dimensional accuracy. The phenolic-urethane cold box (PUCB) process is among the most common core materi- als used in aluminum foundries. This is due to its relatively low cost (although the price of the resins are linked to that of oil), high productivity and high availability of binders, versatile properties, and ability to work with any type of sand.6,7,9
The process uses a two-part binder system that con-
tains a phenolic resin (Part I) and a polymeric isocyanate (Part II). The sand is coated with components from Parts I and II and blown into a pattern at room temperature. Amine catalyst is introduced through vents in the pattern to harden the contained sand mix. The catalyst gas cycle is followed by an air purge cycle that forces the amine through the sand and removes residual amine from the hardened core. The hydroxyl group, from the phenolic resin in Part I, react with the isocyanate groups, from Part II, in the presence of the amine catalyst to form solid urethane resin that bonds the sand grains together.6,7,9
The aim of this work is to present the results of a study con- ducted to evaluate the characteristics and properties of mix- tures used in the production of PUCB sand cores. The studies were carried out with resins from two different sources and with two types of rounded silica sand. Fine grained sand is required when a smooth casting surface (low roughness) is de- sired, although the permeability of the core may be impaired.
Experimental Procedure
The study was conducted on silica sand cores produced by the PUCB process. These cores are installed within cylinder heads and engine block moulds to produce internal cavities and conduits and are subjected to strict mechanical properties and dimensional control standards. Parts I and II of two dif- ferent binder resins were analyzed; the resins will be identi- fied as either A or B in this work. The refraction index (RI), viscosity, average molecular weight (MW
) and polydispersity
index (PI) of the resins was recorded. Viscosity can be defined as the resistance of a fluid to flow, and it can be related to
polymer. Bigger macromolecules will have more difficulty moving than smaller ones. PI is related to the distribution of macromolecules of different molecular weight and takes the value of 1 when the distribution follows a narrow Gaussian distribution. RI is related to the degree of bonding between the macromolecules. Thermogravimetric analyses (TGA) were also carried out; the heating rate in these tests was 10C/ min (18F/min) over the range of 30C to 800C (86F to 1472F), and a flux of 50 ml/min of N2
Mw was used to avoid combustion.
Silica sand of two different size distributions was used. The sand was quartered following standard procedures and sieved to determine grain size distributions as recommended by AFS.11 Acid demand value (ADV), pH and moisture were recorded. Samples of new sand were mixed with the different resins. Experimental cores were prepared with 0.8, 0.9 and 1.15% of each binder; the proportion between Parts I and II was kept as 1:1 in all trials. The bench life of the mixture and the develop- ment of resistance of blown samples were evaluated with stan- dard blown tensile samples. Additional tests were carried out at 150C and 300C (302F and 572F) on cores that were placed into a high convective furnace installed in a universal testing machine, to evaluate the strength of the cores at temperatures above room temperature (Figure 3). The values from the tensile tests correspond to the average of nine individual tests.
The amount of gas evolved during heating was recorded by placing mixtures of sand and resin into a chamber kept at 900C; the volume of gas evolved from the cores was record-
as it reflects the size of the macromolecules that form the
Figure 2. Examples of silica sand cores produced by the PUCB process.
42
Figure 3. Experimental set up for tensile testing at above room temperatures.
International Journal of Metalcasting/Winter 11
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