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How to Purchase Steel Castings From a Foundry

Steel castings begin to solidify at the mold wall, forming a continuously thickening envelope as heat is dissipated through the mold-metal interface. The volumetric contraction that occurs within a solidifying cast member must be compensated by liquid feed metal from an adjoining heavier section, or from a riser which serves as a feed metal reservoir and which is placed adjacent to, or on top of, the heavier section. The lack of sufficient feed metal to compensate for volumetric contraction at the time of solidification is the cause of shrinkage cavities. They are found in sections which, owing to design, must be fed through thinner sections. The thinner sections solidify too quickly to permit liquid feed metal to pass from the riser to the thicker sections.

Pattern equipment design and the resultant costs can constitute a major source of misunderstanding between customer and supplier foundry. The need to construct new pattern equipment when existing equipment is available, a requirement for a full split core box in place of a half core box, pattern material, and mounted or loose patterns are but a few of the many areas of discussion that effect the cost of the equipment. Invariably, the lowest casting cost and highest casting quality evolve from the more sophisticated pattern equipment, which generates the highest pattern cost.

Patterns - Minimum Section Thickness

The rigidity of a section often governs the minimum thickness to which a section can be designed. There are cases, however, when a very thin section will suffice, depending upon strength and rigidity calculations, and when castability becomes the governing factor. In these cases it is necessary that a limit of minimum section thickness per length be adopted in order that liquid metal will completely fill the mold cavity in these thinner sections.

Molten steel cools rapidly as it enters a mold. In a thin section, close to the gate, which delivers the hot metal, the mold will fill readily. At a distance from the gate, the metal may be too cold to fill the same thin section. A minimum thickness of 0.25 in (6 mm) is suggested for design use when conventional steel casting techniques are employed. Wall thicknesses of 0.060 in (1.5 mm) are common for investment castings and sections tapering down to 0.030 in (0.76 mm) can readily be achieved.

Patterns - Draft

Draft is the amount of taper (or the angle) which must be allowed on all vertical faces of a pattern to permit its removal from the sand mold without tearing the mold walls. Draft should be added to the design dimensions while maintaining minimum metal thickness.

Regardless of the type of pattern equipment used, draft must be considered in all die casting finishing. (Draft can be eliminated by the use of cores; however, this may add significant cost.) In cases where the amount of draft may affect the subsequent use of the casting, the drawing should specify whether this draft is to be added to or subtracted from the casting dimensions as given.

The necessary amount of draft depends upon the size of the casting, the method of production, and whether molding is by hand or machine. Machine molding will require a minimum amount of draft. Interior surfaces in green sand molds usually require more draft than exterior surfaces. The amount of draft recommended under normal conditions is about 3/16 in. per ft. (approximately 1.5 degrees), and this allowance would normally be added to design dimensions.

Parting in one plane facilitates the production of the pattern as well as the production of the mold. Patterns with straight parting lines (that is, with parting lines in one plane) can be produced more easily and at lower cost than those with irregular parting lines. Casting shapes which are symmetrical about one center line or plane readily suggest the parting line. Such casting design simplifies molding and coring, and should be used wherever possible. They should always be made as "split patterns" (separate cope and drag) which require a minimum of handwork in the mold, improve casting finish, and reduce costs.

A core is a separate piece (often made from molding sand) placed inside the mold to create openings and cavities which cannot be made by the pattern alone. Every attempt should be made by the design to eliminate or reduce the number of cores needed for a particular design to reduce the final cost of the casting.

The minimum diameter of a core which can be successfully used in steel castings is dependent upon three factors:

1. The thickness of the metal section surrounding the core
2. The length of the core
3. The special precautions and procedures used by the supplier foundry.

The adverse thermal conditions to which the core is subjected increase in severity as the metal thickness surrounding the core increases, and the core diameter decreases. These increasing amounts of heat from the heavy section must be dissipated through the core. As the severity of the thermal conditions increases, the cleaning of the castings and core removal becomes much more difficult and expensive.

The thickness of the metal section surrounding the core, and the length of the core, both affect the bending stresses induced in the core by buoyancy forces and, therefore, the ability of the supplier foundry to obtain the tolerances required. If the size of the core is large enough, rods can often be used to strengthen the core. Naturally, as the metal thickness and the core length increase, the amount of reinforcement required to resist the bending stresses also increases. Therefore, the minimum diameter core must also increase to accommodate the extra reinforcing.

The cost of removing cores from casting cavities may become prohibitive when the areas to be cleaned are inaccessible. The casting design should provide for openings sufficiently large to permit ready access for the removal of the core.

Tolerance refers to the dimensional accuracy achievable for a given production method. For the green sand casting process, mold expansion, solidification shrinkage, and thermal contraction all influence the tolerance of the finished part. Consequently, there are limits for tolerances in an as-cast part. Subsequent machining is commonly employed when a tighter tolerance is required.

In the final analysis the supplier foundry is responsible for giving the designer a cast product that is capable of being transformed by machining to meet the specific requirements intended for the function of the part. To accomplish this goal a close relationship must be maintained between the customer¡¯s engineering and purchasing staff and the casting producer. Jointly, and with a cooperative approach, the following points must be considered:

The casting process, its advantages and its limitations. Machining stock allowance to assure clean up on all machined surfaces. Design in relation to clamping and fixture devices to be used during machining. Selection of material specification and heat treatment. Quantity of parts to be produced.

It is imperative that every casting design, when first produced, be checked to determine whether all machining requirements called for on the drawings may be attained. This may be best accomplished by having a complete layout of the sample casting to make sure that adequate stock allowance for machining exists on all surfaces requiring machining. For many designs of simple configuration that can be measured with a simple rule, a complete layout of the casting may not be necessary. In other cases, where the machining dimensions are more complicated, it may be advisable that the casting be checked more completely, calling for target points and the scribing of lines to indicate all machined surfaces.