Potential Pitfalls in Ultrasonic Plastic Welding
Ultrasonic welding is fast, requires no consumables, and can be applied to a wide variety of applications. Additionally, equipment cost is relatively low. Ultrasonic welding can create strong, hermetic welds that are visually appealing. However, some design considerations must be taken into account to ensure optimum results.
Surface damage and fracturing of features can occasionally result from application of ultrasonic energy to an assembly. Additionally, some design choices may preclude strong and consistent welds from being formed. Here are some common ultrasonic difficulties and how to prevent them.
Gouging occurs when the fit between the sonotrode and the part is not correct. If the part is contoured, it’s important that the contour of the sonotrode closely matches the contour of the part. If the part and sonotrode are shaped differently, the sonotrode will squeeze on the part surface, causing marking. Also, the edges of sonotrodes can leave witness marks.
To prevent this kind of marking, it can help to put a generous radius on the horn edges and have the contour of the sonotrode polished to match the part. Please check with the sonotrode manufacturer before making any modifications to the tool, however. A sonotrode is an acoustic instrument and, as such, any removal of material may interfere with its function.
If the sonotrode is being used to contact a textured surface, that texture could melt during welding. Ultrasonic energy concentrates at small contact surfaces, and a textured surface provides just that. This makes it likely that some of the part surface will melt when exposed to ultrasonic vibrations. To help prevent this, place a film between the sonotrode and the part as a buffer.
Burning can be caused when ultrasonic vibrations cause the part to act like a diaphragm and bow inwards and outwards as vibrations travel through the part. This cyclical strain can lead to internal heating, which can result in a burn mark in the middle of the part. Often called “diaphragming,” this phenomenon is more likely to occur if the part being contacted is thin.
Damage to the part surface can also be caused by cavitation of the air trapped between the sonotrode and part. Air that is captured between the horn and the part surface can heat up enough to burn the part under the rapid compression and expansion that occurs when the horn is vibrating.
These effects can often be prevented by the use of a nodal plunger, which can dampen the middle of the part during welding and prevent any trapping of air between the horn and part. A nodal plunger is a spring-loaded dampener that is mounted inside the sonotrode.
As the parts vibrate during ultrasonic welding, any areas of stress concentration are susceptible to heating. For this reason, a small feature with sharp radii may break off the assembly during welding. Thin walls can also be burned. This is why ultrasonics can be used to degate parts from a runner by contacting the sprue with a sonotrode.
Prevent this by designing the radii as large as possible and avoid using thin walls to connect large pieces of the assembly.
There are a few potential causes of uneven welding. The most common is inconsistent energy at the joint due to variations in the sonotrode contact surface or joint surface. Ultrasonic energy travels as a wave through the sonotrode into the part. If the surface where the sonotrode contacts the part is uneven, the amplitude of the ultrasonic wave could be uneven as well.
Amplitude is the measure of peak to peak displacement, which is directly related to how much ultrasonic energy is being applied to the part. When this varies, the heat in the joint varies, and so does weld strength or quality. This can also occur when the sonotrode contact surface is flat and even, but the distance from this plane to the part is inconsistent. The ultrasonic energy quickly dissipates as it travels through the part, so variations in this travel distance will lead to uneven welding.
Another thing to look out for is providing a consistent joint design. If the energy director height or shear joint width is not the same throughout the part, the weld strength will not be the same either. In fact, some areas may not weld at all.
Finally, make sure there are no voids in the part design between the horn contact surface and the joint. Ultrasonic energy will not travel through air