Figure 5. Box plot of waterjacket core length over time. Core Shrinkage
In some critical casting applications, such as the casting plant’s block, every potential cause of dimensional varia- tion needs to be understood and controlled. When the cold- box process was developed in the 1960s and 70s, a “rule of thumb” was developed that suggested a core shrinkage factor of 0.010 in. (0.25 mm) per foot (304.5 mm). This is slightly less than 0.1%. While known to some through word- of-mouth, it has not been well publicized in the foundry in- dustry that coldbox sand cores will undergo this shrinkage and dimensional change following coremaking. It has been assumed that this shrinkage is generally reproducible for a particular operation and stabilizes within a period of one to two days. The amount has been assumed to be small and generally compensated as part of an overall “shrink factor” in the initial corebox dimensions. However, little has been published about testing methods or the factors that can affect core shrinkage.
Coldbox cores are produced by coating sand with a liquid binder, blowing the sand into a corebox, and curing the core with a gas catalyst/co-reactant. The sand grains are bonded together by resin “bridges” that form when the liquid coating on the surface of one sand grain touches the liquid surface of another grain and surface tension causes a link or bridge to form between the two liquid layers. When the core is cured, these bridges harden and form a network of bonds that hold the sand grains in place and provide core strength.
Liquid binders contain thermoset resins that cure by po- lymerization and cross-linking. As this polymerization occurs, the binders go through a volumetric change, caus- ing the resin bridges to shorten and pull the sand grains together. This means that the core dimensions are slightly smaller than the corebox dimensions immediately after
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curing. Estimates of the shrinkage have been suggested at about 0.015%1
or 0.02 in./ft (1.5 mm/m). The curing
process continues over time, and cores do not reach full strength until after approximately 24 hours, depending on temperature and other conditions. The volumetric changes continue as the curing continues, and volatile solvents are lost through evaporation. This causes the resin bridges to continue to shorten and core dimensions to decrease as the sand grains are pulled tighter and tighter. This could result in additional dimensional changes that are time dependent, so casting dimensions could vary depending on the age of the core. Subsequent operations, like coat- ing and drying the cores or storing the cores under hot or humid conditions, may also impact core and casting dimensions.
Test Methods
Part of the difficulty in understanding core shrinkage is in measurement. Sand cores have somewhat rough surfaces and measurement points are not completely smooth. They also contain draft so that flat, parallel surfaces may not be available for easy measurement. Even the act of position- ing the measurement device on the surface of the core may dislodge sand grains and interfere with subsequent measure- ments. To determine possible measurement error, two fresh- ly made 1 x 1 x 8-in. phenolic urethane coldbox (PUCB) cores produced under the “standard” condition used at the casting plant were measured using a vernier height gauge on a granite table as shown in Figure 6. One of the cores was cut and ground “flat” to provide a second height for measure- ment. After 24 hours the bars were remeasured eight times to check for shrinkage.
The data indicated that while there may have been some core shrinkage over 24 hours, the amount of that shrinkage was
International Journal of Metalcasting/Summer 10
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