Page 296 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 11.3 Expendable-mold, Expendable-pattern Casting Processes 275

FIGURE |l.l5 Cross section and microstructure
of two rotors: (top) investment cast; (bottom)
conventionally cast. Source: Courtesy of ASM
International.

CASE STUDY I l.2 Investment Casting of Total Knee Replacements

With great advances in medical care in the past few pressing them against the sprue until the patterns are
decades, life expectancies have increased considerably, held in place. The final assembled tree is shown
but so have expectations that the quality of life in later in Fig. 11.17a, which contains 12 knee implants
years will remain high. One of the reasons for arranged into four rows.
improvements in the quality of life in the past 40 years The completed trees are placed in a rack, where
has been the great success of orthopedic implants. they form a queue and are then taken in order by an
Hip, knee, shoulder, spine, and other implants have industrial robot (Section 37.6). The robot follows a
resulted in increased activity and reduced pain for set sequence in building up the mold; it first dips the
millions of people worldwide. pattern into a dilute slurry and then rotates it under a
An example of an orthopedic implant that has sifting of fine particles. Next, the robot moves the
greatly improved patient quality of life is the total knee tree beneath a blower to quickly dry the ceramic
replacement (TKR), as shown in Fig. 11.16a. TKRS are coating, and then it repeats the cycle. After a few
very popular and reliable for the relief of osteoarthritis, cycles of such exposure to dilute slurry and fine
a chronic and painful degenerative condition of the particles, the details of the patterns are well produced
knee joint that typically sets in after middle age. TKRs and good surface finish is ensured. The robot then
consist of multiple parts, including femoral, tibial, and dips the pattern into a thicker slurry that quickly
patellar components. Typical materials used include builds up the mold thickness (Fig. 11.16c). The trees
cobalt alloys, titanium alloys, and ultrahigh-molecular- are then dried and placed into a furnace to melt out
weight polyethylene (UI-IMWPE; Section 7.6). Each and burn the wax. The trees are then placed into
material is chosen for specific properties that are impor- another furnace to preheat them in preparation for
tant in the application of the device. the casting process.
This case study describes the investment casting A mold ready for investment casting is placed
of femoral components of TKRS, which are produced into a casting machine. The mold is placed upside
from cobalt-chrome alloy (Section 6.6). The down on the machine, directly over a measured
manufacturing process begins with the injection volume of molten cobalt chrome. The machine then
molding of patterns, which are then hand assembled rotates so that the metal flows into the mold, as shown
onto trees, as shown in Fig. 11.16b. The patterns are in Fig. 11.16d. The tree is then allowed to cool and
spaced properly on a central wax sprue and then are the mold is removed. The cast parts are machined
welded in place by dipping them into molten wax and from the tree and then are further machined and

(continued)