Cost Estimating Tips
Due to the many different types of metalcasting facilities, the variety of metals cast, and the wide range of casting processes employed, it is difficult to offer a generic formula for estimating the cost of castings. However, general guidelines can be established to assist the buyer or designer in determining factors that impact the cost of castings.
Metal type plays a significant role in the cost of a casting. Metal type may be predicated by design or application requirements; in some instances, though, certain metals can match engineering characteristics of other materials but at reduced cost. An example is the potential use of various grades of austempered ductile iron as alternatives to certain grades of cast and forged steel or aluminum. Generally speaking, regardless of metal type, casting cost is directly linked to process and production requirments.
|Green sand||Permanent Mold||Diecasting||Ceramic / Investment||Shell, CO2, Nobake|
|Typical dimensional tolerances||
+/- 0.015 in.
|Relative cost in quantity||Low||Low||Lowest||Highest||Medium high|
|Relative cost for small #||Lowest||High||Highest||Medium||Medium high|
|Permissible casting weight||Unlimited||100 lb.||75 lb.||100 lb.||Shell-250 lbs.; CO2 and nobake: tons|
|Thinnest castable section||1/10 in.||1/8 in.||1/32 in.||1/16 in.||1/10 in.|
|Relative surface finish||Fair to good||Good||Best||Very good||Good|
|Relative ease of casting complex design||Fair to good||Fair||Good||Best||Good|
|Relative ease of changing design in production||Best||Poor||Poorest||Fair||Fair|
|Range of alloys that can be cast||Unlimited||Al-base and Cu-base preferable||Al-base preferable||Unlimited||Unlimited|
Per-Unit Cost Factors
Table 1 compares various characteristics offered by a variety of molding processes. As illustrated, molding processes offer varying capabilities and contraints, with certain processes better suited to specific casting designs and applications. Even within a given process, different methods may exist that lend themselves to greater efficiencies at different production levels. Also, processes utilized for tight dimensional tolerances, thin sections and casting complexity tend to be more costly, thus casting design complexity and application have a substantial impact on cost. Designs requiring tight tolerances and minimal draft may require extensive coring or special molding processes and, as a result, generally cost more.
Casting End Use
Before making any major purchasing decisions, casting buyers and designers can gain a clearer picture of both design complexity and application by asking: Is the casting to be used as-cast? Will the casting be heavily machined? What is the casting application? Answers to these questions can assess casting form, fit and function, and design options should be analyzed to determine realistic tolerances prior to selection of a molding process. While no process is considered perfect, one or two will typically meet the majority of design and application requirements, and a final selection can be made with an eye toward balancing potential costs to alternative process benefits.
In addition to assessment of tolerances, design consideration should be given to include allowance for draft and parting line and the effect of these on dimensional tolerances as well as the potential impact of each on the positioning of machining locators. When considering design options and potential process requirements, the intent should be to produce the most casting-friendly and machine-friendly design possible. Ease of manufacturing generally will result in lower costs.
There are a number of variables that can impact tooling cost:
- Pattern type (plate-mounted impressions or pressure cast impressions, separate cope and drag pattern vs. matchplate design);
- Pattern size and/or number of impressions (single vs. multiple impressions);
- Cored vs. noncored (if cored, additional tooling in the form of one or more coreboxes will be required);
- Tooling material for pattern and/or coreboxes (wood, plastic, aluminum, steel or composite);
- The type of molding machine used to make the molds;
- The type of core machine or coremaking process used to produce the cores.
Whether tooling costs are to be considered separately or amortized on a per-piece basis, they should be considered as tooling modification or replacement may be required to accommodate the changes. Depending on the tooling selected, cost of the changes can range from minimal to very high depending on the tooling.
Projected Casting Demand
Volume plays a significant role in cost, both in terms of annual demand as well as total demand over the lifecycle of the part. Again, tooling costs must be considered when calculating the per-unit cost of the casting, and the lower the volume, the larger the allocated tooling cost will be for each piece. Also, potential setup costs must be factored. Higher volumes typically mean fewer setups and longer production runs leading to economy of scale, thus processes or operations that lend themselves to high production offer attractive individual piece costs. However, these high-volume operations may not be suitable for lower volume parts due to frequent setups that result in lower productivity and reduction in machine revenue. Thus, when allocated setup cost per piece is considered, the per-unit costs of such operations are generally higher.
Casting Facility Production Capabilities
Different metalcasting facilities producing the same metal type and using the same molding process may appear similar yet they may vary substantially in production capabilities due to differences in equipment, layout and operation. Many jobbing companies specialize by size, weight, and even market focus, so it is important to open the lines of communication to and from the casting facility when requesting or reviewing quotes. Even if a metalcasting facility’s capabilities are known, it is important that a strong interface exist between the designer, buyer and the casting facility—from initial quotation through production—so that the most cost-effective designs are considered. Every effort should be made to fit the job to the most suitable production facility and to look at the total unit cost rather than focusing only on the cost of an individual casting.
--AFS Industrial Engineering Committee (1-E)