Amplitude, Time and Pressure

In order for thermoplastic to be ultrasonically welded, there must be a correct balance between amplitude, time and pressure.


Amplitude is the vertical, vibratory, peak-to-peak movement produced by the convertor, modified by the booster and fine-tuned by the horn. This vertical motion (usually between 20-100 microns) applied under pressure causes molecular friction at the interface of the thermoplastic parts being joined. This friction causes the plastic to melt and flow through the joint. When the amplitude and pressure are released from the parts, the plastic resolidifies and the parts are welded. Certain ultrasonic welding applications require different amounts of amplitude in order to get a desired result. For example, ultrasonic staking requires more amplitude then ultrasonic inserting. Thermoplastics require different amplitude settings in order to get them to weld. As a general rule, amorphous polymers require less amplitude then crystalline polymers because of the difference in molecular structure.


Time is in reference to weld time and hold time. Weld time is the amount of time, usually measured in tenths of seconds, that amplitude and pressure are applied to thermoplastic in order to get a desired weld. Weld times vary according to the ultrasonic application. Small thermoplastic parts require shorter weld times than large parts, and staking requires more time than near-field welding. For most applications, a good starting point for weld time would be between .02 – 2.0 seconds. Hold time refers to the amount of time, after ultrasonics has been applied to thermoplastic parts, that pressure is held on the parts to assure that the melted plastic has solidified. As a general rule, hold times are usually half the weld time.


The actual force being applied to the thermoplastic parts or metal inserts is a critical variable needed to ultrasonically assemble the product. The pressure needed for ultrasonic welding will vary. For example, it takes more pressure to insert metal into plastic than it does to stake plastic over metal. Large thermoplastic parts take more pressure then small parts. Good starting pressures range from 20 PSI to 40 PSI depending on the size of the parts and the application being used.