penetration defect, as well as stiff sand during compaction. A good molder will notice when the sand is starting to stiffen up, indicating it is curing too fast and the catalyst should be reduced.
Measuring Work Time In the sand lab at ASK Chemicals,
Dublin, Ohio, researchers developed an interval testing method to deter- mine work time. After the sand was mixed with the binder, laboratory core specimens were produced at 3-minute intervals until the cores were too weak to hold. Each specimen was tested for tensile strength and compared (Fig. 1). Te experiment was performed with the three most common nobake binders used in the North American metalcasting industry—1% furan nobake, 1% phenolic urethane nobake and 1.25% ester-cured phenolic. Tese systems were run with catalyst or core- actants to provide the same 20-minute strip time for each binder chemistry. Tensile strengths of the specimens at each interval were recorded and the percentage of tensile loss calculated. It was determined that a binder had exceeded its work time when it expe- rienced more than 25% tensile loss. In the case of the experiment, furan nobake achieved 25% tensile loss in 3 minutes, resulting in a 3-minute work time. Phenolic urethane and ester- cured nobake had longer work times of approximately 9 minutes (Fig. 2). Te results were compared with the
results of a B scale hardness gauge test run to determine the work time. Te commonly used readings of 45 or 60 for the B scale hardness gauge were used to determine work time.
Making a mold or core past its work time can lead to penetration defects in the casting due to poor density in the mold.
The interval and gauge tests gave
conflicting results. For instance, the gauge test indicated the amount of time before the tensile strength of the furan nobake binder would be too low was 14 minutes. However, the interval test indicated the sand mix produced with the same chem- istry had just 3-6 minutes before tensile strength would be too low for proper core or moldmaking. Disparity of work times between
the two tests occurred for the other binders, as well. According to the experiment results, metalcasting
Poor mold strength can lead to cracking during handling or pouring.
FOOD FOR THOUGHT: MIXING WORK TIMES
Typically, nobake casting facilities set the work time and strip time of its molds and cores to allow enough time to fill the mold and then wait for it to cure before removing it from the pattern. With a more accurate way of determining work time, along with the sophisticated sand mixing equipment available today, it might be possible to adjust the catalyst of the sand as it fills the mold.
By using a slow catalyst in the first sand to fill the mold,
then adjusting that catalyst to work more quickly so the last sand has the fastest reactivity, theoretically, the molder could have the whole mold set at once. This would reduce the time needed to wait for the mold to cure and increase the amount of molds or cores made in a shift. While the idea has not been put to the test yet, with the computer controls available today, the possibility does not seem too far out of reach.
October 2012 MODERN CASTING | 25
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