Excerpt from the Metal Casting Institute website.
Green sand casting is generally considered the easiest casting system because it is possible to use an existing part as a pattern for a one-off project. However, it is also used for some high volume projects also. The major benefit of this process is that the sand is easily recycled with low smell and a low ecological impact. When used to form parts out of aluminum alloys, the sand mixture has an impact on the final quality due to the potential for hydrogen gas pick up in the metal from the moisture in the sand. Therefore, with aluminum, green sand is generally used to form simple shapes and parts that aren't susceptible to leakage.
The process has four primary factors considered important:
- Sand Quality
- Pattern precision and alignment between cope and drag
- Gating and riser design
- Metal Control
Green Sand Quality
The description of "Green Sand" has no bearing on the color of the sand. The name comes from the fact that the sand mold is not "set" it is still in the "green" or uncured state even when the metal is poured in the assembled mold. The molding sand used in this process relies on a well-proven binder system that includes water, clay and some organic compounds. A typical recipe would be:
- Sand Mixture: silica sand (SiO2), chromite sand (FeCr2O4), or zircon sand (ZrSiO4), 75% to 85%, sometimes with a proportion of olivine, staurolite, or graphite.
- Bentonite clay (primary binder): 7% to 13%
- Water: 2% to 4%
There are many Green Sand recipes that vary the proportion of clay, water and sand grains, but they all try to produce a sand mixture that is compactable, yields a good surface finish and provide the ability of the hot molten metal to degas. Green sand casting for non-ferrous metals typically does not use coal additives (as found in an iron foundry) since the CO created does not prevent oxidation. Green sand for aluminum typically uses olivine sand (a mixture of the minerals forsterite and fayalite, which is made by crushing dunite rock).
Quality Control Variables for Green Sand
Measuring and controlling the KPIV's (Key Process Input Variables) should be confirmed by any supplier quality engineer or foundry management audit. The following factors are the most common factors for a typical green sand system used for aluminum casting:
This test is the most frequently performed evaluation at the molding station to measure the compression of a mulled sand as "the percentage decrease in the height of a loose mass of sand under the influence of compaction." Compactability is probably the most common green sand test and can be performed by a lab test method known as "ram compactability." This test is used to confirm how wet or dry the green sand is and the test results indicated whether moisture must be added or reduced. Maintaining a compaction is directly related to the sand will perform in the molding operation and reflects the degree of temper of the green sand. Because it is so important, compactability testing has also been incorporated into automated sand mulling systems.
The moisture test is used to determine the actual percentage of moisture in the molding sand. While this test is not performed as frequently as the compactability test, it's one of the most important KPIV's to the overall "recipe." This is because the moisture content affects the plasticity of the clay bond, which binds the sand during mold filling and prevents sand-related defects. For this reason, the moisture content is maintained within a narrow range.
Active Clay Percentage:
The "Methylene Blue" clay test value is the measurement of the amount of active, or "live," clay present in the evaluation sample from the molding sand. The test determines the amount of exchangeable ions present in the active clay by adsorption of the methylene blue dye. Clay that still has ion exchange capability will contribute to green, dry and hot strength properties of the green sand.
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