Casting answers & advice.
Background: There are various materials added to the typical green sand system to provide the desired properties. Although there has been substantial research on the effects of additives, such as different bentonite types, seacoal and many others, there has not been much research on the effect of water as a sand additive. Therefore, the effect of water variations on green sand properties has not been well documented.
Answer: The AFS Foundry Sand Additives Committee (4-H) conducted a research study to determine whether variations in water chemistry or pH level would affect green sand lab tests results. Seven different water samples were sent from metalcasting facilities in various locations in the U.S. and Canada to an independent sand testing lab to determine if waters of various mineral contents would significantly affect green sand characteristics or the swelling properties of the bentonite.
All water samples were mixed into the same green sand mixture (a clay-bonded sand sample from a production metalcasting facility) using the same mixing procedures. The sand was mulled dry for one minute then for three minutes after the temper water was added. After routine checking, the sand mixture was mulled an additional seven minutes. The sand mixture then was tested for moisture, compactibility, green strength, dry strength, friability, cone jolt, permeability and specimen weight using AFS sand test guidelines. Although there seemed to be a minor effect on dry compression and minor variations in friability, there appeared to be little significant effect between sand samples mixed with the various water samples.
The committee then ran a second set of tests to determine if the water pH had an effect. During this testing, they altered the pH of a known water sample and then ran the green sand tests. Sand was tempered to 40 compactability utilizing deionized water and a test water, both "spiked" to set pH levels. The water was spiked to 3, 4, 5, 6, 7, 8, 9, 10 pH levels utilizing sodium hydroxide and repeated using potassium hydroxide. Then it was added to the same green sand mixture. The sand was tested for compactability, green strength, green compression, permeability, methylene blue, LOI, split tensile and clay using AFS sand test procedures. Three samples for each test were used to validate results.
The results indicated that the altered pH levels did not show any significant changes in the green sand properties in a laboratory setting. However, because the tests did not include multiple repetitions, they may not have addressed the cumulative effect of using the "test water" in the same batch of sand over many temper-dry-temper cycles or the effects over time in an actual green sand system versus a laboratory setting. Additional research will be required for a more definitive answer to the long-term effect of water changes.
Recommendations: Based on the data generated in the 4-H committee study, it does not appear that differences in water significantly affect green sand properties. The fact that the tests show no short-term effect of water variations should indicate that water characteristics have little effect on green sand properties if the sand in a metalcasting firm's green sand system is replaced rapidly, such as through new sand additions or core sand dilution. However, because the research did not address the cumulative effect of water variations, if a green sand system is not rapidly replaced, the cumulative effect of water additions, could, over time, have an effect on your sand system.
Some metalcasting facilities have reported that they experienced casting defects, such as scabbing, that coincide with seasonal changes that create significant variations in temperature and moisture. This may be due to the amount of runoff in the surface water they use for the green sand.
It is unclear if a change in water source or quality could impact green sand properties and affect casting quality over a period of time. Any change in water or any other material made to a sand system should be done carefully. All changes should be fully documented, and detailed testing should be conducted to determine any kind of effect.
The information submitted in this column was supplied by AFS Foundry Sand Additives Committee (4-H). If your metalcasting facility has noticed an effect to its sand system due to water changes, please contact the AFS Technical Dept. at 800/537-4237.
|Printer friendly Cite/link Email Feedback|
|Date:||Nov 1, 2005|
|Previous Article:||Do you qualify? You think your metalcasting firm can provide a casting superior to your competitors? Prove it.|
|Next Article:||2005 Foundry Executive Conference lays out building blocks.|