Ultrasonic welding of thermoplastics has been widely used by the medical industry to assemble plastic parts and components in just seconds without additional consumables. Development of servo-driven ultrasonic welders introduces unique levels of control to the process. Historically, pneumatic ultrasonic welding systems controlled ultrasonic horn movement indirectly by relieving pressure from air cylinder, which lacks precision, provides less consistent weld results, and compromises weld strength. Today’s servo-driven ultrasonic welding systems control the process through a closed-loop servo position control that dynamically seeks to meet the desired position. The ability of servo systems to program independent speeds for up to 10 different segments of the weld, along with the ability to dynamically sense when melt is being initiated at the beginning of the weld process, yields consistent and stronger weld results with less residual stress.
Weld Velocity Profiling
During a typical ultrasonic welding cycle, most of the plastic melt takes place in the energy director body, and its molten material forms a bond. Generating maximum weld strength when using pneumatic welding systems typically requires that the weld distance be set close to the nominal energy director height, so the energy director can be completely melted. Failure to achieve full melt often results in lower strength, incomplete welds, and poor appearance of welded assemblies. As the actual height of the energy director varies (because of the variation in the molding process), there is always a risk that some of the parts with a shorter energy director will have excessive flash, and if the programmed weld distance is reduced to avoid flash, then there is a risk of generating non-hermetic welds.
Servo-driven ultrasonic welding technology introduces weld velocity profiling, which is capable of achieving strong and reliable welds without fully collapsing the joint while minimizing the risk of excessive flash. For difficult medical applications, it is advantageous to profile the speed during the weld process in order to match the natural melt rate of the material. In addition, servo systems provide the ability to sense when the plastic resin is changing from the solid state to the molten state at the trigger stage of the process when it senses a drop in force indicating the precise moment to begin the weld process.
Effect of Weld Velocity
Experiments were completed in which the weld velocity was varied, and the resulting strength and appearance of the welds were evaluated against the stringent requirements of the medical industry. Analysis of welded cross sections suggests that higher weld strength was associated with a linearly increasing weld velocity profile.
During these experiments, three different velocity profiles were examined: 1 mm per second constant profile, 0.5 mm per second constant profile, and a linearly increasing profile from 0.25 to 0.4 mm per second. Samples for each profile were fractured on a tensile testing machine, and the load at break was recorded (see Figure 1).
The welds produced using the linearly increasing velocity profile were strong despite the fact that the energy director collapse during weld was limited to 68– 70 percent of the nominal height. Worthy of note is the fact that significant numbers of these welds failed in the parent material, not the weld joint, and that the failure load of others was close to parent material strength. An example of failure through parent material is shown in Figure 2.
Parts were cross-sectioned through the weld area and the shape of the heat-affected zone was measured on a microscope using imaging software. Cross-sectional images of the weld areas resulting from different velocity profiles show a strong correlation between the shape of the zone and the strength. A smaller, bean-shaped weld zone, not completely covering the full width of the contact area between the parts, showed less strength. These results were consistent with constant velocity profiles, as in Figures 3 a and b.
A larger and more uniform melt layer, proliferated into both parts, was characteristic of linearly profiled velocities and produced stronger welds (see Figure 4). The depth of penetration was observed at 361 μm for the sample produced with a uniform melt velocity of 1 mm per second, while the zone for the sample produced with a linearly increasing weld velocity profile was 690 μm, or nearly twice as deep. It appears that the precise control of the melt achieved with a linear profile allows greater heat propagation early in the weld cycle, and results in a deeper, more consistent and therefore stronger weld.
In welds performed with a linearly increasing velocity profile, the melt layer buildup takes place not only in the volume of the energy director, but also within the mating part surfaces. This forms a uniform melt layer in the contact area of both parts and is apparent in the melt zone shape observed under the microscope (see Figure 4). This zone extends into both parts of the assembly, and its size correlates strongly with high strength welds.
Benefits of Linearly Increasing Weld Velocity Profile in Joining Medical Devices
Elimination of excessive weld flash. The opportunities afforded by linearly increasing weld velocity profile for producing strong, consistent welds, reducing scrap and reject rates, and opening the option of ultrasonic welding to previously inaccessible parts are exciting. This advance control provides a prolonged low force at the early stage of the welding cycle, allowing increased melt propagation in the depth of material and melt-layer growth at the interface. In addition, welding to a distance less than the height of the energy director allows for a larger weld process window and has less propensity for generating excessive weld flash. Elimination of excessive weld flash is a key concern when welding medical parts. Given the variation of size and shape of energy directors that can occur both over molding runs (typical dimensional variations) and over the life of the molding tool, any change allowing a wider process window is useful.
Stronger Welds. Microscopic investigation of welded parts using linearly increasing weld velocity provides insight into the physical characterization of the weld regions. The length and depth of the weld zone correlates closely with the tensile strength of the samples, with a larger zone producing higher tensile strength values. Samples that had full coverage of the contact area, as well as resultant deeper penetration, showed high strength. These larger, more uniform melt regions penetrated well into the surrounding material.
Less Residual Stress. Observation of the samples with a polarized light source shows an additional improvement in the welding process afforded by a servo-driven welder. Optimizing the weld speed throughout the cycle allows the molecules to become less oriented and retain more of the amorphous structure that yields higher strengths. A reduction in number of colors, as well as the number of fringes, is evidence that these samples have less residual stress resulting from the welding process (see Figure 5).
The lessening of residual stress levels will be a key factor for medical device assemblies. High levels of stress can accelerate failure of plastic parts when subjected to a wide variety of environmental factors, such as ultraviolet exposure, chemical attack, and sterilization processes, as well as normal wear. These factors all hasten the failure of a welded plastic assembly, and any process that can minimize residual stress levels caused during welding will help mitigate their impact. This stress reduction can be considered a safety improvement in many products.
In summary, associating a specific weld velocity profile with formation of a homogeneous melt layer in the interface of the assembly offers a key approach to selecting optimum welding parameters. The significantly enhanced capabilities of servo-driven welders in controlling material flow and the rate of material displacement during every stage of the welding cycle, their high repeatability and accuracy, and the optimal implementation of these tools and features enable users to develop a robust joining process with high strength, lower occurrence of welding flash, and lower residual stresses. This approach is beneficial to the welding of small parts typical in the medical device and electronics industry, where strict requirements for strength and dimensional consistency are critical factors.
This article was written by Jason Barton, National Sales and Marketing Manager for Dukane, St. Charles, IL. For more information, click here .