Design guideline for hot riveting
The existing situation with regard to the standards concerning riveting processes for plastics no longer offers an adequate design basis today. Moreover, they do not make any statement about the temperature range in which a hot riveting operation must be carried out.
For this reason, bdtronic GmbH (formerly BARTEC Dispensing Technology GmbH) has conducted a fundamental investigation together with a renowned manufacturer of engineering plastics and the Chemnitz University of Technology. One objective of the investigation was to find the optimum process values for the caulking pressure, the temperature range, the rivet dome geometry, the stamp shape and the heating time using the example of the typical PBT and PA66 which have glass fiber proportions of 30 % and are utilized for electronics. With the "BHS processes", the exact temperature measurement and control made it possible for the first time to precisely portray the process and the relevant data and thus to deduce reproducible process windows because these materials belong to the group of the semi crystalline thermoplastics which have very small process windows between solid and liquid. In order to achieve the best possible structural properties and pull-off values, the comparative test series showed that, subject to an optimum choice of the process parameters and to the process course, the "BHS hot stamp process" as a contact riveting process obtained the highest pull-off values with over 750 N for a solid rivet with a diameter of 3 mm. This result is surprising in so far as experience shows that, as materials with poor conductivity, plastics should be heated up to the forming temperature very homogeneously in order to melt a maximum volume and thus to be able to achieve the optimum structural composition. The hot air process with which the homogeneous plasticization of the entire rivet dome is enabled is normally used for this purpose. In contrast, the hot stamp process with common process sequences allows merely the complete melting and dis-placement of the material via the con-tact face at the rivet tip. The created rivet head thus exhibits weak spots if the sequence is not optimum. The process sequence and the correct choice of the geometry may lead to an improvement in the strength by more than 300 %.As a result, it was possible to deduce anew design guideline which is an optimum compromise between a minimum cycle time and a maximum strength. And one remark should still be made in conclusion: the external appearance does not provide any statement about the strength of the riveted joint. bdtronic offers the hot riveting processes called "BHS hot stamp" (hot stamp riveting) and "BHS hot jet" (hot air riveting). With both systems, even semi-crystalline thermoplastics can be processed in a reliable, energy-efficient and reproducible way in process windows of ± 5 - 10°C using active dynamic temperature control. As single-cycle processes with an integrated cooling system and a Z stroke, they do not need any axial movements, can be integrated in a space-saving way and do not permit any risky distortion between heating and forming. The systems can be utilized on electronic products and are equipped with a quick-change stamp with a long service life and a non-stick coating.
Published in Joining Plastics in June 2013