Linear Vibration Welding
Vibration also can be applied in either a linear or rotational fashion. Linear welding is by far the oldest and most commonly used variety with thousands of machines in use. It provides the best value and a wide range of sizes. Whereas orbital vibration operates best at off-resonance frequency, linear vibration welding is used at resonance frequency. The manner in which the parts are driven is dependent on its shape and rigidity. The driving force is applied on the external shoulders in Figure 2a. However it also can be applied from a recess in the part as shown in Figure 2b. When internal ribs or midwalls are to be welded, a fixture such as the one in Figure 2c. would be required where pressure is applied over the internal joint.
 

The equipment for linear vibration consists of a vibrator located in the upper section of the machine frame, consisting of a vibrating platen suspended from springs and driven by electromagnets. In the lower section of the machine frame there is a clamp plate, which is operated by a hydraulic cylinder that lifts it to meet the vibrating platen and supplies the pressure for welding. Fixtures that are custom-made to fit the contours of each application are attached to the vibrating platen and clamp plate.

The cycle begins with the loading of one of the parts to be welded in the nest or fixture of the lower clamp plate of the machine as illustrated in Fig. 3. The mating part is then placed in position on the lower part. The lower clamp plate is then raised to the vibrating platen in which rests the upper fixture. It fits the mating part closely and fixes its location. The two parts are clamped together under a controlled load and the part in the vibrating platen is vibrated through an amplitude ranging from 0.75 mm (0.030 in) to 5.0 mm (0.200 in) at a frequency from 120 Hz to 300 Hz to create frictional heat at the interface between them. The pressure is important because, without the pressure, no heat is generated. Low weld pressures result in increased cycle times. Sophisticated systems can optimize the process control by varying the pressure during the cycle.

Basically, there are four phases to vibration welding. In the first phase, the vibration of the rigid members creates Coulomb friction, which generates heat at the joint interface. No penetration (movement of the parts toward each other) takes place in this phase. When the glass transition temperature is reached and viscous flow occurs, the second phase begins wherein heat is generated by viscous dissipation in the molten polymer. Lateral flow of the polymer permits the penetration to take place. In the third phase, the melt and flow have reached a steady state at which heat losses through the wall due to flash equal that being generated. At this point, melt is flowing laterally and weld penetration increases linearly with time. This part of the cycle is application-dependent and will require 0.5 to 10 seconds; about two-thirds of the welding cycle. The penetration required to reach steady-state condition increases with the wall thickness of the part, but decreases with increasing weld pressure.

When sufficient heat has been generated to melt the material at the interface, the vibrations are halted and the fourth phase commences. Weld penetration continues because the clamping pressure causes the molten polymer to flow until it solidifies. The parts are held clamped in the desired final position while they cool sufficiently to withstand handling, a period ranging from 0.5 to 5.0 seconds. This is critical because there is no means of controlling lateral location of the two parts to each other beyond the final position of the vibrating platen. Once this is accomplished, the lower clamp plate is lowered to its original position and the completed assembly is removed. The equipment is now ready to commence a new cycle.

Advantages of Vibration Welding
1. NO ADDITIONAL MATERIALS - Vibration welding uses no additional materials such as fasteners, inserts, electromagnetic preforms, adhesives or solvents. Therefore, it is inherently lower in cost than methods, which do use additional materials, and is less expensive to disassemble for recycling.
2. LOW SURFACE PREPARATION - Vibration welding is relatively insensitive to poor surface preparation.
3. EASE OF ASSEMBLY - Vibration welding requires only the placement of the two parts in the fixture of the vibration welder.
4. ENTRAPMENT OF OTHER PARTS - Additional parts can be captured between the two parts to be vibration welded provided they are located such that they do not interfere with the welding.
5. PERMANENCE - Vibration welding creates permanent assemblies that cannot be reopened without damaging the parts. Owing to material limitations, differences in thermal expansion and moisture absorption are rarely of concern once the parts are welded. .
6. INTERNAL JOINTS - In some cases, internal walls can be vibration welded provided they meet at the welding plane.
7. SHAPE FREEDOM - Vibration welds can be made with parts of practically any shape so long as the horizontal joining surfaces can be created within the required limits. Vibration welding is not dependent on the vibration transmission qualities of the plastic, therefore it can weld parts that are not well suited to ultrasonic welding because they are too flimsy, contoured, have holes in the walls or do not have a surface well suited to ultrasonic horns.
8. HERMETIC SEALS - Vibration welding is capable of creating hermetic seals.
9. ENERGY EFFICIENCY - Relative to other welding processes, vibration welding is highly energy efficient. This also means that there is no excess heat that must be removed from the workplace.
10. CLEAN ATMOSPHERE - Unlike adhesive and solvent joining systems, no ventilation equipment is necessary for the removal of toxic fumes.
11. IMMEDIATE HANDLING - Assembled parts can proceed on to other operations at once without waiting for parts to cool and for adhesives or solvents to set.
12. HIGH PRODUCTION RATES - Depending on the application, vibration welding is capable of production rates of 4 to 30 parts per minute based on a single weldment per cycle and not taking into account the part handling time which varies with each application. Higher rates are possible if multiple parts are welded with each cycle.
13. PROCESS FREEDOM - Parts made from virtually all the thermoplastic processes can be vibration welded.
14. LARGE PART CAPABILITY - Equipment is available that can weld parts up to 1016 mm (40 in) by 2032 (80 in). A 1524 mm (60 in) automobile bumper has been successfully vibration welded.
15. PRECISION CONTROL - Vibration welding permits precision control of process variables.
16. QUICK CHANGEOVER - Vibration welding equipment can be quickly changed from one job to the next.
17. CONTROLLABILITY - This process is readily controlled and is not likely to result in surface degradation due to overheating.

Disadvantages of Vibration Welding
1. SHAPE LIMITATIONS - There must be a flat, horizontal welding surface.
2. DAMAGE TO ELECTRONIC COMPONENTS - Vibrations can damage some electronic components or their assemblies.
3. ALIGNMENT - Locating pins or other devices cannot be molded into the part. Alignment between the two parts is set by the final resting place of the two parts.
4. MATERIAL LIMITATIONS - Materials for vibration welding are limited to compatible thermoplastics.
5. SOUND CONCERNS - The foghorn noise associated with vibration welding (90 to 95 db) makes the use of sound enclosures commonplace. These reduce the sound level to approximately 80 db.
6. EQUIPMENT COST - Vibration welders are more expensive than hot plate welders and cost considerably more than ultrasonic welders or spin welders, a consideration that tends to limit their use to applications, which are too large or poorly configured for these techniques.

Materials for Vibration Welding
Virtually all the thermoplastics can be vibration welded. In that respect, this process is similar to hot plate and spin welding. This process is particularly useful for the crystalline thermoplastics, which are difficult to join with ultrasonic welding. As with ultrasonic welding, the amorphous resins are more readily welded than the crystalline resins. However, the crystalline thermoplastics are more readily vibration welded than ultrasonic welded. Materials with low coefficients of friction may require higher vibration frequencies for optimum weld quality. The process is claimed to have been successfully employed with thermoplastic rubber and elastomers. High temperature engineering thermoplastics have been successfully vibration welded.

Filled and reinforced resins are readily welded with vibration welding. However, only the polymer welds, so the strength of the bond is that of the resin itself and the strength of the joint is further reduced by the percentage given to the filler. In particular, glass fibers may protrude through the surface of the weld at its center. Conversely, this process is less affected by mold release and other surface contaminants than either hot plate or ultrasonic welding and it is somewhat less affected by moisture content, although highly hygroscopic polymers like nylon must still be welded with care. Moisture can cause bubbles in the joint due to the formation of water vapor. Pre-drying the parts can reduce bubble formations and welding time. A high joining pressure can prevent the formation of bubbles in the weld. At 50 percent RH, Nylon 6 weld strengths can drop to around half those of dry welds. Nylon 66 fares somewhat better, dropping less than a third.

Comparison with Ultrasonic and Hot Plate Welding
Vibration welding can be described as the process that picks up where ultrasonic welding leaves off, particularly in terms of part size. That it is somewhat similar in principle is true. Consequently, it shares many of the same attributes. In addition, the equipment is made by manufacturers who also produce ultrasonic welding equipment, so the two processes compliment each other nicely. However, due to the considerable cost of the equipment, one normally uses vibration welding only for those applications that are unsuited to ultrasonic welding. There are some significant differences between the two techniques.

While both processes use high-frequency vibrations to create, they apply them in a different manner and at much different frequencies. Ultrasonic welding vibrations are vertical, range from 15,000 Hz to 72,000 Hz and create heat through molecular friction. Vibration welding is horizontal with frequencies between 120 Hz and 300 Hz and creates heat through surface friction as it physically moves the whole part. The net result is that vibration welding can handle much larger parts than ultrasonic welding and does not require the energy directors that largely limit ultrasonic welding to parts made by the injection molding process. While thermoset parts cannot be vibration welded, parts made from virtually all of the thermoplastic processes can be joined by this process.

Hot plate or fusion welding also can be considered a competitor to vibration welding because its joint strengths and material compatibilities are approximately equivalent. However, while it has much lower equipment cost and a greater range in size and shape, it has a much longer cycle time and higher equipment, fixture, energy and maintenance cost than vibration welding.
Linear vibration welding has numerous applications in the automotive field including manifolds, dashboards, bumpers and taillights. Appliance applications include blower and pump assemblies, refrigerator bins and chain saw housings. Examples of orbital vibration welding applications are taillights and electrical housings.

This article is composed of partial excerpts from the chapter on vibration welding in Jordan Rotheiser’s book, “Joining of Plastics, Handbook for Designers and Engineers.” The book may be purchased from Hanser Gardner Publications at www.hansergardner.com.

Jordan Rotheiser is president of Rotheiser Design Inc. and the author of the critically acclaimed, best selling “Joining of Plastics - Handbook for Designers and Engineers”, the product design chapter of the “Modern Plastics Handbook” and the plastics chapter of McGraw Hill’s “Handbook of Materials for Product Design”.