Discussion
is higher than for resin B (Table 2), and this can only be explained in terms of a higher tendency for cross-linking of Part I of resin B, which will result in higher viscosity.
It was mentioned before that the relationship between the parameters measured on the resins did not explain the high viscosity of Part I of resin B as these parameters reflect the size and distribution of the macromolecules, not the tenden- cy for cross-bonding. The temperatures and the amounts at which the different components from the resins are lost are summarized in Table 3. It can be seen that a high amount of low molecular weight constituents from Part I of resin A are lost at a lower temperature than those from resin B (105.5C [221.9F] and 146.0C [294.8F]). In a correspondingly way, the high molecular weight components of Part I of resin A are lost at a lower temperature (423.7C [794.7F]) than those from resin B (436.6C [817.9F]); moreover, the amount of high molecular weight components from resin A is lower than from resin B, although the Mw
from Part I of resin A
Figures 12 and 13 compare the strength of the samples pre- pared with either type of sands. The dashed line in these two figures corresponds to equal value of strength, and the pointed lines correspond to reductions in strength of 10 and 20%. Figure 12 shows the data corresponding to bench life, whereas the values for the development of strength are shown in Figure 13. It can be seen in Figures 12 and 13 that most samples prepared with the coarser sand do not reach the levels of strength that the samples with the finer sand reach.
The results shown in Figures 10 and 11 indicate that the core strength increases with the amount of resin. The type of resin used in making the cores affects the strength, as it is seen that resin A produces stronger cores than those obtained with resin B. Figure 11 shows that the strength increases until around 16 hours (960 min); after which the strength decreases when the cores are kept at room temperature. Such reduction will not be considered critical until 32 hours (1920 min) has elapsed. These results are of interest to the casting shop, as they indi- cate the time that the cores can be left after blowing before being used. Figure 10 shows that resin B is less sensitive than resin A to bench life, which will be important when the mix- ture is processed by batch continuously.
Figure 1 shows how the temperature within thin cores can reach temperatures close to melting; therefore, it is of in- terest to study core strength above room temperature. This was done by testing sand cores blown into standard tensile samples in a tensile testing machine with a high convec- tion furnace (Figure 3). The tests were carried out at room temperature and at 150C and 300C (302F and 572F); the samples were tested just after blowing to avoid the increase of strength shown in Figure 11. Figure 14 shows the varia- tion of strength as a function of temperature and amount of resin added to the sand. It can be seen that the strength of the material is affected directly by the amount of resin, as was mentioned previously. It can also be appreciated that cores prepared with resin A are stronger than those prepared with resin B, with the exception of the values reported for small additions at 300C (572F). The tests indicate that the strength of cores prepared with the finer sand (AFS GFN 72) is higher than in those prepared with the coarser one.
Figure 14 shows that testing at 150C (302F) diminishes the core strength and this can be attributed either to the reduction in strength of the polymer or to the mushy state of the mix- ture, as this temperature corresponds to the range at which different solvents and additives of low molecular weight boil
Figure 12. Bench life strength in samples prepared either with fine or coarse sand. The dashed line corresponds to equal values; the pointed ones to reductions in 10 and 20%.
46
Figure 13. Development of strength in samples prepared with fine and coarse sand.
International Journal of Metalcasting/Winter 11
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