Page 280 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 11.1 Introduction 259

TABLE I I.l
Summary of Casting Processes
Process Advantages Limitations
Sand Almost any metal can be cast; no limit to part size, Some finishing required; relatively coarse
shape, or Weight; low tooling cost surface finish; Wide tolerances
Shell mold Good dimensional accuracy and surface finish; Part size limited; expensive patterns and
high production rate equipment
Evaporative pattern Most metals can be cast, with no limit to size; Patterns have low strength and can be
complex part shapes costly for low quantities
Plaster mold Intricate part shapes; good dimensional accuracy Limited to nonferrous metals; limited
and surface finish; low porosity part size and volume of production;
mold-making time relatively long
Ceramic mold Intricate art sha es' close-tolerance P arts; Limited part size
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good surface finish
Investment Intricate part shapes; excellent surface finish Part size limited; expensive patterns, molds,
and accuracy; almost any metal can be cast and labor
Permanent mold Good surface finish and dimensional accu- High mold cost; limited part shape and
racy; low porosity; high production rate complexity; not suitable for high-melting-
point metals
Die Excellent dimensional accuracy and sur- High die cost; limited part size; generally
face finish; high production rate limited to nonferrous metals; long lead
time
Centrifugal Large cylindrical or tubular parts with Expensive equipment; limited part shape
good quality; high production rate

materials, pattern production, molding processes, and methods of feeding the mold
with molten metal. The major categories are as follows:

l. Expendable molds, which typically are made of sand, plaster, ceramics, and
similar materials and generally are mixed with various binders (bonding agents)
for improved properties. A typical sand mold consists of 90% sand, 7% clay,
and 3% Water. As described in Chapter 8, these materials are refractories (that
is, they are capable of withstanding the high temperatures of molten metals).
After the casting has solidified, the mold is broken up to remove the casting.
The mold is produced from a pattern; in some processes, such as sand and
shell casting, the mold is expendable, but the pattern is reused to produce several
molds. Such processes are referred to as expendable-mold, permanent-pattern
casting processes. On the other hand, investment casting consumes a pattern
for each mold produced; it is an example of an expendable-mold, expendable-
pattern process.
2. Permanent molds, which are made of metals that maintain their strength at
high temperatures. As the name implies, they are used repeatedly and are de-
signed in such a Way that the casting can be removed easily and the mold used
for the next casting. Metal molds are better heat conductors than expendable
nonmetallic molds (see Table 3.1); hence, the solidifying casting is subjected to
a higher rate of cooling, which in turn affects the microstructure and grain size
Within the casting.
3. Composite molds, which are made of two or more different materials (such as
sand, graphite, and metal) combining the advantages of each material. These
molds have a permanent and an expendable portion and are used in various
casting processes to improve mold strength, control the cooling rates, and op-
timize the overall economics of the casting process.