Purchasing and design professionals have to account for many variables when sourcing metal castings.

An MCDP Staff Report

(Click here to see the story as it appears in the July/August issue of Metal Casting Design & Purchasing.)

The metalcasting process requires designers to consider a specific list of requirements prior to purchasing. Casting specifications drive the process, material and ultimately cost. Purchasing agents must evaluate the many important factors that directly influence a casting’s total cost, which can be grouped under the general headings of quality, service, price and delivery. Because a universal formula isn’t possible, each buyer will determine the relative importance of these factors based on particular circumstances.

For the purchasing agent, the total amount of metal in the casting hardly determines its price. Understanding the additional variables and how they impact price is imperative in successfully procuring the best castings at the best price. Unnecessary costs will be incurred if specifications for a casting are more demanding than necessary. Insufficient demands will lead to increased waste and castings that do not meet quality requirements. Following is a look at 10 considerations when examining your casting design:

1. Geometric Complexity

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?

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 special molding processes and, as a result, generally cost more.

For example, cored castings are more costly. If a design calls for cores that form complex internal cavities, the mold may need to be larger. Cores must be made ahead of time, transported to the molding department and placed into the mold. Also, cored castings often require extra cleaning and trimming.

Similarly, wall thicknesses are another important design feature in metal castings related to design complexity. The part’s configuration plays 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 to 0.25 in. Depending upon the part’s size and the area involved, thinner wall sections, down to 0.08 in., are possible over limited areas (about 10 sq. in.).

Castings with unnecessarily complex geometries, both internally and externally, will increase costs without increasing benefits to the end-user.

2. Type of Metal

Casting cost will vary in three ways based on the alloy used:

Inherent Cost—This depends on the baseline cost of the metal and the alloying elements.

Purity—Alloys with fewer impurities are more expensive. Lower impurity levels mean fewer gating and risering returns are allowed in the melt charge, which can
increase waste.

Castability—Of these three considerations, castability has the greatest effect on total cost. In steel and aluminum alloys, different compositions have differing levels of fluidity, weldability and castability. With reduced castability, additional metal must be poured into the mold to ensure a sound casting.

3. Dimensional Allowances and Tolerances

Buyers and designers must have a working knowledge of the major factors influencing dimensional allowances and tolerances. Tolerances that are more rigid than necessary increase the costs and lengthen delivery schedules. Tolerances that are too loose lead to more extensive machining and usually have a heavier section size than required. Here are five considerations when determining allowances and tolerances:

•  Shrinkage Allowance—Metal contracts as it solidifies and cools. To obtain a casting of desired size, the pattern is made slightly larger. This addition to the dimensions is called a patternmaker’s “shrinkage allowance.”

•  Draft Allowance—This term describes taper on the vertical faces of a pattern, which allows it to be removed without tearing the mold. Draft normally varies between one and three degrees of taper. Small or machine-drawn patterns require minimal draft. Large patterns, inside pockets on patterns and hand-drawn patterns generally require greater draft. Maximum draft should be provided where it does not interfere with a casting’s desired geometry. The metalcaster should be consulted to minimize or eliminate draft allowance on surfaces that must be machined. 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 a more expensive pattern and higher labor costs in molding and coremaking.

•  Machining Allowances—Additional stock added to the surface of a casting so it can be machined is called machine-finish allowance or machining allowance. The thickness of this allowance will depend on the casting’s size and type, the required surface finish, how it is to be machined and how accurately the casting can be made. Large and/or low volume castings generally will have larger machining allowances than smaller and/or high volume castings. With newly designed cast parts, a full machining allowance is usually added.

•  Distortion Allowance—Distortion may occur when one area of a casting cools more rapidly than another. Designers should avoid sharp changes in adjacent sections. When this is not possible, distortion often can be minimized by managing heat extraction or heat treatment.

•  Tolerances—Casting requirements such as surface smoothness, dimensional tolerances or machining allowances cannot be stated in a general specification for all castings. These requirements usually are established through mutual agreement between the customer and the metalcaster. The pattern equipment used directly affects dimensional tolerances, particularly when machine molding or coremaking methods are employed. Metal patterns and coreboxes that are machined to size often provide closer tolerances than cast-to-size metal or wooden patterns.

The factors that most affect dimensional tolerances are:

• The shrink factor in pattern design, especially with larger castings.
• Variability in the pattern equipment and coreboxes.
• Mold and core variability due to inconsistencies among employees, equipment and materials.
• Variability in cleaning and finishing operations.
• Distortions due to the heat treatment.

 

4. Cast Surface Finish

Cast components do not have standard surface finish specifications. 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 as-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 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. Finer finish requirements require special molding media or secondary operations, such as grinding or machining and adds to costs.

5. Quality Requirements and Assurance

“Quality” really indicates how well the part meets the purchaser’s requirements and how consistently they are met during production. Consistent quality from order to order and from the first to last casting of an order should be a major consideration in choosing a casting supplier.

The main parameters of cast metal quality are:

• Properties of the metal.
• Soundness of the casting.
• 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) or the Society of Automotive Engineers (SAE). Specifications are available for different alloys and for a wide range of quality levels required for differing applications.

Unnecessarily high quality requirements increase the total costs. The primary concern should be defining and meeting acceptable quality levels. At the same time, decreased cost is not a valid reason for purchasing castings that are below required standards. Any initial savings are likely to be offset by increased finishing cost or performance problems.

The function of cast parts may require different quality levels. Commercial castings normally are inspected only for surface defects. However, for critical applications, visual inspection is not enough. They may require X-ray and fluorescent penetrant inspection for flaws in the part. To obtain the right product at the right price, request for quotes should list all the requirements as thoroughly as possible.

Good quality begins with good design for performance and manufacturability. In the end, the metalcasting facility is responsible for turning that design into a quality casting, which requires process control throughout the steps of production.

6. Order Quantity

Order quantity is an important factor in selecting appropriate tooling. Production efficiency is greatly influenced by quantity. When an order is issued subject to release, the total quantity is important in the selection of metalcasting facility tooling, but each release is normally considered as a separate order by the metalcasting facility because each production order requires separate processing.

The purchasing manager can determine the most economical lot size for cast parts when he knows the casting requirements in advance. 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 rate are major factors in selecting the casting process and type of mold pattern/tooling. Different metalcasting processes have different production rate capabilities, tooling costs and design change flexibility. Sand cast components often are preferred because design changes can be made with relative ease.

The quantity of cast parts can determine tool material selection. Quality wood tools will generally produce up to 100 parts within about a year. Wood and plastic tooling should be usable for up to 500 parts in a moderate time frame (one to five years) with periodic refurbishment. Metal tooling is recommended when more than 2,000 castings are required annually.

7. Packaging

Often customers specify bulk packaging and are then upset when castings arrive damaged, dinged or dirty. Look at the total cost when specifying packaging. Layered or corrugated packing often costs more on paper but reduces cost of scrap, rework and handling. Kitting should also be considered (that is, packaging side by side for delivery to an assembly line).

8. Technical Services

Close cooperation between the metalcaster and customer will yield the greatest benefit for both buyer and supplier. But for the more complex engineered cast components, an additional amount of service or technical assistance may be required from the metalcasting facility, commonly in development of new products. The metalcaster may provide assistance with component design, information on properties and metallurgy, troubleshooting, pattern assistance or pattern models, and experimental castings.

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.

9. Lead Times, First Article Delivery and Prototypes

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.

Different metalcasting processes offer designers and purchasers a wealth of tooling and no-tooling options to economically produce short runs of components.

From a tooling perspective, a hardwood, plastic or aluminum pattern stored at the metalcasting facility can be ideal for production runs from 10-100 parts/year. When purchasers compare the cost of a hardwood tool to that of the jigs and fixtures (and possibly floor space and inventory) required for weldments and assemblies, the simple pattern looks attractive. With the proliferation of CAD and CNC machining, metalcasting tooling can be produced for 30% less than it was even five years ago with higher dimensional accuracy.

From a no-tooling perspective, if a lead time of less than two weeks is required and an operation is looking for a short run of components (typically less than 10) or prototypes to test for form, fit and function, the metalcasting industry and rapid prototyping technologies have teamed up to offer a variety of alternatives for no-tooling metal component production.

10. Value-Added Operations

Often the metalcasting facility 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 and inspection are typical additional operations that may be efficiently performed by a metalcasting facility. For production runs, castings may be justified or targeted in a gaging fixture to provide accurate locating points for subsequent machining operations. This assures the correct amount of stock is available to be removed on each critical finish surface.