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1.4 Historical Background

Die casting is relatively speaking a very forgiving process, in which after tinkering with the several variables one can obtain a medium quality casting. For this reason there has not been any real push toward doing good research. Hence, all the major advances in the understanding of the die casting process were not sponsored by any of die casting institutes/associations. Many of the people in important positions in the die casting industry suffer from what is know as the ``Detroit attitude'' which is very difficult to change. ``We are making a lot of money so why change?''. Moreover, the controlling personnel on the research funds believe that the die casting is a metallurgical manufacturing process and therefore the research has to be carried out by either Metallurgical Engineers or Industrial Engineers. Furthermore, this come as no surprise - that people-in-charge of the research funding fund their own research. One cannot wonder if there is a relationship between so many erroneous models which have been produced and the personnel controlling the research funding. A highlight of the major points of the progress of the understanding is described herein.

The vent system design requirements were studied by some researchers, for example Suchs, Veinik, and Draper and others. These models, however, are unrealistic and do not provide a realistic picture of the real requirements or of the physical situation since they ignore the major point, the air compressibility. Yet, they provide a beginning in moving the die casting process towards being a real science.

One of the secrets of the black art of design was that there is a range of gate velocity which creates good castings depending on the alloy properties being casted. The existence of a minimum velocity hints that a significant change in the liquid metal flow pattern occurs. Veinik poro:veinik1 linked the gate velocity to the flow pattern (atomization) and provide a qualitative physical explanation for this occurrence. Experimental work [#!poro:maier!#] showed that liquid metals, like other liquids, flow in three main patterns: a continuous flow jet, a coarse particle jet, and an atomized particle jet. Other researchers utilized the water analogy method to study flow inside the cavity for example, [#!poro:firstWateranalogy!#]. At present, the (minimum) required gate velocity is supported by experimental evidence which is related to the flow patterns. However, the numerical value is unknown because the experiments were poorly conducted for example, [#!poro:stuhrke!#] the differential equations that have been ``solved'' are not typical to die casting.

In the late 70's an Australian group [#!poro:davis75!#] suggested adopting the diagram for die casting in order to calculate the gate velocity, the gate area and other parameters. As with all the previous models they missed the major points of the calculations. As will be shown in Chapter [*], the Australian's model produce incorrect results and predict trends opposite to reality. This model took root in die casting industry for the last 25 years. Yet, one can only wonder why this well established method (supply and demand theory) which was introduced into fluid mechanics in the early of this century reached the die casting only in the late seventies and was then erroneously implemented.

Until the 1980 there was no model that assisted the understanding air entrapment in the shot sleeve. Garber described the hydraulic jump in the shot sleeve and called it the ``wave'', probably because he was not familiar with this research area. He also developed erroneous model which took root in the industry in spite of the fact that it never works. One can only wonder why the major die casting institutes/associations have not published this fact. Moreover, NADCA and other institutes continue to funnel large sums of money to the researchers (for example, Brevick from Ohio State) who used Garber's model even after they knew that Garber's model is totally wrong.

The turning point of the understanding was when Prof. Eckert, the father of modern heat transfer, introduced the dimensional analysis applied to the die casting process. This established a scientific approach which provided a uniform schemata for uniting experimental work with the actual situations in the die casting process. Dimensional analysis demonstrates that the fluid mechanics processes, such as filling of the cavity with liquid metal and evacuation/extraction of the air from the mold, can be dealt when the heat transfer is assumed to be negligible. However, the fluid mechanics has to be taken into account in the calculations of the heat transfer process (the solidification process).

This proved an excellent opportunity for ``simple'' models to predict the many parameters in the die casting process, which will be discussed more fully in this book. Here two examples of new ideas that mushroomed in the inspiration of prof. Eckert's work. It has been shown that [#!poro:genickreac!#] the net effect of the reactions is negligible.The development of the critical vent area concept provided the major guidance for 1) the designs to the venting system 2) when the vacuum system needs to be used. In this book, many of the new concepts and models, such as economy of the runner design, plunger diameter calculations, minimum runner design, etc are described for the first time.


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