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facilities depending on nonscien- tific methods to determine work time, including the B scale hardness gauge, in particular, run the danger of attempting to produce molds and cores past their window for optimal tensile strength. Figure 3 shows a comparison of


Fig. 1. Three core specimens produced with the major binders used in the U.S. were tested for tensile strength in the lab. In the test, a strip time of 20 minutes was used.


the percentage tensile loss in the interval test versus the B scale mold hardness gauge measurement for the furan nobake. Tis graph suggests that a much lower B scale gauge read- ing of 20-25 corresponds to a 25% loss in tensile strength.


On the Shop Floor


Te advantage to the gauge test is its simplicity, and it could still be used on the shop floor, but with different indicator readings. Metalcasting facili- ties may perform the interval test once for each type of binder used to lay the foundation for work times and adjust the indicator amount from the B scale hardness gauge accordingly. Metalcasters also should remember


the following to ensure the proper production of molds and cores: • Keep work time as short as possible for better productivity.


• Size the sand mixer to the size of the mold or core.


Fig. 2. Work times determined with the interval test are significantly less than that of correlating work times determined with the B scale mold hardness gauge.


• Slow down the cure time using slower catalysis if necessary for a longer work time.


• A B scale mold hardness of 60 is not a good work time measure. Work time is much less on the B scale gauge, depending on the binder.


• Furan nobake binders have a short work time compared to the strip time (the time it takes to fill the corebox and then cure before the core is ready to be removed).


• Phenolic urethane nobake and ester-cured phenolic binders have work times of about half the strip time.


ONLINE RESOURCE


Fig. 3. Shown is a comparison of the percentage tensile loss in the interval versus the B scale mold hardness gauge measurement for furan nobake binder.


26 | MODERN CASTING October 2012


Visit www.moderncasting.com for a a copy of the full presentation on which this article is based, which was given at the 2012 AFS Sand Casting Conference.


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