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A contribution to understanding the heating and welding behavior of thermoplastic polymers in microwave welding

Even though the utilisation of microwave radiation would offer various possible applications within polymer processing it’s employment did not spread in large scale up to now. Advantages like the non contacting, volumetric heating are opposed mainly by the complex and difficult to control microwave technology. gen. In principle the heating of all polymers with polar properties or conductive modification is possible. The welding with microwave radiation can be divided into two varieties: The welding with and without additional welding material. Within the scope of this paper investigations into both are presented. Concerning the direct microwave welding the MW-heatability and -weldability of the technical polymers PA6, ABS, PMMA, PET, PBT, POM and PVDF was evaluated by means of basic systematic investigations. After all only the materials PVDF, POM and PBT were to be rated as suitable. For these the temperatures obtained within the material as function of heating time are given as criteria fort heatability.

By the means of welding trials the achievable weld strengths as well as their dependency of the welding parameters microwave power, heating time and welding pressure are shown. These results are confronted to the measurements of material properties like dissipation factor tanδ and dielectric loss εr’’. It shows that a quite good correlation exists. Furthermore the correlation between weld strength and the resulting joining displacement is shown. For the materials PVDF and POM it can be shown that a minimum joining displacement for good weld strengths exists. For the direct MW-welding the characteristic ratio L0/d and sF/L0 are determined and compared to the optimization criteria known for the hot tool welding. Within the scope of the investigation into the indirect MW-welding different colorants, additives, fillers and reinforcements are evaluated which were considered to be suitable modifiers for in pure state inactive polymers. All of those were characterised by their heatability by microwave irradiation in a waveguide system. It is shown that most of these common compounding materials only possess a minor reaction in the microwave field. A technically relevant heatability is found only for a few. For the heating of those materials in the microwave field the reached temperatures and the temperature curves versus time are given.

As essential influence the correlation between the concentration of compounding material and heatability is described. In general the heatability increases with increasing concentration. By means of the aluminium pigments investigated the correlation of particle size and heatability of the compound is shown. As indication to the complexity and difficulty of controlling of microwave technology phenomena like thermal runaway and spark-over are described and explained with system and material specific properties. To these the phenomenon of awayconduction of MW-power in areas far from irradiation adds with conductively modified materials fort the indirect microwave welding. For all modified materials the named phenomena are influenced by the quality of mixture of compounding materials within the matrix material. Concluding a comparison of energy consumption in relation to the joining surface of the direct MW-welding is given to the hot-tool and vibration welding. It is shown that the direct MW-welding is relatively energy saving which is an additional advantage to other welding processes. By the means of the results presented a material now can be selected specifically for the use of Microwave technology. The correlation of results to the dielectric properties also allows the transfer to other materials.

However the problems and difficulties described in this paper show that microwave welding still is a process that is not jet suitable for an industrial employment on large scale. Nevertheless is becomes clear that potentials are existing which are to be exploited during the further development. Not until the technology of emitting and coupling of microwave radiation reaches a maturity sufficient for the welding of polymers the results presented will show their full benefit.

Lesen Sie die deutsche Zusammenfassung auf Kunststoffe.de
Author
 Odo Karger

Odo Karger
Institut für Kunststofftechnik
Universität Paderborn

Information

Free keywords: Mikrowellenschweißen, Mikrowellenerwärmung, Schweißen von Kunststoffen, dielektrischer Verlustfaktor, Permeabilität, Hohlleiter, Aktivierng, Additive
Institute / chair: Fakultät für Maschinenbau der Universität Paderborn
Language: German
Technical consultant for expert services: Prof. Dr.-Ing. Helmut Potente (Betreuer), Prof. Dr.-Ing. Ortwin Hahn
Publication year: 2005
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) / Kunststoffe.de

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