Impingement mixing - a fluid mechanical approach.

Reaction injection molding (or RIM) is a process in which two liquid resins are rapidly mixed and delivered to a mould, where they react and cure to form a part. RIM is used to make items such as automobile fascia and business machine housings. The mixing part of the process is usually carried out by an impingement mixer. This is simply a small chamber connected to a runner or directly to the mould cavity, in which streams of the fluids to be mixed impinge. The impingement process results in turbulence and mixes the fluids. The advantages of the impingement mixer are that it can mix large amounts of viscous liquids very quickly, and can easily be cleaned of the reacting mixture afterwards. flow rates may be as high as 17 lbs/sec., and viscosities are usually in the range of 500 centipoise. At present, there is very little understanding of the impingement mixing process, and mixers must be designed on a purely empirical basis. This paper is an attempt to understand the process in a fundamental way. It is hoped that such knowledge will prove useful in the design and scale-up of equipment, and also in approaching new problems such as the reaction molding of reinforced materials. It is concluded that the mixing quantity of an impingement mixer increases continuously with Reynolds Number and decreases with increasing mixer size. Momentum ratio seems to have very little effect when the fluid delivery scheme does not permit crossfeed. A simple theory based on dimensional analysis and the Kolmogoroff micro-scale of turbulence is presented to show how mixing quality might scale with Reynolds Number. Experimental data are in good agreement with the theory. (from paper)