, with the kiln's exterior reaching over 150°C.
0:16:12 Designing an Improved Kiln: Unsatisfied with the thin-walled design, the creator builds a second, more efficient kiln. By using slightly smaller 130mm rings, he is able to fit a double layer of insulation (50mm total) inside the microwave for better heat retention and safety.
0:17:54 Crafting a Better Mold Holder: A clever method for creating a stable tray for the casting mold is demonstrated. Small rings made of sand are used to elevate a custom-formed aluminum foil tray, which holds the mold securely inside the kiln.
0:21:12 The Importance of Controlled Burnout: The creator emphasizes that using a cycle timer and an infrared thermometer provides essential control over the burnout process, allowing for gradual, repeatable temperature ramps that are crucial for preventing mold failure, especially with delicate investment materials.
0:23:28 Successful Casts & Final Thoughts: Both iron casting attempts are successful, producing two high-quality brackets. The creator concludes that the soluble fiber blanket works very well for kiln construction, despite showing some minor wear after several high-temperature uses.