Page 319 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

2 8 Chapter 12 Metal Casting: Design, Materials, and Economics

Selecting the Casting Process. Casting processes cannot be selected separately
from economic considerations (see Section 12.4). However, Table 11.1 lists some of
the advantages and limitations of casting processes that have an impact on casting
design.

Locating the Parting Line. A part should be oriented in a mold so that the large
portion of the casting is relatively low and the height of the casting is minimized.
Part orientation also determines the distribution of porosity. For example, in casting
aluminum, hydrogen is soluble in liquid metal, but is not soluble as the aluminum
solidifies (see Fig. 1O.15). Thus, hydrogen bubbles can form during the casting of
aluminum, float upwards due to buoyancy, and cause a higher porosity in the top
parts of castings. Therefore, critical surfaces should be oriented so that they face
downwards.
A properly oriented part then can have the parting line specified. The parting
line is the line or plane separating the upper (cope) and lower (drag) halves of molds
(see Fig. 11.4). In general, the parting line should be along a flat plane rather than be
contoured. Whenever possible, the parting line should be at the corners or edges of
castings rather than on flat surfaces in the middle of the casting, so that the flash at
the parting line (material squeezing out between the two halves of the mold) will not
be as visible. The location of the parting line is important because it influences mold
design, ease of molding, number and shape of cores required, method of support,
and the gating system.
The parting line should be placed as low as possible (relative to the casting) for
less dense metals (such as aluminum alloys) and located at around mid-height for
denser metals (such as steels). However, the metal should not be allowed to flow verti-
cally, especially when unconstrained by a sprue. The placement of the parting line has
a large effect on the remainder of the mold design. For example, in sand casting, it is
typical that the runners, gates, and sprue well are placed in the drag on the parting line.
Also, the placement of the parting line and orientation of the part determine the num-
ber of cores needed, and it is preferable to avoid the use of cores whenever practical.

Locating and Designing Gates. Gates are the connections between the runners and
the part to be CHSI. Some of the considerations in designing gating systems are as
follows:
° Multiple gates often are preferable and are necessary for large parts. Multiple
gates have the benefits of allowing lower pouring temperature and reducing
the temperature gradients in the casting.
° Gates should feed into thick sections of castings.
° A fillet should be used where a gate meets a casting; this feature produces less
turbulence than abrupt junctions.
° The gate closest to the sprue should be placed sufficiently far away from the
sprue so that the gate can be easily removed. This distance may be as small as a
few millimeters for small castings and up to 500 mm for large parts.
° The minimum gate length should be three to five times the gate diameter,
depending on the metal being cast. The gate cross section should be large
enough to allow the filling of the mold cavity and should be smaller than the
runner cross section.
° Curved gates should be avoided, but when they are necessary, a straight section in
the gate should be located immediately adjacent to the casting.

Runner Design. The runner is a horizontal distribution channel that accepts
molten metal from the sprue ancl delivers it to the gates. Une runner is used for sim-
ple parts, but two-runner systems can be specified for more complicated castings.