1.2 Designed/Undesigned Scrap/Cost

There are many definitions of scrap. The best definition suited to the die casting industry should be defined as all the metal that did not turn-out to be a product. There are two kinds of scrap/cost: 1) those that can be eliminated, and 2) those that can only be minimized. The first kind is referred to here as the undesigned scrap, and the second is referred as designed scrap. What is the difference? It is desired not to have rejection of any part (the rejection should be zero) and, of course it is not designed and this is the undesigned scrap/cost. However, it is impossible to eliminate the runner completely, and it is desirable to minimize its size in such a way that the cost will be minimized. This is minimization of cost and this is the designed scrap/cost. The die casting engineer must distinguish between these two scrap components in order to be able to determine what should be done and what cannot be done.

Science can make a significant difference; for example, it is possible to calculate the critical slow plunger velocity and thereby eliminating (almost) air entrainment in the shot sleeve in order to minimize the air porosity. This means that air porosity will be reduced and marginal products (even poor products in some cases) are converted into good quality products. In this way, the undesigned scrap is eliminated. This topic will studied further in Chapter .

Two different examples of designed scrap/cost and undesigned scrap/cost have been presented. There is also the possibility that a parameter which reduces the designed scrap/cost will, at the same time, reduce the undesigned scrap/cost. An example of such a parameter is the venting system design. It can easily be shown that there is a critical design below which air/gas is exhausted easily and above which air is trapped. In the later case, the air/gas pressure builds up and results in a poor casting (large amount of porosity).. The analysis of the vent system demonstrates that a design much above the critical design and design just above the critical design yielding has almost the same results,- small amount of air entrainment. One can design the vent just above the critical design so the design scrap/cost is reduced to a minimum amount possible. Now both targets have been achieved: less rejections (undesigned scrap) and less vent system volume (designed scrap).