Figure 12. Shrinkage of cores from L4 DOE.
perature and humidity. It also could indicate problems with the test method, although the initial trials with cores from the same mix showed good agreement and relatively low varia- tion. However, one thing is clear. Even though the different trials in the DOEs used very different materials and process- ing conditions, the shrinkage was relatively the same over time. For instance from the L8 DOE, at 1 hour, the shrink- age averaged 0.002 in. (0.06 mm) with a range of +/- 0.008 in. (0.02 mm) for all eight trials. At 24 hours, the shrinkage averaged 0.0061 in. (0.155 mm) with a range of +/- 0.001 in. (0.03 mm). The effects of time greatly outweighed the ef- fects of any other variable or combination of variables. Once the shrinkage for a specific core mix (sand, binder, binder % additives, processing conditions, etc.) is established, the ef- fects of variations in material levels or processing conditions are relatively minimal. While the age of the core may need to be considered in critical applications, normal process varia- tion does not seem to be all that important within normal operating ranges.
Effects of Coating and Drying
Cores at the casting plant might be used immediately or placed in a “buffer line” and used after several hours or days. The dimensions of the core could depend on the age and storage conditions. In other operations, the core may be coated with a refractory coating and dried prior to use or further storage. Testing was conducted to determine the effects of coating and drying on the core shrinkage. Because the cores needed to be handled for coating and drying, the measurements were taken using the vernier calipers with a locator pin on the top of the cores. Ten cores were blown using the customer “standard” conditions. All 10 were initially measured, and five were coat- ed and placed into a 350° F (177 C) oven with the other five uncoated cores. After drying for 45 minutes, the cores were removed and allowed to cool to room temperature, then re- measured. Both the coated and uncoated cores showed about the same level of shrinkage with only slightly more apparent shrinkage after 24 hours. Test results are shown in Table 5.
A second series of tests was conducted to confirm that little additional shrinkage occurred after oven drying. A set of eight cores were produced using “standard” conditions. Two
International Journal of Metalcasting/Summer 10 Table 3. L4 DOE
cores were immediately placed in the dial indicator fixture to serve as a “control”. Two cores were coated, dried at 350° F (177 C) for 45 minutes, allowed to cool to room temperature, and placed in the fixture and “zeroed”. Two more cores were coated, dried at 250° F (121 C) for 45 minutes, allowed to cool to room temperature, and placed in the fixture and “ze- roed”. A final two cores were coated, dried at 350° F (177 C) for 45 minutes, and placed into the fixture while still hot and “zeroed”. The results are shown in Table 6. The cores that were not coated and dried showed the same relative shrink- age as seen in previous tests. The cores that were coated and dried at either 350° (177 C) or 250° F (121 C) showed no further shrinkage. The cores that were coated and dried and placed in the fixture while hot showed significant shrinkage during the first hour as they cooled to room temperature, but then showed no further shrinkage.
Effects of Humidity During Storage
The effects of high humidity have been well documented for PUCB core strength. Humidity can reduce core strength sig- nificantly, but drying generally can reverse the effects. Tests were conducted to determine the effects of humidity on core shrinkage (or growth). Initially, coated and uncoated cores that had been produced earlier and oven dried, and then stored in the Controlled Temperature & Humidity (CT) room, were placed in the fixtures and stored for 24 hours at 90% Relative Humidity (RH). The cores showed an average expansion of 0.001 in. (0.025 mm). Next, eight cores were produced from
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