From Science and
From Science and Research Welding of biobased plastics Plastics based on renewable raw materials are more and more used to manufacture complex and more durable products. This becomes possible through intensive research and development work, which is focused on the optimisation of the processing and usage properties of those materials. For this reason, new developments of injection moulded parts made of bioplastics have repeatedly been found in fields such as the automotive, electrical, sports or stationery industries. The economical production as well as the realisation of new products requires the choice of suitable joining technologies. In order to meet the increasing requirements of the modular component design, the knowledge of joining processes for the reliable joining of components made of biobased plastics is gaining in importance. The well-established series welding processes, e.g. the ultrasonic welding, offer the only economical and demand-oriented solution for many applications. While extensive process and material knowledge of plastic welding is documented and accessible to processors of petrochemically produced plastics, there are few reliable findings on welding of injection moulded components made of bioplastics. Current research activities include, for example, the influence of joining speed and joining path in correlation to weld strength during ultrasonic welding of injection moulded PLA specimens [1]. The Institute of Plastics Processing (IKV) at RWTH Aachen University has fundamentally investigated the welding behavior of each two types of polylactide-blends (PLA), cellulose acetate (CA) and castor oil-based polyamides (Bio-PA) using an industrially important series joining process, ultrasonic welding. In ultrasonic welding, the heating of plastics occurs due to internal and external frictional effects. Furthermore, the infrared welding process was used to assess the weldability of the materials. The material is heated up to the joining temperature by the absorption of the radiation in the joining area. The objective was to systematically investigate the influence of injection moulding processing parameters (melt temperature, mould temperature and injection flow rate) on the subsequent joining process and the resulting weld seam formation for novel bioplastics. The work was carried out as part of a “Feasibility study on the weldability of new types of biobased plastics for injection moulding applications“ (BMEL research project No. 22002516). A summary of the performed investigations and the main findings on the welding behaviour of the various plastics are presented below. For further information, it is referred to the publicly accessible final report on the research project [2]. Experimental and results The following criteria were taken into account when selecting materials for the welding tests: • Biobased or semi-biobased plastics • Processable by injection moulding • Suitable for long-lasting applications • New products potentially realisable by further processing with a joining process Table 1 gives an overview of the six different test materials. The PLA and CA types are from FKuR Kunststoff GmbH, Willich, Germany. The two PLA blends are experimental types that are not commercially available. Both Bio-PA are materials from the Evonik Industries AG, Marl, Germany. A systematic variation of the process parameters followed during the production of weld test specimens by injection moulding. The melt temperature, the mould temperature and the injection flow rate were varied in accordance to the respective manufacturer’s instructions for processing the materials. The subsequent welding tests were carried out using the ultrasonic or infrared welding method. For each material welding parameters were experimentally determined. As a first result, it can be stated that all of the test materials can be welded. Conspicuities could be observed, especially for the CA types, depending on the joining methods used. During infrared welding of test specimens using a medium-waved metal foil radiator of Krelus Infrared AG, Oberentfelden, Switzerland, the CA 55 material proved to be problematic during infrared heating. The process window is to be considered small compared to the other test materials. The heating time could only be varied within a range of 2 s using a constant radiator output and defined emitter distances (determined in preliminary tests), so that a material-locking connection resulted without exceeding the flash point of the material. The CA 95 tends to bubble formation in the heat-affected zone during infrared heating (Figure 1), which could be traced back to an unspecified volatile substance (e.g. plasticiser) by Thermogravimetric analysis (TGA). With the help of variance analyses, the influence of the injection moulding parameter variation with regard to the mechanical failure of the joint under short-term tensile stress was evaluated using the measured forces at break. Depending on the joining method and the respective bioplastic, in some cases clearly different effects with a 32 bioplastics MAGAZINE [05/18] Vol. 13
From Science and Research By: Christian Hopmann and Martin Facklam Institute of Plastics Processing at RWTH Aachen University (IKV) Aachen, Germany static significance resulted. For example, an increase of the melt temperature from 190 to 220 °C in the previous injection moulding process tends to reduce the forces at break for the ultrasonic-welded specimens made of the PLA types (V14 and V15). While no statistically significant main effects were observed after the ultrasonic welding of the test specimens from the Bio-PA, a significant increase in the forces at break was observed for the PA 6.10 due to an increase in melt temperature from 240 to 270 °C. For the second Bio-PA type (PA 10.10), the weld test specimen used in many cases (30 % of the sample size) reached its mechanical limits, so that a tensile-test of the weld seams did not produce representative results. Overall, the expected weld seam quality can be evaluated as promising when results will be transferred into practical components. Conclusion In summary, fundamental findings on the welding behaviour of the test materials have been gained based on the joining tests carried out for ultrasonic and infrared welding with a broad spectrum of injection mouldable plastics based on renewable raw material. Depending on the plastic welding method, significant differences in the material specific weldability of and the injection moulding parameters, which influence the quality of the weld seam, could be observed. Table 1: Overview of the biobased test materials Abbreviation Material Type of plastic Biodegradable PLA V14 Bio-Flex V 143621 PLA + PLA V15 Bio-Flex V 150701 PLA + CA 55 Biograde C 5508 CA + CA 95 Biograde C 9550 CA + PA 6.10 Vestamid Terra HS 18 Bio-PA - PA 10.10 Vestamid Terra DS 18 Bio-PA - Figure 1: Bubble formation during infrared heating of the material CA 95 Visible material degration Microscopic image of the joining area References 1. Klinstein, L.; Frantz, J.; Grewell, D.; Lebron, K.: Welding of PLA. Proceedings of the 75th Annual Technical Conference of the Society of Plastic Engineers (SPE). Anaheim, California, USA, 2017 2. N.N.: Machbarkeitsstudie zur Schweißbarkeit von neuartigen biobasierten Kunststoffen für Spritzgießanwendungen. Vereinigung zur Förderung des Instituts für Kunststoffverarbeitung (IKV) in Industrie und Handwerk an der RWTH Aachen e.V., Schlussbericht zum BMEL- Vorhaben Nr. 22002516, 2018 Welding bead Bubble formation over the circumference of the material CA 95 www.ikv-aachen.de Acknowledgement The research project (No. 22002516) has been sponsored due to an enactment of the German Bundestag from the German Bundesministerium für Bildung und Forschung (BMEL) via the Fachagentur für nachwachsende Rohstoffe e.V. (FNR) as project management agency of the BMEL for the Renewable Resources funding program. Info: Download link for reference [2] (German language only): https://tinyurl.com/bioplastics-welding bioplastics MAGAZINE [05/18] Vol. 13 33
