How is ultrasonic welding used in injection moulding?

One of the main benefits of injection moulding is its ability to create complex parts easily. Ultrasonic welding can help to join these complex parts together without having to use bolts, nails, adhesives or other potentially aesthetically unpleasing methods.

The design of a part means that it’s sometimes not possible to join each part of a complex component together simply through the injection moulding process. So here we explain where ultrasonic welding comes into.

How does ultrasonic welding work?

The two parts to be joined together are placed securely into a custom-made fixture. A vibrating tool called a horn then lowers and contacts the parts. Next, pressure and ultrasonic frequency, otherwise known as high-frequency pressure and sound waves, are applied. The horn transmits this pressure to the parts, usually at a frequency of 20 or 40kHz.

These pulses cause the two plastic parts to vibrate against each other. The vibrations then heat and fuse the parts together. This is the result of converting high-frequency sound energy into high-frequency mechanical energy. The heat and friction that melt the plastic flow together to create a solid weld.
Once the parts are joined, there is a hold time to allow the material to harden. The horn then retracts and the welded part is removed.

Ultrasonic welding explained

Ultrasonic welding makes sense when the parts that need joining are complex or too expensive to be moulded in one piece. The process is clean and precise, meaning it doesn’t cause any aesthetic damage. It’s also flexible because the two parts can be joined at pretty much any point, as well as being a fast process.

Typical applications that rely on ultrasonic welding include appliances and electronics, automotive valves and bodies, medical devices, such as hearing aids, and packaging. Wherever the part is small and complex, or needs to be aesthetically pleasing, ultrasonic welding is a joining method of choice.
Can different plastics be joined together?

With ultrasonic welding, it is possible to join plastics made of blends or alloys from different resin families, as long as their molecular structure is compatible. Crystalline or semi-crystalline plastics, such as nylon, only weld to each other. Also, stiffer materials tend to weld together easier because of the compactness of their molecules.

Ultrasonic weld joint design

There are certain joint types that would be preferable to use with ultrasonic welding. This is something to consider when creating the parts that need to be joined together as, wherever possible, joints should be designed specifically for ultrasound welding. Depending on the end use, joint designs to consider are:

• a step joint, which produces visibly flawless, high-strength and air-tight welds, as well as being able to support self-centring parts
• a tongue and groove joint, which produces the greatest strength and requires thick walls• a mash joint, ideal for joining semi-crystalline plastics with thin walls – this typically results in air-tight and high-strength welds
• double V joint, which is recommended for a wall thickness of less than 1.5 mm.

By selecting one of these joints and incorporating it into a design, manufacturers can make sure that the join itself is as strong and aesthetically pleasing as possible in the final component.

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