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Improved quality through ultrasonic metal welding
General information
Figure 38: Wire-splice welding machine with monitor for electronic quality control Figure 38 shows a welding system with electronic quality control. All machine-relevant data are entered via the keyboard and/or mouse via monitor. Reasons for quality variations Quality deviations may occur, for example, when workpieces have different dimensions, when the material quality (batches) differs and when contamination is excessive. A wrong insertion of the workpiece by the operator can also be identified by a quality control system. Quality influences and quality parameters The first task is to monitor the set machine data whereby welding pressure and amplitude are proportional in size to the supplied generator power output. Multiplying the power output with the welding time gives the required welding energy. Welding in the energy mode Welding in energy mode, i.e. with a constant energy setting, is known from ultrasonic plastics welding and can also be used for ultrasonic metal welding. To achieve a constant quality the welding time is automatically adjusted. Although this type of quality control is good with ultrasonic plastics welding, the approach has to be more carefully applied when it comes to ultrasonic metal welding.
Control of welding energy is not always sufficient Considering that were applications which require only minimum welding energy, the total of all system-caused losses amounts to some 80% of the total necessary welding energy (leaving 20% effective welding energy), it becomes evident that small modifications in the workpieces to be welded require only a minor adjustment to the welding energy. The control of the welding energy in respect of welding time does not take into account possible insufficiencies and variations of the machine components. The tolerances are even higher when the workpieces to be welded are of different quality and have different service conditions. All these varying influences can have a considerable effect on the welding quality. Mechanical compression as a quality parameter As the parts to be welded may deform during the welding process, the mechanical compression of the parts after they have been welded together may constitute an important quality measure. Contact-free measuring devices in the welding equipment determined the degree of compression by measuring the movement (stroke) of the welding tools during the process. The starting point of this stroke is the position of the sonotrode under pressure before the ultrasonic is triggered. Reasons for errors The registered mechanical compression is often directly linked to the strength of the weld. In addition, the following deficiencies can be easily determined:
During process control the degree of final product compression is directly determined by the welding tools. Instead of indirect data (time, energy) to determine the quality, the actual condition of the parts is assessed by measuring the degree of compaction. All possible secondary influences are considered so that any degree of compression can be adjusted. This procedure is proven in practice when it comes to welding wires. It is so precise than even single strands which may have been cut during stripping can be identified.  Figure 39: Relation between compression rate and tensile strength Contamination can affect the welding result When parts are contaminated or excessively oily, their surfaces will be cleaned by the oscillating energy before the weld is made. The time required for this process depends on the amount of impurity and can therefore not be predicted. The remainder of the set process time (welding time) reduces by exactly this amount of cleaning time and is therefore not available for the actual welding process. Due to this, the necessary degree of compression is not achieved, which is in turn indicated by the control device (figure 39). Parameter control Although the following process parameters are adjustable, they do not change during the welding process and are therefore not controlled:
Process-variable parameters However, the following variable parameters can be controlled:
For ultrasonic metal welding equipment, various electronic monitoring devices are available. Most of them are also able to control the welding equipment and make it possible to readjust the welding time, so that the required energy and compression values will remain constant in production (figure 40). The quality control devices offer the following possibilities:
-- at a fixed welding time
The faults shown are based on wrong or unfavorable settings. With the use of a quality control monitoring system unfavorable machine settings will automatically be identified by a cross-check with pre-programmed basic data. New settings are calculated according to the memorized settings of good welds. An incorrect programming can therefore be excluded.
-- at fixed welding energy
-- at fixed rate of compression Most of the quality control monitors and command units not only register the measured data, but also analyze them for statistical process control. In addition, all machine's relevant and preset quality parameters can be stored for many different applications and easily recalled.
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Height-volume production requires the strict adherence to predetermined quality parameters. This implies not only exact machine operation based on preset data, but also a simultaneous quality check of the weld against previously determined quality criteria. Ultrasonic metal welding offers the best prerequisites for optimum quality control as there are only a few specific process parameters which are easily measurable and thus controllable. Quality deviations are identified and usually offset during the welding process, i.e. they are adjusted.
