Balancing the Runner System in Multi-Cavity Injection Molds

1 Balancing the Runner System in Multi-Cavity Injection Molds 

Inevitably, in multi-cavity injection molds, some of the mold cavity gates are located further than others from the sprue that delivers the melt from the plasticating unit. If the runner system that distributes the melt from the sprue to the individual cavities has a constant 

Cumulative residence time distribution for a stirring tank with perfect mixing. 

Schematic diagram of an eight cavity mold system with various runner lengths and 

Sample balancing problem. Let us consider the multi-cavity injection molding process shown in Fig. 6.54. To achieve equal part quality, the filling time for all cavities must be balanced. For the case in question, we need to balance the cavities by solving for the runner radius R2. For a balanced runner system, the flow rates into all cavities must match. For a given flow rate Q, length L, and radius R, solve for the pressures at the runner system junctures. Assume an isothermal flow of a non-Newtonian shear thinning polymer. Compute the radius R2 for a part molded of polystyrene with a consistency index (m) of 2.8 x 104 Pa-s and a power law index (n) of 0.28. Use values of L = 10 cm, R = 3 mm, and Q = 20 cm3/s. 

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