MS Thesis Presentation by Kalyanjit Ghosh
Friday, July 2, 2004

(Dr. Srinivas Garimella, Chair )

"Thermal Models and Energy Saving Strategies for Rotational Molding Operations"

Abstract

Transient heat transfer phenomena in the rotational molding of plastic parts are modeled in this study. Natural convection and radiation from the furnace and flue gases to the mold housing are analyzed. Other models include transient heat transfer through the mold, single-phase conduction through the particulate plastic material prior to phase change, melting of the plastic and heating of the liquid pool. Subsequent staged cooling and solidification of the mold and plastic using a combination of free and forced convection and radiation, is also modeled. Information about the properties of the plastic in powder, liquid and solid forms is obtained from the literature. Assumptions about the behavior of the plastic powder and the molten plastic during the rotational operations are also made in accordance with the available literature. The mold wall, melt and solidified plastic regions are divided into a number of finite segments, to track the temperature variation with time during the molding process. The corresponding variations in masses and thicknesses of the melt and solidified plastic regions are also estimated. This information is used to estimate the energy consumption rates for various phases of the process. The model is applied to a specific molding process in a commercial rotational molding plant. Parametric studies of the effect of heating and cooling durations on the plastic temperatures and the energy consumption rates are also conducted. These analyses provide insights about opportunities for optimization of the heating and cooling schedules to reduce overall energy consumption and also improve throughput. The heat input from the furnace reduces from 1.53 MJ for the baseline to 1.33 MJ for the optimized process for a single mold. Optimal natural and forced convection cooling schedules are also proposed based on the analysis.


The overall energy and gas consumption for the rotational molding process, taking into consideration the thermal mass of the auxiliary housing (steel) required to hold the molds, is estimated on a per-batch basis. In addition, a preliminary design for an alternative system for heating and cooling the molds using a high temperature heat transfer fluid (HTF) flowing through jackets integral to the molds is proposed in this study. The HTF provides efficient heat transfer to and from the mold and reduces energy consumption, particularly due to the absence of the auxiliary mass of the housing that must be heated and cooled by flue gases in the conventional process. Two different options for the alternative system are considered. In the first option, the hot HTF from a system reservoir heats the batch of molds directly. In the other, in which thermal energy from the heat transfer fluid is recuperated, hot HTF from the previous cooling batch is used to pre-heat the molds, thus reducing external heat addition. For each of these two options, further energy savings are also realized by implementing waste heat recovery during the cooling stages. This recovered energy is available for supplying process heating needs in the plant. From this study, it was found that for a batch of 14 molds, the energy consumption of the base-line furnace-heated molds is 80 MJ, while 73 MJ is required in the optimized case. For this optimized case, of the 73 MJ required, 54 MJ is required for the heating of the auxiliary mass of the rotational molding apparatus. By implementing the liquid heated and cooled system, this energy consumption per batch reduces to 25.2 MJ, much of it due to the elimination of the need to heat and cool the auxiliary mass. In addition, nearly 27% of the energy input for the jacket heating cycle (25.2 MJ) is recovered through recuperation from the cooling process and used for pre-heating the next batch. Finally, the waste heat recovery scheme enables recovery of 200 kg of water at 56oC to be used in other processes in the plant.