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7.4.2.1.3 effects

The change in the gate area increases the resistance to the flow via several contributing factors which include: the change in the flow cross section, change in the direction of the flow, frictional loss due to flow through the gate length, and the loss due to the abrupt expansion after the gate. The loss due to the abrupt expansion is a major contributor and its value changes during the filling process. The liquid metal enters the mold cavity in the initial stage as a ``free jet'' and sometime later it turns into an immersed jet which happens in many geometries within 5%-20% of the filling. The change in the flow pattern is believed to be gradual and is a function of the mold geometry. A geometry with many changes in the direction of the flow and/or a narrow mold (relatively thin walls) will have the change to immersed jet earlier. Many sources provide information on for various parts of the designs of the runner and gate. Utilizing this information produces the gate velocity as a function of the given geometry. To study further this point consider a case where is a simple function of the gate area. = 90 true mm
Figure: as a function of gate area, A3
When is very large then the effect on are relatively small. Conversely, when the resistance, . The simplest function, shown in Figure [*], that represent such behavior is
    (7.25)

and are constants and can be calculated (approximated) for a specific geometry. The value of determine the value of the resistance where effect is minimum and determine the range (point) where plays a significant effect. In practical, it is found that is in the range where gate area are desired and therefore program such as DiePerfect are important to calculated the actual resistance.




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