PRELIMINARY MODEL FOR RIM IMPINGEMENT MIXING.

Impingement mixing of fast polymerizing reactants is the unique aspect of the RIM (reaction injection molding) process. Mixing is essential to the sucess of the process but relatively little basic work has been done on impingement mixing. Present mixheads are designed by trial and error. In actual RIM processing mixing quality is usually judged by visual appearance of a molded part. This interpretation has proven effective for mixhead evaluation but can be confounded by imbalanced stoichiometry, particularly at the beginning or end of a shot (leadlag), and possibly by air entrainment during molding. The previous results show that Reynolds number is the most important variable in impingement mixer performance, but the functional relation between mixing. Re (Reynolds number) and mixhead design is not yet clear. In this study the authors present further results primarly using the adiabatic temperature rise method. They also develop a preliminary model for impingement mixing which includes aspects of both the fluid flow and the polymerization. It is concluded from mathematical analysis that the preliminary impingement mixing model, the combination of local stretching flow and fast, lamellar polymerization, has the virtue of simplicity and seems to give the correct magnitude for striation thickness and correct trends with mixhead geometry and Re dependence. However it cannot quantitatively model the adiabatic temperature rise curves. On the other hand, the adiabatic temperature rise method is a simple and quantitative measure of mixing quality. It should be very useful in practical RIM studies. More quantitative modeling of these data will require measurements of D//I and C//I and some understanding of their dependence on reaction conditions like temperature and viscosity.