CASTING ANSWERS & ADVICE
Q A
Moisture Mischief, Inadvertent Heat Treating T e percent
Our test results for green sand percent moisture have been running lower recently.
Could this be due to a techni- cian error or changing tem- peratures in the summer?
As with any test, your green sand percent moisture examinations should be performed as consistently as pos- sible for them to be
useful tools. T e percent moisture test is sensitive to testing conditions and can be one of the most diffi cult sand tests to reproduce. It depends on tem- perature and time and is sensitive to leveling the sample in the pan, cleanli- ness of the screen and sample cooling time. It is not uncommon for sand labs to see several tenths of a percentage diff erence between operators. Even if each operator is consistent in his or her testing procedure, a slight diff erence in testing technique can arise. Several principles will help
improve the accuracy of green sand percent moisture test so it can be properly utilized: Sand Temperature and Drying—
T e sand must be kept moist and not allowed to dry out. Many facilities put the sand into a covered, moisture- proof container with a thermometer inserted to measure sand temperature when transporting it from the mold- ing line to the testing lab. T e operator should obtain a quick and accurate sample weight. To minimize dry- ing time, sand experts recommend a weight between 50 and 70 grams. An exact weight is not necessary. Screening—It is not necessary to
run the sand screen prior to running the percent moisture test (unless there is a large amount of foreign material in the sand). Screening the sand can dry it out. Other Issues—If you are experi-
encing lower moisture test results, be careful to look for other potential
42 | MODERN CASTING June 2011
moisture test is sensitive to testing conditions and can be one of the most diffi cult sand tests
to reproduce.
causes beyond testing protocol. Is the moisture to clay ratio down? Is the methylene blue clay level lower? What is the diff erential between the two? Correlating Test Results—How
do you use your test results to help control the green sand system? Some metalcasting sand personnel recom- mend using the percent moisture test not as a control factor but as an indicator of the health of the sand system, particularly the fines in the system and/or methylene blue clay. For example, if methylene blue clay is the main control factor for the sand system, perhaps with compat- ibility measurement as a reference, the clay content will affect the mois- ture level required to achieve proper temper point. If compactability is in the normal range, a low or high moisture reading may indicate a need to raise or lower bond levels or make adjustments to raise or lower fines in the system.
Q
We bake our 356-aluminum castings, which do not require heat treatment,
in the oven for three hours at 900F (482C) to remove the sand cores. Will this affect the casting properties?
for T6 solution heat treatment and 650F (343C) for two hours for an- nealing. At a 900F temperature for three hours, the bake-out process will have some annealing eff ect on cast- ing properties. When the castings are air cooled, they will become annealed, which will change the fi nal properties. Most likely, the yield strength will be reduced, and the percent elongation will increase. T e service requirements of the fi nal component will determine if this change is acceptable. If the component does not require heat treat- ment and the mechanical properties of the as-cast condition (F temper) are satisfactory, these reduced properties may not be signifi cant. However, the practice raises the
A
following concerns: Furnace Temperature Accuracy—
How accurate is the temperature of your core bake-out oven, and how is it measured? What is the thermal profi le in the oven? Some are simple, direct- fi red gas ovens without the forced air circulation common in heat treatment furnaces. In this type of furnace, there may be large temperature variances within the furnace, and it might not be safe to use a temperature as high as 900F. If the temperature measurement is inaccurate, some parts could reach temperatures above the eutectic, where some incipient melting might occur. Distortion—If the part temperature
reaches 900F, there could be distortion during cooling. T e casting should be checked for dimensional accuracy prior to shipping to the customer. Alternative Binders—Many new sand binders are designed to break down at the lower temperatures of aluminum casting and provide easier shakeout. Switching to this technology could eliminate the need for the core bake-out oven.
Recommendations are the opinion of the AFS Technical Department. If you need assistance with a technical issue, email
moderncasting@afsinc.org.
T e American Foundry Society’s book Alumi- num Casting Technol- ogy recommends a temperature of 1,000F (538C) for 12 hours
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