Casting answers & advice.

Q: We are a small foundry making aerospace and precision castings in aluminum. We are using oil-based green sand and find there is little information on testing and technical support for this sand. Can you provide us with any information on this subject?

Background: Oil-bonded sand often is used for castings requiring an exceptionally fine surface finish. Oil-bonded sand is similar to traditional water-bonded green sand--a mulled mixture of sand, liquid and clay--but is mulled using oil as a tempering agent instead of water. A specialized clay-bonding agent also is used.

Water expands significantly when turned to steam as it contacts the hot metal in the mold cavity, so sand foundries use sand permeability and venting as a regular part of their sand molding process. Oil is vaporized with a lower expansion volume and thus creates less gas than water. This allows the use of finer sands with less permeability (120-180 grain), often producing a finer casting finish and greater casting detail.

Oil also produces a reducing mold atmosphere and aids casting surface finish. Oil sand can be stored indefinitely and requires only moderate maintenance--primarily aerating and riddling.

Re-mulling and sand reconditioning also is required with oil-bonded sand to maintain adequate green strengths (typical green strength range for oil-based sand is 7-12 psi). Green strength is influenced by metal type and sand to metal ratios.

Recommendations: Although oil-bonded sand often is used as facing sand to enhance casting finish and detail, it also can be used in complete sand systems. When used in a full sand system, regular testing is helpful to monitor changes in the system sand. Correct sand type and grade and the correct oil are critical ingredients for proper mixing.

As in water-bonded sand, moisture content, clay content, compactibility and green compression are helpful tests and can be run using conventional sand testing equipment. Other useful tests include permeability, percent oil, percent clay and oil-to-clay ratio. Although some of these tests may require specialized equipment or procedures, they will provide the information to help ensure sand system consistency. Your oil-banded sand provider may have the equipment and facilities to assist with testing.

Q: We are a small foundry having casting problems that appear to he sand related. We perform little testing of our green sand system. What types of testing should we consider, and how do we use test results to help control our green sand system?

Background: Green sand testing often is characterized incorrectly as a means of sand control. A green sand molding medium in a sand system is continuously changing in physical composition and quality because of the variable effects of the metalcasting process.

The primary purpose of sand testing is to obtain data--facts about the current status of the green sand sample tested. Testing provides known values and some verifiable properties of the green sand at the specific time of testing. The various sand tests either quantify the amount of an individual component (such as the moisture or clay content) or qualify a physical property (such as the strength of the prepared green sand).

While testing is only a small portion of a sand control program, proper testing is the foundation for a comprehensive sand system control program. In addition to standard sand tests, all facets of the sand production system should be known and monitored.

Recommendation: Table 1 provides an overview of sand system control. This chart is broken into tiers for a better understanding of what should be accomplished at each level.

Tier 3 is the data collection stage. This is the "grass roots level" of your sand control program where data is collected through sand testing. For results to be accurate and consistent, testing requires trained personnel following clear procedures and using equipment that is calibrated and well maintained.

Tier 2 is the analysis and evaluation stage of sand control. Once test data is obtained, considerable effort should be devoted to sand analysis. The test results become a history and database for reference that will help determine the significance of test data and trends. Careful analysis will let foundry personnel know the current status of the sand system and lead to corrective action.

Tier 1 is the decision-making process based on the analysis of the data. Once the test results are obtained, sand processing parameters are detailed and the analysis of the data completed. The foundry can make the necessary adjustments to maintain the desired sand system properties. Corrective action may include muller controls, core sand additions, equipment, testing procedures, personnel and/or training.

The information in this column was supplied by the AFS Green Sand Test Committee (4-D).

Table 1. Suggested Sand Control Program

Tier 1  Sand System Operation

           Muller Controls
           Additive Rates & Formulations
           Temperature
           Sand-to-Metal Ratios
           Fines Control
           Core Sand Influx Sand Analysis

Tier 2  Statistical Process Control

           Trend Analysis
           Graphs of Results
           Process Variation
           Data Significance
           Green Sand Properties

Tier 3  Sand Testing

           Personnel
           Training
           Procedures
           Equipment