Moulds, Polymers, & Machinery
The rotational moulding process requires moulds, polymers (or resin), and the appropriate machinery. An overview of each is presented below:
Types of Moulds
Moulds used in the rotational moulding process are typically manufactured from either stainless steel or aluminum. Although thicker than equivalent stainless steel moulds, Aluminum's thermal conductivity is many times greater than that of steel and thus the rotational moulding cycle time is not significantly affected.
Aluminum moulds are the most common used in the rotational moulding industry. With cast aluminum moulds manufacturers can produce complex shapes with intricate detailing in a wide range of sizes - from very small products (such as ear bulb syringes) to large industrial items (such as waste water holding tanks). According to manufacturers, a cast aluminum mold is preferred for its heat transfer, malleability, strength, design flexibility, and how it faithfully reproduces the product appearance.
Types of Polymers
According to recent reports 80% - 90% of all material used in the rotational moulding industry is polyethylene (HDPE, LPDE and LLPDE). Although PVC, nylons, and polypropylene compounds are also used. Most manufacturers decide upon polyethylene as the resin of choice because of its availability, ease of use, and suitable properties.
A paper written by J.D. Ratzlaff of Chevron Phillips Chemical Company LP in 2004 entitled "Polyethylene: Process Sensitivity in Rotational Moulding" presents the results of a study of the impact sensitivity of polyethylene to processing conditions and discusses methods to maintain high impact standards.
Learn More Download PaperTypes of Rotational Moulding Machines
There are two types of rotational moulding machines commonly used: Independent-arm and Fixed-arm turret machines. In Fixed-arm turret machines all the arms (typically 3 or 4) index at the same time. This requires that heating, cooling, and servicing operations must be done at the same time for each arm. Independent-arm machines provide process flexibility by allowing one arm to index while other arms can remain stationary.
