The thread-coining screw for microcellular foamed thermoplastics
The implementation of innovative lightweight design solutions is of particular importance not only for the automotive and aerospace industries, but also for various other branches. Due to the increasing demands on resource conservation and energy efficiency, the respective companies are consistently pushing the topic. Besides many other lightweight materials available on the market, especially foamed thermoplastics produced via the MuCell® process* are becoming increasingly popular amongst designers and component developers. For this ambitious application area, the fastening technology specialist EJOT has developed a particularly efficient fastening element - the Cell PT® screw.When plastic components are developed, foamed thermoplastics are increasingly considered in the technical specifications for weight reasons. This innovative material allows a significant cost saving during production because it is easily processible and thus makes the whole production process much more efficient. In practice, two different foaming processes are distinguished: physical or chemical. They differ in the production process as well as in the structure of the finished plastic part, which is reflected in different cell sizes. The result of both processes is always a microcellular foam structure in the component (even for wall thicknesses of less than 1 mm) and a dense outer layer.
Besides the production-related advantages of this innovative process, the undisputed weight advantage is the main reason for its increasing popularity. Depending on the component, weight savings of 5 to 15 percent are possible. This offers a wide range of possible applications for thermoplastically foamed components. For the automotive sector, for example, the whole interior is predestined for their use. The range of possible applications is virtually unlimited: from the instrument panel to covers, lids, door and seat panels, electronics and control housings to handles or covers.
Since the lower density of the foamed thermoplastic, which ultimately accounts for the weight advantage, has a direct effect on the stability, the challenge for the product developer is the structural consideration of this point in the component design. Along with this, the fastening concept must also be reconsidered, since conventional self-tapping screws are not designed for these altered material properties and therefore quickly reach their limits. For this application, the fastening technology specialist EJOT has developed the Cell PT®. This new screw features a special thread contour because of the specially formed thread point in combination with a sophisticated flank angle and radius combination in the threaded area.
This special thread contour of the EJOT Cell PT® was developed in a comprehensive test series especially for cellular foamed thermoplastics. Contrary to self-tapping geometries, this contour allows to coin the thread by elastic / plastic deformation inside the plastic boss without damaging the material. Through the thread-coining process, the dense outer layer of the core hole is deformed by the thread root and not destroyed - it is thus preserved. The resulting high torque and load transmission in (MuCell®) foamed thermoplastics guarantees the highest fastening reliability. In the case of blind hole screw joints, the specially formed point additionally penetrates into the plastic material at the bottom of the blind hole and a further increase of the stripping torque is achieved.
* MuCell® is a registered trademark of Trexel Inc., USA.
* MuCell® is a registered trademark of Trexel Inc., USA.
