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Determining Your Binder Work Time


A common nobake binder work time testing procedure may not be as accurate as you think. GREG STURTZ AND JEAN SZABO, ASK CHEMICALS, DUBLIN, OHIO


to determine the work time of your binder. Te test, which was origi- nally designed to measure green sand hardness and now is used for nobake molds, is one of several common tests used across the metalcasting industry to measure work time offered by the AFS Mold and Core Test Handbook. However, few of the tests are scientific ways of measuring work time. In nobake molding, sand is mixed


I


f you work in the nobake molding department of your metalcasting facility, it is likely you are using the B scale mold hardness gauge method


with the binders and then packed into a pattern. Tis step must be finished quickly before the binders start to react, resulting in stiff sand, poor mold strength and, eventually, flawed castings. Determining how long the molder has to pack the pattern before the sand mix loses its optimal molding properties is necessary to keep molds or cores from breaking or producing defective castings.


Work Time Defects Te consequences of exceeding the


work time of your binders are costly. Compacting the sand too long after


curing begins could result in: • Reduced strength. • Mold breakage during handling. • Mold cracking or core failure at casting.


• Poor mold and core density. • Penetration defects and poor sur- face finish in the casting. Unfortunately, many mold and


casting problems also can be attrib- uted to other issues in the casting process, so exceeding the worktime may not be recognized as the root cause. Telltale signs that your facility is compacting its molds too late after curing include weak molds and the


Many metalcasting facilities rely on these tools to determine work time. 24 | MODERN CASTING October 2012


ASK Chemicals produced dog bone core specimens and tested them over intervals to determine how quickly tensile strength deteriorated.


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