According to DIN 8583, extrusion belongs to pressure forming and thus to the family of forming processes . This is a massive forming process that creates both hollow and solid bodies through a one-step or multi-step manufacturing process.
Extrusion is a forming process in which there is predominantly compressive stress on the workpiece to be formed. The extrusion processes are described in more detail in DIN 8583-6 and are further subdivided there on the basis of the cross-sectional shape before the start of forming. If the cross-section of the blank is not reduced by recesses, it is referred to as full extrusion; if the cross-section is reduced, it is referred to as hollow extrusion. It is also possible to distinguish between the direction of flow of the material relative to the direction of movement of the punch. The following distinctions have emerged:
- Forward extrusion = material flow and direction of punch movement are the same
- Backward extrusion = material flow and punch movement direction are opposite
- Cross extrusion = material flow perpendicular to the direction of movement of the punch
A combination of these three extrusion processes is possible.
The extrusion process can be further characterized with regard to the temperature at which the workpiece is fed to the forming process. According to DIN 8582, it is decisive whether the workpiece was heated above room temperature before forming. If this is not the case, one speaks of cold forming . If the workpiece is heated above the recrystallization temperature, one speaks of hot forming or forging . If the workpiece is heated prior to forming, but not beyond the recrystallization temperature, this is called warm forming.
In general, it can be assumed that the deformability of a material increases with increasing temperature. For this reason, hot forming is often carried out when very large material distributions are required to manufacture the component. Since the strength of most materials decreases as the temperature rises, forging also requires less effort. For this reason, very large components, for example the rotor of a steam turbine , are produced by hot forming, since the otherwise required forces could not be applied by any forming machine or press .
There are also extrusion processes with active media (e.g. hydroforming ). This includes hydrostatic extrusion . It is a forward extrusion in which the punch does not press directly on the workpiece, but a liquid that surrounds the workpiece. The required pressure (15,000–20,000 bar) is achieved using a pump or press.
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Tool technology in extrusion
Extrusion tools usually consist of a die and a punch. In most cases, the punch carries out the movement required for forming and the die is stationary.
Extrusion die dies
With most dies it is necessary to reinforce the die due to the high internal pressures caused by the forming process. This reinforcement is known as reinforcement. In this process, at least one so-called reinforcement ring is placed around the die. The inside diameter of the reinforcement ring is slightly smaller than the outside diameter of the die. There is consequently an excess . If the two parts are now joined together, the reinforcement ring is mainly subjected to tension, whereas the die is mainly subjected to compression. At the beginning of the forming process, there is consequently a compressive stress state in the die. Before the die can be subjected to tensile stress at all, this compressive stress state must be overcome. Ideally, the interference fit should be dimensioned in such a way that at no point in the process do any positive expansion components occur in the die. Reinforcing the matrices can increase their resilience.
Extrusion die punches
With the punches of extrusion tools, one moves in the field of tension between sufficient compressive strength and toughness. If you choose a very hard stamp material, the pressure resistance is excellent, but the very low toughness can lead to the stamp breaking.
Materials for dies and punches
During extrusion, depending on the workpiece material, internal pressures in the die or normal contact stresses on the punch can exceed 5000 MPa. Because of these high internal pressures, a large number of different wear mechanisms occur. Since the production of tools is very expensive, the tool materials listed below are mainly used in order to minimize wear as much as possible.
In the group of tool steels in particular, tool steels produced by powder metallurgy are increasingly used today, as they are clearly superior to their conventionally melted counterparts in terms of wear resistance and fatigue strength . However, tool steels have a lower pressure resistance and lower wear resistance than hard metals. Therefore, hard metals are increasingly being used in order to further increase the tool life of the tools used. Compared to tool steels, however, due to their significantly higher hardness , cemented carbides are also much more brittle, so that when used as a die material, it is imperative to ensure that no positive expansion components occur in the die during the process, as otherwise very rapid failure due to fatigue would occur. Modern hard metals achieve compressive strengths of over 8000 MPa.
Materials for reinforcement rings
As already mentioned, reinforcement rings are mainly subjected to tension. Therefore, these materials must have a particularly high fatigue strength. For this reason, hot-work steels are used in most cases . A common steel is 1.2343 or 1.2344. However, compressive stresses occur on the inside, so it is important to ensure that these steels are sufficiently hard. As a rule, hardnesses of around 42 HRC are sufficient. In exceptional cases, however, hardnesses of up to 50 HRC can be used, which, however, makes a precise design of the tools necessary.
As a rule, tools must be designed for the intended application in order to achieve the maximum economic tool life. While design nomograms and simple analytical calculations were used in the past, FE-based design is increasingly used . In the analytical methods, Lamé's equations are mainly used . These equations are sufficiently precise for simply rotationally symmetrical cross-sections, but the precision decreases significantly with difficult shapes, so that the informative value must be regarded as too low. For this reason, the FE-based design is becoming increasingly important. The advantage here is the fact that complex three-dimensional workpiece geometries can also be modeled with it and the tools for producing them can be designed. In addition, a large number of different result parameters are available, which play a decisive role in tool design. These are among others:
In general, when designing tools, care should be taken not to create any sharp-edged transitions in the tools, as this can have a negative effect on the tool life due to the risk of transverse cracks. It is therefore desirable to provide transitions with the largest possible radii or chamfers .
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