DE4321874A1 - Process and device for the open-loop and closed-loop control of process parameters in ultrasonic welding - Google Patents

Abstract

In ultrasonic welding, until now optimisation of the process has largely been carried out empirically. The interrelationships between the quality-relevant process parameters and the weld quality were until now described only partially and incompletely. However, it has been shown that the quality of the welded joint depends on which flow conditions exist in the polymer melt at the instant at which the welding time is ended. The flow movements of the polymer melt can be represented by the joining path curve or its time derivation (joining rate), in conjunction with the force curve. This makes it possible to keep a check on the weld quality. According to the invention, the machine is provided with a measuring and controlling system for the joining path and the joining force, in order to establish in a simple way when various flow states of the polymer melt are reached and in order to perform an open-loop or closed-loop control of the joining rate at various instants in the process or over a period of time in the process. <IMAGE>

Description

The invention relates to a method and an apparatus for Control and regulation of process parameters in ultrasonic welding according to the preamble of claim 1. With the Ultra the parts to be welded are welded into an oscillating lon longitudinal oscillating movement offset and as long as the Ultra exposed to sound until the material in the contact melting zone of the two parts to be joined and melted Can flow into each other in the contact area. After that the ultrasound is switched off, so that a cohesive connection can arise.

In contrast to other plastic welding processes, such as. B. the Heating element welding, rotary friction welding and the Vibration welding is the main reason for ultrasonic welding and the joining phase, with the resulting composite liability and liability processes, in time. Whereby the Heating mechanisms from hysteresis losses in the plastic cyclic ultrasound exposure and that at the beginning of the process It combines to some extent occurring interface friction Zen. This results in new process conditions and influencing factors which, especially for this welding process, must be considered sen.

In order to achieve a defined melting in the joining plane, special joining seam geometries must be provided for ultrasonic welding. A distinction is made here between energy direction sensor and pinch seam geometries ( FIG. 1). The ultrasonic welding process can be divided into different process phases, particularly in the case of ultra sound welding with energy direction sensor geometry. These can be clarified using the time-dependent path of the joining path ( Fig. 2a). The various process phases of the ultrasonic welding process correlate with the achievement of certain weld seam quality levels. But also when ultrasonically welding squeeze seam geometries, the path of the joining path is decisive for the achievable weld seam qualities. Here, however, it is not always possible to divide the welding process into different phases due to the more complex seam geometry requirements. But here, too, the connection properties correlate with the course of the joining path or the speed. So that the joining speed can also be used as a control variable during the welding process to achieve certain connection qualities. In Fig. 2b, a Fügewegverlauf is shown in welding a Quetschnahtgeometrie example.

A distinction is made between joining parameters in ultrasonic welding force from which the joining pressure results, and the joining path, the from the melting movement of the parts to be joined in the direction of joining gives. Both parameters are time-dependent. The joining force is in the Usually kept constant. The time course of the joining path of the the two parts to be joined, however, result from the geometric shape the joining level and from the existing process conditions such as Vibration amplitude, vibration frequency, joining pressure and Welding time.

In order to achieve firm connections, the material must be sufficient be melted and there must be a corresponding joining pressure be so that homogenization and mixing of the melt zestream can take place. In practice, these demands often taken into account by complying with a minimum joining path becomes. However, this procedure can only be used for simple Joining part geometries with extremely low tolerances are leading to the goal.

In the case of joining parts produced by injection molding, these have usually inevitable manufacturing tolerances, so that here Quality assurance during the welding process through the specification a minimum joining path is no longer possible or this is too great Must take values in order to get all connections with high quality manufacture. In addition to the geometric manufacturing tolerances the internal structure of the injection molded parts to be impacted on the course of the process in ultrasonic welding. As a result  different injection molding conditions, the interior The structure of the parts to be joined differ greatly, so that the same Welding production parameters different welding results result.

Furthermore, the specification of a specific Ultra sound generator, energy given off during the welding process used as a criterion for good connection quality. As Studies, however, have shown that the generator correlates energy delivered not with the weld quality. So that too the specification of a certain welding energy does not lead to quality fuse during the welding process.

They are decisive for the quality of a connection Flow conditions in the joining plane when welding and the subsequent temperature and pressure conditions with the resul flow conditions of the cooling melt in the fol holding phase. But the process is also economical the quality-determining parameters are determined by the effective energy input adapted to the joining task and Conversion determined. Important manufacturing parameters are here Amplitude of the ultrasonic vibration and the joining force or at Relation to the joint plane surface of the joint pressure. Under the prerequisite an amplitude that is largely constant during the process the seam quality is of the realization of a certain one Joining speed with a given joining force in some areas or a joining pressure determined.

EP 0 421 019 A1 describes a method and an apparatus for Joining plastic parts by ultrasound, in particular Welding, riveting, flanging or shaping, known in which according to a predetermined lowering speed Material deformation is achieved, characterized in that the Lowering speed of the sonotrode depending on the is controlled or regulated in the plastic part building force. Furthermore, the possibility is considered that for the realization of a desired force curve the lowering speed of the Sonotrode is specified as the time profile or the specified Force profile via regulation of the lowering speed of the sonotrode  always taking into account the deformation of the parts to be joined is maintained.

In contrast, the object of the invention here is the course of the Flow speed in the joining plane and certain joining paths The joining parts run over the sizes joining force and sonotrode influence sink rate.

This results in the object on which the invention is based, to specify a method and an apparatus with which the Parameters in ultrasonic welding can be selected so that good connection properties result and firm connections can be produced reproducibly. The procedure and the In this case, the device must match all geometries of the parts to be joined and Joining part materials can be used.

The stated object is involved in the method according to the invention solved the features in claim 1. Advantageous training extension of the method and a device according to the invention characterized in further claims.

The following is an example of ultrasonic welding The geometry of the process and the process itself resulting possibilities of influencing the Weld quality shown.

The course of the parameters of the joining path or joining speed mentioned at the outset divides the ultrasonic welding process when welding with energy direction sensor geometry into four different process phases ( FIG. 2a), here it is assumed that the joining force and the vibration amplitude of the sonotrode are approximately constant during the welding process:

• 1st phase:
  Melting of the energy direction sensor by interfacial friction and hysteresis losses due to the vibration deformation. The melting speed drops steadily due to the widening area of the energy direction and the consequently decreasing joining pressure.
• 2nd phase:
  Coupling between the upper and lower part. The melting rate is constant for a certain time. The material that has cooled due to contact with the cold surfaces on the side of the energy direction sensor is melted again. The result is a renewed increase in the melting rate.
• 3rd phase:
  Stationary melting behavior. A constant melt layer thickness forms in the seam. That is, in the third phase the joining path increases in proportion to the time, provided that the joining pressure is constant.
• 4th phase:
  Hold phase. Here the melt cools down due to the elimination of the ultrasonic vibration.

This course can also be seen in a modified form changing joining force and amplitude. According to the invention Investigations as a prerequisite for good connection properties, that depending on the material used and quality requirements certain phase must be reached. According to the invention In any case, depending on the time of the ultrasonic welding process the quality-relevant parameters of joining path and joining force measured, to enable the different process phases to be precise separated from each other and the introduction of energy into the parts to be joined can be controlled. Particularly suitable as an indicator the time derivative of the Joining path, i.e. the joining speed. In the third phase z. B. In the first approximation, the joining path increases linearly with time. The Derivation is therefore a constant. Studies have shown just by reaching the third phase at constant Joining force, or in this phase also constant joining pressure, a Minimum strength in the connection is reached, which is at progressing welding time no longer changed. In this phase reproducible welding results can be expected. The sub Searches have further shown that as a prerequisite for good ver binding properties a defined flow rate in the Melt layer must be present. This is about the welding process material and geometry specific flow rate with a  defined joining speed. This makes it possible the joining speed which can be determined from the joining path as To use the quality criterion for the ultrasonic welded joint. For this purpose, the joining speed is used as a controlled variable during the Welding process. A certain joining speed can either with the joining pressure or the joining force which the parts to be joined are moved towards each other by a drive be regulated so that a certain setpoint for the Joining speed is reached. However, that is the only one Reaching a certain joining speed is not a measure of the in the flow velocity of art present at the joining plane melted fabric and therefore not a sole measure of the achieved weld quality. The flow rate is below other very strongly by the joining force and the associated Energy introduction and melt formation processes determined. Out For this reason, the joining force must also be monitored.

In Fig. 3, the course of the process variables joining force and joining path is shown as an example for an energy direction sensor geometry. In case A welding is carried out with constant joining force, in case B however the joining speed is regulated in the third process phase by specifying a joining force profile. One of the goals here is to achieve a fast passage through the first two phases through the initially high force and to ensure optimal melt formation after reaching the third phase. The lowering of the joining force leads to a reduction in the joining speed. By regulating the joining speed via the joining force as a manipulated variable, defined flow conditions of the plastic melt are induced in the weld seam, which in turn correlate with the achievement of certain connection qualities. When specifying the joining force as a manipulated variable, it should be noted, however, that the joining force may not assume any values here, but only those where good energy introduction into the parts to be joined can continue, because the welding force has a decisive influence on the energy conversion and melt formation processes. This also underlines the importance of joining force monitoring during the welding process.

This results in the control or regulation of the Joining speed the possibility of the connection quality of the To influence ultrasonic welding in a targeted manner. This is at previous process control concepts not possible. Were missing here So far, the necessary devices and methods for metrological recording of the quality-relevant process variables as well as their control and regulation.

In the case of ultrasonic welding of pinch seam geometries, it is not possible to divide the welding process into different phases due to the more complex seam-geometrical requirements. But here, too, the connection properties correlate with the course of the joining path or speed. So that the joining speed can also be used as a control variable during the welding process to achieve certain connection qualities. In addition to the seam geometries shown in FIG. 1, modified seam geometries or special shapes are also frequently used in practice. In general, the relationships shown are valid for all seam geometries. There tend to be different joining paths here. However, the decisive factor for the quality of the welded connection is the course of the joining path in connection with the joining force course.

In Fig. 4, a displacement sensor 1 is attached to the movable travel unit not shown Darge the ultrasonic welding machine. This moving unit is moved by means of a drive which either allows the variation of the joining force or the direct specification of defined joining speeds to the housing-fixed joining part, which results in the joining path in the z direction of FIG. 1.

In the case of a pneumatic drive, for example, a certain pressure is exerted on the parts to be joined by the pneumatic cylinder, ie the machine is force-controlled. The signals of the Wegauf participants are amplified in an amplifier 2 and then fed to a Dif ferenzierer 3 . The output signal of the differentiator thus represents the time derivative of the joining path and thus the joining speed. A comparator 4 is connected downstream of the differentiator, in which the joining speed signals are compared at short time intervals. The differentiator 3 is given a defined joining speed or a temporal joining speed curve as the target variable. If the deviation between the measured and the specified speed falls below a certain tolerance, the comparator 4 emits an output signal “ultrasound off” that switches off the ultrasound oscillation of the welding machine via a relay 5 . Providing a constant joining force, it is thus ensured that defined flow conditions are present in the plastic melt at the end of the process.

In Fig. 5 the position transducer 1, the amplifier 2 and the differentiator 3 is again shown. Again, the output signal from the differentiator 3 provides information about the current joining speed. In this case the ultrasound is shown in FIG. 5 is not switched off, but the output signal of the differentiator 3 supplied to a controller 6 and is compared here with a set value for the mean joining velocity. The output of the controller 6 is fed to a proportional valve 7 . The proportional valve 7 is controlled by the controller 6 so that the force exerted by the cylinder and thus the joining speed reach the setpoint. As soon as the measured joining speed has reached the specified target value within a certain tolerance band, the machine switches off the ultrasound via relay 5 . The advantage of this circuit concept is that by regulating the joining speed, tolerances in the joining parts, such as. B. geometric tolerances or different internal structures can no longer negatively affect the welding process. The different joining path profiles caused by these tolerances can be compensated for by adjusting the joining speed to a size that is optimally recognized for the respective joining part and the material. This enables reproducible seam qualities to be produced. The prerequisite for this is that the control force is varied during the control process in such a way that, in addition to achieving a predetermined joining speed, similar melt layer thicknesses and thus similar rheological and thermal conditions occur in the seam plane as a result of the energy introduction conditions depending on the force. A further prerequisite for reproducible weld seam qualities is that the joining speed is reached uniformly in all joint seam areas.

In the control concept shown in FIG. 5, it must be taken into account that, due to the variable joining force acting as a manipulated variable, different parts of the joining part are set during the control process. These have an impact on the measured joining speed, which then no longer corresponds to the actual joining speed due to the melting movement in the joining plane. The joining part information must be taken into account when determining the joining speed in the differentiator 3 .

Claims (13)

1. A method for controlling and regulating process parameters in ultrasonic welding, the parts to be joined being set into an oscillating longitudinal oscillating movement by a vibrating sonotrode until the material melts in the contact zone and is moved towards one another with application of a joining force and covering a joining path, whereupon, after the ultrasonic welding process has ended, a material connection is formed when cooling. Characterized in that are time dependent for the production of the characteristic parameters during the ultrasonic welding process is measured and reaching the process phase in which a, for the material and the respective joining part geometry is determined with respect to rules of the achievable weld quality, optimal melt flow in the joining plane, whereupon the ultrasonic welding process is ended. 2. The method according to claim 1, characterized in that for Determination of a process phase of the joining path and / or the joining path speed can be measured. 3. The method according to claim 1 or 2, characterized in that the joining speed when covering the joining path is telt and after reaching a predetermined joining speed the ultrasonic welding process is ended. 4. The method according to claim 3, characterized in that the Joining path measured and derived from the joining path in time the joining speed is determined. 5. The method according to claim 3 or 4, characterized in that a defined joining speed or a joining speed profile by comparing the determined from the joining path Joining speed with a constant value or a time the joining speed curve is determined. 6. The method according to any one of claims 3 to 5, characterized  records that after reaching the defined joining speed speed within a previously defined period or Joining speed curve over a certain period of time, the ultrasonic welding process is switched off. 7. The method according to claim 6, characterized in that the Joining speed is controlled so that a predetermined Flow rate of the material melt is reached. 8. The method according to claim 7, characterized in that the Regulation of the joining speed by the force takes place with the parts to be pressed together. 9. The method according to claim 8 in connection with one of the An sayings 1-7, characterized in that for monitoring of the energy introduction into the parts to be joined Joining force is measured during the welding process. 10. The device for performing the method according to any one of claims 1-9, characterized in that a displacement transducer ( 1 ) is provided for measuring the joining path, to which a differentiator ( 3 ) and a comparator ( 4 ) are connected, in which the Signal from the differentiator is compared with a setpoint. 11. Device for performing the method according to claim 9, characterized in that a for measuring the joining force Load cell is provided. 12. The apparatus according to claim 10, characterized in that a controller ( 6 ) is connected to the comparator ( 4 ) which, after reaching a defined joining speed by comparison with a target value, carries out a regulation of the joining speed and the ultrasonic welding process after reaching a certain joining speed is ended within a previously defined period. 13. The apparatus according to claim 12, characterized in that the controller output is guided to an actuator ( 7 ) for a drive in the joining direction. The drive can be pneumatic, electric or hydraulic. The joining force exerted by the drive on the sonotrode serves as the manipulated variable.