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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.

The ability to produce dimensionally accurate cast components depends on your understanding of the physical 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 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.

PUCB Binders

Results

Tensile strength decreased with increasing temperature 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 degradation 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 90% of its original tensile strength but only 70% of its original modulus at 212F (100C). The sample lost approximately 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, particularly 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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EACB Binders

Results

Testing showed a rapid decrease in tensile strength as the samples were heated. The most pronounced decrease in strength was seen at 212F (100C). At this temperature, approximately 80% of the tensile strength and modulus were lost through heating the sample. This loss continued until 392F (200C), where a slight strengthening was observed. Similar to the PUCB, the EACB had a linear relationship between the tensile strength and the modulus of the samples tested at elevated temperatures.

The Bottom Line

EACB systems have the quickest loss of strength due to temperature of the commercially available sand binders. This may cause thermal distortion in thin unsupported core sections.

Who Should Use Them?

The EACB system has an almost unlimited bench life and improved shakeout over the PUCB system due to rapid binder breakdown. Automotive metalcasters, for example, can benefit from these characteristics when shaking out engine cylinder head and block castings where cleanliness is critical.

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Resin Coated Sands

Results

Resin coated shell sands had the lowest thermal degradation of the three sands tested. Since the sand is thermosetting, it was expected there would be little if any thermoplastic deformation at elevated temperatures. The samples were almost unaffected at 212F (100C) and retained a large quantity of their original strength until complete thermal degradation occurred. A slight strength increase occurred at 392F (200C). All other temperatures showed a slight but constant degradation of tensile properties with extended times. Although no increase in tensile strength as a result of temperature was observed, an increase in modulus did occur. This increase in modulus almost doubled the original stiffness of the test sample, although the strength was approximately 55%. Because the system is thermosetting, a linear relationship between the modulus and the elevated temperature tensile strength did not exist.

The Bottom Line

Resin coated systems exhibit high strengths and stiffness over a wide temperature range and then degrade quickly as temperature exceeds 752F (400C).

Who Should Use Them?

Metalcasters seeking greater dimensional accuracy benefit from resin coated sands, as they maintain their strength throughout the casting process. Heavy iron or steel sections may not be possible due to the quick breakdown at temperatures exceeding 752F (400C).

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How the Sands Were Tested

Compression samples of phenolic urethane cold box (PUCB), epoxy acrylic cold box (EACB) and resin coated sand were produced using standard industry tooling and comprised of cylinders 1.125 in. in diameter and 2 in. long. The bonded, sand specimens were heated in a muffle furnace using a neutral atmosphere consisting of a 7 to 1 ratio of carbon dioxide and carbon monoxide. (This mixture replicates the mold gas atmosphere found in chemically bonded molds.) Three soak times of 15, 30 and 60 minutes were used to provide time dependent physical properties. The test cores were removed after the desired soak times and immediately placed in a fixture for testing. The samples were compressed until fracture. Compression strengths and Young's modulus calculations resulting from the deformation under load were compared to known values for bonded sands at room temperature.

A second testing methodology involved the use of a universal testing machine with integrated extensometer. Fig. A illustrates the test setup using the extensometer. The standard dog bone tensile specimen was modified for the 1-in. gage length necessary for strain measurement.

[FIGURE A OMITTED]

The samples were produced using addition levels and ratios provided by the manufacturers. The PUCB and EACB test samples were produced at a 1.2% binder level, while the resin coated sand was bonded by the manufacturer at a 1.9% resin level. Each of the samples was allowed to cure for a minimum of 24 hours in a low humidity environment before testing. High temperatures for the samples were provided with the muffle furnace. Samples were placed in aluminum foil and held in a nitrogen atmosphere for the required time at temperature.

The three times were chosen to give values corresponding to the temperatures involved in the casting process. Because sand is an insulator, the slow transfer of heat makes it difficult if not impossible to test the tensile stress and strain under casting conditions. A constant loading rate was used for all testing.

Jerry Thiel, University of Northern Iowa, Cedar Falls, Iowa

Larry Stahl, General Motors Corp., Saginaw, Michigan

Shelly Dutler, Magma Foundry Technologies, Schaumburg, Illinois
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Title Annotation:phenolic urethane coldbox
Author:Thiel, Jerry; Stahl, Larry; Dutler, Shelly
Publication:Modern Casting
Geographic Code:1USA
Date:Feb 1, 2010
Words:1093
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