The concept of prestressing is widely used in the industry and for many different applications. The general idea is to bring a certain tool component (for example a die) into a compressive state at tool assembly for lowering the level of tensile stresses at maximum process load. Prestressing of high-loaded dies in precision metal forging applications, high-pressure synthesis of industrial diamonds, and high-pressure experiments are well-known examples of such industrial applications. Prestressing is one of several design parameters but is normally a key to achieving high-performing tooling systems including elimination or postponement of die cracks, chipping, wear, or loss of required part tolerances.
The technology was originally developed by Dr. Jens Grønbæk and started as a new internal cold forging business unit at Danfoss in the 1980’ties.
Today, STRECON is an independent company and the original developer and leading manufacturer of prestressing tool systems based on the stripwinding technology, which offers several merits in terms of high strength, stiffness, extended lifetime, and high reusability compared to conventional tools.
Prestressing of tools can be understood as the compressive stress (typical radial), which intentionally is being devoted to a loaded tool, and for example a forming die. The purpose of prestressing is to reduce the level of critical tensile stresses, which the tool will see under fully loaded conditions in the production or testing process. The higher the prestressing, the deeper the tool will be pushed into a compressive state as illustrated in the figure.
Traditional prestressing has been focused on tool stress. However, it is equally important to look at the corresponding strain resulting from tool loading. Strain is a measure of how much the tool deflects under loading. Principally, tool deflection is acceptable or even preferable as long as it does not exceed the yield point of the tool material itself. In that case, the material starts cracking or plastically expanding. However, no, or very limited tool deflection is pursued in certain industrial applications of high precision products like gears and components for the driveline and steering column as well as diamond / CBN products made under extreme pressure and temperature conditions. The strain characteristics of the tool system are determined by the Youngs Modulus of the applied materials, also known as the E module. The higher the E Module, the higher the stiffness of the tool system, and the lower the tool deflection. In general, tungsten carbide (540 GPa) has a stiffness that is more than twice as high as tool steel (225 GPa).
Often prestressing would be needed in both radial and axial directions, and the goal is to apply the optimal level of prestressing. This requires special software for analytical calculations but often FEA is also needed to simulate and understand the complex nature of multi-directional stresses and strains in high-loaded tooling.
The higher the tool load, the higher the level of critical tensile stresses in the tool. Consequently, the importance of prestressing as an effective countermeasure increases with the tool load.
In the industry, there is a general trend toward increasing tool loads, and reflects the demand for products with higher strength, robustness, precision, and reduced weight. However, the need for improving the prestressed condition of the loaded tools is limited with the conventional tool and material technologies. Normal prestressing tool systems are designed as one and two rings, and a multiple ring system is used for manufacturing industrial diamonds / CBN products. However, standard prestressing tools are usually made with standard tool materials like 1.2344/H13 and with varied hardness levels, resulting in a tool system with a maximum loadability of half to two-thirds of the strength capability of a stripwound container.
The STRECON prestressing technology is based on a different constructive tool design, materials, and manufacturing process (see below section), and in general 50-100% stronger than normal prestressing tool systems. Furthermore, and in addition to the strength capability, the stiffness of STRECON’s prestressing tool system itself is about twice as high as normal prestressing ring systems when the inner ring of the prestressing tool is made with a carbide core, namely 400 GPa compared to 225 GPa.
STRECON has developed a prestressing tool based on the stripwinding principle. A special heat-treated steel strip material is wound around an inner ring of high-alloyed tool steel or tungsten carbide while fully controlling the back tension of each strip layer until the preset winding diameter has been achieved. On the outer diameter of the strip material, a ring of hardened tool steel is mounted by heat shrinkage. The three elements of the inner ring (called the winding core), the steel strip material, and the outer casing define the basic concept of the stripwound prestressed container, which normally is called a STRECON container. The figure to the left provides a graphic explanation of the difference between a STRECON container and a normal ring system. As can be seen, the stripwound container offers a constant stress distribution across the full radius of the stripwound body. This feature is unique to the stripwinding technology.
The steel strip material has a thickness of 0,1 mm, it is fully elastic up to approx. 2000 MPa, and it can be stressed up to 1% before elongation initiates. In other words, the steel strip material is very strong and elastic which also explains the very long service life (i.e. reusability) of the container itself.
Recently STRECON expanded the technology base for precision forging applications by developing a full tool system in which the STRECON container is one of several integrated elements. This system solution has been named Prestressing Tool System (PTS), which fits directly into the press machine. PTS offers both radial and axial prestressing of the forging die, which is ensured with a top lid that is prestressed with a number of specially designed extension bolts.
Normal tooling has limited loadability due to the materials and system design available. The prestressing tools used in metal forging and high-pressure experiments are traditionally simple ring tools made as hardened one or double rings whereas the prestressing system in high-pressure synthesis is made of 5-7 rings.
The STRECON prestressing system solutions are based on the merits of the stripwinding technology and adapted to both the application field and customer requirements. STRECON has a strong competence and experience in designing, manufacturing, and commissioning prestressing tool solutions for high-loaded tools like precision forging, high-pressure synthesis, and high-pressure experiments but can also be adapted to other application fields like powder pressing, deep drawing, etc.
- Innovations in Cold Forging Die Design (PDF)