What is High Pressure Die Casting?
High pressure die casting is a versatile process for producing engineered metal parts by forcing molten metal under high pressure into reusable steel molds. These molds, called dies, can be designed to produce complex shapes with a high degree of accuracy and repeatability. Parts can be sharply defined, with smooth or textured surfaces, and are suitable for a wide variety of attractive and serviceable finishes.
Die castings are among the highest volume, mass-produced items manufactured by the metalworking industry, and they can be found in thousands of consumer, commercial, and industrial products. Die cast parts are important components of products ranging from automobiles to toys. Parts can be as simple as a sink faucet or as complex as a connector housing.
The die casting process (video below) has evolved from the original low-pressure injection method to techniques including high-pressure casting- at forces exceeding 4500 pounds per square inch- squeeze casting and semi-solid die casting. These modern processes are capable of producing high integrity, near net-shape castings with excellent surface finishes.
The Advantages of Die Casting
Die Casting is an efficient, economical process offering a broader range of shapes and components than any other manufacturing technique. Parts have long service life and may be designed to complement the visual appeal of the surrounding part. Designers can gain a number of advantages and benefits by specifying die cast parts.
- High Speed Production: Die casting provides complex shapes within closer tolerances than many other mass production processes. Little or no machining is required and thousands of identical castings can be produced before additional tooling is required.
- Dimensional Accuracy and Stability: Die casting produces parts that are durable and dimensionally stable, while maintaining close tolerances. They are also heat resistant.
- Strength and Weight: Die cast parts are stronger than plastic injection moldings having the same dimensions. Thin wall castings are stronger and lighter than those possible with other casting methods. Plus, because the die castings do not consist of separate parts welded or fastened together, the strength is that of the alloy rather than the joining process.
- Multiple Finishing Techniques: Die cast parts can be produced with smooth or textured surfaces, and they are easily plated or finished with a minimum of surface preparation.
- Simplified Assembly: Die castings provide integral fastening elements, such as bosses and studs. Holes can be cored and made to tap drill sizes, or external threads can be cast.
Die Casting Process Overview
The basic die casting process consists of injecting molten metal under high pressure into a steel mold called a die. Die casting machines are typically rated in clamping tons equal to the amount of pressure they can exert on the die. Machines sizes range from 400 tons to 4000 tons. Regardless of their size, the only fundamental difference in die casting machines is the method used to inject molten metal into a die. The two methods are hot chamber or cold chamber.
Hot chamber machines are used primarily for zinc, copper, magnesium, lead and other low melting point alloys that do not readily attack and erode metal pods, cylinders and plungers. Cold chamber machines are used for alloys such as aluminum and other alloys with high melting points. The molten metal is poured into a “cold chamber”, or cylindrical sleeve, manually by a hand ladle or by an automatic ladle. A hydraulically operated plunger seals the cold chamber port and forces metal into the locked die at high pressures.
A complete die casting cycle can vary from less than one second for small components weighing less than an ounce, to two-to-three minutes for a casting of several pounds, making die casting the fastest technique available for producing precise non-ferrous metal parts.
In addition, there are several variations on the basic process that can be used to produce casting for specific applications. One of which is squeeze casting. Squeeze casting is a method by which molten alloy is cast without turbulence and gas entrapment at high pressure to yield high quality, dense, heat treatable components.
Aluminum Die Casting vs. Other Metals
Aluminum is a lightweight alloy which possesses high dimensional stability for complex shapes and thin walls. Aluminum has good corrosion resistance and mechanical properties, high thermal and electrical conductivity, as well as strength at high temperatures.
Other cast materials include zinc, magnesium, copper, and lead and tin.
Choosing The Proper Aluminum Alloy
Varying aluminum alloys are composed of many elements, including silicon, iron, copper, manganese, magnesium, nickel, zinc, tin, titanium, and others. Although a variety of aluminum alloys can be die cast from primary or recycled aluminum, RCM Industries, Inc. has the capabilities to cast the following alloys:
- 360 Aluminum: Selected for best corrosion resistance and pressure tightness.
- 380 Aluminum: The most common and cost effective aluminum alloy. It also provides the best combination of utility and cost.
- 383 Aluminum: A modification of 380. It provides better die filling but with a moderate sacrifice in mechanical properties such as toughness.
- 390 Aluminum: Selected for special applications where high strength, fluidity and wear-resistance and bearing properties are required.
- 413 Aluminum: Used for maximum pressure tightness and fluidity.
Don’t know which alloy is best for your product or application? RCM’s team of engineers will work with you to make sure that the proper alloy is selected for your project.
Dies, or die casting tooling, are made of alloy tool steels in at least two sections, the fixed die half, or cover half, and the ejector half, to permit removal of castings. Modern dies also may have moveable slides, cores or other sections to produce holes, threads and other desired shapes in the casting. Sprue holes in the fixed die half allow molten metal to enter the die and fill the cavity. The ejector half usually contains the runners (passageways) and gates (inlets) that route molten metal to the cavity. Dies also include locking pins to secure the two halves, ejector pins to help remove the cast part, and openings for coolant and lubricant.
When the die casting machine closes, the two die halves are locked and held together by the machine’s hydraulic pressure. The surface where the ejector and fixed halves of the die meet and lock is referred to as the “die parting line”. The total projected surface area of the part being cast, measured at the parting line, and the pressure required of the machine to inject metal into the die cavity governs the clamping force of the machine.
There are a few different types of dies:
- Single cavity to produce one component
- Multiple cavity to produce a number of identical parts
- Unit die to produce different parts at one time
- Combination die to produce several different parts for an assembly.