http://www.revmaterialeplastice.ro MATERIALE PLASTICE ♦54♦No. 3 ♦2017 461 Research of the Thermal Gradient and Quality of Welded Joints from PVC Thermoplastics Foils at High Frequency Symmetric Welding ION MITELEA, ILARE BORDEASU*, NICOLAIE VARZARU, ION DRAGOS UTU* University Politehnica Timisoara, 2 Piata Victoriei Sq., 300006, Timisoara, Romania Extending of the welding application field by high frequency currents of the thermoplastic polymers requires the full knowledge of the physical parameters which characterize the joining process of the welded joint. Since these engineering materials are extremely sensitive to temperature variations, the present paper highlights the role of the thermal gradient in obtaining of some welded joints with high mechanical strength and free from continuity defects. Regarding this, it is demonstrated that increasing the thickness of the polyvinyl chloride films, from 0.4 to 1 mm, the heat effect in the contact area of the electrodes is kept and the welding process has a high stability. For a well-run welding process is obtained a uniform distribution of the molten material in the joint area with positive impact on the breaking force values. Keywords: high frequency welding, PVC foils, thermal gradient, quality Welding assembly of polymeric materials, currently, knows a continuous development from both material involved and in terms of potential applications [1-3, 9, 10]. In this context, high frequency (HF) welding is a joining process that offers many advantages such as: dimensional accuracy, flexibility, control and automation facility, etc. [4, 5]. During decades, the researches performed in this area aimed both aspects related to welding sources and modeling phenomena occurred at the interaction of the electromagnetic field - polymer. Despite these constant concerns the current level in the control and optimization of process parameters remains one based on experimental tests. The difficulties encountered are related to the knowledge of the materials properties to be joined, in particular, how they respond to the high frequency current action [6,7]. Therefore, this paper aims to study the importance of the thermal gradient on the cross section of HF performed welds of some polymers on their quality. Experimental part By introducing a dielectric material in an electric field, a part of the field energy is dissipated in the substance, being transformed into heat in most cases. Material energy dissipated per time unit under the influence and on behalf of the electric field constitutes the dielectric losses (of the material to be welded). Since the polymers are poorly electrically conductive materials with a high dielectric constant, they act as a dielectric between the electrodes of the welding die; they are heated and under the pressure of the electrodes and after cooling time the welded joint is achieved. Thermoplastic materials that exhibit good weldability by HF are [3, 6, 9, 10]: - polyvinyl chloride (PVC) soft and hard; - polyamides (PA); - polyester (PES); - thermoplastic polyurethane (TPU); - some types of composite materials (TPO). A HF mold welding can use up to two material layers with thickness between 0.2 to 5mm. The materials can be welded without equipment constraints if there are sized to a maximum of 20 cm²/kW. The welding machine used in experiments [7] contains an induction coil which is supplied with radio-frequency electrical currents. It generates a high frequency electromagnetic field that acts as a electric capacitor (welding tool as reinforcements and the material to be welded as dielectric). The symmetrical high frequency (HF) welding has the particularity to use a lower electrode. Generally the lower electrode represents the mirror view of the electrode or with a slight deviation from it. The symmetrical welding was the first application of HF welding process, because the symmetry ensure stabile and constant electrical field and melt material flow. Applications of this process are met in all the industries: - packaging with plastic products as bags, boxes etc, - medical with plastic bags, perfusions etc, - automotive with plastic components (sunblind, carpets etc), - other applications: plastic covers, light plastic structures etc. After HF welding some products must be cut to reach the final product. To reduce production costs has raised the idea of developing a combined technological application to perform welding and cutting plastic sheets in one single step. So instead of simply welding electrodes shown in figure 2, there are used sharp electrodes equipped with cutting edge shown in figure 1. The HF symmetrical welding process can be: - simple presented in figure 1, where the foils are just welded. - combined presented in figure2, where the foils are welded and cut simultaneous. The difference of the process variants, presented above, is the geometry of the electrode. For combined HF welding symmetrical process to obtain an easy cutting process it is applied the cutting edge only to the upper electrode, while the lower electrode has a slot (fig. 2). One of the advantages of symmetrical HF welding is also the visual appearance of the seam which can simulate different geometries. * email: ilarica59@gmail.com, ilare.bordeasu@upt.ro; Phone: 0723650248; dragos.utu@upt.ro; Phone: 0723742252