An alternative method to methylene blue clay test: spectrophotometry could take subjectivity out of the molding sand test for better reproducibility and repeatability.
The principle of using dye absorption for clay mineral measurements originates in the 1940s. In the late 1970s, analysis of dye absorption using a spectrophotometer was proposed, but few metalcasters showed interest at the time. A later paper described using methylene blue as the working chemical for clay detection, but proposed a new method based on spectrophotometry instead of the standard drop test. By ensuring an oversaturated solution, the absorbency of the solution is always greater than zero. It was found adding clay to the solution would pull the methylene blue out of the solution and reduce absorbency. The authors demonstrated a linear relationship between the amount of active clay and absorbency of the solution.
The research ream at WMU used a spectrophotometer to determine the absorption or transmission of visible light by a sample to measure concentrations of absorbing material based on calibration curves. When possible, the WMU team based the methodology on the existing methylene blue clay test. The results showed the proposed spectrophotometry technique surpasses the methylene blue clay test in a Gage R&R measurement system analysis.
The methodology for the proposed method is as follows:
1. Zero the spectrophotometer using distilled water at 490nm.
2. Measure the absorbency of the orange dye solution (at 490nm) using the spectrophotometer, values must be between 1.700 and 1.750.
3. Dry 5.5 g of the sample as determined in the AFS Mold and Core Test Handbook.
4. Immediately weigh out 5 g ([+ or -] 0.05 g) of the dried free flowing sand into a metal beaker with capacity of 250mL.
5. Add 200mL of orange dye solution into the beaker.
6. Ultrasonic scrub and stir the solution simultaneously for seven minutes. Make sure the stirrer is no more than 0.125 in. (1/8 in.) from the bottom of the beaker but not touching it.
7. Immediately centrifuge 50mL of the solution at 2000 RCF (Relative Centrifugal Force or G-Force) for 13 minutes.
8. Immediately, pour the sample directly into a cuvette.
9. Place the cuvette into the spectrophotometer and read the absorbency at 490 nm (Note: make sure finger prints are not on the transparent sides of the cuvette and the cuvette is properly oriented in the holder) (Fig. 1).
Determining the clay content within the mixture depends on several factors, including the dye concentration, the volume of dye solution to be mixed with the sample and the sample size. Additionally, the absorbency depends on the clay types being measured.
Three dyes were tested for the new technique: orange, black and yellow. All three dyes interacted with the clay in the solution, but the black dye solution would precipitate out during the centrifuging process even in the absence of clay, which complicated the determination of clay levels. The orange dye solution showed minimal decrease in absorbency due to centrifuging. Less than 10% of the original absorbency was lost over the course of a month. Comparatively, the black and yellow dyes had relatively poor shelf life, dropping 25-30% after a few days.
Absorbency measurements were conducted at each solutions peak. Due to the nature of spectrophotometry, a given liquid will have different levels of absorbency at different wavelengths. The peak is the wavelength at which absorbency reaches its maximum. The orange dye solutions peak was found at wavelength 490nm.
Initial tests of the dye solution were performed with varying amounts of solution prepared with tap water. The tests demonstrated an increasing amount of clay reduces the amount of dye remaining in the solution, thus reducing absorbency. To have a more reproducible solution, the researchers used deionized water. Hardening the deionized water with calcium chloride allowed the dye and clay to interact. The controlled hardness of water used for the dye solution preparation was maintained using calcium chloride (Ca[Cl.dub.2] as the hardener.
The concentration of the dye in the solution was critical to the proposed tests effectiveness. While the maximum detectable absorbency varied between equipment models, a 0-1 absorbency range was the most sensitive for the equipment used in the study.
Making a Correlation
In the study, the WMU researchers made three assumptions: both Southern and Western Bentonite clays absorb the dye; increasing the amount of clay lowers the dye concentration in solution; and the dye is absorbed by the different types of clays with a different magnitude.
For the third assumption, a series of experiments used constant concentrations of the dye and testing mixture. By varying the amount of clay within the mixture, the maximum amount of the clay necessary to change the absorbency was identified. Finding this point might lead to an opportunity to estimate the proportion of different clay types within the mixture.
The transition point for the orange dye solution occurred at 0.11g of Southern Bentonite per 50mL of dye solution and at 0.04g of Western Bentonite at 50mL dye solution (Figs. 2-3). Several combinations of dye solution volume and sample weight were tested to find the lowest deviation. Using a 5g sample size and 200mL of dye solution provided acceptable accuracy for the dye concentration.
Researchers used four different levels of clay: 4%, 6%, 9% and 12% and each level were tested 10 times. The same number of methylene blue clay tests were performed (Table 1). Results from the methylene blue clay test and the spectrophotometric absorbency technique (using a clay bond of 50% Southern Bentonite and 50% Western Bentonite) showed a strong correlation (Fig. 4).
Table 1. Comparing the Averages of MBT and the Spectrophotometry Test Average % Clay MB Clay Test Spectrophotometry 4 3.8 0.24 6 5.8 0.20 9 8.6 0.17 12 12 0.13
Gage R&R studies were used to measure the repeatability and reproducibility of the spectrophotometric technique. Repeatability indicates the variability between repeated readings of a sample, and reproducibility indicates the variability between different operators (Figs. 5-6).
Source VarComp %Contribution (of VarComp) Total Gage R&R 0.0000002 0.31 Repeatability 0.0000001 0.25 Reproducibility 0.0000000 0.05 Part-To-Part 0.0000494 99.69 Total Variation 0.0000495 100.00 Study Var| %Study Var Source StdDev (SD) (6 * SD) (%SV) Total Gage R&R 0.0003897 0.0023383 5.54 Repeatability 0.0003545 0.0021269 5.04 Reproducibility 0.0001619 0.0009715 2.30 Part-To-Part 0.0070252 0.0421510 99.85 Total Variation 0.0070360 0.0422158 100.00 Number of Distinct Categories = 25 Figs. 5-6. Gage R&R studies were conducted for the spectrophotometry test (left) and the methylene blue clay test (right) Source VarComp %Contribution (of VarComp) Total Gage R&R 0.0037250 16.01 Repeatability 0.0000000 0.00 Reproducibility 0.0037250 16.01 Part-To-Part 0.0195452 83.99 Total Variation 0.0232702 100.00 Study Var %Study Var Source StdDev (SD) (6 * SD) (%SV) Total Gage R&R 0.061032 0.366195 40.01 Repeatability 0.000000 0.000000 0.00 Reproducibility 0.061032 0.366195 40.01 Part-To-Part 0.139804 0.838825 91.65 Total Variation 0.152546 0.91S274 100.00 Number of Distinct Categories = 3
Based on common statistical procedures, the range of acceptability of a measurement system can be determined using the following guidelines. If the total Gage R&R percentage in the %Study Var column (% tolerance, % process) is less than 10%, the measurement system is acceptable. If it is between 10% and 30%, the measurement system is acceptable depending on the application, cost of the measuring device, repair cost or other factors. If it is greater than 30%, the measurement system is not acceptable and should be revised.
If the %Contribution column is less than 1%, the measurement system is acceptable. If it is between 1% and 9%, the measurement system is acceptable depending on the application, repair cost, cost of the measuring device or other factors. If it is greater than 9%, the measurement system is unacceptable and should be improved.
Both the indices are lower than 10% in the spectrophotometry technique (%Study Var = 5.54% and %Contribution = 0.31%).The indices for the methylene blue test were %Study Var = 40% and %Contribution = 16%.
The next phase of the research will evaluate the effect of secondary green sand additives on the data.
Research for the alternative methylene blue clay test was endorsed by the AFS 4H committee. For the original paper (13-1455) presented on the test at the 117th Metalcasting Congress, go to www.moderncasting.com.
RELATED ARTICLE: ONLINE RESOURCE
For more about the methylene blue clay test and proper testing procedures, read "Avoid the Methylene Blue Blues," Modern Casting, December 2008. p. 36-38.
RELATED ARTICLE: MEDIA RESOURCE
Using the Actable App, scan this page to see a video demonstration of the alternative methylene blue clay test. For instructions on how to use the app, go to page 3. To watch online, go to www.metalcastingtv.com.
A. PIKE, DARIUSH MORADINEZHAD, JAN PEKAROVIC AND SAM RAMRATTAN, WESTERN MICHIGAN UNIVERSITY, KALAMAZOO, MICHIGAN