Copper die-casting vacuum applications: characteristics, solutions…

Die casting alloys to contain the commonly used aluminum, magnesium, zinc, and copper alloys, the earlier tin and lead, and the less commonly used steel and iron. Pure copper is very difficult to cast and shape, and its products are prone to surface cracking, porosity problems and the formation of internal cavities. The castability of copper can generally be improved by the addition of small amounts of elements (including beryllium, silicon, nickel, tin, zinc, chromium, and silver) and is used in die casting to make bearings, bushings, gears, fittings, valve bodies, various parts for chemical applications, decorative arts, and motor rotors. Given the narrow condensation range and high beta phase content required, copper dies casting alloys are mostly referred to as brass (usually with lead or silicon) and copper (copper alloy) alloys, the latter being used for electrically conductive products such as rotors for electric motors. Copper die casting is usually used for complex geometries and larger batches of copper alloys.

 

The characteristics of copper die-casting and vacuum applications

Die-cast copper alloys have the following main characteristics

Excellent corrosion resistance

Highly efficient electrical conductivity

Excellent low friction properties

good elongation and tensile strength

Suitable low to medium hardness

This makes copper a versatile alloy in a metal die casting alloys.

Here is an example of a die-cast purple copper motor rotor: Since the invention of the AC motor by the American inventor Tesla at the end of the 19th century, this simple structure, using alternating current, without rectification, the non-sparking motor has been widely used in industrial and household appliances. As early as the beginning of the last century, business people envisaged that induction motors’ electrical efficiency would be improved by replacing aluminum with copper, which has a higher electrical conductivity. The conductivity of copper is about 40% higher than that of aluminum, and the cast copper rotor can significantly reduce the motor’s total loss, thus improving the overall efficiency of the motor and greatly reducing the temperature rise. In recent years, along with the rapid development of die-casting technology and mold technology, the mass production of die-cast copper rotors has been achieved. In many types of electric motors, die-casting copper rotors have been mature and stable applications.

The most significant difference with other die-casting processes is that copper die-casting has poor fluidity and short injection times; high pouring temperatures, 900-1200°C; and other die-casting processes are faced with the common problem of porosity, brought about by incomplete filling, surface defects, internal looseness, poor air tightness, and other defects. The introduction of vacuum exhaust has also become the leading process solution for die-casting enterprises to consider.

 

A die-casting motor rotor die-casting mold cavity volume for 3.8l put the mosaic (silicon steel sheet iterative core) after the volume: 2.2l; molten cup filling rate of 70%; punch seal the sprue mouth position to start slow pressure injection, slow pressure injection time for 0.22s; turn to fast pressure injection, fast pressure injection filling time for 0.08s. It can be seen that slow exhaust time is short and need greater instantaneous exhaust capacity and the whole venting process. If the HV300 system is used, the vacuum tank is 300 liters, and the instantaneous vacuum connection is 13 bar; with a double exhaust plate exhaust, the total exhaust area is 300 mm2, and the exhaust time takes 0.273s.

Vacuum copper die-casting die points

As we all know, compared to aluminum die-casting molds, which have a life of 10,000 or more than 100,000 times, copper die-casting molds are counted in thousands, so the copper die-casting process is more challenging. The high temperature and poor fluidity of copper alloys are the fundamental reasons for the complexity, and high cost of copper die casting.

Mould material

At higher pouring temperatures, 900-1000°C for brass and 1000-1200°C for copper, die-casting molds require higher requirements: high-temperature resistance and heat insulation; considering the poor flowability of copper, the gate area is slightly larger compared to aluminum molds; the mold materials are usually INCONEL alloy 617 and alloy 625, and Haynes alloy 230. INCONEL alloy 617 is a 22% Cr, 12.5% Co alloy solid solution reinforced with 9% Mo; alloy 625 has 21.5% Cr, 9% Mo, and 3.65% Nb; Haynes alloy 230 is optional material for copper die casting molds and has a slightly higher yield strength and ductility than alloy 617, and is highly weldable and repairable. The life of H13 as dies steel is greatly limited by the decarburization and softening of common die steels at high surface temperatures.

Temperature control

In the die casting of high melting point copper melts, high temperatures, high heat of melting, high latent heat, and high thermal conductivity all combine to maximize, resulting in a large temperature difference between the colder interior and the mold surface. The resulting thermal shock and thermal fatigue cause rapid mold failure. Temperature control is essential when using molds at higher temperatures to enable the mold material to retain the necessary strength despite prolonged exposure to high temperatures. The mold temperature controller is its initial preheating measure, requiring over 200°C for copper alloys compared to the 150-180°C preheat holding setting for aluminum alloys. For mold preheating temperatures of 350-500°C, the capacity of the oil warmer is inadequate. The degree of temperature difference can be reduced by preheating the mold insert (resistance heater) to control mold failure due to thermal shock. The same preheating insulation is required for the molten cup. The addition of cooling water is also a means to maintain mold temperature equilibrium, for example, to ensure a mold operating temperature of 600-650°C, but excessive cooling will be counterproductive. A dry powder release agent should be used before each press shot.

Mould design

In the copper die casting process, the design of the mold is a fundamental aspect of the whole manufacturing process, and all the attributes of the mold affect the final product. The quantity of product to be produced, the size and weight of the casting, the shape and complexity of the product, the amount and quality of finishing required, the surface finish required, the internal requirements specified (e.g., sealing), the type and level of treatment and inspection to be carried out, the amount of variability in dimensional accuracy of individual parts, the castable properties of the copper alloy specified or selected, etc. are the initial conditions for the mold design. Usually, how the copper alloy flows depends on the designer’s understanding and interpretation of the copper die casting technology and process, including the parting surface setting, the setting of the gates, the barrel and runner set, the determination of the feed direction, and the insulation and cooling channel arrangement, and finally the vacuum exhaust setting, including the corresponding mold sealing. In other words, for high-pressure copper die casting, the molten copper will flow under external high pressure. All details involved, from the entry into the cavity to the completion of the filling, must be carefully considered. Vacuum venting is always the last aspect to be considered in mold design.

 

Copper die-casting vacuum system

Copper die-casting vacuum principle: Several manufacturing companies have adopted vacuum technology for copper die-casting. The usual drawback is that it is prone to the problem of porosity, which is due to air being trapped in the casting during the die-casting process. Over the years, vacuum die casting has proven to be the best way to eliminate the problem of porosity.

Normally, the cavity of a steel mold must form a relatively sealed environment. The vacuum vent is positioned close to the top or side of the mold; due to the introduction of vacuum, the molten copper is injected into the mold cavity through the kinetic energy of the press injection at a back pressure below atmospheric pressure; as the press injection time progresses and the molten copper continues to fill, the pressure in the cavity is gradually reduced until the filling is complete and the press injection process is finished. This process repeats itself cyclically.

The main advantages of vacuum copper die casting technology include

Good solderability of die castings.

High mechanical strength of die castings.

Suitability for high volume production.

Minimal porosity.

High product denseness and uniform conductivity.

High airtightness of the product.

Improved product form-filling.

Ensures the precision of the product.

The disadvantages of vacuum copper die casting technology has the following main disadvantages.

Relatively high initial investment costs, including molds and equipment and hermeticity.

Setting up the vacuum die-casting process is tedious, both in terms of the copper alloy’s die-castability and the venting’s feasibility.

There are many factors affecting the efficiency of vacuum venting, in addition to the vacuum machine and venting components, the die casting machine, the mold, the quality of the alloy, the temperature of the alloy, the spraying technique, and so on, all have to be considered in unison.

For die-casting components and molds, including parting surfaces, melt cups and punches, ejectors, and even slides, sealing must be considered. This will ensure the efficiency and reliability of the vacuum exhaust.

To obtain high product repeatability and precision, it is important to control the pressure and speed profile of the die casting machine to be stable.

The following factors must be optimized: die coating, punch lubrication, copper melt alloy quality, and die temperature control.

Achieving an optimized vacuum process for copper die casting can be a delicate and challenging process. Each process should be optimized to achieve a high process vacuum.

Vacuum venting solutions: Vacuum venting needs to be set up in conjunction with the die-casting process, including selecting the vacuum machine and identifying the venting elements. Despite the high efficiency of the vacuum valve, the exhaust block is often the most suitable option in combination with the copper alloy’s high pouring temperature and the exhaust element’s cost. The poor fluidity of copper alloys and the high filling rate of the molten cup, with its low speed at 0.25-0.5m/s and high speed from 0.8 eventually reaching 3m/s, give relatively little opportunity for vacuum exhaust, so exhaust elements often require a larger exhaust area.

Considering that the temperature of the gas in the cavity is very high, instantly reaching even 500-600°C or more, which poses a problem for the sealing of the vacuum system, a gas water cooling device is usually considered for the pipeline between the mold and the vacuum machine to reduce the gas temperature and improve the life o

Summary

Brass die-casting for the solution of complex shapes and large quantities of copper products provides a broader and more effective means of production. Among them, brass die-casting products applied to decorative parts and wear-resistant parts have been very mature; there are brass die-casting transmission shift systems, control rods, pump-turbine parts, etc.; purple copper die-casting products applied to the motor rotor more broad prospects, rotor copper weight from a few grams to dozens of kilograms, its permanent magnet performance, life and cost are better than the current use of permanent magnets for electric motors. With the increasing market rate of electric and hybrid vehicles, copper die-casting of electric motor rotors will have greater prospects. As the vacuum copper die-casting process becomes more mature and widely used, it will provide strong support and guarantee for high-quality copper die-casting products.

If you are interested in copper die casting, please contact us.

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