What is Rotational Molding?
- Rotational molding is a plastic casting technique that produces a hollow, seamless, and double-walled part by heating a powdered resin in a hollow mold tool while being slowly rotated and cooled to solidify. This process started hundreds of years ago, was popularized in the mid-20th century, and is still developing. This process has a variety of applications.
- The rotational molding equipment consists of a mold, ovens, and cooling chamber. The type of rotary molding machine is distinguished by the direction in which the mold travels according to the sequence of the process.
- The types of rotational molding equipment are clamshell machines, turret machines, shuttle machines, vertical wheel machines, rock and roll machines, and open-flame machines.
- Loading is the first step wherein the fine, homogenized, and dried powdered resin is placed in a mold coated with a mold release agent. The resin is heated while being slowly rotated in a bi-axial direction until it is molten and covers the entire tool. The molded part and the entire mold are gradually cooled by convection to a room temperature wherein it can be safely removed.
- Critical parameters of the rotational molding process are heating time, cooling rate, and rotation ratio. These settings determine the mechanical properties and dimensional quality of the parts to be produced.
- The polymer resin must have high thermal stability, flow easily at its molten state, and antioxidant groups in the molecular structure. The resin must be easily ground to powdered form. Polyethylene is the most commonly used polymer.
- The advantages of rotational molding are the creation of parts with uniform wall thickness, inexpensive tooling, flexibility of production, less downstream process involved, and less wastage of the resin. Large and double-walled parts are also easily created.
- The disadvantages of rotational molding are a high cycle time attributed to long heating and cooling cycles, limited material options, short tool life, and difficulty of molding of some details.