ed as a function of time and corrected for standard tempera- ture and pressure (i.e. 0C and 1 atm). The evaluation was carried out in fresh mixtures and in samples that were heated for either 30 or 60 min at either 70C or 150C.
Results
Figure 4 shows the size distributions for each type of sand. The coarser sand resulted in an AFS grain fineness number (GFN) of 53 that is distributed in three meshes, with the high- est amount in mesh 70. The finer sand has an AFS GFN of 72, and it is distributed within four meshes, with the highest pro- portions in meshes 70 and 100. The physical characteristics of both sands are shown in Table 1. ADV and pH are monitored in sand bonded by chemical components as high values of either of these parameters reduce resin reactivity and decrease bench life and strength development, affecting the molding procedure.
viscosity of Part I of resin B.
Table 2 shows the physical characteristics measured in sam- ples from Parts I and II of both type of resins (identified as A and B). It can be seen that the viscosity of Part I of resin B is higher than that from resin A, indicating that the use of the former one will confront more problems when mixing. The values reported for Mw
, PI and RI do not explain the high
A series of TGA were carried out in samples from the two resins. Figures 5 and 6 show the changes in weight as a function of temperature of samples from Part I of resin A and Part II of resin B. Figure 7 shows the corresponding curve for the mixture of Parts I and II in a 1:1 ratio of resin A. The inflections and peaks indicated in Figures 5 to 7 correspond to the loss of the various solvents and additives that form part of the resin. The initial and final weights of each test are indicated in each diagram. Table 3 summarizes the results of these TGA.
The amount of resin added to the sand is critical, as the cores have to sustain pressures and forces during pour- ing and solidification, but, as the amount of resin in- creases, so will the gases due to vaporization or burning of the various solvents and components added to the res- ins. Figures 8 and 9 show the amount of gas registered in mixtures of sand of AFS GFN 53 with either 0.80 or 1.15% of resin added. The samples were either placed in the chamber directly from mixing or after heating them for a period of 30 or 60 min at either 70C or 150C (158F and 302F). Reheating the cores may not be of practical use in a foundry, but in the study it showed the amount of gas that was due to the loss of low molecular weight additives.
Figure 4. Size distribution of both sands. Table 1. Properties of the Two Different Sands
Table 2. Characteristics of the Resins
International Journal of Metalcasting/Winter 11
43
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