Five Factors Influencing a Successful Ultrasonic Weld
by Brian Gourley
Sonics & Materials, Inc.
FEA diagnostic outputs showing welding horns before (inefficient weld) and with design improvements after (optimized weld)
The basic principle of ultrasonic assembly involves conversion of high-frequency electrical energy to high-frequency mechanical energy in the form of reciprocating vertical motion, which, when applied to a thermoplastic, can generate frictional heat at the plastic/plastic or plastic/metal interface.
In ultrasonic welding, this frictional heat melts the plastic, allowing the two surfaces to fuse together; in ultrasonic staking, forming or insertion, the controlled flow of the molten plastic is used to capture or retain another component in place (staking/forming) or to encapsulate a metal insert (insertion).
To accomplish this assembly, ultrasonic welding systems are available in a variety of configurations, power levels and frequencies. There are bench-top presses, handheld welders, stepper or servo motor-driven systems, heat staking and inserting machines, as well as ultrasonic kits and stack components for use by OEM systems’ integrators.
Likewise, numerous factors influence a successful ultrasonic weld: tooling, materials, frequency, etc. We’ll touch on some of those factors in the rest of this article to expand understanding of the welding process.
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.
Whether the application involves near-field and far-field welding also must be taken into consideration. Near-field welding refers to welding a joint located ¼ of an inch (6mm) or less from the area of horn contact; far-field welding refers to welding a joint located more than ¼ of an inch (6mm) from the horn contact area. The greater the distance from the point of horn contact to the joint, the more difficult it will be for the vibration to travel through the material and for the welding process to take place.
Depending upon the material and the other variables mentioned, certain processes will be better suited for each application – spot welding, staking/swaging and/or inserting.
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.
Typical joint design options include the following:
- Butt joint with energy director. This is the most common design used in ultrasonic welding and the easiest to mold into a part. It features a small 90- or 60-degree triangular-shaped ridge – the energy director – molded into one of the mating surfaces to focus the ultrasonic energy.
- Step joint with energy director. It molds readily and provides a strong, well-aligned joint with a minimum of effort. Material flows into a vertical clearance.
- Tongue and groove joint with energy director. This is used primarily for scan welding, self-location of parts and prevention of flash, both internally and externally.
- Shear joint. It is generally recommended for high-strength hermetic seals on parts with square corners or rectangular designs, especially with crystalline resins.
- Scarf joint. This is generally recommended for high-strength hermetic seals on parts with circular or oval designs, especially with crystalline resins.
To determine which joint design would work best for a particular application, consult with ultrasonic equipment engineers or salespeople.
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.
Ultrasonic horns can be designed with an FEA (Finite Element Analysis) computer simulation program that allows engineers to observe the vibratory action of the horn prior to actual fabrication. FEA simulations show in “live” animation a horn’s amplitude and stresses during a weld through the use of visual indicators, such as color and line patterns, as well as numeric values. In this way, 3D horn models can be designed for uniform amplitude and low stress levels before any machining takes place.
This same technology often is used as a valuable diagnostic tool for pinpointing the cause of ineffective ultrasonic welds or cracked horns.
Another important consideration in tooling manufacture is symmetry – horn symmetry is vital. An unbalanced horn may vibrate in a non-axial direction. Radial motion may increase stress levels to the point of failure. Ultrasonic vibrations that are not applied efficiently to the individual parts may not properly weld. If needed, higher amplitude boosters or half or multi-wave extenders (smaller horns) can be attached to a larger primary horn. As an example, applications that are tall or difficult to reach may need small, localized horn contact.
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.
To select the best processes, equipment and techniques for a particular application or parts production demands, consult directly with the ultrasonic equipment manufacturer. Take advantage of the manufacturer’s unique experience in this specialized field. Working with applications engineers and perhaps even using FEA (Finite Element Analysis) simulation to test the weld will enhance the success achieved on the assembly line. There are a lot of factors to be considered even before finalizing parts’ make-up and configuration, let alone their assembly methods.