How To Purchase Steel Castings From A Foundry
The ordering of metal castings takes a certain amount of time and energy to qualify a potential supplier foundry
. To get the best value from the metal casting also requires a cooperative effort on the part of the customer and the supplier foundry from the early stages of the design through to the end manufacturing process. Good planning ahead of time will pay dividends for both you (the customer) and your supplier foundry.
The purpose of requesting a quotation for a steel casting is basically to determine the lowest purchased casting cost. The customer then must weigh all of the provisions of the quotation including exceptions taken to drawings, specifications, and processing requirements, as well as supplier foundry experience, tooling requirements, tolerances, finish allowances, and delivery. Such factors as reduced machine work, better tolerances, improved delivery schedules and reliability are particularly important to determine the lowest end cost of the casting.
To avoid misunderstandings, reduce costs, and expedite the processing of quotations, all or some of the following information should be included in a request for a quotation:
Design - What is the part? See DESIGN below.
Quantity - What is the anticipated or required volume, both present and future?
Material and inspection requirements; what should the part be made of, and how should the part be tested before delivery? ASTM or other nationally recognized specifications should be used whenever possible to identify the material and inspection requirements. See MATERIAL SPECIFICATIONS and SOUNDNESS below.
Actual or estimated casting weight. Actual weight information is preferred. Estimates can be provided by the supplier foundry in the absence of actual weight information, but this may require offering prices that are subject to changes based on the actual weight of the casting(s) in question at the time of production.
Drawing. Machine drawings are preferred over casting drawings. Drawings or sketches are mandatory if samples or patterns are not available. Drawing should include dimensional tolerances, indications of critical areas and surfaces to be machined. See MACHINING below.
Pattern. If patterns and core boxes are available, the request for a quotation should indicate the type, condition and set up of the equipment. See PATTERNS below.
Production/delivery schedules required. Present and anticipated need should be included in quotation requests.
Beyond these basics, there are levels of customer requirements that could include supplier foundry liabilities, which affect the casting cost drastically. These could include receiving inspection acceptance and back charge policy, casting return policy, expediting procedures, and sophisticated controls not normally associated with the standard inquiry. A complete understanding of these areas is best developed by an open relationship between the customer and the supplier foundry representative, and the professional attitudes and experiences that both can provide during the quotation evaluation phase.
DESIGN
To achieve the most efficient production and the highest quality product, the part should be designed to take advantage of the flexibility of the casting process. The supplier foundry must have either the designer's drawings or pattern equipment and know the length of the run (number of parts to be made).
Castings are generally furnished with un-machined as-cast surfaces, unless otherwise specified. To take advantage of the casting process, the supplier foundry should also know which surfaces are to be machined and where datum points are located. The acceptable dimensional tolerances must be indicated when a drawing is provided. Tolerances are normally decided by agreement between the supplier foundry and customer. Close cooperation between the customer's design engineers and the supplier foundry is essential to optimize the casting design.
MATERIAL SPECIFICATIONS
Industry standard specifications provide the casting customer with the tools necessary to establish criteria for almost any casting application. These specifications do not preclude special requirements that the customer's technical staff members may require. Variations from standard specifications can result in misunderstandings, higher costs and disqualification of potential supplier foundries. If exception is taken to a provision in the main body of a specification requirement (as opposed to taking exception to a supplemental requirement of a specification), the resulting casting cannot be held to compliance with that specification.
Mechanical properties may be verified by the use of test bars cast either separately or attached to the castings. The mechanical properties obtained represent the quality of the steel, but do not necessarily represent the properties of the castings themselves, which are affected by solidification conditions and rate of cooling during heat treatment, which in turn are influenced by casting thickness, size and shape. In particular, the harden ability of some grades may restrict the maximum size at which the required mechanical properties are obtainable. Short of destructive testing of an actual casting sample, the use of a test bar is the best measure of the steel quality.
SOUNDNESS
Soundness of metal components refers to the level of freedom from impurities and/or discontinuities such as sand inclusions, slag inclusions, macro porosity, and shrinkage.
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.
Testing ensures that the material meets the requirements of the specification; consequently, testing may be mandatory. More frequent testing or other tests may be imposed, by use of supplementary requirements of material specifications or general requirement specifications. In addition to specifying test methods, acceptance criteria must be agreed upon between the purchaser and the foundry. The more testing and tighter the acceptance criteria the more expensive the product will be - without necessarily increasing quality or serviceability. Hence, the extent of testing and acceptance criteria should be based on the design and service requirements.
It is impossible to produce a defect free casting, only castings with defects of varying degrees of acceptability. The acceptance and/or rejection of such castings can only be determined by examination and analysis of parts (in accordance with internationally recognized standards such as ASTM) based on customers' formal engineering requirements. A defect in one application may not be a defect in another application and it is impossible to make a casting without some kind of flaw. The size of flaw(s) can vary significantly, and what is acceptable and what is defined as a REJECTABLE defect depends on agreement between the supplier foundry and the client prior to production. Large cavities often exist in thick-section castings and can be perfectly acceptable depending on the application and the location within the casting. On the other hand, some applications are very critical and tiny flaws (or even micro-porosity - as defined by a specific NDT process and acceptance/rejection level) may be considered as defects that may be detrimental to the intended use of the product.
Acceptance and rejection criteria for castings production must be determined at the time of quotation and certainly at the time of order, as such criteria affect the price of castings, as well as the production procedures and processes used to produce the castings.
PATTERNS
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 casting designs. (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.
Patterns - Parting Line
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.
Patterns - Cores
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.
MACHINING
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.
by: Brad Done
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