EXPERIMENTAL-STUDY OF HIGH REYNOLDS-NUMBER FLOWS BETWEEN NARROW GAPS

High Reynolds' number flows between narrow gaps are very common in the reaction injection mold filling process (RIM) where low viscosity polymers are injected into thin mold cavities in relatively short time intervals. The resulting short cycle times have given RIM advantages over other manufacturing techniques in the polymer industry. In addition to the lower temperatures encountered during the RIM process, the low material viscosities enables mold filling using smaller injection pressures and mold clamping forces than in the conventional thermoplastic injection mold filling process. Considering the low viscosity of the injected fluid, short injection times and considerably large parts, such as automotive body panels, it is clear that in RIM processes inertia effects are not negligible. In fact Reynolds' numbers in such processes can reach 10 and above. This paper presents an experimental study performed on the high Reynolds' number flows between narrow gaps. First, the general equations that govern such flows are derived and written in dimensionless form. The magnitudes and influence of inertia forces, surface tension and viscous forces are analyzed in terms of Reynolds' number, capillary number and Weber number. The theoretical background is followed by a short explanation of the experimental set-up. Several experiments are shown in different mold configurations and at various mold filling speeds. The results showed that inertia greatly influences the mold filling pattern and capillary effects are a major cause of air bubble entrapment.