Can Metalcasters Better Monitor Sand Properties by Adopting Additional Testing Methods?
Originally published in the September 2015 issue of Modern Casting.
by Sam Ramrattan, Mo Khoshgoftar and Hiroyasu Makino
Sand property testing tells a metalcasting facility how well it is controlling its green sand system. Many of these current tests do not provide as timely results as desired, but certain laboratory tests are essential in properly controlling processes. Due to the nature of the variables that are being monitored, some test data, such as loss on ignition (LOI) values, tend to change slowly, while other test results, such as green compression strength and compactability, may fluctuate more abruptly. Current sand testing provides insight into sand quality, but the metalcasting industry as a whole would benefit from methods to better and more quickly quantify the effectiveness of sand-clay bond strength in green sand molds.
The green sand samples subtly change color after thermal erosion tests (TET) for various temperatures.
Research was conducted to determine if a metalcasting facility could improve its monitoring of clay and moisture levels in green sand with non-standard testing methods, such as a thermal erosion test (TET) and modified cone jolt toughness test. Using both ambient and elevated-temperature testing procedures may potentially provide metalcasters with more relevant information about green sand system control.
Methodology & Preparation
All green sand data was generated in Western Michigan University’s Metal Casting Laboratory. Physical, mechanical, and dynamic measurements were taken on the samples using common testing procedures, though the major focus was on the thermal erosion test and modified cone jolt toughness tests. The five bentonite clays used to develop 15 specimens for each green sand systems studied included:
A. 100% Western bentonite.
B. 100% Southern bentonite.
C. 75% Western bentonite, 25% Southern bentonite.
D. 50% Western bentonite, 50% Southern bentonite.
E. 25% Western bentonite, 75% Southern bentonite.
AFS sand tests measured properties of each green sand system. The key variable was compactability, a mechanical property related to the percentage decrease in height during compaction at the molding machine. Moisture and compactability are the key tests for controlling water additions. Density, specimen weight, permeability and grain fineness are considerations when determining new sand additions.
Procedures & Results
The erosive flow of molten metal is a major cause of casting defects. The brittleness of green sand will only serve to augment this issue. Four green sand tests are indicative of green sand brittleness.
- Friability test
- wet tensile strength test
- modified cone jolt test
- thermal erosion tests
Wet Tensile Strength (WTS) Test: This is useful for determining the quality of incoming bentonite consignments. A higher WTS value means the sand blend will resist scabbing. During casting, water from the sand adjacent to the molten metal is driven back, creating a condensation zone between the dry and wet sand. The strength of the sand in this layer is considered the wet tensile strength.
Friability Test: Friability measures the sand’s surface brittleness and abrasion resistance on mold edges, corners, parting lines and abraded mold surfaces developed during molding, core setting and handling. Loose sand can result in sand inclusion defects on casting surfaces. In general, friability is inversely related to compactability. A small drop in compactability, or a brief air-drying period, will produce a large increase in friability. Higher clay levels will reduce friability. As moisture content in the green sand increases, surface brittleness decreases in the green sand for the same level of bentonite.
Finally, the friability test does not represent the ratio of metal to sand. If the ratio is wrong, the sand could be overheated, taking the moisture out of the sand and potentially altering the clay. These changes will cause the sand to become more friable.
Thermal Erosion Testing: The thermal erosion test is similar to the friability test in purpose, but the information acquired is different. It measures the bulk surface abrasion characteristics of a specimen at elevated temperature. It records the time, temperature and amount of abraded sand. The discoloration of the sand due to elevated temperatures—a sign of dead clay related to heat damage—is typically observed after the test.
Modified Cone Jolt Test: This test measures bulk brittleness and is related to difficulty in pulling deep pockets in a pattern. The modified cone jolt toughness test is directly related to compactability and measures the sand’s ability to absorb energy. A computer and data acquisition system is used for controlling, monitoring and plotting graphs of jolts versus displacement of a specimen. A standard AFS cone jolt specimen is placed between the base and the cone. When the test is initiated, a solenoid is cycled to automatically pick up and drop the specimen, while a linear voltage displacement transducer measures longitudinal displacement. The data acquisition system automatically logs and plots the jolts versus displacement curves. The test stops automatically when the specimen splits or displaces 0.05 in. (1.25 mm) vertically.
As compactability increased, the number of jolts sustained increased, meaning increased strength. Green sand system B (100% Southern bentonite) was superior in ultimate strength for every compactability level tested. Similarly, toughness increased alongside compactability. Green sand system B (100% Southern bentonite) was able to absorb more energy for every compactability level tested.
The thermal erosion, wet tensile strength, modified cone jolt toughness and the friability tests were able to differentiate among various compactability levels. Friability and thermal erosion tests (both ambient and elevated temperature) indicated that 100% Western bentonite (sand system A) had superior abrasion resistance. In support of this finding, the green sand system A had the higher wet tensile strength.
The researchers’ results show a potential path to improving new green sand controls via thermal erosion and modified cone jolt tests. In addition to currently widespread testing methods, non-standard procedures including ambient and elevated temperature testing may provide a two-pronged approach to sand testing. Thermal erosion and modified cone jolt toughness tests may improve the measurement and control of thermomechanical properties of green sand systems.
(Originally printed in the September 2015 issue of MODERN CASTING.)
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