Simple visualization techniques for die casting part and die design. Final report, July 1, 1995--September 30, 1997 Page: 16 of 220
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for smooth metal flow will yield better overall castings (NADCA 1992; Ramalingam,
Zhao, 1987).
Past experience is the principal basis for design of die castings and die casting dies.
However, geometric parameters can be used to improve current industrial practice with
respect to filling (Ramalingam, Zhao, 1987) and solidification.-
In the next section, geometric characteristics drawn from the literature and/or used by
experienced die casters to prevent defects will be identified. The correlation of these
geometric characteristics with manufacturing problems and an efficient approach to
extract the geometric data from a voxel model will be discussed
2.3 Geometric Characteristics Related to Defects
2.3.1 Thermal Mass and Pre-mature Solidification Related Problems
The production rate, among other things, depends on the "thickness" of a casting (Barton,
1981). In general, wall thickness is not directly responsible for meeting the functional
requirements including strength. The skin layer of a casting, formed at the initial contact
where the metal splashes against the die surface, has stronger mechanical properties than
the inner bulk portions of the casting. This is due to its finer grain structure and lower
porosity. From this perspective, in cases where a thin wall cannot provide adequate
strength, it is better to use strengthening ribs rather than increasing the wall thickness
(NADCA DDC, 1988; Barton, 1981). On the other hand, if the wall is too thin, metal
may not have enough time to completely fill the cavity before it freezes. Therefore, the
wall thickness of a part should be designed so that it promotes good metal flow, but at the
same time does not unnecessarily delay the solidification.
From a qualitative point of view, wall thickness should be kept as uniform as possible.
Abrupt changes in wall thickness may result in isolated thermal masses and pre-mature
solidification that can cause casting defects such as shrinkage voids. This scenario is
demonstrated in Figure 2.1 (Herman, 1979).
Pre-mature solidification is responsible for many die casting defects, including
incomplete fill and woody areas. A thin section of a part contains a relatively small
amount of heat and hence may solidify earlier than surrounding regions. The pre-mature
freezing may block the metal being pushed into the rest of the casting to fill it completely
or to compensate for shrinkage. The presence of non-uniformly thin sections can also
cause a perturbation in the metal flow and thus increase the amount of turbulence and
entrapped gas. From the perspective of castability, relatively thin sections can cause as
many difficulties as the isolated heavy sections. They both significantly complicate die
design and process control.
There may be a quantitative range of preferable wall thickness for different die casting
alloys, depending on the size of the machine a company possesses. Within this range, a
part can be manufactured most economically with standard die casting practice. Thick
parts usually incur a longer cycle time. A casting can be regarded as too thick when its
cycle time is increased to the point where die casting is no longer the most economical
manufacturing process. Conversely, a large machine is usually needed to die cast a thin7
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Miller, R.A.; Lu, S.C. & Rebello, A. B. Simple visualization techniques for die casting part and die design. Final report, July 1, 1995--September 30, 1997, report, May 1, 1998; Columbus, Ohio. (https://digital.library.unt.edu/ark:/67531/metadc690873/m1/16/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.