Measurement of Parameters and Automatic Selection of Optimal Modes during Ultrasonic Welding of Thermoplastic Materials

Transcription

1 Measurement of Parameters and Automatic Selection of Optimal Modes during Ultrasonic Welding of Thermoplastic Materials Vladimir N.Khmelev, Senior Member, IEEE, Alexey N.Slivin, Roman V.Barsukov, Sergey N.Tsyganok, Igor I.Savin, Member, IEEE, Andrey V.Shalunov, Student Member, IEEE, Sergey V. Levin, Student Member, IEEE, Alexey D. Abramov, Student Member, IEEE Biysk Technological Institute (branch) of Altay State Technical University named after I.I. Polzunov, Biysk, Russia Abstract - Article is devoted to research of changes of electric parameters of ultrasonic oscillatory systems (UOS) during ultrasonic welding of thermoplastic materials. In article the basic problems interfering development of systems of automatic definition of quality of ultrasonic welding are analyzed. The technique performance researches which are directed on improvement of quality of welded connection at ultrasonic welding of thermoplastic materials is described. I I. INTRODUCTION n the present time is released by the industry the big quantity of various products which are manufactured from polymeric thermoplastic materials. One of stages, at manufacturing a final product from thermoplastic materials, reception of a welded joint is. The primary goal now is development of automatic ways of a welding and improvement of quality of a welded joint of products from thermoplastic polymeric materials. Ways of a welding of thermoplastic materials can be divided into two groups. In the first group to contain ways of a welding in which use heat of external sources of heating: 1) The heated up gas; ) The heated up tool; 3) The heated up additional material. In these methods warmly it is transferred to a junction of materials on means of convection, thermal conductivity and a radiation. In the second group to contain ways of a welding of thermoplastic materials in which warmly it is formed inside materials owing to transformation of various aspects of energy. Thus energy is used: 1) Friction; ) H.f. currents; 3) Ultrasonic oscillations; 4) Caloric radiation. At a choice of this or that way of welding it is necessary to take into account specificity of technological process, industrial expenses and physic mechanical properties of thermoplastic materials. For example, welding by the heated up gas and the heated up tool is the most economic process. At its use of the characteristic of durability of welded connection are high enough. But these ways are dangerous at work with inflammable substances. Using these ways of welding pollution of surfaces of materials reduces durability of welded connection. The welding h.f. current has high efficiency, but is not applicable for a welding of not polar plastic: polythene, polypropylene, a fluoroplastic, it is dangerous at work with inflammable substances, etc. At a welding h.f. currents of containers from the polyvinylchloride, filled by liquids, observe electric punctures which result in destruction of a product and deterioration of a welded joint of these thermoplastic materials. The welding the heated up filler material is expedient for applying to reception of seams of the big extent. To the help of glue connect of thermosetting plastics. Thus receive high-quality connection from homogeneous and diverse materials. However polymeric glues are toxic, some kinds of glues eventually grow old and as a result durability of connection is reduced. Now all over the world the ultrasonic welding is the most popular way of the joint of thermoplastic materials. Application of a ultrasonic welding: 1) To make a welding at temperatures smaller temperatures of decomposition welded thermoplastic materials. Allows to increase reliability of a welded joint and to improve appearance of a final product; ) To make a welding without preliminary clearing welded surfaces of various pollution;

2 3) To exclude excessive heating welded thermoplastic materials, owing to local allocation of heat in a weld zone; 4) To gain the joint of thermoplastic materials at a welding at a great distance from a point of feeding into of energy. It allows carrying out joints in remote places; II. PRODUCTION OF THE PROBLEM To receive a qualitative welded joint of thermoplastic materials change the following parameters: amplitude of oscillation of radiating surface UOS, magnitude of welding force and duration of ultrasonic affecting. Change of last parameter has received the widest application. At an automated ultrasonic welding restriction of duration of welding affecting is carried out in several ways: 1. The way «on the fixed time» assumes use of ultrasonic affecting of constant duration at a welding of the same products. Duration of ultrasonic affecting is defined from preliminary experiments and set on all welding cycles equal, is usual with the help of the electronic time relay.. Way «with the fixed deepening» at which magnitude of deepening UOS in a welded detail (deformation of a zone of a seam) is kept by a constant. When the specified value of deformation is attained, provided the best characteristics of a welded joint, there is an automatic shut down of ultrasound and restriction of moving UOS. At use of this way duration of a time of a welding in each separate case can be various. 3. A way with use of additional external sensing transducers, which carry out the control of an initial state of a material (no uniform thickness, impurity of surfaces, etc.) and various destabilizing parameters. At use of a way of a welding «on the fixed time» the probability of emersion of a unsound welded joint is defined by the following parameters: 1) Instability of a resonant frequency and amplitude of oscillation UOS; ) A deviation of initial thickness of welded materials from optimum; 3) Instability of conditions of a welding (temperature of a waveguide and magnitude of welding force). 4) Impurity of welded surfaces. In a way of a welding «on the fixed time» sampling of duration of a time of a welding is not connected to formation of a welded joint. The way of a welding «on the fixed upsetting» in part is connected to kinetics of formation of a welded joint. In this case the probability of emersion of an unsound welded joint is defined only by instability of initial thickness of welded materials. Use a way of a welding «on the fixed deepening» assumes the certain deformation of a zone of a seam. It sometimes happens is inadmissible because of a decline of a packaging of a product. This way is not applicable at a welding of film materials of small thickness. It is necessary to solve the following problems of that any of ways used now does not meet the requirements of stability of process of a ultrasonic welding of thermoplastic materials and reliability of a weld seam: 1. To reveal the basic electric parameters of ultrasonic oscillatory system determining change of properties of welded polymeric materials during a welding at agency of external destabilizing factors.. Practically to implement the circuit of measurement of electric parameters of ultrasonic oscillatory system. 3. To lead measurement and research of agency of the properties, welded materials and destabilizing factors on electric parameters of ultrasonic oscillatory system. 4. To develop a new way and a control algorithm process of the welding, taking into account agency of the varying properties, welded materials and destabilizing factors. III. THE CIRCUIT OF MEASUREMENT Reception of a qualitative welded joint is defined: stabilization during a welding of varying parameters of a compression pressure, a resonant frequency and an amplitude of oscillation of ultrasonic oscillatory system, quantity of thermal energy, which depositing in a weld zone. As the quantity of the energy depositing in a weld zone, is connected to quantity of the energy brought to the welded material there is a necessity of a metering-in last. The way of a metering-in of a brought vibration a) b) Fig.1. - The physical equivalent circuit of ultrasonic oscillatory system. energy defines the circuit of management of process

3 of the ultrasonic welding, consisting of the devices which are carrying out measurement of electric parameters, properties of welded materials taking into account change. Then the metering-in of ultrasonic energy is carried out depending on the varying properties, welded materials, regulating of amplitude of the ultrasonic oscillations absorbed in a weld zone, turning on and shut down of ultrasonic oscillations during the necessary moment of a time. At management of process of an ultrasonic welding it is offered to control a wave resistance of welded materials (medium) varying in a time. For measurement of a wave resistance of welded materials it is necessary to carry out measurement of electric parameters of a voltage and a current of the ultrasonic oscillatory system used for a ultrasonic welding of thermoplastic materials. Thus, itself UOS represents itself as the sensing transducer of parameters varying during a welding of thermoplastic materials. As the circuit of measurement of electric parameters the circuit of management of ultrasonic process (the Patent 47544) has been accepted for the prototype. For the explanatory of process of measurement of a wave resistance we shall analyzed the physical equivalent circuit of ultrasonic oscillatory system on the basis of the piezoelectric elements, shown on fig. 1a. The branch containing consistently established coil with inductance L М, the capacitor with electric capacity C М, and the resistor with nominal resistance R М, is the equivalent circuit diagram of mechanical oscillatory system. Elements L М and C М simulate weight and pressure of a material of oscillatory system, accordingly. The element (resistor) with nominal resistance R М characterizes an energy loss in a material of oscillatory system and resistance to radiation OUSE of oscillations on processable medium. Resistance R М is proportional to a wave resistance of medium with M. Its depends on a design of oscillatory system (coefficient of amplification) and an emitting area. The element (the capacitor with electric capacity) С К corresponds to own electric capacity piezoelectric the elements included piezoelectric in the converter. Generally the wave resistance of medium will consist of two components - the valid and imaginary parts. The valid part characterizes amplitude of ultrasonic oscillations and in an observed way is informative parameter. The imaginary part characterizes changes of a resonant frequency of the acoustical-electrical transducer and in model to oscillatory system can be presented as "additives" to rating values of elements L М and C М. Work of oscillatory system is meaningful only at conformity of working frequency of the oscillator to own mechanical frequency of oscillatory system. On a resonant frequency of oscillatory system, i.e. on own frequency of the oscillatory circuit consisting of elements L М and C М, their resistance decreases up to null. For exclusion of agency of an imaginary part of a wave resistance of medium presence of system of the automatic frequency control providing a permanent job of system on frequency of a mechanical resonance is necessary. Hence, the equivalent circuit of oscillatory system can be presented as, the shown fig. 1b. From the equivalent circuit in fig. 1b follows, that, on frequency of a mechanical resonance it is possible to measure only the valid part of a wave resistance and for this purpose it is necessary to measure a current proceeding through resistance to radiation of oscillatory system R М, and to display an anode drop on it. As it is necessary to control dependence of change of a wave resistance of medium on a supply voltage of oscillatory system, instead of to measure its exact numerical value there is no necessity for definition of a coefficient of proportionality of M for determination of an exact relationship between resistance to radiation and a wave resistance of medium. Measurement of the picked parameter - a wave resistance of medium, is carried out by means of the device including the differential amplifier providing allocation of a signal, AFD which corresponds AFD a current in a mechanical branch (Lм, Cм, Rм) and the transformer of a voltage by means of which is measured a voltage on oscillatory system. In fig. the circuit of the device of measurement of a wave resistance of medium, with an opportunity of compensation of change of electric capacity С К presented the block. The presented circuit will consist of two blocks: the block of measurement of a crest value of the current proceeding in mechanical branch L М, C М, R М, and the block of measurement of a peak voltage on oscillatory system. On the basis of the received data from the block of measurement of a crest value of a current and the block of measurement of a crest value of a voltage on oscillatory system the block of microprocessor CPU counts value of a wave resistance of medium and produces the corresponding control action for maintenance of a metering-in of necessary ultrasonic energy of the oscillator.

4 IV. RESULTS OF MEASUREMENT OF THE WAVE RESISTANCE OF THE MATERIAL For measurement of a wave resistance of polypropylene varying during an ultrasonic welding the continuous control of amplitude of a current of a mechanical branch of ultrasonic oscillatory system and amplitude of an electric voltage was provided. From dependences presented in fig. 3 it is visible, that at a single welding (a curve 1) value of a wave resistance is kept to constants during all time interval of ultrasonic affecting. Full curves show change of magnitude of a wave resistance of polypropylene during ultrasonic affecting. Such change of a wave resistance during an ultrasonic welding is connected to varying properties of a material of polypropylene in a weld zone (a softening of a polymeric material and transition in viscous plastic a condition). the condition steps through various intervals of a time. And, the more welding force at equal values of power, the steps the moment of transition of a material in viscous plastic a condition earlier. It speaks maintenance of the best acoustical contact of welded surfaces of polypropylene due to increase in welding force during the initial moment of a time. V. THE CONTROL ALGORITHM OF PROCESS OF THE WELDING Management of process of an ultrasonic welding ρ c, m kg s A C B D 0 t 1 t t 3 t 4 Fig. 3 - Dependence of a wave resistance of polypropylene during a welding with various welding force at one level of power. 1 t,с Fig. - The block of measurement device circuit of wave resistance of medium. At a softening of a welded material under act of ultrasonic oscillations, acoustical contact of a working surface of an emitter to a material and as consequence is reduced good quality of an ultrasonic vibrator improves. All schedules presented on fig.3. have a well defined maximum which corresponds to transition of a material in viscous plastic a condition. As a result of conducting assemblage of experiments by definition qualities of a weld seam it has been established, that in points of the maximum value of a wave resistance the best are attained strength characteristics of a weld seam. From dependences presented in fig. 3 it is visible, that at various welding force the moment of transition thermoplastic a material in viscous plastic is based on the continuous control of a change of state of medium over scaling magnitude of a wave resistance of medium to which the radiating surface of the working tool contacts. At measurement of amplitude of a current and a voltage define values of a wave resistance of acoustical loading of oscillatory system. Thus, each of the received values of a wave resistance compare with previous or accept in the capacity of a reference value at the first measurement. In a case if value of a wave resistance starts to increase that, it testifies about a volume, that between welded of thermoplastic materials, owing to their softening, and oscillatory system the acoustical contact resulting in decrease of good quality and a damping of oscillatory system improves. At achievement of the maximum value of magnitude of a wave resistance (the best acoustical contact between welded materials and radiating surface UOS is attained) it is necessary to switch off ultrasonic affecting. Management of process of an ultrasonic welding occurs as follows:

5 The oscillator controlled by voltage, produces a Fig. 4 - The block diagram of managerial process of ultrasonic welding. low-voltage rectangular signal of ultrasonic frequency which acts, on the one hand, on an entry transistor of half-bridge the inverter 3, and on the other hand, acts on one of entries of the phase comparator 1. From an exit of the inverter 3 rectangular signal of ultrasonic frequency with the amplitude set by a regulator of a voltage 5 and laying limits 0 300V, acts on agreeing LC the filter 4 which gates out a first harmonic from a rectangular signal and will coordinate an output impedance of the inverter 3 to an entrance impedance of the converter 13. The unit 6, by means of the differential circuit (presented on fig. 4), gates out a signal which bears in itself the information on amplitude of a current in a mechanical branch of the converter 13. This signal acts on discriminators 8 and 9. On an exit of the discriminator 8 there is a rectangular digital signal with the same phase and frequency, as a current proceeding in a mechanical branch of the converter 13 which acts on the second entry of the phase comparator 1. On an exit of the discriminator 8 there is a signal, directly proportional to amplitude of the current proceeding in a mechanical branch of the converter which acts on one of entries of unit of scaling of a wave resistance of medium 11. The unit 7 by means of the transformer of a voltage (fig.4.) gates out a signal which carries in itself the information on a voltage on electrodes of piezoelectic elements of oscillatory system. This signal acts on the discriminator 10 on which exit there is a signal, directly proportional to a peak voltage on oscillatory system. This signal acts on other entry of unit of scaling of a wave resistance of medium 11. The entry of a wave resistance of medium in a digital aspect acts on an entry of the circuit of management 1. By means of a control system 1 there is a management of the apparatus as a whole: turning on and shut down, management of a regulator of a voltage 5, gauging of the differential circuit 6. The circuit of management 1 produces an error signal which is defined by a difference between the received value of a wave resistance and its previous value and acts on a regulator of the voltage 5, the feeding inverter 3. At equality of the received value of a wave resistance and its previous value, the regulator of a voltage 5 fixes the current voltage on the exit. VI. CONCLUSION As a result of the lead researches the new way of management by process of a ultrasonic welding of thermoplastic the materials, set up on measurement of a voltage and a current proceeding through ultrasonic oscillatory system, measurement of a wave resistance of welded materials varying in a time under act of ultrasonic oscillations is offered. During the lead researches the following private problems have been solved: 1. The basic electric parameters of ultrasonic oscillatory system which are taking into account change of properties, welded polymeric materials are determined during a welding at agency of external destabilizing factors.. The scheme of measurement of electric parameters of ultrasonic oscillatory system is practically implemented during an ultrasonic welding. 3. Measurements are lead and agency of the properties, welded materials and destabilizing factors on electric parameters of ultrasonic oscillatory system is investigated. 4. The new way of management by process of the welding, taking into account agency of varying properties of welded materials and destabilizing factors is developed during an ultrasonic welding. REFERENCES [1]. Wolkov S.S., Cherniak B.JA. Welding of plastic using ultrasound, Chemistry. []. Holopov JU.V. Ultrasonic welding of plastic and metals. "Mechanical engineering", [3]. RU, patent, kl. A 61 B 18/00. A mode of steering of process ultrasonic liposaction / Khmelev V.N., Barsukov R.V., Tsyganok S.N., Slivin A.N., Rascals А.В. [4]. Patent the USSR Control mode of a ultrasonic welding.