| Asymmetric flow in a naturally-balanced runner system was investigated using a selected group of polymeric materials and two runner designs. As in previous studies, the asymmetric filling of the cavities depended on the material, injection velocity, and runner design, reaching a stable value at high shear rates. In short shots, however, symmetric flow was observed after the first and second tee junctions, while asymmetric flow occurred after the 90° corners with some materials and in the cavities with others. Although flow fronts measured at the tee junctions and corners exhibited preferential flow along the outside walls of the runners, the flow fronts became symmetric well before the next junction in the runners.; Process anomalies, including runner and vent dimensions, mold surface roughness in the primary and secondary runners, mold temperature, and gravity (i.e., mounting position of the mold) had little effect on the asymmetric flow. Rheological phenomenon, including wall slip effects, melt elasticity, shear thinning, shear heating, melt viscosity, and corner effects had far more influence on the asymmetric filling of naturally-balanced runners. The asymmetric flow appeared to have two regimes. At high shear rates, the degree of asymmetric flow increased with melt viscosity and the temperature sensitivity of the melt viscosity. Overall the level of asymmetric flow correlated with the logarithm of the Deborah number, power law index, and activation energy for flow. The ability of a melt to slip along or adhesion to the mold walls also influenced asymmetric flows, with slip effects being more pronounced at higher shear rates.; These results suggest that flow asymmetry arises from the flow of the melt through the three turns in the runner system. The symmetric splitting of the two tee junctions, itself, does not seem to produce the asymmetric flow. As the materials exit the 90° corner, however, the slippage along the cavity walls may asymmetrically accelerate the melt while melt elasticity may force the polymer to the cavity walls. The balance of these forces and internal shearing of the polymer melt may determine whether asymmetric flow occurs in the corner or arises from expansion of the melt in the cavity. |
