with 170 kg/cu.m density. Samples had a cross section of 4
sq.in. (2,580
sq.mm) and a thickness of 1 in. (25.4 mm) or 2 in. (50.8 mm). Tey subjected the foam to compression test- ing to determine the elastic modulus after aging at 212F (100C) for 24 hours. Termal dilation during aging was measured with a laser-assisted dilatometer. Foam samples were cut into 1.97-
2
Procedure Te research-
ers focused their study on poly- urethane foam
Fig. 1. This investment casting shell was built around a foam pattern.
Fig. 2. The finite element model for the foam pattern and ceramic shell used the mesh shown here.
in. (50-mm) long, 0.71-in. (18-mm) diameter cylinders, and two thin aluminum disks were placed on both ends of the foam and inserted into a quartz glass tube submerged in an oil bath. Te end of the tube had a small hole through which oil could flow for improved heating of the sample. Another quartz tube was placed on the upper aluminum disk. Te researchers monitored the
expansion of the foam sample through the linear movement of the upper tube using a laser proximity probe and determined the average temperature of the foam samples. Te heating rate of the foam was approximately 33.8F (1C)/minute. Samples were held at various aging
temperatures and times, cooled back to room temperature and heated again until softening. One sample was heated and held at different tempera-
tures in a stepped fashion. Te samples were stabilized by first heating from room temperature to 356F (180C) and then quenching in liquid nitrogen and held for one minute. Te quenched sample also was tested. To study shell construction and its
properties, the researchers used simple patterns (2 x 2.5 x 2.5 in. [50.8 x 63.5 x 63.5 mm]) to test shell cracking dur- ing burnout (Fig. 1). Te patterns were submerged into the slurry coating until completely covered and then removed and suspended over the slurry for approximately 50 seconds. During this time, the pattern was rotated to pro- mote even coating. Te lab technicians then applied a uniform distribution of stucco using the rainfall method. Shells were fabricated with one
prime coat, three or five backup coats, and one seal coat. After the seal coat was applied, the samples dried for another 24 hours. Half of the patterns were aged. After sample preparation, the
shells were fired in an electrical resis- tance box furnace using flash firing in a furnace preheated to 1,112F (600C). Researchers determined the
maximum stress at rupture and elastic modulus of the five- and seven-layer shells using three-point bend testing of shells at room temperature. A nonlinear finite element model
was developed to study the crack formation in the shell during pattern removal. Te model accounts for both mechanical and thermal loadings and assumes a fixed interface between the pattern and shell. Te mesh of the finite element model for the shell and foam pattern is shown in Fig. 2. Te researchers used a smeared
crack model to describe the response of the ceramic material when a crack initiates. Cracking is assumed to occur when the stress reaches a crack detec- tion criterion surface. When a crack has been detected, its orientation is stored for subsequent calculations.
Fig. 3. These graphs show an example of an aging test of polyurethane foam at 212F (left), a maximum expansion/shrinkage of polyurethane foam after aging 24 hours at various temperatures (middle), and the final shrinkage of polyurethane foam after aging for various amounts of time at 212F (right).
August 2012 MODERN CASTING | 41
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