Simulation And Experiments Research On The Ultrasonic Bonding Mechanism Of Microfluidic Chips

It's a modern thing that introduces ultrasonic plastic welding technology to the field of microfluidic chip encapsulation,which has some advantages,such as external substance free, higher bonding strength,shorter bonding time,wider applying material,etc.To the ultrasonic plastic welding technology,however,the joining mechanism is not well understood.Research on the mechanism helps understand the melting behavior and the effects of welding parameter on the welding process,which would greatly benefit welding quality improvement.Aiming at providing theories to the advanced implication of the technology,the dissertation focuses on researching the joining mechanism of ultrasonic plastic welding.Firstly,a practical viscoelastic model to the polymer material is proposed.Based on generalized Maxwell model and Boltzmann superposition principle and TTEP (Time-Temperature Equal Principle),the dynamic modulus could be expressed as the function of temperature and frequency.However,when shifting relaxation modulus segments of PMMA(polymethyl methacrylate) to gain a master curve,inaccuracy appears,so the normal Time-Temperature Shifting Factors is modified in the dissertation.The viscoelastic thermogenesis equation of viscoelastic material under periodical strain load is deduced.From the numerical solution it could be concluded that,viscoelastic heat is generated in a very short time in specified temperature range,and viscoelastic heat is not apparent at low temperature. Other heat generating mechanism may exist at temperature below T_g.Subsequently,kinetic simulation was proceeded to the Finite Element Model with a rectangular energy director, which bears both welding pressure and high-frequency vibrating loads.In the simulation, corner of the energy director produces high instantaneous friction stress and relative sliding velocity.So viewpoint was derived that the heat generating mechanism at low temperature is friction heat right at the interface.Simulating strategy to the friction heat and viscoelastic heat was put forward.The results showed that temperature field on the rectangular energy director is nonuniform,and viscoelastic heat makes the temperature rises in a very short time.To validate the proposed heat generating mechanism and the simulating method,experiments were proceeded with measurements of temperature tendency of energy director during welding process.Simulation of the temperature field when the welding parameters,that is welding pressure and amplitude,varies is proceeded,indicating that temperature rise rate tends to increase with the applied welding pressure and amplitude.At last,two kinds of energy director shape,triangular and rectangular,for the encapsulation of microfluidic chips is presented,and their feasibility is discussed theoretically.Encapsulating experiments of the two kinds of microfluidic chips were proceeded.