Excerpt from the May 2020 article from the Engineering.com article by Dr. Jody Muelaner
A key process for close-to-market manufacturing of high-value products.
Products are getting more complex, with organic shapes increasingly being specified to achieve the required strength while minimizing material use. There is also a trend for re-shoring and producing goods closer to the market where they are sold. The usual business case for close-to-market manufacturing is being able to respond quickly to increasingly dynamic markets. Reducing the carbon footprint involved in transporting goods and making supply chains more resilient are also important drivers. The current need for an urgent ramp-up in ventilator production has sharply brought into focus the need to respond rapidly to increasing demand with a resilient supply chain. Additive manufacturing (AM) plays into both trends. It is making people believe that products can be any shape we like and that they can be locally produced. In reality, AM has little scope to replace conventional production in the foreseeable future. Issues with material properties, feedstock costs, machine costs and build speeds mean that AM will remain a relatively niche process.
This article gives a detailed overview of another highly automated manufacturing process that can produce complex shapes in high-strength alloys. Die-casting may not be a new technology, but it is very well-suited to many modern products. I recently reported on how European bicycle manufacturers are re-shoring production as demand for high-quality e-bikes ramps up rapidly. These are sophisticated machines that use aerospace-grade materials with correspondingly high prices. In that article, I noted the importance of automation when re-shoring production into high-wage economies and identified high-pressure die casting as the most highly-automated process for high-quality bicycle frames. High-pressure die casting is a highly automated process that can economically produce parts with very complex shapes. It is typically suited to high-volume production. This article explores the process in detail, looking at tooling requirements, breakeven volumes, material properties and surface finish.
Die Casting Process Basics
The die-casting process uses a permanent metal mold or die. Molten metal is forced into the die cavity at a pressure of between 0.7 MPa and 700 MPa. Die casting is essentially the same process as injection molding. The term injection molding refers to the production of plastic parts while die casting involves production in metals. Die casting is most suited to softer alloys. In the past, tin and lead were popular materials for die-cast parts, such as toy soldiers. Today, zinc-, aluminum- and magnesium-based alloys are most common. High-strength structural automotive and aerospace components are produced, as well as many consumer goods.
Each injection, is known as a shot. A shot may be made up of more than one casting or part, as well as the scrap material that is produced during the casting process. Scrap includes the sprue, where the material enters the die; the runners, which distribute material to multiple part cavities; and the gates, where the material flows into individual part cavities. Sprues, runners and gates are found in other types of casting as well as in injection molding. If you’ve ever made an Airfix model, you will remember receiving the parts still attached to the shot. You may have also sometimes had to trim away flash, a thin layer of material that has leaked into the interface between the two halves of the mold. The parts are attached to the gates and joined together by the runners. In industrial casting processes, the individual parts must be separated from the scrap. This process is known as shakeout and may be carried out using a trim die in a press.
The die is made in two halves so that the shot can be removed. One half of the die is fixed and contains a hole through which the molten metal is injected. The other half is closed with a press that must be able to resist the pressure of the metal being injected. This pressure can be considerable, resulting in die casting machines being very large and heavy. Molten metal enters an injection cylinder, known as a shot chamber, and a piston then injects it through a nozzle into the die.
Die casting is normally categorized into two basic types, depending on whether the shot chamber is heated:
- In Hot-Chamber die casting, the shot chamber is located within a large crucible containing molten metal, and it is ejected through a gooseneck that rises up out of the molten metal and into the nozzle. This is suited to lower pressures, of up to 35 MPa, and low melting temperature alloys of zinc and magnesium. Cycle times are typically 200 to 300 shots per hour, although very small mass-produced parts, such as zipper teeth, can be cast at 18,000 shots per hour—five every second!
- In Cold-Chamber die casting, the shot chamber is not heated, and the molten metal is poured into it. Pressures can be as high as 150 MPa. This process is suited to higher melting point alloys of aluminum, magnesium and copper. It is even possible, although unusual, to cast steel using this process.