
The components and processes that affect ultrasonic welding:
- The Welder (Generator and Press)
- The Stack (Converter / Booster / Horn)
- The Tooling (Horn and Fixture)
- The Part
- The Ultrasonic Setup
ULTRASONIC WELDERS
- Ultrasonic machines have come a long way since the first machine produced many years ago.
- Newer Generators
- Systems have gotten more powerful.
- Operating frequencies have been added. The most common are 15kHz, 20kHz, 30kHz, 35kHz, and 40kHz. Higher frequency usually means smaller, more delicate parts being welded.
- Generators are better at tracking the frequency of the stack , which is the Converter/booster/horn combination. The are more efficiency, which means lower power used.
- Generators have better overload protection when something goes wrong.
- There is a lot of ways to track the process, so you can monitor changes in the whole process.
- Closed loop control systems, monitoring and controlling many parameters, such as distance, energy, time, and velocity.
- Older Generators
- Most pre mid 80’s equipment need to be manually tuned to the stack. This is a process of adjusting the tuning knob or tuning potentiometer when any of the components are changed in a stack, or the job is starting up in the morning. In most equipment, when tuning the power supply to the stack, the lowest reading on the power meter is the desirable one. The generator may also need to be re-tuned if the stack gets warm or hot after a few hours of running. When a stack gets hot, the frequency can change. This can make the generator and stack less efficient which can affect the weld process.
- Presses
- Most all presses have some degree of flex in the column. High pressure applications will have more flex than lower pressure applications. This can affect the parallelism of the horn to the part, making marking or welding along one side more prominent. This is why all machines either have leveling in the base plate or the fixture.
- A lot of presses are pneumatic in operation. Some newer ones use a servo motor to control the press. Servo’s can definitely have an advantage over pneumatic presses when it comes to the control of the welding process.
- Some of the more common methods of leveling the part to the horn are:
- Leveling the fixture using its own leveling screws, usually built in, one in each corner of the fixture.
- Using a built in base leveling (if the machine has it).
- Trying tape in a fixture to lift a certain area.
- Or using shim stock under the fixture.
- Newer Generators
THE STACK (Converter / booster / horn combination)
- Converters (transducers) convert the electrical energy produced by the generator into mechanical movement. As converters age, the frequency tends to go up. Heat will also affect the converter. Too much heat and the crystals within the converter can crack. Most 20 kHz converters have about 20 microns of movement at the face. Most converters are also are insulated from the housing by O-rings. O-rings do allow some movement or play of the “driver” within the housing. The driver is also pinned thru the housing so they don’t rotate which keeps the high voltage lead and ground lead from being torn off. Care should be given so the driver is not rotated within the housing when removing or putting on boosters. Never grip the housing, only use spanner holes provided on the driver of the converter to hold it.
- Boosters are used to change the amount of gain (or amplitude) going to the horn. In most cases more gain is required to weld plastic successfully. Boosters are also mounted by rings in the middle (nodal point, where the least amount of movement is), by O-rings. Again, this can allow for some movement of the booster and are pinned to keep the booster from rotating. Age of the booster, and more importantly, the condition of air supplied to the machine (if oily) can affect the O-rings and therefore affect the amount of play or movement. Boosters come in ratios of gain, the most common are (low to high): 1:.6, 1:1, 1:1.5, 1:2, and 1:2.5. Anything more than 1:2.5 is considered very high gain, and will typically shorten the life of a horn.
- Horns are designed to resonate at the frequency that the machine is made for. Typically this is 20 kHz. There are many different shapes, but only a few different materials used for horns. Aluminum, titanium and steel are the most common materials used.
- Aluminum: used for mostly for larger horns and cost savings, they can be plated to help reduce marking and wear.
- Titanium: used for longer runs, higher gain applications. The size of the horn usually limits the use of titanium due to cost.
- Hardened Steel: used primarily for smaller horns, usually driving metal inserts into plastic and high wear applications. Size and amplitude limitations do exist with steel horns. Medium to small size horns in steel are good for wear when glass or other fillers are used in plastics and if the part can be welded at lower amplitude.
PARTS, HORNS AND FIXTURES
- Contoured horns are typically cut from the part files. They are typically cut .005” per side greater than the part to allow for differences, flex, and expansion of the part when welding.
- Welding multiple cavities can have slightly different results, because the parts can vary some.
- The stack up tolerances of the parts inside the housings has an effect on the part welding. They dampen the vibration of the horn and therefore the vibration to the joint. This can be why there is a difference in strength of the weld joint.
- Horns are vibrating at very high velocity. The way the ultrasound moves at the face is affected by contours. A vertical wall in the horn is actually scuffing the side of the plastic which can cause marking if long weld times are required.
- Make ready or shimming the part in the fixture is a common practice in the industry, but is more likely to be used on parts that are not symmetric, parts that have different levels, and parts that have internal components that keep the two halves from matting evenly or consistently each time.
- Thin walls in parts allow them to burn thru where there are sharp edges. Ultrasound concentrates in corners and produces heat very rapidly. Radius as much as possible in these areas.
- Curved walls do not transmit energy as well as vertical walls, which will reduce the weld in those areas.
- Long thin walls will vibrate sideways instead of up and down reducing the weld at the joint.
ULTRASONIC WELDING SETUP
- A lot of applications weld by time. But there are other methods, that can provide better or more consistent results. Here are the main categories of parameters. There are some underlying categories, such as Weld by Absolute Distance (Part of Distance), or Weld by Peak Power (part of Energy). Explanation will be given of them as well.
- Time – open loop control: The controller will weld to a set time regardless of other settings.
- Distance or Collapse – closed loop control: The system will try to go to a distance entered into the controller after the trigger force is made or some other type of start parameter is reached.
- Pre-Trigger: Will start ultrasound, just before hitting part, or it can start when the press head starts traveling in some machines.
- Absolute Distance: From home or up position, the system will turn off sonics when a distance has been reached.
- Energy – closed loop: The controller will shut off ultrasound when a certain energy is reached.
- Peak Power – closed loop: Sonics will turn off once the set value of wattage has been achieved.
In short, each part with its own stack up differences, possible multi-cavities, and design, along with machine construction and setup can make the welded parts vary. Some things in the design can be done, if the wall thickness is great enough to limit flash. Newer equipment can help, with closed loop processing. Tooling can be chrome plated, to take away some of the abrasiveness of the aluminum against the plastic. If we have 1 to 2 dozen parts when producing tooling, tooling can modified in areas that mark the plastic Additionally, a plastic barrier can be used between the horn and part to help eliminate or reduce marking. All of these do typically add some cost to the project.
If you have any questions, please give us a call to discuss your application. 630-769-1886
For other information, please visit our website at: accusonics.com – May you always stay in Resonance!
Dan Wuthrich