Binder behaviors examined.

The 16 presentations and four panels in the Molding Methods & Materials Div., touched on sand testing, sand additives, defect studies, green sand basics, coatings, and sand reuse and reclamation. Several sessions focused on improving sand binder performance.

W.A. Gorby, WHEMCO, and L.W. Markow and D.L. Winters, Borden, described WHEMCO's experience with ester-cured phenolic binders.

A steel foundry, WHEMCO converted from a sodium silicate binder system to a conventional (potassium-based) ester-cured phenolic (ECP) nobake for reclaimability and other reasons. While tests confirmed reduction of hot tears and improved surface finish, WHEMCO discovered surface shrinkage defects on larger castings. The foundry then experimented with a new sodium-based ECP, and after a year in use, found it reduced hot tears, improved surface finish, and offered longer work life and less odor.

The new ECP reduced surface shrinkage defects through faster heat dissipation in support of directional solidification. The authors found the rate of heat transfer for molds made with a conventional ECP was lower than that of an alkyd-oil urethane binder. "This correlates directly with an increased tendency for surface shrinkage defects to arise in heavy sectioned areas of large steel castings when using the conventional ECP systems unless other measures are taken to prevent them," they reported.

The new ECP binder's heat transfer rate, however, was higher than conventional ECPs, and was about the same as that of alkyd-oil urethane binders. Its performance with surface shrinkage defects is also similar to the alkyd-oil urethane and sodium-silicate.

To help foundries select a binder to meet molding and handling requirements in a [CO.sub.2] sand binder system, S. Ahmed and S.N. Ramrattan, Western Michigan Univ., compared handling properties of organic and inorganic binders.

While the organic process has historically performed better than inorganics in flowability, breakdown and storage, the inorganic process has made strides and renewed interest because it's environmentally safe. In addition to organic binders' environmental concerns, they have metallurgical problems and require special processing equipment.

Conducting tests on dog bone samples, the authors proved organic binders provide a higher surface hardness, higher maximum strength and ultimately better handlability than inorganic silica sands. They also noted that liner silica grain size mixtures with both binders provide higher surface hardness values and higher maximum strengths; a correlation exists between scratch hardness and ultimate tensile strength for each sand/binder system; coarser sand/binder mixtures provide higher flowability; and [CO.sub.2] activated binder systems lose strength over time.

D.M. Churches, Precision Castparts Corp., and K.B. Rundman, Michigan Technological Univ., undertook a study to better understand core breakdown in phenolic urethane bonded cores and evaluate the extent of thermal degradation of a binder in different casting situations.

Following shakeout, the loose binder-coated sand from chemically bonded cores/molds returned to the sand system has little or no impact on green sand properties. However, the carbon coating found on some sand grains - a by-product of organic binder combustion known as lustrous carbon - adversely affects the clay bonding mechanism when returned to the green sand system.

"Ideally, the binder in all sections of a core would burn out completely due to the heat of the castings, and the gaseous combustion products would be subsequently removed by the ventilation system," they said. In actuality, the fate of the binder depends on type and amount of binder used, amount of heat, proximity to molten metal, availability of oxygen for combustion and gas flow within the mold and/or core.

Examining in situ thermal combustion, breakdown with early shakeout, phenolic urethane coldbox (PUCB) combustion studies and oxidation/reduction, they found that the gases generated by the PUCB and phenolic urethane nobake (PUNB) binders are responsible for forming carbonaceous condensate, leading to the deposition of lustrous carbon. The type of gases formed in green sand under the conditions of this study didn't lead to lustrous carbon formation.

Combustion of carbon in PUCB cores/molds caused the carbon content to decrease to 50-60% of the original value, independent of shakeout time and whether the PUCB was present as a core or on the outside of the casting. Meanwhile, they discovered lustrous carbon formation in PUNB cores occurs in a reducing atmosphere at temperatures as low as 1000F (538C) and between 15 minutes and three hours.

Outside of the binder presentations, V.S. LaFay, S.L. Neltner and J. Burgasser, Hill and Griffith Co., determined an optimal preblend formula for an aluminum foundry that wished to decrease friable edges and reduce preblend consumption. After performing a design of experiments, the authors suggested a preblend that included adding western bentonite and medium-solubility dextrin while reducing overall southern bentonite. In addition to improving its casting quality, the foundry reduced its active clay requirements in its molding sand, and decreased friability and brittleness in its green sand molds.