Green sand casting is generally considered the easiest casting system because it is possible to use an existing part as a pattern for a 1-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 does have 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 4 primary factors considered important:

1. Sand Quality
2. Pattern precision and alignment between cope and drag
3. Gating and risering design
4. 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 hat 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:

Compactability: 

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.

Moisture Testing: 

 The moisture test is used 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.  

Permeability Testing:

Permeability is a test to evaluate the ability of the sand mixture to allow gases formed during mold filling to permeate the sand, thereby venting the gases away from the metal front and through the rammed green sand mold.  In this case, the most important factors in regulating the degree of permeability include sand grain size, shape, distribution and type.  Also, the binder and how it combines with the sand fines will also inhibit the permeability after it has been activated with the moisture.  With this KPIV, there are both Low and High limits established to control the process.  While low permeability can result in a smooth casting surface finish, it can also result in cause blows, pinholes and expansion defects.  Similarly, high permeability reduces gas pressure but can result in mechanical penetration and a rough surface finish.

Green Compression Strength:

Green compression strength indicates the maximum compression stress the sand mixture is capable of sustaining and is used to control the rate of clay addition to the green sand system.  Similar to compactability, it evaluates the overall performance from the degree of mulling,  clay content, and additive affects.  Again, with this KPIV, there are both Low and High limits established to control the overall molding process.  While a low green compression strength provides good flowability of the sand around a pattern shape, it can also result in broken molds and poor draws.  Other factors related to a low strength reading are low clay content, dry sand and poor mulling.  A high green compression strength will provide stronger molds but will also make a difficulty in shakeout, result in poor casting dimensions, require high ramming resistance and higher cost. High clay content is the primary reason for high strength readings.

Pattern precision and alignment between cope and drag

In the most simple sense, a machined prototype part, or even an existing part, can but used to form an impression in the green sand mold.  However, most serial production green sand foundries using traditional cope and drag mold set rely on a pattern match plate to create the impressions for both cavity and runner sections.  These types of patterns can be made from Wood, Aluminum, Plastic, Iron, or Steel.  The investment in the pattern is the most expensive tooling component for molding in green sand.  However, if internal cores are required, these pieces are made from a resin-bonded sand molded in a machined core-box.  Resin bonded cores are produced separately and stored until needed in the green sand molding line.