Dynamic Testing of Green Sand
Can metalcasters better monitor sand properties by adopting additional testing methods?
Sam Ramrattan and Mo Khoshgoftar, Western Michigan University, Kalamazoo, Michigan; and HIroyasu Makino, Sintokogio Ltd., Aichi, Japan
(Click here to see the story as it appears in the September issue of Modern Casting.)
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.
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 Univ.’s Metal Casting Laboratory. Physical, mechanical, and dynamic measurements were taken on the samples using a number of common testing procedures, though the major focus was on the thermal erosion test and modified cone jolt toughness tests.
Metalcasting facilities often use additives to solve green sand problems such as flow. Additives can affect the sand, such as waterproofing the clay. The researchers kept the bonding formulation simple to reduce potential errors in preparing the green sand batch and simplifying the analysis. Apart from sand, clay and water, no additives were introduced to the green sand systems. The five bentonite clays used to develop 15 specimens for each green sand systems studied were:
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 (Table 1). 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. Before proceeding with the other tests that require a standard test specimen, the specimen weight needed to be determined. Compacted density can be determined simultaneously. It is important to record the specimen weight because the weight provides useful information regarding changes in sand composition. If the specimen weight increases, the sand’s silica content has increased, because silica is the heaviest component of the sand. If it decreases, either the additives have increased or the amount of dead clay and ash has increased. In this way, it can be used as a guide for determining the need for 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 and wet tensile strength tests are currently used in the industry, the modified cone jolt and thermal erosion tests may allow metalcasters to improve the control of their green sand systems, in a more timely manner.
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.
As compactability increases (increasing moisture) for the same level of clay in the green sand system, WTS increases until 39% compactability, after which, the data indicates a decline (Fig. 1). The green sand system A (100% Western bentonite) had the higher WTS compared to the green sand system B (100% Southern bentonite) but a more dramatic decline was observed in system A. The point of inflection corresponds to the thermal erosion test data.
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 expected, compactability and friability have an inverse relationship. As moisture content in the green sand increases, surface brittleness decreases in the green sand for the same level of bentonite (Fig. 2).
As friability increases, the ability to draw deep pockets decreases, causing the sand to fall away resulting in defects such as sand inclusions. The AFS friability test is run at room temperature and does not add heat or pressure to the sand specimen, which doesn’t mirror a green sand mold experiencing heat and erosive flow from molten metal. When clay in the sand is heated at the mold metal interface, it loses moisture and may go through a phase change. When organic additives to green sand, such as starch, dextrin and cereals, are heated at the mold metal interface, they burn-off. The current test does not recognize that once this interface has changed from the heat, its properties change, leading to increased sand friability. This often results in the creation of casting defects.
In addition, the current friability test does not consider pressure created when pouring from height. Head pressure always comes from the molten metal when it is poured into the mold.
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. Therefore, the current friability test of two specimens being rubbed together for one minute at room temperature does not accurately depict what is happening in a casting situation. The lack of information and realism in this test provide a rationale to improve this green sand test.
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. Figure 3 shows a green sand sample tested at various temperatures. The room temperature sample remains black, the green sand tested at 572F (300C) has a gray color, and the sample tested at 1,292F (700C) appears light gray.
When thermal erosion data is examined as compactability increases for the same level of clay in the green sand system, the percent green sand loss decreases at room temperature (Fig. 4). However, more losses in green sand system B (100% Southern bentonite) occurred for every compactability level tested but especially at the lower compactability levels.
When green sand systems A and B were tested at an elevated temperature the same general trend was seen as tested at room temperature. As compactability was increased, both systems showed less loss. However, the major difference was that a greater percent loss occurred at all the compactability levels but the losses were less drastic with green sand system A (Western bentonite) (Fig. 5).
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 (Fig. 6). Green sand system B (100% Southern bentonite) was superior in ultimate strength for every compactability level tested. Similarly, toughness increased alongside compactability (Fig. 7). 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 modified cone jolt toughness test was able to identify differences among green sand systems at the target compactability. At ambient conditions, the 100% Southern bentonite (sand system B) was able to sustain a greater number of jolts and was stiffer and tougher than sand system A. Further, results from blends (green sand systems C, D and E) indicated no significant improvement in the mechanical properties over sand system A.
The researchers’ results show a potential path to improving new green sand controls via thermal erosion and modified cone jolt tests. In addion 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.
This article is based on the paper, “Dynamic Testing of Green Sand” (15-091), which was presented at the 119th Metalcasting Congress in Columbus, Ohio.