Evaluating Ultrasonic Welding of Medical Plastics: Part 2

For ultrasonic welding (USW) processes, it is essential to understand how key parameters and material properties can influence the results of the process. A previous article, “Understanding Ultrasonic Welding,” described the theoretical framework for USW of thermoplastics, including welding process pros, cons, joint types, part design influencers, and general knowledge.¹ This article presents the key parameters and materials influencing an ultrasonic welding process, so that USW can be accomplished successfully. The study provided fundamental knowledge about the USW behavior of CYROLITE^® poly(methyl methacrylate (PMMA) copolymers, polycarbonate (PC), methyl methacrylate acrylonitrile butadiene styrene (MABS), and styrene acrylonitrile (SAN). It showed the key parameters used, the influence on the welding joints, and the compatibility of these materials in an ultrasonic welding process.

CYROLITE^®, with balanced optical transmission and mechanical robustness offers chemical resistance to isopropyl alcohol (IPA), lipids, blood, disinfectants, and oncology drugs, necessary for safe bisphenol A (BPA)-free, infusion therapy, diagnostic, and blood applications. Ultrasonic welded applications include filter housings, as shown in Figure 1, and many other applications.

Before the evaluation of ultrasonic welding can be understood, it is important to recognize the different defects that can be seen throughout this process — whether it’s breakage on the jig, a bad weld, or just a general issue with testing after the fact.

Figs. 2-4 - Tongue-and-groove joint design; nonconforming welds from various process material incompatibility issues; homogeneous welded tongue-and-groove part.

Figure 2 shows the initial design of the weld joint (tongue and groove). The nonconforming welds in Figure 3 show different issues throughout the weld, which can be seen from improper low temperatures, welding times, or welding pressures, all of which can lead to insufficient heat at the contact joints. This phenomenon is known as frosting or sticking. Material incompatibility can cause non-conforming welds that can be a potential mechanical failure for the product. The final welded conforming part is shown in Figure 4. This part, in particular, uses two different materials (hence the color difference in the cross section), but it still shows what a homogeneous mixture looks like when welded properly.

Testing

The test equipment used for the study was an ultrasonic welding machine with a frequency of 20 kHz, a tensile testing machine specially equipped for the test specimens used in the welding trials, and a microscope to examine the weld.

The test procedure was as follows: top and bottom part of the test specimens were welded together; the weld quality was examined with the microscope and in each case by tensile tests. The study varied two of the crucial machine parameters — amplitude and weld force — while keeping weld distance locked 0.35 mm due to the test specimen design. The influence of two important machine parameters were tested to the outcome of the ultrasonic welding process: the amplitude (20, 25, and 30 μm), welding force (200, 300, and 400 nm). This resulted in nine different parameter pairs for each material combination.

For each parameter pair, 10 welds were performed followed by tensile testing. A total of six material combinations were tested. The results were statistically analyzed, and the parameter groups with the best average tensile forces were extracted. Based on the best parameter groups, recommendations for amplitude and weld force were made. An optimal weld exhibits high tensile strength with low variation; another consideration was the machine parameter processing window, which is explained further. The objective of the study was to determine an optimized process of ultrasonic welding different CYROLITE^® grades, welding them to themselves and to other competitive materials.

The study tested CYROLITE^® G-20 welded to CYROLITE^® G-20, CYROLITE^® GS-90 welded to CYROLITE^® GS-90, and CYROLITE^® Med 2 welded to CYROLITE^® Med 2. Furthermore, the ultrasonic welding behavior was tested, welding CYROLITE^® GS-90 to medical-grade MABS, SAN, and PC.

CYROLITE^® materials are well known in the medical device industry and have been used in various applications for decades. CYROLITE^® is often used for medical consumables for infusion therapy, blood management, and diagnostics. USW is a well-established joining technique in these application areas, and the use of USW of CYROLITE^® for specific applications is well documented. What was missing was systematically generated information that would allow basic statements to be made about the USW of CYROLITE^® molding compounds for medical applications, as well as the added information of joining other materials that may have different uses.

An inherent problem of plastics testing is that most properties can only be measured at processed parts and not directly on the raw materials. This also applies to the quality of ultrasonic welding. Tensile testing is the most common and intuitive approach to determine the quality of an ultrasonic weld by measuring the tensile strength required to tear or break the welded parts. USW test specimens are needed for this purpose; there is ISO and/or ASTM guidelines for testing many plastic properties, which also precisely define the test specimens.

For USW, there is, for example, the AWS G1.2M/G1.2, “Specification for Standardized Ultrasonic Welding Test Specimen for Thermoplastics” and in Germany, the DVS 2216-1 guideline.

These documents describe test specimens for USW. The AWS test specimens are elongated pieces that look like small T-beams. An advantage of these specimens is that a uniaxial stress state is set during the tensile test. A disadvantage is that these parts are prone to injection molding defects and are difficult to fix during the USW process. The rotationally symmetric DVS specimen is easier to injection mold and can be easily fixed during the USW process. The disadvantage of the DVS specimen is the difficulty of homogeneous force distribution during tensile testing.

In this study, the rotationally symmetric test specimen, Hexagon 2.0 (see Figure 5) from Hermann Ultrasonic, was used. This specimen has a circular tongue-and-groove design and an energy director. As the name suggests, the lid and base of the test specimen are hexagonal. This shape allows easy fixing during welding and enables tensile testing of the weld with little bending of the lid, as the pull point is close to the weld line, as shown in Figure 6.

Study Purpose

The purpose of the study was to gain general knowledge about the USW behavior of CYROLITE^® and other medical plastics. Since such studies can easily generate an enormous complexity, it is important to carefully design the structure of the study. For this reason, this study tested the USW behavior of three CYROLITE^® grades with themselves and additionally the USW behavior of one CYROLITE^® grade with three other transparent medical-grade plastics. The following CYROLITE^® materials selected for testing: G-20 HIFLO, GS-90, and Med 2.

CYROLITE^® G-20 HIFLO is a low-viscosity material with a high melt flow index, particularly suitable for highly complex designs and thin-walled and intricate parts. CYROLITE^® GS-90 is a material with superior clarity and easy flowability, and it is specifically designed for high-energy radiation sterilization. For example, GS-90 shows no significant color shifts after gamma sterilization. CYRO-LITE^® Med 2 has a higher ductility and provides excellent chemical resistance to a wide range of substances occurring in the medical environment, such as lipids, oncology drugs, or complex disinfectants. Other materials selected included MABS, SAN, and PC medical grades. CYROLITE^® GS-90 was used as the material for the USW tests with the other materials. To make all tests consistent and comparable, GS-90 was always used for the bottom part and the other material was always used for the top part.

Table 1. Material used and material combinations tested.

Material properties are crucial in the ultrasonic welding process. The ability to efficiently absorb and transmit vibrations is important. This is reflected, for example, in the tensile modulus. The higher the modulus, the better the energy is transmitted through the material. In some cases, however, the amount of modulus and other inherent properties showed inferior results due to the parts being more brittle. A test matrix and basic material descriptions are given in Table 1 to show the main experiment setup.

Fig. 7 - The results for G-20 HIFLO, GS-90, and Med 2.

The results for the CYROLITE^® grades G-20 HIFLO, GS-90, and Med 2 are shown in Figure 7. For CYROLITE^® G-20 HIFLO, the best results (highest tensile strength) were obtained with all three welding forces with amplitude 25 μm. For 400 N welding force, the pull force at amplitude 25 μm was also the optimum. At amplitude 30 μm, the tensile strength decreases. For 200 and 300 N weld forces, the tensile strengths are slightly higher at 30 μm. Overall, it seems to be a good starting point for USW of CYROLITE^® G-20 HIFLO to use 300 N weld force with an amplitude of 25 μm.

For CYROLITE^® GS-90, the welding force is a more influential factor. At the lowest welding force (200 N), the pull forces are generally low. At 300 and 400 N weld force, the results are in a comparable range at amplitude 20 and 25 μm and decrease at 30 μm. For CYROLITE^® GS-90, high welding force and low amplitude seems to be a good starting point.

For CYROLITE^® Med 2, the maximum pull forces achieved are generally somewhat lower than for G-20 HIFLO and GS-90. This is in accordance with results from tensile testing (ASTM D638), but for all tested amplitude/weld force combinations, the resulting pull forces are approximately in the same range. In addition, the variance in the test series is very low. This means that CYROLITE^® Med 2 enables consistent weld qualities with high reproducibility in a very wide process window. The results for the USW tests of CYROLITE^® GS-90 with MABS, GS-90 with SAN, and GS-90 with PC are shown in Figure 8.

Fig. 8 - GS-90 with MABS, GS-90 with SAN, and GS-90 with PC results.

For GS-90 to MABS, the USW tests clearly show that lower amplitudes are beneficial for this material combination. A good starting point appears to be low amplitude and medium weld force. SAN is the only plastic in this study that is brittle rather than ductile. SAN has by far the highest young modulus of all the plastics used in this study. Due to the brittleness of SAN, many of the SAN parts showed cracking at the edges of the lids during the USW process. However, this did not significantly affect the pull testing or weld strength.

When designing a part made out of SAN for USW, brittleness should always be kept in mind. And the test results clearly indicate that low amplitudes and low weld forces should be used whenever possible.

Table 2. Recommended processing conditions from this study.

The PC to GS-90 combination showed great results. Using medium conditions (25 μm amplitude and 300 N weld force) should always be a good starting point. The recommended processing conditions for each material combination are shown in Table 2. Processing results from such tests under idealized conditions cannot be directly transferred to real applications. However, they give a good indication of the general behavior and for optimizing the process conditions. Overall, the CYROLITE^® materials performed very well in all tested material combinations, as shown by the pictures of good welds in each example above.

CYROLITE^® Med 2 showed an exceptionally wide process window with extremely low variance in pull forces results. CYROLITE^® GS-90 and PC showed an even stronger result than a potential PC to PC weld was showing. These phenomena demonstrate why a material like CYREX (PMMA/PC combination) is also available for medical applications.

This article was written by Dr. Dirk Heyl and Andrew Sneeringer, Technical Marketing Managers for CYROLITE^® advanced medical acrylics. For more information, visit here .