The molds used in rotational molding are shell-like constructions. They are normally made in two halves, although complex parts may require molds to separate into three or more pieces. The molds are held closed at the parting line by clamps. The mold almost always has a vent tube (“breather”) to ensure equalization of pressure between the inside of the molded part and the external environment. The positioning of the vent tube depends on the nature of the plastic part – for example, the fill port of a tank is a convenient place to locate the vent.
The most common mold materials are cast aluminum or fabricated sheet steel. The latter is favored for large articles such as tanks, whereas casting is used for smaller parts that contain complex details, or where several identical molds are required. Electroformed or vapor-formed nickel plate molds are also used, particularly for PVC parts. In recent years, the use of CNC machined molds is becoming common, and this is resulting in exciting improvements in mold quality, particularly at the parting line. Molds undergo high thermal stresses as they are regularly cycled from room temperature to over 300˚C (over 600˚F), and finite element analysis of CNC machined molds ensure that high performance can be maintained over long periods of time. The desirability of having a small positive pressure inside the mold can also be realized more easily in computer-designed molds. Automation of mold opening and mold filling is also helping to reduce cycle times and improve consistency in the molded part.
In the latest types of ‘Leonardo’ machines developed in Italy, the mold and the machine are a single unit. This enables very precise control over mold rotation, as well as the temperature and pressure inside the mold.