Copper and Copper Alloy Casting Methods

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Introduction to Copper Alloy Casting Technologies

The copper-based alloy casting industry employs numerous manufacturing methods, each offering specific advantages for different applications. This article explores the six main casting processes and additional specialized techniques, highlighting their unique characteristics, benefits, and limitations.

Sand Casting and CO₂ Process

Conventional floor molding and bench molding methods remain widely practiced in the copper-based alloy casting industry. These traditional approaches offer significant cost advantages for small production runs due to lower pattern expenses. However, they cannot achieve the dimensional accuracy possible with other methods.

Two factors contribute to these inaccuracies when using loose patterns: first, patterns require reasonable "draw" to facilitate removal from the sand mold; second, patterns typically need to be rapped to withdraw them from the mold, causing dimensional variations in the final castings.

Machine molding, often called plate molding, dominates production for smaller and medium-sized castings and likely accounts for the largest tonnage of castings produced. Modern molding machines enable high production rates and, with precisely manufactured metal patterns, deliver significantly better dimensional accuracy than pre-WWII techniques. These patterns still require appropriate "draw" allowance for clean mold separation.

The CO₂ process, a recent innovation in foundry work, offers substantial advantages and can replace most dry sand molding and much green sand molding. Rather than using natural clay-bonded sand, this process mixes sodium silicate with silica sand, which is converted during CO₂ gassing to form silica that binds the sand grains. A key benefit of the CO₂ process is its compatibility with existing pattern equipment while providing improved accuracy. This improvement stems from the ability to gas and partially harden the mold before pattern withdrawal, reducing the amount of rapping required.

Shell Molded Casting

Developed over the past 50 years, the shell molding process uses synthetic thermosetting resin mixed with high-grade silica sand to produce sand castings with much tighter tolerances. This machine molding process requires metal pattern plates capable of withstanding temperatures of 250–300°C. Shell molding patterns need less "draw" than conventional patterns, and the process eliminates pattern rapping for mold ejection.

Shell molding offers several significant advantages:

• Improved surface finish and definition
• Ability to mass-produce complex castings without skilled labor
• Much tighter tolerances

While some dimensions can achieve tolerances as precise as 0.003 inches (≈0.1 mm), foundries should be consulted regarding specific tolerances, particularly for dimensions crossing the mold joint line.

Pattern plates for shell molding typically cost more than conventional plate molding patterns due to precision requirements. The process also requires more expensive resin and high-grade sand, generally resulting in higher per-pound costs compared to conventional sand castings.

Despite higher initial costs, shell molded castings often eliminate machining operations and reduce machining allowances, resulting in lower metal weight and substantial advantages in many applications. The process also delivers greater consistency in dimensions, surface finish, and soundness once proper running and feeding techniques are established.

Shell molded castings exhibit mechanical properties similar to sand castings, and virtually all copper base alloys (except specific die-cast specifications) work well with this process.

Die-Casting

Gravity die-casting has long been employed for copper base alloy castings, while pressure die-casting has more recently proven viable for certain applications. Die-castings offer several advantages:

• Excellent surface finish and close dimensional accuracy, reducing or eliminating machining requirements
• Higher mechanical properties allowing reduced casting sections, saving weight and material costs

Not all copper base alloys are suitable for die-casting. The primary alloys used in die-casting include aluminum-bronzes, certain brasses, and high tensile brass.

Complex-shaped castings can present significant challenges for foundry operations, either because alternating thick and thin sections complicate feeding and create unsoundness, or because the castings become difficult to remove from the die.

Chill Casting

Chill casting has been utilized in the copper base alloy field for over 50 years and applies principally to tin bronzes, including phosphor bronze, gunmetal, leaded bronze, and leaded gunmetal.

As its name suggests, this process accelerates solidification, which increases the proportion of hard delta constituent in tin bronze castings. For leaded bronzes, it also promotes finer distribution of insoluble lead throughout the matrix. With the possible exception of elongation and reduction of area, chill casting enhances all mechanical properties, with hardness values potentially 50% higher than sand castings.

The process is used both for producing chill cast stick and rods and for individual irregularly shaped chill castings. Both applications use permanent (metal) molds similar to die-casting molds but made from different metals. Chill casting can produce components ranging from a few ounces to hundredweights. Design considerations for die-casting apply equally to chill casting.

Centrifugal Casting

Centrifugal casting can employ either permanent molds (similar to those used in chill casting) or sand molds. The process leverages centrifugal force to create denser, more reliable castings with improved soundness.

Like chill casting, tin bronzes work well with centrifugal casting. Additionally, high tensile brasses and aluminum bronzes are suitable for this process. Centrifugal casting typically serves larger casting applications and was originally developed for producing gears, worm wheels, and cored bars.

The mechanical properties achieved through centrifugal casting generally match those of chill casting. However, leaded bronzes may develop increased lead content at the periphery compared to the remainder of the section. Horizontal centrifugal casting techniques are sometimes employed for smaller bearings.

Continuous Casting

Continuous casting represents a relatively recent development for copper base alloys, with most work focusing on tin bronzes, phosphor bronze, leaded bronze, gunmetal, and leaded gunmetal.

When production quantities justify this approach, continuous casting offers distinct advantages over chill casting:

• Exceptional soundness, with continuously cast bars rarely containing foundry defects
• Denser material resulting in superior overall mechanical properties
• Clean external surfaces and, for tube production, clean internal surfaces
• Accurate sizing and straightness sufficient for automatic lathe use with bar feed
• Reduced machining allowances compared to other production methods

The process can also produce square, rectangular, and other regular sections.

Specialized Casting Processes

Precision Investment Casting

Precision castings see increasing industrial use due to their close tolerances and excellent surface finish. These qualities reduce or eliminate costly machining and polishing operations while providing dimensional consistency. Since casting quality cannot exceed pattern quality, high-quality pattern equipment is essential for this process.

Plaster Mould Casting

These processes use non-expendable patterns, typically metal but sometimes plastic or wood. Resulting castings can range from ounces to several hundredweights. The Shaw process substitutes a special refractory slurry for plaster, creating expendable molds. Semi-permanent ceramic molds made with special refractory slurry can produce 100 or more relatively simple castings with good surface finish and close dimensional tolerances.

Selecting the Optimal Casting Method

When choosing a casting process for copper alloys, several factors require consideration:

• Required production volume
• Dimensional tolerances needed
• Surface finish requirements
• Component complexity and design
• Mechanical property specifications
• Cost considerations
• Alloy compatibility with the process

For low-volume production with moderate tolerance requirements, traditional sand casting may prove most economical. For high-precision components requiring minimal machining, processes like shell molding, die-casting, or investment casting offer superior results despite higher initial costs.

Conclusion

The diverse range of casting methods available for copper alloys provides manufacturers with flexibility to balance performance requirements against production economics. Understanding the capabilities, limitations, and appropriate applications of each process enables optimal selection for specific component requirements. As technology continues to advance, these processes will likely see further refinements to improve quality, reduce costs, and expand the practical applications of copper alloy castings.