Rotational molding, or rotomolding, is a cost-effective way of making large, joint-free polymer containers. “Rotationally Molded Transit Cases: 8 Advantages” discussed what makes this process so useful. Here, we’ll dive deeper into the details of the process.
The principle behind rotomolding is pretty simple. A molten polymer is spread over the internal surfaces of a closed mold. It hardens to forms a hollow container where the outside shape mimics the inside of the mold. Wall thickness is dictated by the quantity of polymer powder used.
The rotomolding machine
The most common design appears in this video “Pelican Rotomold Case Technology”. It consists of three horizontal arms rotating around a vertical capstan. A mold tool is held in a cradle at the end of each arm where motors rotate it in two axes, usually at different speeds. As the tools themselves can be very large, these are big machines.
Around the capstan are three stations: one for loading (charging) and unloading, one for heating/melting, and one for cooling. The capstan rotates slowly, carrying the tools through each station.
The video shows powder being spread inside one half of the mold tool. This must be weighed precisely to maintain consistency from piece to piece. And, to ensure there’s enough powder to form the required wall thickness.
Particle size distribution is important. Smaller particles melt first, coating the walls of the mold and forming a base for larger particles to bond to. The largest particles settle last and determine the internal appearance of the molded part.
After charging the mold is closed and moved into the heating stage. Hot air blows over the rotating tool melting the polymer powder inside. As the tool turns, the polymer builds up a coating of the internal surfaces.
Cooling speed influences final appearance and properties. In many applications, water is sprayed over the tools to ensure the polymer cools quickly enough to provide the toughness required.
Once the polymer has solidified, the mold is opened and the container removed. Then the cycle starts again.
As in the video, after excess mold flash is trimmed away the case is flame polished. This entails plying a gas torch carefully over the surface to locally re-melt the polymer. It’s a highly skilled job that results in a smooth and attractive appearance. Finally, hinges, handles and gaskets are added to make the case ready for use