The resistance in the bend is created because a change in the momentum and the flow pattern. Engineers normally convert the bend to equivalent conduit length. This conversion produces adequate results in same cases while in other it might introduce larger error. The knowledge of this accuracy of this conversion is very limited because limited study have been carry out for the characteristic of flows in die casting. From the limited information the author of this book gadered it seem that it is reasonable to carry this conversion for the calculations of liquid metal flow resistance while in the air/liquid metal mixture it far from adequate. Moreover, ``hole'' of our knowledge of the gas flow in vent system are far more largebad english, change it please. Nevertheless, for the engineering purpose at this stage it seem that some of the errors will cancel each other and the end result will be much better.
In the light of the for going discussion, we left with two parameters that control the resistance, the angle, , and the radius, As larger the angle is larger the resistance will be. In the practice today, probably because the way the North American Die Casting Association teaching, excessive angle can be found through the industry. It is recommended never to exceed the straight angle (). Figure made from a data taken from several sources. From the Figure it is clear that optimum radius should be around 3to continue after the figure finished