3 Binders Evaluated
Metalcasters have long guessed at the relative strengths and weaknesses of chemical sand binders at elevated temperatures. New test results remove the guesswork.
Jerry Thiel, University of Northern Iowa, Cedar Falls, Iowa Larry Stahl, General Motors Corp., Saginaw, Michigan Shelly Dutler, Magma Foundry Technologies, Schaumburg, Illinois
The ability to produce dimensionally accurate cast components depends on your understanding of the physi- cal characteristics of the molding materials. To further that understanding, a study recently was conducted on three commercially available chemically bonded sands. Phenolic urethane cold box (PUCB), epoxy acrylic cold box (EACB) and resin coated sands each were
PUCB Binders Results
T
ensile strength decreased with increasing tempera- ture until 572F (300C), where a slight increase in tensile strength was observed in the 30 and
60 minute cycle times. The results of further testing showed the same trends in strength levels at the higher temperatures, although they were not as pronounced at the 15 and 60 minute cycle times. At 212F (100C), the samples exhibited approximately 70% of their original tensile strength. At 932F (500C), the samples had lost 90% of their original strength. The strength of the PUCB binder increased with time at the 572F and 752F (400C) temperatures while experiencing degrada- tion at all other temperatures. This increase in tensile strength was degraded if held at the temperature in excess of 30 minutes. In general, the PUCB binder degraded proportionally with the increase of both time and temperature. The PUCB retained approximately
evaluated during 15, 30 and 60 minute heating cycles for high temperature strength, expansion and stiffness (see sidebar for more on the methods used to evaluate the core sands). Following is a look at the results of those tests and what
they mean for the use of the binders in commercial metal- casting facilities.
90% of its original tensile strength but only 70% of its original modulus at 212F (100C). The sample lost ap- proximately 75% of both its strength and stiffness after 30 minutes at 392F (200C). When the tensile strength was charted as a function of the modulus, a linear relationship was discovered.
The Bottom Line
PUCB systems lose strength and stiffness quickly with moderate to high increases in temperature. Secondary strengthening occurs between 572F (300C) and 752F (400C) with prolonged heating.
Who Should Use Them? PUCBs fit best in general metalcasting applications, par-
ticularly for iron and steel metalcasters, because of robust curing and ease of use. Dimensional accuracy problems may occur if used in, thin unsupported lacy core sections.
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MODERN CASTING / February 2010
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