July 2007 Design Tip

Good Vibrations — Ultrasonic Welds

There's no shortage of ways to join plastic parts. There are bolts and screws, moulded-in clips and snaps, and a variety of adhesives. But for simplicity and permanence, nothing beats sonic welding.

The ultrasonic weld joint is a method for joining two parts. They are designed with a small amount of extra plastic in the area you want to weld. The two parts are placed together and in contact with an ultrasonic generator which causes them to vibrate many thousands of times a second. Friction at the joint liquefies the extra plastic and small adjoining areas on both parts. As the melted material on the mating parts cools, the two parts become essentially one. If you can live with its permanence, Ultrasonic welding is the best of all possible solutions. It eliminates the loose parts and painstaking insertion of threaded connectors. It avoids the geometric complexity of moulded-in plastic snaps, and it does away with the chemical problems and mess of adhesives.

The weld is ideal for permanently sealing maintenance-free devices like batteries or assembling a non elastic cup seal to a piston. It is a solution to encapsulating something within plastic when overmoulding is not allowable. Sonic welding is also useful for preventing tampering that could void a warranty.

Protomold does not actually perform the sonic welding of parts, but is often called upon to mould parts that will be joined using that process. Weld interfaces have different configurations from simple to complex. Here are three that work well within the Protomold process and Ultrasonic welding in general:

The Shear Joint

The Shear Joint

The Shear joint is a very strong, self-aligning joint that is particularly useful for creating hermetic seals and right-angle joints. It is ideal for crystaline materials such as Nylon, PPS, and PPO, and can also be used with larger parts made of amorphous materials. Note that this joint can leave flash when parts are joined.

The Step Joint

The Step Joint

The Step joint is a stronger, self-aligning joint that provides an excellent appearance. It is suitable for use with amorphous materials.

The Tongue and Groove Joint

The Tongue and Groove Joint

The Tongue and Groove joint eliminates flash caused by the welding process as the weld occurs between two walls and is an excellent choice for hermetic seals. Not recommended for thin walled parts.

Ultrasonic welding design is based on part geometry, including wall thickness and required strength. For more information on designing parts for ultrasonic welding, see:

Joint Design A Critical Factor In Strong Bonds (PDF) from Engineering Design.

The Rule on Joint Design from Sonitek.

Joining of Plastics, Handbook for Designers and Engineers from Hanser Gardner. (Also available through Protomold. Call your sales representative for details.)