Fig. 3.4 Suggested design modification to avoid abrupt section changes: a) incorrect; b) and c) correct.
Minimize the number of sections. The goal is to design the casting well with a minimum numberof sections together at one point. A simple wall section will cool freely from all surfaces, but simply adding a section forming a T or intersect shape also creates a hot spot at the junction, and it will cool as would a wall that was 50% larger.
The way to avoid this is to stagger the ribs (Fig. 3.5b) and thereby maintain uniform cross-sections. Staggering sections minimizes hot-spot effects, thus eliminating weakness and reducing distortion.
When two or more uniform sections intersect they create a region of heavy cross-section, resulting in the problems mentioned earlier. One way to minimize this is to core the intersection by a hole, similar to a hub hole in a wheel with spokes (Fig. 3.5c).
Fig. 3.5 Schematic illustrations of minimize number of section: a) incorrect; b) correct (stagger section); c) correct (core intersection by a hole).
Flat areas. Large flat areas (plane surfaces) should be avoided. They are difficult to feed and it is difficult to develop in them good directional solidification, and may wrap or develop poor finish surface because of uneven metal flow. Lightener holes eliminate the problem of isolated flat areas; they also save weight. But they need to be placed in “nonstructural” areas (Fig. 3.6).
Fig. 3.6 Suggested design modification to avoid flat areas: a) incorrect; b) correct.
Shrinkage. Molten metals contract during cooling. To avoid thermal stress and cracking of the casting during cooling, the cavity is usually made oversized. To account for shrinkage, pattern dimensions should also allow for shrinkage during solidification. These shrinkage allowances are only approximate, because the exact allowance is determined by the shape and size of the casting. Table 3.1 gives the approximate shrinkage allowance for metals that are commonly sand cast.
Table 3.1 Shrinkage allowance for some metals cast in sand molds
Draft. Draft is the amount of taper, or the angle, that must be allowed on all vertical faces of a pattern to permit its removal from the sand mold without permitting tearing of the mold walls; draft is also an issue in the removal of a casting from a die. (Fig. 3.7). The standard draft for sand casting is 1 degree per external side. The angles on the inside surfaces typically are twice this range.
Fig. 3.7 Draft angle: a) incorrect; b) correct.
Dimensional tolerances. Dimensional tolerances depend on the particular casting processes, row casting basic dimension, casting materials, and casting tolerance grades. The International Standard (ISO) is defined for16 casting tolerance grades designated CT1 to CT16. Table 3.2 gives dimensional casting tolerances for sand casting, machined molded.
Machining allowance. On raw castings, some extra material should be left to permit the removal of the effects of casting on the surface by subsequent machining and to allow the achievement of the desired surface texture and the necessary agreement with design dimensions.
Part identification. In the final cast drawing, the design engineer in final cast drawing needs to include some form of part identification. These features can be sunk into the casting or can protrude from the surface.
b) Locating the Parting Line
Parting in one plane facilitates the production of the pattern as well as the production of the mol. The term parting line is a bit of a misnomer. Perhaps the term parting surface would be better. However, the first term is commonly used, which is why we retain the term parting line in this book. Parting lines are the lines on the component where the mold or die halves come together. Parting in one plane facilitates the production of the pattern as well as the production of the mold.
Table 3.2 Dimensional Tolerances for Sand Casting, Machine Molded.
* The tolerance zone should be symmetrically disposed with respect to a basic dimension, i.e. with one half on the positive side and one half on the negative side
Patterns with straight parting lines (that is, with parting lines in one plane) can be produced more easily and at lower cost than those with irregular parting lines.
Casting shapes that are symmetrical about one centerline or plane readily suggest the parting line. Such casting design simplifies molding and coring and should be used wherever possible. The patterns should always be made as split patterns, which require a minimum of handwork in the mold, improve casting finish, and reduce costs.
Parting lines that cross a feature with tight tolerances may lead to decreased yield due to mold mismatch. Since parting lines will be noticeable as a bump on the surface of the part, they should not be located on a sliding surface. Similarly, parting lines should not be located on a sealing surface, where a bump or mismatch would prevent the seal from making complete contact. The placement of the parting line and orientation of the part determine the number of cores needed, and it is preferable to avoid the use of cores whenever possible.
c) Location and Design of a Gating System
The primary requirement for the gating system is that it accomplishes an extremely careful transport of the liquid metal to the mold cavity. Gating system location is defined on the basis of the geometry of the mold, method of casting, and economic aspects.
The gating system guides the poured liquid metal, prepared in the foundry, to the mold cavity and has direct and indirect effects on the quality of the casting process. Redundant the designer and the foundry team should take these aspects into account right from the design phase in order to guarantee trouble-free processing and constant casting quality.
In the design of a gating system the focus is on the casting part. Factors that influence the design of the gating system include:
•size
•targeted surface quality
•mechanical requirements
•complexity of the geometry
•economic efficiency.
The gating system can be divided into the following:
•pouring cup
•sprue
•runner
•riser
•gate.