Casting Answers & Advice Advice.

Q Lately, we have had some minor problems with our green sand system, but our sand tests do not show any significant changes. We are concerned that the sand testing is not being performed properly or consistently. What is the proper procedure for sand testing and what should we do to ensure that we are getting accurate data and information from our sand technicians?

Background--A previous installment of this column was devoted in part to answering this question about sand testing procedures. In answer to the question regarding procedures, we referred to the procedures for sand testing that are set forth in the AFS Mold & Core Test Handbook. These procedures are the result of conceptual and experience-related performance practiced by AFS committee members engaged in the daily production of quality casting. The tests and procedures contained in the handbook are presented as "AFS Standards" and are to be considered as guides for process control whether a company already has an established testing program in place or is contemplating augmenting an existing testing program.

The solution for maintaining test accuracy is more difficult to provide. Sand testing must be done with tight procedures. If tests are not performed the same way each time, then the variability of the test procedure may be responsible for any changes in test value and the test may not represent a change or trend in the sand system. As long as testing components have been tested and accepted for use and procedures are followed closely, there should be minimal disagreement in results submitted from the individual technicians responsible on each shift, and the sand test should be a useful tool in tracking the sand system. If test comparisons between shifts show disagreements beyond the "normal" acceptable limits, a refresher course might be in order for all technicians as a group to ensure consistency of the testing.

Additional Advice--In response to the previous answer, a letter to the editor from Dieter Leidel, Tanoak Enter prises, provided further insights on sand testing. He indicated that the published answer might be somewhat misleading regarding the relevance of sand testing to sand system control. He refers to work performed by an AFS sand committee ("Sand System Functions: Requirements and Specifications," 1980 AFS Transactions). Leidel says, "In 1980, an AFS committee published a comprehensive report on green sand control which[ldots]has never been amended or superceded by newer work. One of the conclusions reads: 'Standard green sand testing does not generate enough significant data to design or control sand systems.'"

Leidel says that sand testing is intended to be part of an "audit system," not necessarily a system diagnostic, and refers to the Mass Balance" process of sand system control ["Green Sand System Control: An Inventory (Mass Balance) Update," G. J. Vingas, 1997 AFS Transac tions, adapted in modern casting in February 1998]. Leidel continues that sand testing alone does not give the information necessary to design or control the sand system, and the foundry must be careful not to make major additions or changes to the system based solely on a few changes in test results. Sand testing is designed to provide benchmark information that may indicate the need for system maintenance or change. Changing values in the sand tests can indicate impending problems or changes in the sand system, but sand testing does not necessarily indicate what corrective action needs to be made. That often is complex. In other words, Leidel said, testing may indicate that something is wrong but does not tell how to correct it. Sand testi ng is only one factor in the overall measures required to achieve proper process control.

Q We are an aluminum permanent mold foundry and have had problems with the coating coming off the mold and sticking to the casting surface. What causes this and how can we prevent it?

Background--Beyond the necessity of protecting the iron mold from the aggressive attack of molten aluminum, mold coatings also provide casting surface finish and insulating properties that can aid directional solidification of the casting. The coating properties that are most desired are adhesion, insulating capability, wear resistance and surface texture. Any study of mold coating will lead you to the work of Franco Chiesa, Centre de Metallurgie du Quebec, Trois-Riviers, Quebec, Canada

The insulating property of a mold coating is its ability to limit the transfer of heat generated by the molten metal into the mold material, thus keeping more heat in the casting. Chiesa has shown that a prime factor in insulation is the porosity of the applied coating--a more porous coating has higher insulating properties. The porosity is higher closer to the mold surface where the coating dries faster. A thicker coating exhibits reduced porosity because the insulation of the previous coats decreases the drying time. Thermal conductivity decreases (thus insulation increases) up to a coating thickness of 200 microns. After that, there is little change in insulating properties due to this decreased porosity, so a thicker coating provides only slightly more insulation.

Recommendations--The following suggestions are given to improve the application and effectiveness of mold coatings:

1. Coating should be applied by spraying onto a hot mold of 400F (204C) from a distance of 9-12 in., according to Chiesa. Spraying from a farther distance can cause the coating to dry before contact with the mold surface, decreasing the coating adhesion. This dry-sprayed coating will pull off the mold and stick to the casting surface.

2. Because porosity is a key to insulation, polishing the coating by rubbing with steel wool (which decreases the porosity) increases the heat transfer and can produce a "chilling" effect in the mold. In one study, Chiesa found that "the polished surface's reduced roughness produced a 40% increase in heat transfer across the coating. Consequently, polishing the mold coating to control hot spots should be quite effective."