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Brass and Aluminum Die Casting - An Essential Manufacturing Process

Die casting is a vital manufacturing process used to produce precisely dimensioned, sharply defined metal parts. This process involves forcing molten metal under high pressure into reusable steel molds called dies. The metal quickly solidifies in the die, after which the part is ejected and can go through secondary finishing operations if required.

Die casting offers many benefits compared to other manufacturing techniques. It allows for the economical production of complex, high-tolerance parts with fine details and excellent repeatability. Thin walls, tight tolerances, and smooth cast surfaces can be achieved. It is also a fast production method, making it ideal for high-volume manufacturing.

Two of the most commonly die cast metals are brass and aluminum. Both offer unique advantages and are suitable for different applications.

Brass Die Casting

Brass is an alloy composed primarily of copper and zinc. It is valued for its high strength, excellent corrosion resistance, attractive gold-like color, and ability to be machined, polished, and plated. Brass die castings account for about 20% of all die cast products manufactured.

Brass is exceptionally suitable for die casting because it has high fluidity when molten, allowing it to flow easily into dies. It also solidifies rapidly, allowing fast production rates. Brass parts can be cast with very thin walls while still retaining strength, down to approximately 0.015 inches for simple shapes. More complex shapes with ribs and bosses can be cast with wall thicknesses around 0.02 to 0.025 inches.

Brass die castings maintain tight dimensional tolerances. Precision grade brass alloys have tolerance capabilities of ±0.001 to ±0.002 inches per inch. This helps reduce the need for secondary machining operations after casting.

Brass is heavier than other die casting metals like aluminum or magnesium, with a density around 0.307 pounds per cubic inch. This gives brass parts a solid, quality feel. The higher density also improves heat and vibration dampening capabilities relative to lighter metals.

Brass die castings have an excellent surface finish right out of the die, usually around 125 Ra microinches. This allows products like industrial valves and plumbing fittings to be used without polishing or other surface treatment. When required, brass parts can be plated, painted, powder coated, or chrome plated for appearance and corrosion protection.

Common applications of brass die castings:

- Automotive parts like locks, pilot bushings, and wiring connectors
- Plumbing valves, fittings, and manifolds
- Medical equipment components
- Door hardware and architectural fixtures
- Musical instrument parts like valves, tuning slides, and mouthpieces
- Decorative trim pieces and costume jewelry

Aluminum Die Casting

Aluminum is well known for its light weight, corrosion resistance, ease of machining, excellent thermal and electrical conductivity, and recyclability. It is the most widely used non-ferrous die casting metal, accounting for about 50% of all die cast products.

Aluminum has a low melting point and high fluidity when molten, making it easy to die cast thin, complex shapes. It solidifies faster than any other common die casting alloy. This extremely fast solidification allows aluminum parts to be die cast with very thin wall thicknesses down to approximately 0.020 inches for simple forms, and 0.060 inches for complex shapes with ribs or bosses.

Aluminum die cast parts can be produced with tight tolerances of ±0.005 inches per inch to ±0.002 inches per inch depending on part size. They have excellent surface finish right out of the die, usually around 150 Ra microinches.

A huge benefit of aluminum die casting is that it is lightweight - almost three times less dense than brass. Aluminum has a density of only 0.098 pounds per cubic inch. This provides customer appeal and substantial weight savings over alternative manufacturing methods.

Being lightweight makes aluminum easier to handle during secondary operations like surface finishing, painting, or assembly. The lighter weight also means lower material costs and shipping costs.

Aluminum provides excellent strength for its low weight. Thin walled castings allow complex shapes to be die cast while retaining strength and rigidity. Aluminum alloys offer a good strength-to-weight ratio, and some alloys can be heat treated after casting to further enhance strength.

Aluminum die cast parts offer excellent corrosion resistance due to the naturally occurring oxide film present on aluminum surfaces. This is essential for functional components like housings, covers, and brackets exposed to the elements. Aluminum is not easily plated, so is often anodized or powder coated to improve corrosion and wear properties if required by the application.

Common applications of aluminum die castings:

- Consumer electronics like smartphone and computer housings
- Automotive parts like transmission housings, wheels, and engine components
- Aerospace industry structural components
- Medical equipment housings
- Military and defense components like weapon frames or ordnance parts
- Appliance housings and hardware

The Die Casting Process

While brass and aluminum are both commonly die cast metals, the actual die casting process used is essentially the same regardless of material. There are two main types of die casting - hot chamber die casting and cold chamber die casting.

Hot Chamber Die Casting:

This method is used for metals with lower melting temperatures like zinc, magnesium, and lead alloys. The furnace holding the molten metal is connected directly to the die casting machine. The metal is kept molten in the machine and ladled from the furnace into the die as needed.

Cold Chamber Die Casting:

This method is used for higher melting point alloys like brass, bronze, and aluminum. The furnace holding the molten metal is separate from the die casting machine. There are two techniques used:

1. A precisely measured ladle transfers molten metal from the distant furnace into the cold chamber of the die casting machine.

2. In automated operations, a robotic arm takes a casting ladle full of molten metal and tips it into the cold chamber.

For both hot and cold chamber processes, once the molten metal is injected into the die the cycle continues:

1. The two die halves close and metal is injected into the die under high pressure, usually around 10,000 to 40,000 psi.

2. The metal rapidly cools and solidifies, taking only 40-90 seconds.

3. The die halves open and the ejector pins push the finished casting out of the die cavity.

4. The die closes again and the cycle repeats, producing castings one shot at a time.

Die casting production rates vary based on part size and complexity but can reach over 200 cycles per hour.

For small production runs, die casting is done using aluminum prototypes dies. For larger production volumes, tool steel dies are machined to produce thousands to millions of die cast parts over years of service.

Design Tips for Die Cast Parts

There are some helpful design tips to follow when designing parts intended for die casting:

- Add generous internal radii and external fillets to the corners of the part to avoid cracking from high injection pressures.

- Avoid using very thin sections below 0.015 inches for brass and 0.020 inches for aluminum to prevent metal from filling the die.

- Use a draft angle of 1-3 degrees on vertical walls to make the part easier to eject from the die.

- Keep overall wall thickness uniform across sections - changes in cross-section can lead to sink marks or distortion.

- Place heavy sections opposite to gating locations to improve molten metal flow and minimize jetting.

- Locate holes and openings away from the die parting lines. Side cores increase cost.

- Minimize undercuts, sharp angles, and hollow sections to facilitate part ejection.

- Add a small radius or chamfer to any sharp external edges to prevent them from chipping or flaking.

Die casting is an efficient, precision metal forming process suitable for complex, thin-walled parts with excellent finish and dimensional accuracy. Both brass and aluminum are commonly die cast metals, each offering their own benefits. With sound design and proper dies, die castings can deliver outstanding performance and durability across countless industries and applications. CNC Milling CNC Machining