| The rapid development of electronic technology and the consumer’s increasing demand for electronics products have made smart electronic products such as mobile phones,tablets,and computers change with each passing day,and have rapidly developed into diversification,novelization,miniaturization,and intelligence.Its internal electronic components are also becoming smaller and more delicate,especially the appearance of mobile phone assembly processes such as narrow bezels and large screens.The requirements for the precision of dispensing have gradually increased,but at the same time,a glue with higher viscosity and better bonding performance is needed.Hot-melt adhesives are medium-to-high-viscosity adhesives and are currently the most widely used in the field of electronic packaging.In order to inject hot melt adhesive droplets,for all existing piezoelectrically injection valves in the field of dispensing,the high voltage must be used to drive the needle to impact the nozzle vigorously at high speed,but high driving voltage often means a large stroke of the needle,and the large stroke of the needle will cause the larger size of droplets,which can not meet the requirements of the bonding and packaging process.Therefore,we proposes a double-needle type piezo-driven valve,which is upgraded and optimized on the basis of a conventional piezoelectric-driven injection valve.The double-needle type piezo-driven valve is used to dispense high viscosity hot-melt adhesive,which can form a micro size.This device we designed can perfectly match the requirements of 3C industry electronic packaging in today’s era.This paper combines the knowledge of hydrodynamic,vibration mechanics,and elastic mechanics to systematically analyze the injecting system,and uses software such as Fluent,Matlab,Ansys,and Ls-dyna to simulate and verify the model.The specific research work is as follows:(1)Hydrodynamic Analysis of Jet Dispensing ProcessFirstly,the flow pattern of the hot melt adhesive in the molten state and the constitutive equation of the medium studied in this paper are determined.Then,the mechanism of droplet injection is analyzed through theoretical derivation,and the droplet ejected velocity must be greater than a certain value,and then theoretically derive the influencing factors of droplet ejected velocity and obtain the corresponding mathematical model.The volume of droplet ejection molding is calculated theoretically and thecorresponding mathematical model is obtained.The impact of the velocity and displacement of the needle on the droplet ejection effect is analyzed qualitatively based on two mathematical models.Finally,the mechanism of droplet ejection molding is simulated with Fluent software,and the effect of the needle velocity on the droplet ejection velocity is studied,the impact of the displacement on the droplet ejection volume is also studied,which provides a solid theoretical basis for the design of the high impact velocity and small motion displacement of the needle in the impact structure of this paper.(2)Dynamic Analysis of Double Impact Structure with Double-NeedleFirstly,the structure and working principle of the hot-melt adhesive injection valve are introduced in detail,and the advantages of the design are explained.Then,the system is divided into a driving subsystem and an execution subsystem,and a vibration mechanics model is established to solve the transfer function and natural frequency of the two subsystems.The two systems are simulated by Matlab to guide the piezoelectric stack,needle,spring of the needle to design and selection.Then,an impact mechanical model of the double needle structure is established,and the the impact force and impact velocity at the terminal and at the input end is calculated by characteristic line method and the law of conservation of energy.The analysis and comparison show that if the impact velocity of the terminal is greater than the input velocity,we need to appropriately reduce the mass of the needle below or the stiffness of the return spring of the needle below,or reduce the movement displacement of the needle below when designing the structure.However,reducing the movement displacement of the needle below is also conducive to reducing the volume of the injected glue point.After that,Ls-dyna is used to perform a dynamic simulation of the double-needle and the nozzle model,and it is compared with the previous theoretical analysis.Under the conditions we selected in this article,when the upper needle impacts the needle below at a velocity of 0.8 m/s,the needle below will be hit and the velocity is increased to 0.88 m/s immediately,then the velocity will be 0.85m/s when it hit the nozzle,the impact force Impact force increased from 213 N to 560 N.Finally,the temperature control system of this device is designed,and the device is thermodynamically simulated by using Ansys.When the temperature of the heating block is set to 110 °C,the temperature in the area where the glue flows in the main pipe and the area where the glue at the nozzle is to be sprayed is uniformly distributed between 100 °C and 110 °C,it meets the requirements for the hot melt adhesive to maintain the molten state.(3)Experimental study of the injection valveFirstly,a velocity test platform is built.It is experimentally measured that when thedriving voltage is between 40V-100 V,the velocity of the needle below is greater than the velocity of the upper needle,and the difference between the velocities of the two gradually increases.Then the next experiment shows that the greater the needle below,the stronger the ability to inject of the system.When the velocity of the needle below is less than0.32m/s,droplets stick to the nozzle and cause "hanging glue",when the needle velocity is greater than 0.32m/s,the droplets start to be spray-formed.In addition,the weight of the ejected droplet increases with the increase of the displacement of the needle below.It is experimentally determined that under the condition of a driving voltage of 40 V,a needle below radius of 0.75 mm,a nozzle diameter and the inner cone angle of 0.1mm and 90 °,respectively,the average minimum droplet value is 0.01 mg,the average minimum droplet diameter is 234μm,and the accuracy of the injection is within ±2.5%.In addition,the smallest hot-melt adhesive droplets meet the requirements of the adhesion force of the simulated mobile phone frame.Finally,the needle’s impact force test platform was built.It was experimentally measured that when the driving voltage was 100 V,the upper needle hit the needle below at a velocity of 0.68m/s,and the needle below hit the nozzle at a velocity of 0.8m/s.At this moment,the instantaneous impact force was about 593 N,which was close to the previous simulation results. |
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