Definition of "Die Casting"
Die casting is an automated casting process in which the liquid melt is pressed into a mold under high pressure (150 to 1200 bar) and at a high filling speed (up to 540 km/h). Usually alloys with a low melting point are used. This casting process is particularly suitable for series and mass production of components because, unlike sand casting, for example, permanent metal molds are used which do not have to be destroyed after casting. It is possible to produce large and complex components with low wall thicknesses.Molds
The die casting molds, made of high-quality, heat-resistant steel grades, consist of two halves which form a cavity into which the liquid melt is pressed during the casting process. The halves are located on a fixed and a movable machine plate. During the casting process a high pressure is applied to the mold halves, which is why the mold is equipped with latches. In addition, certain parts of the mold are cooled and/ or heated so that the casting solidifies as desired. The production of the molds is very expensive and time-consuming, but several tens of thousands to over a million castings can be produced with just one of them. A further advantage of the reusable casting molds is that the melt cools down quickly.Functionality and Procedures
In die casting, there are two different ways of manufacturing components: hot chamber and cold chamber die casting. In both manufacturing processes, the mold is sprayed with a release agent prior to the casting process in order to ensure that the subsequently cast part can be easily remover from the mold. However, the melt is not poured directly into the mold cavity, but is first filled into the casting chamber of the die casting machine. From there, the alloy is pressed into the mold by a piston (the so-called casting set) through one or more channels. The difference between the two processes lies in the structure of the casting chamber as described below.Hot Chamber Die Casting Method
A characteristic feature of hot chamber die casting machines is that the casting chamber is constantly in contact with the liquid alloy. The melt passes through a valve into the casting chamber, where it is pressed at high speed into the closed die casting mold by the piston. This process is used for alloys with a low melting point, such as zinc, lead or tin.Cold Chamber Die Casting Method
Cold chamber die casting machines are designed in such a way that the casting set is located outside the melt. To produce a component, the alloy is filled into the casting chamber and pressed into the die casting mold through channels. This process is suitable for materials with a higher melting point. These include, for example, aluminum and copper. After the alloy has been pressed into the mold in both processes, the component solidifies under the strong pressure, whereupon the latches of the mold can be opened. The part with gate is removed from the mold by automatically operated ejection pins and can be further processed if necessary. In simple words, the casting process can be divided into the following steps and in practice takes place in hundredths of a second - or even only thousandths of a second:- Fast filling of the casting mold with the alloy
- Curing of the component under high pressure
- Opening of the mold and removal of the component
Materials1
In die casting, non-ferrous metals are used to manufacture components, and the choice of alloy for a particular application depends on budget, weight and material properties.Aluminum is one of the most important materials with a share of more than 80 %, followed by zinc and magnesium. However, copper, lead and tin can also be used. The alloys have different properties. For example, aluminum (600°C) and magnesium (520°C) have a high melting point, zinc (380°C) and lead (320°C) a low melting point.
Die casting alloys offer many advantages:
- High corrosion resistance
- High strength and hardness
- High thermal conductivity
- High electrical conductivity
- Very good EMI/ RFI isolation
- Good processing properties
Low-Pressure vs. High-Pressure Die Casting
Various processes are used in foundry practice. Castings can also be produced without high pressure. In the sand casting process, for example, the alloy is poured into a mold made of sand, which must be destroyed in order to reveal the manufactured component (lost foam). In investment casting, which is used to manufacture very small cast parts, the molds and models (usually made of wax or plastic) are also destroyed after the casting process. Another example is Gravity Die Casting, which uses a permanent metal mold but does not use high pressure to press the melt into the mold. Rather, the casting is manufactured or the mold filled by gravity.
There are also differences in the die casting process. For example, there are processes that use either high or low pressure to produce the components. While high-pressure die casting accounts for around 50 % of light metal casting production, low-pressure die casting only accounts for just under 20 % of total production.2
Low pressure die casting primarily uses alloys with low melting points. It is possible to cast components from 2 to 150 kg. The advantages are that very high strength values and complex geometries as well as improved material utilization and dimensional accuracy can be achieved. The process is less suitable for very thin-walled parts, since only a minimum wall thickness of 3 mm can be obtained. It should also be mentioned that casting cycles using low pressure die casting are slower than those under high pressure.
In high pressure die casting, the melt is pressed into the mold under high pressure and at high speed, thus accelerating the casting cycle. In addition, thinner-walled castings (minimum wall thickness of 1 mm) with smoother surfaces can be produced. The disadvantage of this manufacturing process, however, is that high operating and investment costs are incurred, the strength values are lower and the die casting weight is limited because it depends on the closing force of the machine.
Fields of Application
Die casting is mainly used for large series production, i.e. for many components of the same type to be cast. Despite the high pressure used during the manufacturing process, a high casting quality is achieved. The die casting process is particularly suitable for the production of very thin (up to 1 mm) (lightweight) components.
Most commonly, die cast components are manufactured for the automotive industry, such as wheels, blocks, cylinder heads, valve blocks and manifolds. This sector accounts for around 84 % of the castings produced by German foundries.3 The use of aluminum parts leads to a reduction in the weight of the vehicles and thus to a reduction in fuel consumption. In addition, there are other industries in which die cast parts are used:
- Aerospace
- Domestic Appliances
- Furniture
- Power Tools
- Electronics
- Machinery
- Lighting Technology
In the future, other industries such as electromobility will be of interest to foundries. This offers enormous potential for light metal castings.
History of Die Castings
Die casting came into being in the middle of the 19th century, when publishing flourished. The aim was to be able to print newspapers and books quickly, flexibly and cost-effectively. These include a manually operated device in which an alloy of tin and lead could be cast into letters (1838), the rotary printing press (1846) and finally the Linotype typesetting machine (1886). One of the main elements of this machine was an integrated die casting machine which poured liquid lead into the line molds made of brass letter dies. After casting, the matrices were returned to the matrix magazine and the cast lead lines were ejected. The lead lines were then assembled into pages that served as printing plates for sheet-fed printing or as templates for the round printing plates required for the rotary printing process.
During the first 30 years, die casting was practically exclusively used in the printing industry. The turning point came when Herman H. Doehler founded the Doehler die casting Company in 1908, which in the early 1940s was regarded as the world's largest manufacturer of die cast parts and existed until 1998. The first die casting machine available on the market, which contained all essential components of a modern die casting machine, was developed in 1925 by Joseph Soss and Louis H. Morin in the USA.
During the late 19th century and the first decades of the 20th century, new products came onto the market and industrial production grew rapidly. Manufacturers recognized the advantages of die casting, which could be used to economically produce parts for typewriters, cash registers, watches and electrical appliances. Toy manufacturers also used die casting to produce toy and model cars that were as true to the original as possible. Last but not least, the emerging automotive industry and its suppliers became important buyers of die cast products.
Since the early 20th century, die casting technology has progressed with increasing speed. Initially, lead and tin, two metals with a relatively low melting temperature and good ductility, were mainly used in die casting. Around 1914, work began on zinc and aluminum alloys, which offered higher strengths. Copper and magnesium alloys were added in the 1930s. Magnesium, which with a specific density of 1.74 g/cm3 weighs a third less than the light metal aluminum (2.75 g/cm3) and is used today mainly in the automotive industry, experienced its first boom as a die casting material between 1946 and 1978: Engine and gearbox of the VW Beetle were made of this material. However, die casting technology did not linger on this development level. The machines were equipped with refined control technology and became more and more powerful. Tool steels with improved properties came onto the market for mold making, and the physical properties of die casting alloys were optimized so that today cast parts with both very thin wall thicknesses and high mechanical properties can be produced. Current topics include automation, energy and raw material efficiency, complex shapes, hybrid parts, process automation and digitalization (Industry 4.0).
References
1) Dynacast, die cast metals, available on https://www.dynacast.com/en-gb/
2) The Metal Casting, available on http://www.themetalcasting.com/pressure-die-casting.html
3) BD Guss, 2016
Further References
KUG BD Guss, 2009, Die Casting - Series production for highly complex thin-walled light metal castings, available on https://www.kug.bdguss.de/giessverfahren-inhalte/druckguss/
Powerguss - Haus der Gießerei-Industrie, Die casting - how does it work?, available on https://www.powerguss.de/was-ist-giessen/druckguss/
Lechuga, G., 2016, Die Casting Facts Infographic, available on https://www.slideshare.net/GermnLechuga/die-casting-facts-infographic
Dynacast, die casting, available on https://www.dynacast.com/en-gb/
