The first concept was an attempt to use the motion of the turntable to create a continuous wobbling effect when a bowl is placed on a plastic plate supported only by a single point in the center. The idea was that the contents of the bowl would be agitated gradually when placed on the wobble plate. While this concept consists of only a single part that can be used with a variety of bowls, it proved to be not very effective.

The possibility for harnessing the heat of the microwave seemed promising, so a device was prototyped that would float inside the bowl and use steam propulsion to drive mixing paddles. This prototype consists of a sealed floatation chamber and a separate volume where water can be stored with several ports for emitting steam. The thought was that the water would boil inside the microwave creating a teapot effect at each of the ports, spinning the mixing paddles. After testing, it took too long for the contained water to heat up and generated very little force.

The thought of just inserting a spoon or spatula into the bowl during turntable rotation was worth a try. A model was made with an articulating arm fixed to the interior of the microwave with a suction cup. The articulating arm is adjustable to accommodate a variety of bowl shapes. The end could potentially be detachable and interchangeable. This simple idea did not provide enough of a mixing effect.

Using bits of the previous prototype, we then created an array of paddles attached to the lid of a bowl that would turn when impacting a point held fixed with a suction cup. The wings at the top of each paddle could impact anywhere along their length allowing for some flexibility with suction cup placement. The results were promising so we decided to pursue this further and create a version with gear driven paddles that would spin more continuously.

The following video describes this concept in more detail:

After a conversation with the inventor Michelle B. last Friday, it was agreed to move forward with the geared paddle concept. In addition to mechanism refinement, we will be considering possibilities for optional paddle shapes that may be interchangeable for use with different types of food. Our goal for the short term will be to focus on a simple mechanical solution with a minimum number of parts made from microwave safe materials. In the long term we will continue to research other more advanced and technically challenging ways of stirring inside a microwave oven.

If you have any comments or suggestions, feel free to add them in the comments of this blog post.

Far and away the most common process we use is injection molding – it’s probably the most common manufacturing method for any plastic or rubber. Molten plastic is shot into a steel mold at high pressure, allowed to cool slightly, and then ejected from the mold. The steel mold is called the tool, and tooling costs tend to be pretty high for injection molding ($1K-$80K) because steel is expensive and labor intensive to cut. On the other hand, the part cost (basically the cost of the plastic used) is pretty low, because it’s pretty easy to automate. The most basic mold would have two sides and no moving parts (except opening and closing). For more complex geometries, or to add features like threads, mold makers add more parts to the mold (which, of course, adds more cost).

While the plastic is cooling, it also shrinks, and that’s the basis of most of the limitations involved in injection molding. One example is wall thickness. Because the cooling occurs at different rates depending on how thick the part is, parts need to have more or less even thickness – otherwise the part warps, because the middle of thick sections are still liquid while the thinner sections are trying to shrink. If you want to know more about different design constraints for injection molding, just google it – there are tons of guides out there.

Two of the most powerful things you can do with injection molding are overmolding and insert molding. For overmolding, you mold two different materials in the same mold – that’s how you get all of those objects that have rubber grips permanently attached attached to hard plastic. Overmolding is very popular in toothbrush and sneaker design. Insert molding is where you take something and put it in the mold before the process starts, so that it ends up embedded in the part. For example, Cordies has an insert molded steel piece that adds weight to the ends.

There are lots of books out there on injection molding, and lots of slight variations on the process, but that’s the general gist of how it works.

Addendum from Ben: How does this all work in the real world? Check out our awesome PowerCurl molds!

Despite my techie background, I’m a blog virgin, so be gentle on me.

Working on Pen Zen reminds me of my knife block, which is awesome for those of us still in the phase of life where nothing comes in sets: www.kapoosh.com.