CHARACTERISTICS OF PHENOLIC-URETHANE COLD BOX SAND CORES FOR ALUMINUM CASTING


R. González and R. Colás


Universidad Autónoma de Nuevo Léon, San Nicolás de Los Garza, N.L., Mexico A. Velasco


Nemak, S.A. de C.V., Garcia, N.L., Mexico S. Valtierra


Owens Corning, Santa Catarina, N.L., Mexico Copyright © 2011 American Foundry Society Abstract


This work presents the results of a series of tests carried out on mixtures of sand and binders used in phenolic-urethane cold box cores. Two resin binders and silica sand with two grain size distributions were analyzed; the cores were blown with blends of sand that had resins within the range of 0.8 to 1.15%. The mechanical properties of the mixtures were obtained by blowing them into conventional tension test specimens. The strength of the material was recorded at room temperature and at 70C and 150C (158F and 302F).


Introduction


Most automotive engines are manufactured from aluminum alloys due to the reduction in weight and enhancement in power rating.1


Changing from iron to aluminum has not been


an easy task for the industry. The higher thermal conduc- tivity of aluminum, compared with that of iron, allows for higher temperatures within the combustion chamber, which imposes stiffer working conditions as they enhance the re- duction in strength while at the same time promoting ther- mal fatigue.2,3


Heat produced by combustion in the chambers of the engine block is extracted by refrigerating of chilled liquids that flow within internal cavities and conduits.1,4


The cores must be rigid enough to These internal chan-


nels are made from cores made from blends of different types of sands and binders.2,5-8


withstand the stresses generated by the static pressure of the liquid metal during pouring. The gases that are generated within the core, either by thermal decomposition of the bind- er of boiling of the different solvents and additives, must have ample means of escape without blowing into the metal and causing defects. During shake-out, the core binder must be destroyed by the heat released from the liquid metal as it solidifies.5-7


The amount of temperature that the core has to


resist depends on its size and location and can be higher than the melting point of the alloy. Figure 1 shows the tempera-


International Journal of Metalcasting/Winter 11


A gas evolution chamber was used to measure the amount of gas that is produced when the mixture of sand and resin is burned. These measurements were carried out on fresh samples that were heated for 30 or 60 minutes at either 70C or 150C (158F and 302F) prior to burning. This study was carried out to determine which resin fulfills the conditions critical to obtaining sound aluminum castings.


Keywords: casting, binders, aluminum, sand cores, cold box


ture cycle to which a thin core was subjected during filling and solidification of an automotive cylinder head.


The cold box process is widely used in industry for produc- ing a wide range of cores (Figure 2). The process was in- troduced about 40 years ago, and it allows the production of cores with good tensile strength, hot strength, and high


Figure 1. Temperatures recorded during filling and solidification of an engine cylinder head. Three thermocouples were placed within a thin core; the remaining thermocouple registered the temperature of the molten metal.


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