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Ultrasonic welding at prototype stage is being increasingly requested. It’s a secondary process that’s suited for testing and development especially when parts need functional testing in production intent material. At RP Technologies, we’ve developed a close partnership with our trusted supplier, Sonics and Materials (UK) Ltd, meaning we can now provide this service for prototype and low volume production projects.

What is Ultrasonic Welding?

Ultrasonic welding is a technique where high-frequency ultrasonic acoustic vibrations are applied to components being held together under pressure to create a solid-state weld. The process converts high frequency energy into high frequency mechanical energy, which causes heat and friction to melt the plastic which then flows together to create a solid weld.

Typical applications for this type of welding is automotive valves and bodies, medical devices, toys, electronics and appliances as the process is very clean, fast and precise. Ultrasonic welding is often chosen when parts are too complex or expensive to be moulded in one piece.

The process begins by placing the two parts into a custom-made fixture. Once securely in place the Sonotrode horn lowers and contacts the part, pressure and ultrasonic frequency are applied which causes friction and heat to melt the plastic. Once complete there is a hold time to allow material to solidify, the horn retracts and the welded part is removed.

Depending on the parts being welded and the volume of the order there are a variety of different ultrasonic welding machines that can be used. For example, a bench mounted electro-press provides repeatable welds with a position control of +/- 0.5mm so is perfect for higher numbers, whereas a hand welder is quick and easy to change which allows for rapid cost effective component changes, most suited to prototype or low volume runs.

Design Considerations

To create acceptable, repeatable welded joints there are a few design guidelines that need to be followed:

The contact area between the mating surfaces should be small – this concentrates the energy and decreases the time needed to complete the welding process. This results in less flash and it also ensures the vibrating horn is in contact with the part for the least time possible which reduces potential scuffing.

Aligning the mating parts is also very important. For repeatable alignment features such as steps, tongue and groove or pins & sockets can be used.

The horn must be placed directly over the joint area to transmit the mechanical energy to the weld line. This also reduces the risk of part marking from the horn.

It’s important to ensure that the weld line is supported around the full perimeter on the bottom component. This means that any spouts or outlets must be far enough away from the weld joint to allow adequate support to the component.

We can offer advice for each project individually and make recommendations suited to the customer design and final application requirements.

Joint Design

There are a variety of joint designs depending on what best suits your application. The two major types of joint design are the energy director and the shear joint.

The energy director is typically a raised triangular bead of material moulded on to one of the joint surfaces. The function of the energy director is to concentrate the energy to rapidly initiate the softening and melting of the joining surface.  It permits rapid welding while achieving maximum strength as material within the director generally flows throughout the joint area.

The energy director can be applied to a variety of different joint designs including:

Butt joint with energy director – this is the most common joint design and is the easiest design to incorporate into mouldings.

Step joint with energy director – this provides a strong, well aligned joint.

Tongue and Groove with energy director – this joint design is excellent for self-alignment and it also prevents flash from escaping weld area.

With a shear joint, welding takes place by melting the small, initial contact area and then continuing to melt with a controlled interference along the vertical walls as the

parts telescope together. This type of weld gives strong structural and a hermetic seal as the molten area doesn’t come in to contact with the surrounding air. However, to obtain a successful shear joint rigid side wall support is needed and a minimum wall thickness of the bottom part should be 2mm to prevent buckling.