Applied Technology Leader
Edited by Martha K. Raymond
Thermoplastics have been replacing metals and thermosets in components for autos, for the lawn and garden, and in power tools. Many of these parts are hollow and ideally could be designed as one piece. However, most parts still require assembly.
There are a number of thermal and mechanical joining processes for thermoplastics. But those used most often are the welding processes such as linear and orbital vibration, spin, hot plate, electromagnetic, ultrasonic, infrared, and laser. One process that stands out from the group is vibration welding because it produces highstrength joints in nylon-based thermoplastic that can be melted and fused. Plus vibration welding efficiently handles large parts and ones with complex shapes. Among the anticipated benefits are decreased production time and weight reduction. This is why several industries are testing linear and orbital- vibration welding for thermoplastics in a variety of applications.
STEERING TOWARDS VIBRATION WELDING
In the past, automotive manufacturers used complex and expensive processes to produce many hollow parts, such as air-intake manifolds, resonators, valves, and fluid reservoirs. One method, lost core injection, required a number of steps including forming, melting, and metallic core removal. Other ways of making hollow parts included welding combined with fastening, and overmolding technologies. Neither were as flexible as linearvibration welding.
The equipment, tools, and fixtures for linear and orbital-vibration welding are much less expensive than those for earlier more elaborate methods. Moreover, the welding process itself isn’t as time consuming. Linear and orbital-vibration welding are PC-controlled with sensors that scan the part’s position during the process and automatically report key process parameters critical for both welding technologies.
These benefits have led automotive manufacturers to use more and more thermoplastic parts for complicated under-the-hood components, such as air-intake manifolds, air-filter housings, and resonators. But the strength of the welded joint is a major concern when selecting thermoplastic for a part. Parts such as plastic air-intake components and manifolds need to match the mechanical performance of aluminum. To do so, they must be produced from thermoplastics with tensile strength values of 50 to 60 MPa at a 23°C ambient. Several types of thermoplastics, such as 30 to 35 wt. % glass-reinforced (GF) nylon 6 and 66 products, meet the tensile strength requirements and provide beneficial chemical, physical, and mechanical properties.
Molded nylon parts resist creep, fatigue, and repeated impact better than parts made of many less rigid materials. Welded nylons are used in automotive air induction components, power train systems, and fluid reservoirs because they typically weigh up to 55% less than alternatives and may cost less.
Designing these critically stressed welded components requires advanced structural analysis, as well as noise, vibration, and harshness testing. Another part of design is to examine short and long-term strength and life criteria.
PUTTING IT TOGETHER
Linear-vibration welders are complete plastic assembly systems that often don’t require extraneous equipment and machines. They are designed to join large or irregularly shaped thermoplastic parts up to 1.7 3 0.7 m. Typical cycle times are up to 30 sec, including both welding and cooling time. This is only 1⁄50 the cycle time of other methods. The joining process is fast, with welding cycle times ranging from 0.5 to 12 sec. Less time is required because practically all the energy converts into heat. This makes vibration welding considerably more cost effective than hot plate or electromagnetic methods.
In a typical linear-vibration welding machine, two surfaces of the part are mechanically clamped together. One component is held stationary while the other vibrates against it, creating friction which heats and melts the parts at the interface. This vibration is a high-amplitude, low-frequency, reciprocating linear motion. Typical nominal frequencies are from 120 to 240 Hz, with amplitudes ranging from 0.5 to 5.0 mm.
The melt zone undergoes a series of phases. The first is premelt consisting of unsteady melting and material flow in the lateral directions. The next phase establishes a melt-zone at steady-state conditions. Then materials flow unsteadily and solidify at the weld zone. When vibration stops, the melt zone cools and the parts are permanently welded in their clamped alignment.
Certain processing and weld interface parameters are critical for reliable joints. One is the temperature in the weld interface at the beginning and end of the vibration. Temperature depends on processing parameters such as time, clamp-pressure, amplitude, frequency, and meltdown, and also depends on the physical qualities of the materials. However, most standard linear and orbital-vibration welding equipment doesn’t directly control temperature in the joint interface during welding. If it’s necessary to determine temperatures in the weld interface during the process instead of only before and after, it usually takes advanced thermal imaging infrared methods with computerized systems.
The next concern is clamp pressure. It must be high enough to initiate part fusing during premelt. But clamp pressure too high can deform parts the under heat. After heating, it’s a matter of holding the parts together until they’re cooled and sealed. Also important is the interface thickness, which should be narrow and only micronsthick. The interface is created from the preheated and the meltdown layers of the material.
The linear-vibration direction can be longitudinal or perpendicular to the wall thickness. But linear-vibration welding doesn’t always produce a strong joint in welds where an unsupporting thin wall is perpendicular to the base. However, orbital vibration welding does indeed create strong joints for these thin walls.
Orbital-vibration welding allows motion in many directions and also provides more freedom in designing the weld areas. The technique overcomes some drawbacks of linear-vibration welding and is also based on friction. It uses an electromagnetic drive to create relative motion between two plastic components. The orbital motion ensures that each point surface of the driven plastic part orbits a different point on the butt joint surface of the stationary part. This orbital motion is continuous and identical for all points on the joint surface. Plus, orbital-vibration welding operates at low vibration amplitudes. This operating mode is less likely to mechanically or thermally damage sensitive electronic components, contrary to other plastic joining methods.
A few basic guidelines ensure that vibration welding produces a quality joint. The first one is to determine if the design has multipiece parts. From this comes the purpose of the joint in the assembly, which determines the type of welded joint. Joints can be butt, shear, or overlap. Other factors to note in joint design are component geometry along with welded joint geometry such as shape, size, and stiffness. Furthermore, additional parameters include the joined parts’ dimensional stability and tolerances.
Designers must determine both internal and external loads. This includes short-term mechanical properties of joints by tensile, shear, flexural, or combined load conditions.
Other factors also effect joint stability. One is fatigue strength of joints undergoing long-term creep. Environmental factors and chemical resistance are important as well.
The complete assembly is subjected to an additional set of guidelines. For example, joints in the assembly must withstand exposure to harsh temperature conditions, noise, and vibration. The quality of the weld is critical, too. Some areas to observe for quality are internal and external surface smoothness and aesthetics. The weld should not have any defects, cracks, voids, or inclusions.
One can also examine the assembly process to find ways of optimizing vibration welded joint performance. For example, the upper fixture and nested part may have design limitations. If this is the case, consider changing the size of the part placed in the upper fixture. There also may be limitations on the weld plane configuration and maximum value of out-of-welded plane angle. To check if these limitations will adversely affect the weld, compare a part made from similar materials that has exhibited good mechanical strength and joint life performance. On the other hand, dissimilar plastics with different melt temperatures can be more difficult to weld.
Overall, linear and orbital-vibration welding processes are less sensitive to dimensional tolerances because they both self-adjust in the contact areas. But it is possible to close up to a 5-mm gap between two joined surfaces with optimized weld clamp pressure.
In many designs, the material determines welded joint performance. This is also true for vibration welding. For example, vibration welded air-intake components and manifolds need a material that guarantees dimensional stability. Some processes such as postmold warpage can change the tolerances of joined surfaces. The impact of the clamp and welding pressure can increase residual stresses.
You can eliminate some warpage by optimizing vibration welding process parameters such as melt down, clamp and weld pressure, and welding and hold/cooling cycle time. Another way to eliminate warpage is to examine tooling design and molding conditions. Sometimes the geometry of joined parts or surfaces causes warpage, so it may be necessary to change the part design.
Other considerations include the type of joining materials, and whether or not they are similar or dissimilar. In general, similar materials provide a more reliable bond than dissimilar ones. In thermoplastics, similar materials have a comparable base matrix, types of fillers, pigments, and reinforcements. Also, optimizing the ratios of the material additives can provide even stronger bonds. For example, a 14 to 24% glass-fiber reinforcement reaches maximum tensile strength compared to materials with other glass-fiber proportions.
In the future, linear-vibration welding will create stronger and more reliable joints from dissimilar materials. The automotive and appliance industries are interested in welding dissimilar plastics, copolymers, and blends. However, molecules in these materials typically don’t mix well at the interface. The reason is that each material has different thermal properties, so one material may melt faster than the other. Also, one of the materials may not even reach its melting point at the interface.
Other drawbacks of joining dissimilar materials are that joints of dissimilar nylons have low tensile strength. Often welds of dissimilar materials aren’t as thick as welds of similar materials at optimized conditions. Parts must be heated independently to increase the tensile strength of these joints. Unfortunately, parts heated with linear and vibration welding selfadjust to one temperature by transferring heat across the interface.
Overall, linear-vibration welding is a reliable and efficient process that creates strong joints in many thermoplastics. Orbital-vibration welding is efficient for nylons and may serve as an alternative to linear-vibration welding and ultrasonic technologies to weld small components.
OPTIMIZING THE WELDED NYLON BUTT JOINT
The tensile strength of welded butt joints is important for critically stressed under-the-hood components. It’s estimated that more than 45 million kg of nylon will be used in under-the-hood applications this year and another 11 million kg for welded air-intake-manifolds and resonators.
Parts made from nonreinforced thermoplastics — such as polyethylene, polypropylene, or polyester — retain 30 to 90% of their tensile strength with various welding techniques. In comparison, the tensile strength of a butt weld joint in nonreinforced nylon 6 and nylon 66 is nearly equivalent to the base material strength.
But the maximum weld strength is usually between 70 and 80% of the base material strength for glass-fiber-reinforced thermoplastics. The reason tensile strength is lower is because the glass fibers change orientation and line up at the welded joint interface, instead crossing the weld line.
In comparison, under optimized vibration welding conditions — including amplitude, pressure, meltdown, and interface thickness — tensile strength increases at the nylon butt joint. Tensile strength is at least equal to or can be up to 14% higher than the base polymer’s tensile strength. Tensile strength increases through optimized welding parameters in linear and orbital vibration welding because some glass fibers orient themselves perpendicular or at an angle to the weld plane across the interface.