Originally published in the June 2018 issue of Modern Casting.
by Bo Wallace, Sairam Ravi, Jerry Thiel
Water-soluble salts or electrolytes are just some of the mineral contaminants found in everyday tap water used by foundries across the U.S for their green sand systems. Bentonite is the main bonding compound of green sand systems, and for it to have quality bonding, it first must be activated with the use of an agent. A good activating agent must fulfill three requirements:
- Be distinctly soluble in water, having a solubility of at least 1 g/100 g of water.
- Be composed of anions, which react with cations (Na+) to give insoluble compounds.
- Supply small, univalent cations for ion exchange.
Previous research studied the effect of these electrolytic salts on the physical characteristics of green sand, in particular, wet tensile strength. Wet tensile strength testing helps determine the tensile strength of the green sand’s condensation zone after the mold has been poured. Previous research has shown that an increase in the electrolytic salts, found in well water, led to a decrease in wet tensile strength when compared to samples of green sand containing distilled water, where the wet tensile was not affected.
Past research led to the question of whether these electrolytes are causing the decreasing strength within the clay-water relationship. This possibly could be due to the anionic (positively charged ions) nature of sodium bentonite clay and its necessary cationic (negatively charged ions) exchange between the electrolytic salts within the water. Anions and cations react together to form neutralized compounds. An example of this reaction is that of Sodium (Na+) and Chlorine (Cl-) where the two react together, forming your basic table salt (NaCl).
Past research conducted on the effect of individual electrolytic salts on the wet tensile strength of green sand showed salts had an immediate effect on the wet tensile strength, with the wet tensile strength decreasing initially. However, as time went on, the wet tensile strength was determined to increase. This was determined to be due to the presence of the particular electrolytes within the system and not the actual concentration. Read More.
pH of Doped Water Samples
The pH of the doped water samples was measured. Adding the elements at their low levels slightly increases the pH, when compared to the control sample. From the pH of samples 2, 3 and 4, it can be seen adding higher levels of Mg, Ca and Cl2 considerably increases the pH. However, the addition of sodium can be observed to have a larger effect on the pH, when compared to the other contaminants. All water samples with high sodium levels were observed to have pH values of approximately 11.65-11.89.
Green Sand Properties
The green compression strength results are shown in Figure 2. Similar strengths were measured for all samples, with the exception of sample 3. Sample 3 was recorded to have an average green strength of 17 psi. Sample 3 can be observed to have a high chlorine level and the other contaminants at low levels.
The wet tensile strength results are shown in Figure 3. A trend similar to green compression strength can be observed. Wet tensile strength results were measured to be similar for all samples, except samples 3 and 4. Samples 3 and 4 were observed to have wet tensile strengths of 0.303 N/cm2 and 0.330 N/cm2. The other samples were measured to have wet tensile strengths in the range of 0.380-0.410 N/cm2. Read More.
This article is based on the paper “Effect of Water Quality on Green Sand Properties” (18-009) presented at the 122nd Metalcasting Congress. The study was an AFS-funded research project.
(Originally printed in the June 2018 issue of MODERN CASTING.)
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