8 Cost Considerations of Iron Castings
Numerous factors influence the total cost of a finished casting, ranging from design to type of metal to the type of price quotation.
Efficient purchasing of iron castings requires using a considerable amount of specialized information. With the generally increased emphasis on value analysis techniques, purchasing managers must evaluate the many important factors that directly influence the total cost of a component part. These are usually grouped under the general headings of quality, service, price and delivery. The relative importance of these considerations will be determined by each buyer. The agent must establish his or her own weighting system of these factors, based on the particular circumstances.
1. CASTING DESIGN
Size, weight and complexity of a casting are generally the most important cost factors because they directly influence the materials and labor required for its production. Weight of the metal in a casting is also important, but it no longer dominates the cost, as was true at one time. Overall size of a casting is also important because several tons of sand and other materials are involved in the casting process for each ton of castings produced. A large, thin casting that must be made in a large mold can be much more expensive than a heavier casting made in a small mold.
Cored castings are also more costly. Cores have to be made ahead of time, transported to the molding department and placed carefully in the mold. But equally important is the extra cleaning and trimming that is usually necessary on cored castings. However, complex castings are not necessarily more expensive than simple castings. Some types of complexity can be molded into the casting without increased production costs.
2. TYPE OF METAL
Certain types of iron are more expensive to produce than others. This is especially true of alloyed irons. In addition to metal costs, some irons are more difficult to cast than others and thus more expensive to specify. Additional metal must be poured into the mold in the form of flow channels and feed heads to assure a sound casting. For some castings, weight of the actual metal in the casting is less than half of the total weight poured. This is especially important when considering the cost of alloyed iron since the alloys must be added to all of the iron poured into a mold.
3. QUALITY REQUIREMENTS
Iron castings are produced in a very wide range of quality, from counterweights to airborne turbine housings. The term quality really indicates how well a casting meets the requirements of the purchaser and how consistently those requirements are met by castings in production quantities. Consistent quality from order to order and from the first to the last casting in an order is of primary importance and should be foremost in the identification of a dependable casting supplier.
Sometimes, the term “high quality” is improperly used to indicate superiority in a particular property, such as high tensile strength or excellent surface finish. But it should be kept in mind that specifying unnecessary quality requirements, which increases the cost of a casting, is contrary to good purchasing practices. The chief concern should be meeting acceptable quality levels. The main parameters of casting quality are properties of the metal, soundness of the casting, accuracy and consistency of dimensions, and smoothness of finish. Excessive requirements for any of these increase the direct cost of a casting because:
- A simplified production method may be available but might not meet the specified quality level.
- More highly skilled workers and/or more production time may be
- More expensive materials and/or equipment may have to be used.
- Additional inspection may be necessary.
- A higher percentage of rejects may result.
However, quality requirements that are necessary should not be compromised. Decreased cost is not a valid reason for purchasing castings that are below the required standard. Any initial savings are very likely to be offset by increased finishing cost or production problems that may arise from using such castings. It is unfortunate that this latter cost is not always evident. Although quality requirements can be designated by specifications or sample castings, this is often difficult to do, and casting quality is more generally thought of in terms of the established reputation of the foundry.
The various quality certifications that individual foundries may have achieved are not used to specify the casting quality; the customer is responsible for specifying the level of quality that the casting must meet to be acceptable. The foundry certification provides the purchaser with a measure of assurance that the processes used by the foundry will be consistent from hour-to-hour throughout the year and, therefore, the castings will be consistent in the quality level supplied.
The type of pattern equipment provided is an important factor in the total cost of a casting. The term pattern equipment includes core boxes, if necessary, and also any jigs or fixtures used for checking or assembling cores, or for checking the dimensions of the casting. Typically, the pattern equipment is owned by the customer with the foundry storing and maintaining the equipment.
In addition to different types of patterns, there are wide differences in pattern quality. Quality differences are especially evident in wood pattern construction. Quality standards for metal patterns also vary greatly with respect to the kind of metal used and the accuracy to which they are made and finished. High-quality patterns that are designed specifically for efficient production of large quantities of castings are very costly as compared to a pattern made for only one, or at the most, a few castings. The important question then, in selecting pattern equipment, is how many castings are to be made over which the pattern cost can be amortized. This becomes a difficult question in the case of developmental work, where the possibilities for future production are uncertain. Under this condition, past experience indicates that it is usually best to plan pattern equipment for only immediate requirements.
Short-time pattern use and early replacement with a better pattern seems wasteful. This alternative is only advisable when it provides an opportunity for modifying or improving the original design and changing the molding equipment to a machine that is more suitable to the annual production requirements. When the replacement pattern can be cast, the original pattern may often be made or modified so that it will also serve as the master pattern for casting the production pattern.
Patterns, particularly inexpensive ones, should not be employed beyond their useful life. When they are, casting quality usually suffers and production costs rise. The type of pattern to be used also must be suitable for the particular foundry in which the castings are to be made. When new patterns are obtained for production quantities, it is often desirable to have quotations based on more than one type of pattern in order to determine which one provides the lowest total casting cost. Where pattern equipment is already available and conditions have changed, it may be possible to modify the pattern to reduce casting cost or improve quality. Patterns should be kept in good repair because off-dimension or irregular castings increase machining and finishing costs and may impair appearance or reduce the quality of the finished product.
5. POST-CASTING OPERATIONS
Often, the foundry can perform additional production operations that may increase direct cost of the casting, but will reduce the total cost of the finished product. Heat treating, painting or coating, and more sophisticated inspection and testing procedures are typical additional operations that may be performed efficiently by the foundry. For quantity production runs, castings may be justified or targeted in a gauging fixture to provide accurate locating points for subsequent machining operations. This assures that the correct amount of stock is available to be removed in machining each surface.
6. TECHNICAL SERVICES
Close cooperation between the foundry and its customer is the rule rather than the exception. But for some castings, an unusual amount of service or technical assistance may be required from the foundry. This commonly occurs in development of new products. Assistance with casting design, information on properties and metallurgy, troubleshooting, pattern assistance or pattern models, and experimental castings may be provided by the foundry.
These services may be billed to the customer, at cost, or included in the price of the castings. But, in either case, quotations from a foundry that stands ready to provide its customers with this assistance cannot be directly compared with quotations from a foundry that does not. However, the latter type of foundry should not be ignored for it can be a good, economical source for castings. This is particularly true where special problems or requirements do not exist and where additional services are not necessary. When a complex casting is to be produced in large quantities, a model casting may be made to assist in planning production equipment. The model casting will assist in establishing the most efficient casting method by providing an opportunity to establish critical factors such as the parting line and core prints on the actual shape. This can also help in planning for subsequent processes such as machining. Solid modeling in computer-aided design (CAD) can reduce the number of make-test-modify cycles in the design and prototype stages of product development.
7. ORDER QUANTITY
An important factor in selecting appropriate pattern equipment is order quantity. Even with a given pattern, the order quantity may have an important influence on casting cost. One reason for this is a number of standard operations in the foundry must be performed for each casting order, regardless of the quantity involved.
Some of these are:
- Quoting, acknowledging and recording the order.
- Scheduling production and issuing shop work orders.
- Getting pattern equipment out of storage, inspecting and transporting it to production departments.
- Setting up patterns for production (i.e., laying out or attaching to machines and lining up proper flasks).
- Delivering core equipment to the core room (if cores are required), making cores, and transporting them to the molding line.
- Melting, pouring, cleaning, finishing, inspecting.
- Shipping and billing.
Efficiency of production operations is also greatly influenced by quantity. When an order is issued subject to release, the total quantity is important in the selection of pattern equipment, but each release is normally considered as a separate order by the foundry, since each production order requires separate processing.
The purchasing manager can determine the most economical lot size for a casting when he or she knows what the casting requirements will be for some time in advance. Just-in-time manufacturing techniques can require the same part to be delivered by the foundry as frequently as twice a week, with some instances of daily and hourly delivery. Possible changes in product demand and casting design or prices may not be equated directly in numbers, but are incorporated as a matter of judgment by experienced buyers.
8. TYPE OF PRICE QUOTATION
At one time, metal was very expensive, in comparison to labor, and castings were commonly sold by the pound. This was a simple buying method and was successful because then the weight of a casting was a more important part of total cost than was the workers’ time to make it. Today this is no longer true.
With the tremendous increase in cost of labor and mechanized equipment in a modern foundry, the value of the metal in most castings has become a secondary factor. Accurate cost accounting must include cost of the workers’ time, value of materials and overhead. This provides price quotations that are based on cost per casting, and a ready reference for the purchaser to arrive at his own unit cost. The cost-per-piece can, of course, be easily converted to cost-per-pound. But such a figure is of value only in general comparison: it cannot be justly compared to other castings that may be ordered in different quantities or require more or less man-hours to be produced.
Some foundries, however, still retain the price-per-pound method of quotation for specific types of castings. Thus, equivalent quotations based on price-per-pound and price-per-piece may show variations when compared for individual castings. This is particularly true when the quotation is based on an estimated casting weight. Variations of actual weight from estimated weight still affect the cost more on a price-per pound basis than on a price-per-piece basis.
This article is an excerpt of Iron Castings Engineering Handbook. It is available for purchase at www.afsinc.org.
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