STAPLA Homepage
Optimized welding parameters and ultrasonics are determined by the manufacturer before a decision on the design of the tool and the fixture is made. The parameters can vary depending on variations in materials, dimensions, and surface contamination. The control and monitoring device stores the defined data with individual tolerances and adjusts the individual parameters during the welding process automatically.
The selected welding parameters for ultrasonic welding are:
Welding pressure or contact pressure
This is the pressure between the anvil and the sonotrode applied on the parts to be welded. The contact pressure must remain constant during the whole welding process. It is usually produced pneumatically and totals up to approximately 400-1500N
Welding time
The welding time is defined as the duration of ultrasonics and can be both a constant parameter or a variable parameter adjusted with the help of quality control devices for an optimum weld. Depending on the application the welding time can be between 0.1 and approximately 1 second.
Trigger point
The starting point of ultrasonics welding is adjustable. This point in time, which is called the trigger point, is usually selected as a function of the clamping and compaction of the parts -- sometimes also as a function of stroke length.
As soon as the sonotrode's work surface touches the workpiece and a certain clamping force has been built up, the process is automatically triggered according to the selected welding time. For some applications it is advantageous to initiate pressure onto the workpieces while the sonotrode is already oscillating or while the contact pressure is still low. For other applications, it is advisable to wait for the total pressure to build up before the oscillation is started.
The adjusting element is a pressure-regulated valve for the activation of the air cylinders.
The trigger point has to be chosen individually and determined by welding experiments. It is one element of the parameter welding time.
Constant amplitude
The amplitude constitutes the oscillation length of the welding tool (sonotrode). The amplitude should remain constant throughout the welding process. With modern welding equipment a relatively large amplitude range is defined mechanically by the booster. Within this range, the fine adjustment of the amplitude is made electrically with a selective switch.
This solution is important not only from the engineering point of view but also for operational reasons. Instead of time-consuming mounting and dismounting of the boosters with different amplitude transmission ratios, the amplitude can be adjusted electrically very quickly and, above all, very precisely. The amplitude at the welding point is usually between 15 and 45 microns for half a period (semi-wave).
The individual magnitude of the amplitude is determined by the shape and construction material of the welded parts. In addition, there is a correlation between welding time, welding energy and welding pressure. The proper magnitude of the amplitude should be determined during test weldings.
High degree of application know-how required
Welding tools
The design of the welding tools requires a considerable amount of process engineering know-how and is often the decisive factor for determining the quality of a weld. If the surface structure on the work surface(s) of the sonotrode begins to wear, it may be judicious to change one or several parameters until a reconditioning or exchange of the tool becomes necessary.
Ultrasonic frequency
The frequency, measured in cycles per second, remains largely constant and depends on the generator's working frequency. It is also decisive for the construction of the machine and the complete transducer system. Most equipment has a working frequency of 20 kHz, but for small-scale applications, equipment between 35 and 40 kHz is available.
Parameter adjustment with the help of test weldings
Whereas the design of the welding tool and the working frequency is fixed according to the type of welder used, the welding pressure, welding time, trigger point and amplitude need to be adjusted according to each application. The parameters to be adjusted are in close relationship to each other. The exact ratios between the individual parameters have to be determined during test weldings with a focus on resultant welding quality.
A low welding pressure can, for example, be offset by using a larger amplitude and/or a longer welding time. On the other hand, a shortened welding time can be offset with a larger amplitude and/or a higher pressure.
Optimum adjustment for each application is usually achieved quickly by using the proper workpieces and observing the requested specifications for a weld. In addition, experience and practice may help in fine-tuning the adjustments for optimum welding results.