Where:
Permeability Number = (V x H) (P x S x T)
V: Percolated volume (ml) H: Height of test sample (cm) P: Air pressure (g/cm2 S: Area of sample (cm2 T: Time in minutes
) )
The permeability of a sand mold or core is affected by several factors, including the sand’s size, shape, and distribution, and the method of sand compaction in the mold or core box. Furthermore, permeability is directly affected by the quantity of resin in the sand. Permeabil- ity testing is one of the sand control tests performed on a regular basis at most foundries.3
The application of a
coating onto the surface of a sand mold or core reduces its permeability considerably, due to the solids depos- ited by the coating. These solids are meant to act as a barrier between the sand and the molten metal flowing through the mold or around the core. They are capable of performing their function even though they form a layer typically about 1 mm thick. These are all indications of the strength of the solids layer, and consequently a coated specimen is likely to be significantly less perme- able than an uncoated one. A Gerosa Simpson perme- ability tester (Figure 11) was used to perform the perme- ability tests conducted in this experiment. However, the specimen holder was designed and fabricated at Western Michigan University. A special rubber gasket was used between the specimen and the holder to provide a seal. Additionally, a plug was used to restrict airflow in order for the Gerosa Simpson machine to detect the perme- ability of uncoated specimens.
The MQI test which is inversely related to permeability, was also performed. The MQI number is a measurement of the resulting back pressure developed from resistance of airflow through a mold or core.3
The MQI unit (Figure 12) is equipped with an air pump, air tubing, and a rub-
Equation 1
ber/foam contact head connected to the end of the tubing. An MQI unit typically is deployed somewhere along the molding line to perform real time measurements on the molds waiting to receive the molten metal. With some modifications to the original rubber contact head, this in- strument was utilized with the Western Michigan Univer- sity specimen holder as a second means of measuring the coating thicknesses.
Procedure:
The specimen was secured into a holder which was then fixed to the permeability tester. The test was started and the permeability measured. The holder with specimen was removed and attached to the MQI unit for measure- ment. This sequence was repeated three times for each specimen prior to placing the specimen back into the des- iccant chamber., The purpose was to investigate how two tests track coating thickness on chemically bonded sand.
Findings:
Higher mold and core venting (permeability) is desir- able in the foundry industry since it can avert some gas related casting defects. The venting characteristics of chemically bonded molds and cores have been difficult to measure. With the addition of refractory coatings to chemically bonded sand, measurement becomes espe- cially challenging. The process of sectioning and mea- suring the coating layer thicknesses using a microscope is tedious, impractical, and time consuming for working foundries. For this reason, permeability was considered as a possible method for a foundry to control its coating thickness. An inverse relation between the permeability and MQI has long been established in the green sand foundry industry. However, the purpose here was to in- vestigate how these two tests track coating thickness on chemically bonded sand.
Uncoated specimens tested using the custom made speci- men holder and plug had a permeability number of 209 ±3; it is important to note that the specimen holder and plug has a permeability number of 221 ±3 with a flow rate
Figure 11. Permeability tester with new permeability accessory attached.
14
Figure 12. MQI with new permeability accessory attached.
Figure 13. Specimen holder accessory with specimen and gasket.
International Journal of Metalcasting/Spring 11
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