Ultrasonic plastic welding is the joining or reforming of thermoplastics through the use of heat generated from high-frequency mechanical motion. It is accomplished by converting high-frequency electrical energy into high-frequency mechanical motion. That mechanical motion, along with applied force, creates frictional heat at the plastic components’ mating surfaces (joint area) so the plastic material melts and forms a molecular bond between the parts.

Material Consideration

To bond two thermoplastic parts, it is necessary that the materials be chemically compatible. Otherwise, even though both materials may melt together, there will be no molecular bond. A good example would be trying to weld polyethylene to polypropylene. Both of these semi-crystalline materials have a similar appearance and many common physical properties. However, they are not chemically compatible, and are therefore unable to be welded to each other. Like thermoplastics (i.e., materials with the same chemical properties) will weld to themselves. For example, one ABS part will weld to another ABS part. Dissimilar thermoplastics may be compatible only if their melt temperatures are within 40ºF (6ºC) and they are of like molecular structure. For example, it is likely that an ABS part could be welded to an acrylic part because their chemical properties are compatible. Generally speaking, only similar amorphous polymers have an excellent probabilty of being welded to each other. The chemical properties of any semi-crystalline material make each one only compatible with itself. When the materials to be welded are compatible, several other factors may affect the weldability of the parts. These factors include hygroscopicity, mold release agents, lubricants, plasticizers, fillers, flame retardants, regrind, pigments, and resin grades.

Joint Design Considerations

The joint design of the mating pieces is critical in achieving optimum assembly results. A particular part’s joint design depends upon factors such as type of plastic, part geometry, and the requirements of the weld. There are many different joint designs, each with its own advantages. Some of these designs are discussed later in this section. There are three basic requirements in joint design: • A uniform contact area • A small initial contact area • A means of alignment