Ultrasonic welding of thermoplastics

Ultrasonic welding (UW) is not only a well-known industrial process but it has also been an active research area. Materials ranging from metals to non-metals e.g. polymers and from virgin materials to non-virgin materials e.g. composites are easily welded using this welding technique. Some research has already been carried out but more thorough analysis is needed on ultrasonic welding of thermoplastics. Two thermoplastics selected for this research are commercially known as Acrylo-nitrile-Butadiene Styrene (ABS) and Polypropylene (PP). ABS belongs to amorphous type of thermoplastic, whereas PP is semi-crystalline thermoplastic. Owing to this dissimilarity in their molecular structure, ultrasonic welding of these two plastics has already been considered to be different. Energy director (ED) is usually protruded on anyone of the samples to be welded. Ultrasonic energy is uniformly driven in the presence of energy director at a localized area between the samples. In this research, triangular (TRI) and semi-circular (SEMI) energy directors (ED) were protruded on the surface of specimen by designing and manufacturing injection molds. Tensile testing in shear was performed for measuring lap shear strength of joints after being welded ultrasonically at constant strain rate of 3.24 mm/minute. Maximum LSS (lap shear strength) of 17 MPa and 6 MPa were found for ABS with TRI ED and PP with SEMI ED respectively. In other words, these LSSs (lap shear strengths) were 34% and 14.63% of base material strength for ABS and PP respectively. In this work, a statistical analysis (General Linear Model) was also used to deduce meaningful information from experimentation for both materials. For different factor settings, percent change in LSS was also calculated. Maximum percent change in LSS was found for weld time e.g. 2682 and 76.67 from low to high level was computed for ABS and PP respectively. Similarly, 55 & 47.2 for amplitude and 41 & 6 for static force were calculated with ABS & PP respectively. Percent change in LSS of 409 and 47 was also evaluated from SEMI to TRI and TRI to SEMI EDs for ABS and PP respectively. All the factors appeared to be significant to affect the LSS but weld time was found to be the most significant to achieve the higher bond strength. After doing the GLM (General Linear Model) analysis, some interesting results were also highlighted. Experimental techniques were used to investigate the main reasons for these findings. Difference in softening temperatures, viscosities, temperature spreads, ED collapses, heat affected zones (HAZ) and fractured surfaces for both materials were used to determine the reasons for interesting effects. For example, low static force was required for ABS in gaining higher LSS due to its lower softening temperature and viscosity drop at weld zone. Thus this work presented new and deeper understanding of the ultrasonic welding of thermoplastics. Various hypotheses were also made after having gone through the literature. Experimental tools were also used to test these hypotheses. These technical tools included differential scanning calorimeter (DSC), melt flow index (MFI) tests, finite element analysis (FEA), high speed video camera (HSVC) and microscopy (optical and scanning electron). These tools helped approve or disapprove the hypotheses. Viscoelastic heating was considered to be crucial mechanism in joining thermoplastics ultrasonically. Viscoelastic heating was depending mainly upon loss modulus, applied frequency and strain amplitude. Weld strength further depended upon the temperature development at weld interface. Apart from above description, simulation based on FEA was also validated for its accuracy and precision. A good match was found between experimental and simulated results.