Rotational Molding or “Rotomolding” is used to make hollow parts of complex shapes. Assuming you had a decent childhood, you’re already familiar with many of the kinds of products that can be created using this method: Chocolate bunnies, playground equipment, action figures…
Rotomolding mainly deals with materials that are solid at room temperature, but turn liquid with a moderate degree of heat – not high enough for liquid metals though (mercury and Cyberdyne Systems Model T-1000 notably excepted). We’re going to assume you’re not in the candy business and instead focus on plastics in this article. Safe assumption since this IS a blog about part manufacturing. Either way, we’ll give you enough detail here to be able to engage with your manufacturer and chat it up at this year’s Rotoplas Expo.
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How is it Made?
To begin, a two or more part mold is assembled together with a specific amount of powdered plastic resin inside. It’s then heated and, wait for it… rotated around in different directions. Centrifugal force (the same force that plasters you to the edge of whichever Tilt-A-Hurl ride your kids make you go on) ensures the plastic evenly coats the whole interior surface before it fuses together.
After enough heat, time and movement, the mold is cooled from the outside to solidify the plastic shell on the inner surface. The mold is then split apart to extract the newly made hollow plastic part before repeating the process to make more of the same.
What is the Rotomolding Process?
In general if a part in mind is a hollow plastic part, it’s likely a good candidate to utilize rotomolding. The process works great for complex shapes that hold liquids such as gas cans and water barrels, or for playground equipment where soft curves and double-walled forms both protect and structurally support children at play. To help determine if it is, Industrial Designers first conceive the part with 2D or 3D sketches. While this serves as the communicative bridge between the design engineers and future consumers for aesthetics, it also highlights what is physically possible.
Design Engineers will develop a CAD model from the concept sketches. In this 3D model, they will identify the parting lines. These are where two or more segments of a mold tool will meet together. Parting lines are important in that they establish tool complexity and how undercuts and draft angles are evaluated. There will also be a small blemish line visible on the outer surface of the final plastic parts where the parting lines trace the surface. Design Engineers also need to determine the specific material to be used in this stage. The material’s strength properties drive requirements for its shape (how much support is needed in different places), while the thermal characteristics determine its size (how the material behaves when heated).
One benefit of this process is that tooling is inexpensive relative to other types of molding. Since this is a low-pressure process, the tools just need to set the shape of the part and don’t require much in the manner of strength. Aluminum is commonly used for rotomolding tooling, though large simple parts can even incorporate steel sheet metal weldments. One drawback though is that a part well-suited for rotomolding (large and hollow with complex geometry) lacks a more economical option for prototyping.
With the amount of tolerance needed for the beginning of the process, a bit of fine-tuning is unavoidable at the end.
Much of it will be calculation: What is the surface area of the finished part? The desired wall thickness? But calculation will take you only so far. There will always be uneven material that has accumulated because of the process. This is where experience and experimentation are required. Perhaps achieving the thickness required in large flat areas may mean that small convex features are a bit thicker. Perhaps the weight of the product will increase because of that. The features of the part will accumulate material somewhat unevenly no matter the design and process.
What’s the Bottom Line?
For those aficionados of pie charts and diagrams, we have your fix coming up. If you’re not one of those, we hope you’ve found something of value in this article. Stay tuned to our blog for more of this type of information.
Here are a few cases of large and small parts showing the development time, production time, and component part costs for a few examples: