Rotational Molding Method Arranging Rotomolding

Rotational molding or rotomolding is a method for producing hollow

, one-piece articles by using heat and biaxial rotation. The kinematics of the mold and the energy exchange of the polymer with its surroundings are the key aspects of the process. Both factors contribute to the process optimizations . To date, the industry enjoys approximately 10% growth rate annually. This is because a relatively low capital cost is required, the availability of suitable raw materials and technical advances, and its process adaptability for manufacturing different sizes . The process cycle begins with placing a premeasured plastic powder with a typical particle size distribution from -30 mesh (500 [micro]m) to 200 mesh (70 [micro]m) in an empty mold. The closed mold is then heated in a hot oven at about 250-375[degrees]C while subjecting to a biaxial motion with a relatively low rotational speed at about 4-20 rpm. The plastic tumbles, melts, and sticks inside the rotating mold. The heating cycle ends when all the powder melts completely and sticks onto the mold surface. Then the cooling cycle proceeds until the plastic achieves its demolding temperature. The rigid part is removed to end the rotational molding cycle. The prediction of these heating and cooling cycle times is generally done by monitoring the internal air temperature, which replaces those ineffective trial-and-error methods as the today optimization tool.

The general motion of powder inside a revolving system can be categorized into three types [2, 3]; avalanche bed flow, steady-state circulating bed flow, and slip-flow, or static bed flow. The movement of the avalanche bed flow and circulating bed flow might show a more realistic enclosed flow pattern of the moving powders. However, these models require tracking the dynamic surface variation and the location of the powder pool inside the revolving mold. In contrast, the slip-stick flow pattern of the static powder bed is more suitable to model with the continuum-based finite element procedure. This is because the powder particles are assumed to remain in continuous contact with their neighbors in any situation. In the context of theoretical and numerical difficulties, the avalanche flow model is the most complex to be represented. This is because the powder flow is in an unsteady-state. The second most difficult flow pattern to be modeled is the steady-state circulating bed flow, while the slip-flow model is the easiest. In addition to the different flow patterns, a revolving mold is often found causing a particle segregation of the particles enclosed in the mold, in which the particles with same size, density, or surface roughness form clusters inside the mold.

In 1972, Roa and Throne developed the first circulating-bed-flow model to predict the mold and plastic temperatures. Later in 1976, Throne published a comparatively simple static-bed-flow model. This model shows better predictions than the circulation model. Sun et al. and Liang have managed to predict more satisfied cycle times and the internal air temperature profiles with their one-dimensional (1D) static models. Wright and Crawford later developed a new combined thermal-kinematic model for the rotational molding process. The kinematic model is to predict the polymer distribution inside a rotational mold only. The thermal model for transient heat transfer was developed based on the energy equation in which the mass of the heated powder changes continually. Their studies have shown that the radiation effect only makes a very

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