Efficient and effective purchasing of cast components requires using a considerable amount of specialized information. With the increased emphasis on value analysis techniques, purchasing agents must evaluate the many important factors that directly influence the "total cost" of a component part. The total cost factors are usually grouped under the general headings of quality, service, price and delivery. Each buyer will determine the relative importance of these factors, based on particular circumstances. Since a universal formula isn't possible, the following information will assist purchasing managers by covering the important criteria that affect the cost of a cast component. It is important that the casting supplier be included during the early stages of the design and purchase planning.
Regardless of the type of foundry, dollar costs will break down into three main categories: materials, direct labor and expenses. Materials may be subdivided into raw materials (primary metal and alloying constituents) and process materials (coke, sand, molding materials, bonding agents, shot, paint, etc). Automation has markedly improved productivity in the foundry industry, but labor remains a high percentage of costs. Expenses are extremely variable depending on the foundry facilities and the cost accounting procedures.
Although there is no magic formula that can be universally applied to foundry costs, it is possible for the buyer to develop a basic understanding of the cost factors for any foundry or particular cast component. Numerous factors influence the total cost of a finished cast component. These are considered in two groups: factors directly affecting the cost of cast component and indirect cost factors for a finished casting. Dimensional tolerances and allowances are also discussed since they have an important bearing on casting costs.
Size, weight and complexity of a metal 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 cast part also is important, but it no longer dominates the cost as was true at one time. Overall size of a casting also is important because several tons of mold material and other expendables are involved in the casting process for each ton of cast components produced. A large, thin casting that must be made in a large mold can be much more expensive than a heavier, simpler part made in a small mold.
When designing a cast component, complexity also is an important factor. In general, the more complex a design, the more difficult it will be to manufacture. This may lead to higher costs. However, this is not an ironclad rule. With careful design, complexity can be molded into the metal casting without increased production cost. To minimize the cost of complex designs, the engineer should call upon the foundry to make manufacturing suggestions at the early design stage. The foundry can help to minimize costs and problems with such things as draft, fillet radius, parting line locations, coreprints, wall thickness and surface finish.
Geometric complexity of a design raises costs because the mold must be much larger in order to contain external coring to form complex outer configurations.
Cored castings are more costly. Cores must 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 cast components. At the same time, the cores make the metalcasting process as unique as they allow for the creation of internal cavities while the external surfaces of the part are produced.
Wall thicknesses are an important design feature in metal castings, with the part configuration and alloy selection playing a major role in determining what wall thicknesses can be cast. As an example, in aluminum castings, the normal minimum wall thicknesses will range from 0.12~0.250 in. Depending upon the part size and the area involved, thinner wall sections, down to 0.080 in. are possible over limited areas (~10 sq in.).
Type of Metal
The cost of a casting will vary in three ways based on the alloy used:
• the inherent cost of the alloy. Cost depends on both the baseline cost of the metal along with alloying elements in the alloy.
• the purity of the alloy. Premium prices have to be paid by the foundries to obtain alloys with fewer impurities. The lower the impurity level requirements of the alloy, the fewer the gating and risering returns allowed in melt charges;
• the castability of the alloy. In steel and aluminum alloys, different alloy compositions have differing levels of fluidity, weldability and castability. With reduced castability, additional metal must be poured into the mold in the form of flow channels and feed heads to assure a sound casting. For such parts, the weight of the actual metal in the finished cast component may be less than half of the total weight poured. This total-metal-poured factor is especially important when considering alloys with costly alloying elements since the alloying elements must be added to all of the metal poured into a mold.
Of the above three considerations, the last one (castability of the alloy) has the most effect on the cost of a cast component.
The type of pattern equipment provided is an important factor in the total cost of producing a metal casting. The term “pattern equipment” includes coreboxes, if they are necessary, and any jigs or fixtures used for checking or assembling cores or for checking the dimensions of the part. Note that part quality is only as good as the pattern quality.
Dimensional Allowances and Tolerances
Because dimensional allowances and tolerances affect both the cost and delivery of metal castings, it is important that part buyers have a working knowledge of the major factors influencing them. Realistic tolerance requirements are of prime importance in minimizing overall costs. Tolerances that are more rigid than necessary increase foundry production costs and lengthen delivery schedules. Those that are too loose result in castings that require more expensive machining and usually have a heavier section size than required. Following is a brief review of the more important dimensional allowances and tolerances that should be considered.
When metal is poured into and takes shape in the mold, the metal is still at or above its liquidus temperature. As the metal solidifies and cools to room temperature, the metal contracts. Thus, to obtain a casting of desired size, the pattern is made slightly oversized. This addition to the dimensions is called a patternmaker's shrinkage allowance.
Another type of shrinkage that occurs while the liquid metal solidifies in the mold is called solidification shrinkage. This aspect of metal solidification is corrected by feed heads, risers or shrink-bobs, which are attached to the casting when they are poured. This is a separate phenomenon and should not be confused with pattern shrinkage allowance.
This is the term describing taper on the vertical faces of a pattern that allows the pattern to be removed without binding or tearing the mold. Draft will normally vary between one and three degrees of taper. Small or machine-drawn patterns require minimum draft. Large patterns, inside pockets on patterns and hand-drawn patterns generally require a greater draft. Under ideal conditions, some patterns as deep as six or more inches may be drawn with almost no draft, using special techniques. Maximum draft should be provided where it does not interfere with desired casting geometry. The foundry should be consulted so that draft allowance may be minimized or even eliminated entirely on surfaces where it must be machined off.
For a no-draft condition, a loose piece in the pattern or corebox can be used. This will increase the cost of the part due to an expensive pattern and higher labor costs in molding and coremaking. Alternatively, the buyer should consider the lost foam process. Cast tooling or datum points should be fixed on designated "no-draft" surfaces.
Additional stock that is added to the surface of a metal casting so that it can be finished to size by machining is called machine-finish allowance. Thickness of this allowance will depend on size and type of the cast component, the type of surface, how it is to be machined and how accurately the casting can be made. Large castings and those made on a low-volume basis will generally have larger machining allowances than smaller castings or components produced in large quantities. With newly designed cast parts, a full machining allowance is usually added. After some experience is gained with the pattern, machining allowances may be reduced to the minimum necessary. To avoid local surface hard spots, a ground finish can be applied directly to a cast surface, with just enough stock so that the surface will clean up well.
When one part of a casting cools more rapidly than another, the part may become distorted. The first line of defense is to pay attention to design details such as avoiding sharp changes in adjacent sections. When this is not possible, distortion often can be minimized by special foundry practice to control heat extraction or by follow-up heat treatment. In some cases, the pattern may be intentionally distorted so that the casting will be true. This is called distortion allowance.
Because castings are individually designed and process details are engineered for the characteristics of each pattern, casting requirements such as surface smoothness, dimensional tolerances or machining allowances cannot be stated in a general specification. These requirements usually are established through mutual agreement between the casting user and the foundry.
Ordinarily, variation in casting dimensions can be held to within a half of the patternmaker's shrinkage allowance. On a simple 12-in. casting dimension, this would be a tolerance of ±0.0825 in. Closer tolerances often are obtained on repetitive production where corrective adjustments can be made. For such cast parts, a tolerance of ±0.05 in. in an 8-12-in. dimension may be considered normal. Special procedures have been developed to produce some cast components with dimensions as close as 0.005 in. However, note that tolerances do not vary proportionally with the component’s dimensions.
Dimensions within each half of the mold are generally more accurate than those across the parting plane or those related to a cored surface. The type of pattern equipment used directly affects dimensional tolerances. This is particularly true when machine molding or coremaking methods are employed. Metal patterns and coreboxes that are machined to size often will provide closer tolerances than will cast-to-size metal or wooden patterns.
High-strength cast components frequently demand tight dimensional tolerances or thin-wall sections. In either, higher production costs are incurred if a systems approach is not used by the OEM to work with the foundry to recognize the true requirements of the component. Frequently, components are over-specified. In other cases, the design can be altered to take advantage of the capabilities of the many casting processes that are commercially available to meet true design requirements.
The factors that most affect dimensional tolerances are as follows:
• use the proper shrink factor in pattern design. This is less of a problem on smaller castings. For larger castings, the foundry must have experience with the type of part required so it can make proper judgments as to what shrinkage factors to use;
• variability in the pattern equipment and coreboxes from nominal blueprint dimensions. To minimize this, quality tooling is required;
• variability in the molds and cores themselves due to people, equipment and materials used in their production. Quality foundries are aware of this and have controls and gauges in place to minimize the effect;
• variability in cleaning and finishing operations performed on the part. The foundry will perform inspections after this process to assure that finishing operations are done properly;
• distortions due to the heat treatment of the castings. The foundry should plan to use heat treat fixtures and special quenchants where the potential for this problem exists.
The quality of both the tooling and the production equipment is of great importance in dimensional tolerances. Even if only a small quantity of castings is required, the tooling must be of prime quality if the dimensional tolerances are to be satisfied.
Designers must be aware that there are additional costs incurred when tight tolerances are specified. Linear tolerances often must be increased for larger casting sizes. Critical dimensions should he identified to the foundry so proper tooling will be built.
Cast Surface Finish
There is no standard finish specification for cast components. Although root mean square values for cast surfaces are sometimes given on drawings, they are of questionable value because there is no dependable method of quantitative surface roughness measurement for unmachined, cast surfaces. However, sample castings of the type required provide a satisfactory way of designating desired surface finish and observing what can be produced.
In some casting applications, the degree of roughness of the cast surface is critical to successful use. Dimensions that are held within close tolerances may be ineffective if the surface is not smooth enough for accurate measurement or proper positioning in a fixture. While a textured surface is desirable or even necessary for some finishing or coating procedures, a surface that is too rough can be detrimental to finish quality and may require extra finishing with increased costs. Although a smooth surface on a cast part often is considered an aspect of its quality, this is not an accurate indicator of the overall quality of the casting.
Surface quality can involve several characteristics that may not be evident or even measurable. Superficial roughness on the casting can be caused by duplicating the rough surface of a pattern in metal or by an improperly made mold. However, most of the surface roughness of iron castings is the direct result of the high-energy abrasive cleaning methods in common use.
Accurately evaluating a cast component's surface roughness is difficult. Instruments and methods that are used successfully for measuring the roughness of machined surfaces are unsuitable for cast component surfaces. A sandcast surface is not composed of regular spaced surface features, as found on a machined surface. The size distribution of the sand mix produces a much more complex and richer pattern of surface features. A finish that varies from location to location on a casting (due to inherent variations in sand compaction around a complex engineered geometry) further complicates the problem of measurement.
The surface texture or roughness is generally specified by the term root mean square (rms) or arithmetic average (AA). The rms gives about 11% higher values than AA, but the difference is not significant. The surface finish may be specified as 300 rms maximum, 115 AA maximum or C50 maximum on a visual comparator. Finer finish requirements will require special molding media or secondary operations, such as grinding or machining. This will add to the cost. Measurement should be made by surface comparator. The profilometer probe diameter acts as a filter for the input data. Fine surface areas should be identified and specified on drawings.
Quality Requirements and Quality Assurance
Cast metal components are produced in a very wide range of parts (ranging from engine crankshaft counterweights to aircraft turbine blades) with different quality requirements. The term "quality" really indicates how well the part meets the requirements of the purchaser and how consistently they are met by castings in production quantities. Consistent quality from order to order and from the first to the last casting of an order is of primary importance and should be foremost in the identification of a dependable casting supplier.
The main parameters of cast metal quality are:
• properties of the metal;
• soundness of the cast component;
• accuracy and consistency of dimensions;
• smoothness of finish.
Quality requirements often are detailed by reference to industry specifications, such as those published by the American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), and government/military and other professional and trade associations. Specifications are available for different alloys and for a wide range of quality levels required for differing applications.
Sometimes high quality is improperly used to indicate superiority in a particular property, such as high tensile strength or excellent surface finish. However, it should be kept in mind that calling for unnecessarily high quality requirements, which increase the total cost of a casting, is contrary to good purchasing practices. The chief concern should be defining and meeting acceptable quality levels. Excessive quality requirements for any of these increase the direct cost of a casting for several reasons:
• a more complex production method may be necessary to meet the specified quality level;
• more highly skilled workers and/or more production time may be required;
• more expensive materials and/or equipment may have to be used;
• additional inspection may be necessary and a higher percentage of rejects may result.
However, necessary quality requirements should not be compromised. Decreased cost is not a valid reason for purchasing cast parts that are below the required standard. Any initial savings are very likely to be offset by increased finishing cost or performance 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 cast components, this is often difficult to do and cast component quality is more generally thought of in terms of the established reputation of the foundry.
An important factor in selecting appropriate tooling is order quantity. Even with a given pattern, the order quantity may have an important influence on casting cost. One reason for this is that there are a number of standard operations in the foundry that must be performed for each part order regardless of the quantity involved. Some of these are:
• quoting, acknowledgment and recording of the order;
• scheduling production and issuing shop work orders;
• getting pattern equipment out of storage, inspecting and transporting it to production departments;
• setting up tooling for production (i.e., laying out or attaching to machines and lining up proper flasks);
• if cores are required, delivering core equipment to coreroom, making cores and transporting them to the molding line;
• shipping and billing.
Efficiency of production operations also is greatly influenced by quantity. When an order is issued subject to release, the total quantity is important in the selection of foundry tooling, 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 cast parts when he knows what his casting requirements will be for some time in advance. Due to usual billing practices, less than one month's supply is seldom practical. Savings realized by larger releases should be weighed against inventory costs, which include investment, space and records. 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.
The production quantity and the production rate are major factors in selecting the casting process and the type of mold pattern/tooling for the casting run. Different metalcasting processes have different production rate capabilities, different tooling costs and different design change flexibility. Sand cast components often are preferred because design changes can be made with relative ease.
Two factors play important roles in tool material selection. The first factor is the total quantity of cast parts needed. The second factor depends on the delivery period involved. Quality wood tools will generally produce up to 100 parts within about a year. Over a longer period of time, the tool may distort and produce castings with dimensional discrepancies. Wood and plastic tooling should be usable for up to 500 parts in a moderate time frame (one to five years) with periodic refurbishment. Whenever small lot quantities are required for more than three years, metal or plastic tooling should be used to assure only minor refurbishment costs. Metal tooling is recommended when more than 2,000 castings are required annually.
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 inspection that are more sophisticated and testing procedures are typical of additional operations that may be efficiently performed 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 on each critical finish surface.
Close cooperation through concurrent engineering between the foundry and its customer is the approach required to yield the greatest benefit for buyer and supplier. But for the more complex engineered cast components, an additional amount of service or technical assistance may be required from the foundry. This commonly occurs in development of new products. Assistance with component design, information on properties and metallurgy, trouble shooting, 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 parts. However, in either case, quotations from a foundry that stands ready to provide its customers with this assistance cannot be compared directly with quotations from a foundry that does not. The latter type of foundry should not be ignored because 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 will be produced in large quantities, a model casting may be made to assist in planning production equipment. The model will assist in establishing the most efficient metalcasting method by providing an opportunity to establish critical factors such as the parting line and core prints on the actual shape. This also can be of assistance in planning for subsequent processes, such as machining.
Types of Price Quotations
At one time, metal was expensive compared with labor and castings were commonly sold by the pound. This was a simple buying method and worked well because the weight of a cast part was a more important part of total cost than was the workman's time to make it. 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 worker's time, value of materials used and overhead costs. This provides price quotations that are based on cost per casting and a ready reference for the purchaser to arrive at unit cost. The cost-per-piece can easily be converted to cost-per-pound, but such a figure 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 quoting for specific types of parts. 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 from estimated weight still affect the cost more on a price-per-pound basis than on a price-per-piece basis.
Lead-Time and First Article Delivery
Essentially, the metalcasting process involves pouring molten metal into a cavity close to the final dimensions of a desired component. It is the most direct and simplest forming method available for metals and often is an essential production factor in rapidly manufacturing finished metal parts.
Lead-time is commonly defined as the amount of time between contract agreement and delivery of the first article, either prototype or initial production. Lead-time is reduced with metalcasting compared to other manufacturing methods by eliminating and/or shortening the time required for tool production, parts ordering and delivery, assembly, finishing and machining.