company news about Five Factors Influencing a Successful Ultrasonic Weld

1. Welding frequencies:

Typical welding frequencies range from the 40kHz range to the 15kHz range. The various parameters of the application will determine the best equipment and frequency to achieve an optimal weld for the parts.

For example, for small, delicate assemblies (printed circuit boards, microelectronic components, etc.) with close tolerances, a higher frequency (for example, 40kHz) is better suited as applied pressure and ultrasonic vibrations can be minimized, along with any marking of Class A surfaces.

Low frequency (for instance, 15kHz) is well suited for medium- to large-size parts and also permits the welding of many softer plastics with greater far-field distances (more on this below) than often is possible with higher frequency systems.

The 20kHz frequency is the most commonly used ultrasonic frequency for plastics assembly and offers maximum flexibility, as it is suitable for a wide range of applications and thermoplastic components.

2. Materials considerations:

In keeping with the basic principle of ultrasonic assembly as outlined above, thermoplastics can be ultrasonically assembled because they melt within a specific temperature range; whereas thermosetting materials – which degrade when heated – are unsuitable for ultrasonic assembly.

Weldability of any thermoplastic depends on its stiffness or modulus of elasticity, density, coefficient of friction, thermal conductivity, specific heat and Tm or Tg.

In general, rigid plastics exhibit excellent far-field welding properties because they readily transmit vibratory energy. Soft plastics, having a low modulus of elasticity, attenuate the ultrasonic vibrations and, as such, are more difficult to weld.

In staking, forming or spot welding, the opposite is true. Generally, the softer the plastic, the easier it is to stake, form or spot weld.

Also as a rule, resins are classified as amorphous or crystalline. Ultrasonic energy is transmitted easily through amorphous resins, which therefore lend themselves readily to ultrasonic welding. Crystalline resins, on the other hand, do not readily transmit ultrasonic energy. For this reason, when welding crystalline resins, higher amplitude and energy levels should be used, and special consideration should be given to joint design.

Variables that can further influence weldability are moisture content, mold release agents, lubricants, plasticizers, fillers reinforcing agents, pigments, flame retardants and other additives, along with actual resin grade.

Likewise, it is important to determine the degree of compatibility of the materials being welded together. Certain materials have some degree of compatibility, but not all grades and compositions may be compatible, and some are not at all compatible.

3. Joint design impact:

Perhaps the most critical facet of ultrasonic welding is joint design (the configuration of two mating surfaces). It should be considered when the parts to be welded are still in the design stage and then incorporated into the molded parts. There are a variety of joint designs, each with specific features and advantages. Their selection is determined by such factors as type of plastic, part geometry, weld requirements, machining and molding capabilities and cosmetic appearance.

4. Tooling and fixtures:

It is difficult to overstate the importance of horns and fixtures when it comes to achieving an effective ultrasonic weld.
There used to be a perception in the industry that horns and fixtures for a particular application needed to be provided by the same manufacturer of whatever welding press was being used. Today, engineers understand they are free to mix and match: The best tooling for the job does not have to bear the same name that’s on the press, as long as the welding frequency matches.
Tooling fabrication material options include aluminum, titanium, hardened steel and stainless steel. Such factors as the type of plastics being welded, the joint size and configuration, weld strength and/or durability will determine the best material for the job. For instance, for increased longevity, hardened steel could be a good choice.
Good fixture design also is imperative. The fixture has two main purposes: to align the parts under the horn and to support directly under the weld area. This support also includes reflecting the ultrasonic energy back to the weld plane, which is why fixtures often are machined from metal.
For added strength and durability, carbide facing or chrome plating can be applied. Contoured fixtures and tools for irregularly shaped parts can be custom designed, along with peripheral devices to clamp, hold and align opposing parts. Segmented and adjustable fixtures also can be built to ensure a secure fit with molded plastic parts.

5. Welding parameters:

During the weld process itself, depending upon the type of system being used, a variety of weld parameters influence the outcome. These include amplitude/pressure, trigger force and tolerance limits, depending upon whether the welding is done by time, energy or distance.
The amplitude setting is used to specify the vibrational amplitude. Fine adjustments of amplitude and pressure settings often can be made on the controller that powers a press, while major adjustments can be accomplished through the use of boosters and pressure controls.
Trigger force pressure settings specify the pressure that needs to be reached to trigger the ultrasonics. Adjustment of this parameter with settings, such as delay timers, pre-trigger modes and force/pressure settings, can affect how long the parts are in contact before the ultrasonics is actually on.
Time settings, such as weld time (the duration of time for which ultrasonic vibrations are actually applied to the parts) and hold time (the duration for which pressure is maintained to ensure proper bonding of the parts, after the actual weld time and with ultrasonics off so the weld can cool), further influence when and for how long ultrasonics should remain on.
Likewise, some systems will allow the user to specify energy settings – with limits and a calibration pulse, for instance – while some also will allow distance settings – such as incremental, pre-trigger, absolute and limits.
As can be seen, a lot of moving parts, if you will, come into play during the ultrasonic welding process. Manipulation of these parameters can mean the difference between a successful weld and an ineffective weld or a cracked horn.