MICROCELLULAR SHEET EXTRUSION SYSTEM PROCESS DESIGN MODELS FOR SHAPING AND CELL-GROWTH CONTROL

The feasibility of shaping a nucleated polymer/gas solution represents a significant advancement for microcellular plastics process technolo gy. Through proper design of the foaming die, nucleated solution flows can be shaped to arbitrary dimensions while maintaining the functiona l independence of cell nucleation, cell growth and shaping. To maintai n functional independence, stringent pressure and temperature design s pecifications, which supersede those of conventional foam processing, must be met by the foaming die design. As a means of aiding the design process, a model is developed for predicting pressure losses and flow rates of nucleated polymer/gas solutions. A comparison of the model p redictions and the actual foaming die design performance shows good ag reement for limited data. These relatively simple models capture the m ajor physics of the complicated two-phase now field and provide a soun d base from which scale-up of the foaming die concept can be achieved. The nucleated polymer/gas solution flow models predict highly nonline ar volumetric flow rates contrasting constant flow rates predicted for the neat polymer now. In addition, a convenient method for classifyin g nucleated polymer/gas solution flow is presented based on a dimensio nless ratio of the characteristic flow rate to the characteristic gas diffusion rate.