Principles of rotational molding

Abstract

Rotational molding is a process by which powdered or liquid plastics are converted into hollow articles. This paper is devoted to the theoretical understanding of the process of rotational molding. There are seven sections:Ths Art, wherein we describe the process, discuss previous attempts at understanding the process, and mention processes that are similar in principle to rotational molding.Transient Heating of Mold Surface, wherein we show that the criterion for selection of mold materials is the ratio of the thermal d8usivity to the thermal conductivity, and present the heating curve for a mold in a rotational mold machine.Melting of Plastic Powder in a Rotating System, wherein we discuss in detail those physical powder characteristics that are necessary for good flow within the mold cavity.Flwid Flow During Rotational Molding, wherein we discuss the velocity profiles within the melt film, point out that there is very little bulk polymer flow possible within the mold cavity under normal processing conditions, and consider capillary flow forces and surface wetting.Sinter-Melting, wherein we compare the Kuczynski-Neuville empirical sintering model with the Lontz viscoelastic model, conclude that the latter is correct for the sintering of materials such as ABS, and apply the Frenkel glass densification theory to the prediction of void disappearance in sinter-melt polymers.Degradation, wherein we compare our experimental tensile strengths of polystyrene, obtained at varying oven cycle times and oven set point temperatures, with values obtained from degradation models given in the literature.Laboratory Simulation of Rotational Molding, wherein we propose two series of experiments, the first series being carried out without using rotational molding equipment, and the second using rotational molding equipment with molds having relatively simple geometries. THE ARTAs far as we can tell, the first materials that were R o t a t i o n a l molding or rotational casting is the process by which hollow plastic parts are formed.This process, in principle, is as follows. Very fine plastic powder is loaded into a split hollow metal mold. The mold is then rotated about the equatorial and polar axes while the powder is heated above its melting point. Once the melted plastic covers the inside of the mold surface, the assembly is cooled to room temperature and the part removed. The process is then repeated. * Present address: University Est. de Campinas, Caixa Postal 655, Campinas; S. P., Brazil. O 0 Present address: Energetics Department, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201.cast in this manner were vinyl plastisols, which were introduced into the molds as liquids and heat-cured. An example of an early application is the vinyl beach ball. This process was well known in the late 1940's 243

Simulation of heat transfer during rotational molding

“…7 Thus for most of the process the polymer in contact with mold forms a film at the mold surface. 7,16 In these conditions the very high melt viscosity in practice prevents any relative motion of the material on the mold wall, thus avoiding convective heat transfer. The BC at the mold-polymer interface can then be written as…”

Modeling the morphology development of ethylene copolymers in rotational molding

“…[6][7][8][9] The energy is conducted through the mold wall to the polymer powder as it melts, densifies, cools, and crystallizes against the mold wall. The heated air in the oven convects its energy through contact with the outer surface of the mold, and the air inside the mold cavity is heated and cooled through convection by the inner polymer surface.…”

“…Theoretical heat transfer models for the entire rotational molding process have been proposed and used to predict the internal air temperature profile as well as the temperature distribution inside the molded part. [6][7][8][9][10] Given the predicted temperature distribution with the heat transfer model, it is possible to simulate two-dimensional crystal growth during the cooling process through the mathematical description and numerical simulation of the modified phase-field theory. Traditional ways of modeling crystallization in polymers are based on the work of Avrami.…”

Experimental study of blade thickness effects on the overall and local performances of a Controlled Vortex Designed axial-flow fan

“…Low-speed axial-flow fans usually have conventional thin blades. However, following motivations such as active control of the blade shape or the use of the rotomoulding process [5,6], the designers would have now to face profiles with thicker blades. The control of blade shape may help to reduce the radiated noise or to extend the operating range and the need for actuators then implies thicker blades.…”