Mold runner balanced

For multi-impression molds, a balanced runner design is preferred generous radii should be incorporated at each runner junction so as to prevent excessive shear heating to the material during mold filling and/or, causing unnecessary pressure drops to occur. 

Balancing the Runner System in Multi-Cavity Injection Molds... 

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. 

Write a ID FEM program using 2-noded tube elements to balance complex runner systems in injection molding. Compare the simulation to the runner system presented in Chapter 6. 

When the mold contains more than one identical mold cavity, it is important that the cavities fill equally. The usual approach to accomplishing this is to balance the flow paths for the plastic, so that distances and geometry, and thus pressure and flow, are equalized. Where nonidentical objects are being produced, this job is even more complex, but that seldom applies in packaging applications. It is also important to design the runner geometry to avoid dead spots, where plastic can accumulate and be subjected to an excessive heat history. 

Balanced runners n. In a multicavity injection mold, the runners are balanced when the injected melt reaches all the cavity gates at the same instant after the start of injection. In practice, with identical cavities whose shape, size, number, and layout permit, all runner branches are given equal cross sections and corresponding branch lengths are made equal. Uniform metal temperature throughout is assumed. 

In 1978, Moldflow introduced commercial software on a worldwide computer time-sharing system. This software enabled users to determine process conditions (melt temperature, mold temperature, and injection time) and to balance flow in cavities and runner systems. Although accepted at the time, this software was difficult to use as it required the user to produce a layflat. ... 

Although runners should deliver melt to each cavity at the same time, they may be naturally balanced or artificially balanced. In naturally balanced runners the distance from sprue to all gates is the same, whereas artificially balanced runners have the same pressure drop from sprue to all gates. Cold runner molds also incorporate cold slug wells at each turn in the melt flow. These collect the cold melt from the sprue plug (melt frozen at the nozzle), thereby preventing this melt from entering the cavities. 

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