Page 320 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 12.2 Design Consideratnons in Castmg 2

Runners are used to trap dross (a mixture of oxide and metal that forms on the sur-
face of metals) and keep it from entering the gates and mold cavity. Commonly,
dross traps are placed at the ends of runners, and the runner projects above the gates
to ensure that the metal in the gates is tapped from below the surface.

Designing Other Mold Features. The main goal in designing a sprue (described in
Section 10.3) is to achieve the required metal flow rates while preventing aspiration
or excessive dross formation. Flow rates are determined such that turbulence is
avoided, but the mold is filled quickly compared to the solidification time required.
A pouring basin can be used to ensure that the metal flow into the sprue is uninter-
rupted; also, if molten metal is maintained in the pouring basin during pouring, then
the dross will float and will not enter the mold cavity. Filters are used to trap large
contaminants, and these also serve to reduce the metal velocity and make the flow
more laminar. Chills can be used to speed solidification of the metal in a particular
region of a casting.

Establishing Good Practices. It has been observed widely that a given mold design
can produce acceptable parts as well as defective ones and rarely will produce only
good or only defective parts. To check for defective castings, quality control proce-
dures are necessary. Some common concerns are the following:

° Starting with a high-quality molten metal is essential for producing superior
castings. Pouring temperature, metal chemistry, gas entrainment, and handling
procedures all can affect the quality of metal being poured into a mold.
° The pouring of metal should not be interrupted, since this can lead to dross
entrainment and turbulence. The meniscus of the molten metal in the mold cav-
ity should experience a continuous, uninterrupted, and upward advance.
° The different cooling rates within the body of a casting cause residual stresses.
Stress relieving (Section 4.11) thus may be necessary to avoid distortions of
castings in critical applications.

I2.2.2 Design for Expendable-mold Casting
Expendable-mold processes have certain specific design considerations, attributable
mainly to the mold material, size of parts, and manufacturing method. Clearly, a
casting in an expendable-mold process (such an investment casting) will cool much
more slowly than it would in, say, die casting, which has important implications in
the layout of molds.
Important design considerations for expendable-mold casting are the following:

Mold Layout. The features in the mold must be placed logically and compactly,
with gates as necessary. One of the most important goals in mold layout is to have
solidification initiate at one end of the mold and progress in a uniform front across
the casting, with the risers solidifying last. Traditionally, mold layout has been based
on experience and on considerations of fluid flow and heat transfer. More recently,
commercial computer programs have become available that assist in fluid flow and
heat transfer analyses. These programs simulate mold filling and allow the rapid
evaluation of mold layouts.

Riser Design. A major concern in the design of castings is the size and placement
of risers. Risers are extremely useful in affecting the solidification-front progression
across a casting and are an essential feature in the mold layout described previously.