Such “holes” cannot be large. T e limit on the size of a hole in the NTA is probably 10 to 20 metal thicknesses in diameter. But even with the size restriction, a hole can be signifi cant and might justify being specifi ed on occasion. Because of the composite nature
Fig. 6: The casting must have a datum system for each die member and the datum systems must be related to each other with dimensions and tolerances.
always happen, even when it should for the function of the casting. Figure 8 shows how casting shape can cause the NTA to be on the casting surface or even outside the casting, not in the middle of the wall. When the NTA is at the surface or outside the casting, the porosity will be on the casting surface. T e position of the NTA can
be strongly infl uenced by the local temperatures in the adjacent die steels. T e illustration in Figure 7 implies the casting solidifi cation pattern results in the solidifying fronts all meeting at the NTA at the same time, through- out the casting. T at might or might not happen. For example, if both die halves were hotter on the left end of the casting in the fi gure and colder on the right end, the casting would solidify fi rst on the right. T e solidifying fronts would meet fi rst at the right end, and the meeting would move progressively to the left. T e right end of the casting would be denser with less porosity than the left end. T e ramifi cations of the NTA and
how to manage and specify it are be- yond the scope of this article. T e NTA can be manipulated by design, but some practical limits cannot be exceeded. T e product designer must learn how cast- ing features, die features and process controls interact to aff ect the NTA. T ink about where porosity can be detrimental to the function of the cast- ing and how to design the casting so its shape will naturally move porosity away from where it would hurt the casting’s function. A designer might even specify the location of the NTA and include it in the product CAD data. But, if the
diecaster does not design and control the die according to the NTA strategy of casting design, the desired perfor- mance will not be achieved.
Casting, Die & Process Controls Interact as a Single System
Identify regions within the casting
where large amounts of porosity will not hurt the function of the casting. Such areas could be outlined with an envelope surface (within the casting) and the region within the envelope specifi ed to be “exempt” of X-ray poros- ity specifi cation. T e NTA can have holes. T ese
holes are places that are forced (by die cooling design and control) to solidify signifi cantly before the surrounding casting and are nearly porosity free.
of the cast structure, standard test specimens and procedures usually are too small to test a casting’s material adequately. As a result, most cast- ings perform better and much more consistently than the test data from specimens cut from the castings would lead one to expect.
Perspective
A successful die casting is one that has been designed, deliberately or by chance, to be within the capability of the diecaster. T ere is no ideal cast- ing design. T ere is, however, an ideal system for casting design to die design process control. T at system can be primitive or sophisticated and still be ideal, if it works. Designing a cast- ing that exceeds the capabilities of the diecasting supplier is asking for trouble. Similarly, a design needing only basic diecasting could be too expensive if it is produced by a sophisticated diecaster.
For more information on designing die castings, see
www.diecastingdesign.org or
www.diecasting.org/education/ webinars/
schedule.htm.
Fig. 7: A die casting has a dense fi ne grained skin and a coarse grained and porous core.
Fig. 8: The neutral thermal axis is not always in the center of the wall thickness. Nov/Dec 2013 | METAL CASTING DESIGN & PURCHASING | 35
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