Primer on Green Sand Molding
The green sand casting process is the most widely-used process for casting ferrous and nonferrous metals. The process has evolved from a manually intensive operation to a highly mechanized and fully automated process that can produce up to 500 molds/hour depending on complexity, size, and foundry capabilities.
A wide range of castings can be produced via green sand casting. In terms of shape, the process can be used to cast parts as simple as ingots and as complex as engine blocks. It is commonly used for ferrous alloys; however, it is used for almost any alloy, including those of aluminum, copper, magnesium, and nickel.
One of the major advantages of green sand casting is its low cost, especially when mass-producing castings on highly automated molding and coremaking equipment. Another major advantage of green sand is its versatility with respect to production. The process can be economically applied to small and large production runs. In addition, the production rate can be increased by making multiple mold cavities in a single mold. Capabilities vary from foundry to foundry so talk with your supplier for more specific information on production volumes, properties, and sizes.
The green sand process uses molds that are made from a compacted or compressed green sand mixture. The green sand mixture consists of sand, clay, water, and various additives used to obtain desired properties, such as improved surface finish.
Silica sand is the most common type of sand used in green sand molding, although zircon, olivine, and chromite sand, as well as ceramic media, are used. The basic steps involved in making green sand molds are:
• The pattern is positioned in the flask.
• The flask is filled with conditioned sand.
• Sand is compacted.
• Pattern is removed from the mold.
• Gating and risering system are completed.
• Cores are placed in the mold, if required.
• Mold is assembled/closed.
Two of the most common automatic green sand molding operations are vertically-parted flaskless and horizontally-parted matchplate molding.
Castings weighing from several ounces to 500 lbs. are common, while some as large as 7,000 lbs. have been made using the green sand molding method.
Green sand castings can be produced with walls as thin as 0.09 in. with no maximum limit (Table 11-1). When thin walls are required, the metalcasting facility will take special precautions with respect to the shrinkage allowance in patterns, mold preparation, venting, pouring technique, and other factors.
In general, the closest tolerances can be kept only for the dimensions that lie entirely in one part of the mold. Greater tolerances are required for dimensions across the parting line, which is subject to variations in the closing of the mold. Casting dimensions also are a function of the degree of compaction of the mold, method of pattern removal from the mold, as well as the pouring temperature and pouring rate.
Parts may be cast with openings or holes as small as 0.19-0.25 in. However, if required, small holes may be more economical to drill than to core. Cores may introduce some problems in a casting, such as shifting or gas porosity, and the designer should evaluate coring vs. machining options. Producing castings with undercuts and special inserts also is possible, but at an additional cost.
Draft on sand casting is on average 0.0625 in./ft., or 1-3 degrees. For manual molding and deeper sections, this allowance may need to be increased. In general, for the convenience of molding, draft allowances on the internal surfaces are larger than the drat allowances on the outer surfaces.
Typical surface finish of green sand castings is in the range of 250-1,000 micro inches, or root mean square (RMS). The surface of the castings depends on physical characteristics of the molding sand, molding method, quality of the pattern surfaces, and the type of metal and pouring temperature. Since the surface finish directly influences the casting cost, designers must be careful not to overspecify an excessively smooth surface.
The amount of machining allowance depends on the type of metal used, casting configuration, casting size, mold characteristics, tendency of the casting to distortion, and the required machining method. Small castings and castings made in large production quantities commonly will have a smaller machining allowance than large castings and castings made in small production runs. For example, typical machine finish allowance for iron castings varies from 0.1-0.4 in. for bores ranging from below 12 in. to more than 80 in. and from 0.09-0.4 in. for outside surfaces in the same dimension range. With more experience, these tolerances can be reduced.
Green sand castings can be heat-treated to reduce residual stresses and improve mechanical properties. Also, by placing chills in the mold, hardness and wear resistance of the casting can be increased at desired locations.
Like other casting processes, green sand casting has its limits. The surface finish of green sand castings is not as smooth as the finish produced from other processes. This results from the fairly rough interface between the sand mold and molten metal. In addition, tolerances in green sand castings are poorer than in other processes due to mold deformation under the metallostatic pressure and influence of heat, mold shift, and unpredictability of casting shrinkage. Thus, some machining is required on castings that are used in assemblies.
Automatic Matchplate Molding
Developed in the mid-1960s to eliminate the operator-specific and backbreaking work of hand molding, the automatic matchplate molding process for green sand produces a mold that is held together without flasks (the frame that typically holds a mold in place) to withstand pouring. When first introduced, the process slashed labor requirements by as much as 50%, significantly reducing production costs.
One of the main reasons that flaskless molding is the most economic automatic green sand molding system is that it takes less initial capital investment and less ongoing maintenance when compared to tight-flask molding systems of equivalent volumes.
The more components the metalcasting facility must maintain in its system (such as flasks, bottom boards, weights, etc.), the more costs it must assume in parts, preventive maintenance, and overall attention to detail in order to maintain capability.
The simplicity of the matchplate molding process also makes it more economical than the cope and drag molding process (in which the two mold halves are produced on separate patterns and separate molding stations). The matchplate cope and drag mold is produced in one station, with a single pattern and one machine squeeze cycle (Fig. 11-2). The reduced complexity reduces the cost of the molding operation.
Vertically-Parted Green Sand Molding
Vertically-parted, flaskless green sand molding creates molds at close tolerances that can be booked together to form a long, straight stack, providing metalcasters the ability to produce high-quality, near-net-shape castings at higher speeds and efficiencies. Since it was introduced more than 60 years ago, this method has become the predominant molding process used in the metalcasting industry for medium to high volume casting production.
In vertically-parted green sand molding, a mold is comprised of six sides: the front (called the swing) and rear (called the ram) pattern plates, the top, the bottom, and the two sides. The positioning of the front and rear pattern plates in the mold chamber can be adjusted to minimize pattern wear and maximize mold density.
Vertically-parted molding can produce high-density molds at rates of up to 500 molds/hour or more without flasks (resulting in significant cost and performance advantages over other processes). Other advantages include:
- Mold thickness is not dictated by a molding line’s “flask size.” Because flaskless molds are made with pattern impressions on both faces, the metalcaster can optimize the mold thickness for the casting(s) being produced. Considerations include the ratio of sand to metal in a mold and the molding rate during production. Cost is minimized as the metalcaster only needs to prepare the sand mixture for what is actually needed.
- Manufacturing operations are simplified.
- The high-density molds produce castings of tight dimensional tolerances with an excellent surface finish.
- The draft angle necessary to draw patterns can be held to a minimum.
- Mold wall movement during pouring and solidification is greatly minimized. This helps reduce the amount of feed metal required during solidification, increasing casting yield.
- Several hundred molds per hour can be produced with great reliability in this process.
- Uptime capability for these vertical molding systems generally exceeds 90%.
Part Selection Criteria
Most castings produced horizontally also can be made in a vertical orientation. The anticipated production volume is a major criterion when considering molding processes for a given job. The vertically-parted molding process is most often considered for medium (10,000-50,000) to high (50,000 and above) production volumes. Even low quantities (down to a single hour’s production) can be produced cost effectively due to the rapid pattern change capabilities of today’s molding machines.
The primary cost concern when considering lower production volumes is tooling cost. To help keep costs to a minimum, the casting engineer can recommend different tooling materials (such as plastics or urethanes) that can accommodate low volume production requirements.
The vertically parted green sand molding process can be considered for most casting configurations within the size constraints of the various machine models. The process is seldom used for complex castings weighing more than 400 lbs. Again, the applicability of a specific casting design is best discussed as early as possible with your metalcasting representative. CS
Interested in reading more from the book on which this article is based? Visit the AFS bookstore at www.afsinc.org to purchase “Designing and Purchasing Metal Castings.”