Balanced runners

This process uses the plasticising and heat advantages of the injection unit to impart good flow properties to the rubber mix. It also offers the advantages of the flexibility of the transfer layout without the sprue and runners of the balanced runner system required by injection moulding. The space used by runners in other systems can be profitably used by more mould cavities. 

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. 

Cavity layout will determine that of the runner system. For best operation, the cavities need to fill evenly and at the same time. Uneven filling often results in components that are light and with runner systems that are heavily webbed with thick flash, which is the result of high pressure build up before filling is complete. Each cavity must have a runner path length that is identical, or balanced . Some cavity layouts have a runner system that will be inherently balanced, whilst others will need to be carefully designed to ensure balance. An example plan view of a balanced runner system is shown in Figure 5.1. 

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. 

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. 

In multicavity molds where close part tolerances are required, a balanced runner system is extremely important to achieve part-to-part uniformity. Fig. 7-3 depicts some multicavity runner systems. 

The runner system is used to convey the plastic melt from the sprue to the gate. It is important in producing identical quality of plastic parts. Cost reduction can be carried out by using multi-cavity mold with a balanced runner system and the mold fills in the cavity at the same time. The runners deliver the plastic into the part cavities (usually multiple). 

Runner layouts should be designed to deliver the plastic melt at the same time and at the same temperature, pressure, and velocity to each cavity of a multicavity mold. Such a layout is known as a balanced runner (Fig. 7.15). A balanced runner will usually consume more material than an unbalanced type, but this disadvantage is outweighed by the improvement in the uniformity and quality of the moldings. Balance in a multi-cavity mold with dissimilar cavities (known as a family mold) can be achieved by careful variation of runner diameter in order to produce equal pressure drops in each flow path. Such balancing can only be achieved efficiently by the use of computer flow simu-... 

ISO 3167 specifically deals with the production of multipurpose test specimens. It requires the use of a balanced, two-cavity mold with a defined gate design to prepare test specimens used to measure properties of plastics materials. Two identical cavities with large gates and a balanced runner system promotes filling and packing consistently and nniform orientation, minimizes shear effects, and produces uniform specimen. Figure 20-2 illustrates differences between old style, multicavity, unbalanced family mold and ISO test specimens (6). 

Figure 4.7 Balanced runner layouts melt has the same length of travel for all impressions...

Sometimes, however, it is not always practieal to have a balanced runner system (family monlds etc.). In these cases uniform filling of the cavities ean be achieved by varying the ruimer and gate dimensions as shown in Figure 4.8. 

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. 

Mold Filling Imbalances in Geometrically Balanced Runner Systems ANTEC 98. John P. Beaumont, Jack H. Young, Matthew J. Jaworski... 

Mold Filling Imbalances in Geometrically Balanced Runner Systems ANTEC 03... 

Since this reverse comer effect imbalance still deals with shear, it is important to understand the sensitivity of the filling imbalance to injection flow rate along with the sensitivity of different materials to these mold filling imbalances in a geometrically balanced runner. 

Beaumont J.P., Young J.H. Jaworski M.J. Mold Filling Imbalances in Geometrically Balanced Runner Systems, ANTEC, pp 599-604, 1998. 

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