Polymer welding plays a critical role in how medical devices are built, even though the welds themselves are rarely seen by clinicians or patients. Across plastic surgical instruments, filters, catheters, drug-delivery systems, diagnostic cartridges, disposable products, and more, these welded joints must perform reliably to support safety, functionality, and to meet regulatory requirements.
A failure at a polymer weld can have serious consequences, including leaks, unintended exposure to bodily fluids or drugs, inaccurate dosing, diagnostic errors, or device fragments remaining in the body. As medical devices become smaller, more complex, and more mechanically demanding, ensuring the fatigue resistance of polymer welds has become increasingly important to prevent these failures.
Why Fatigue Resistance Deserves Attention
Many polymer welds experience repeated mechanical or thermal loading during use, sterilization, shipping, or storage. Cyclic bending, pressure fluctuations, vibration, and temperature changes all contribute to fatigue stresses over time. Even single-use devices can experience meaningful cyclic loading before or during use.
Traditional strength testing typically measures performance under a single, increasing load. While useful, this approach does not fully represent real-world conditions. Welds that perform well in monotonic testing may still fail prematurely when exposed to repeated, lower-level stresses. Testing fatigue resistance provides a more realistic measure of long-term reliability and is especially important for identifying failure modes that may not appear until late in a product’s development.
What Separates a Strong Weld from a Weak One
Fatigue performance is strongly influenced by weld quality. A polymer weld is not simply two surfaces joined together. For a weld to be robust, material from both sides of the joint must soften sufficiently during welding to allow molecular-level bonding, known as intermolecular diffusion, across the interface.
When welding conditions are properly controlled, polymer chains from each side intermingle, forming a joint that can approach the strength of the bulk material. When conditions are insufficient, bonding is limited to surface contact, resulting in a weaker interface that is more susceptible to fatigue cracking. Surface preparation, joint design, applied pressure, heating method, and cooling all influence whether a true structural weld is formed or a marginal bond that may fail under cyclic
loading.
The Role of Material Properties
Material behavior defines the safe and effective window for welding a particular polymer. Thermal properties such as softening temperature, melting temperature, and resistance to thermal degradation of a given material determine how much energy can be applied and how forgiving the process will be.
Material characterization is commonly used to establish these limits. Identifying the temperature range where a polymer becomes sufficiently mobile helps define minimum welding temperatures, while understanding degradation thresholds prevents overheating that could embrittle the joint.
How Welding Parameters Influence Fatigue Life
Welding technologies based on infrared (IR), hot plate, ultrasonic, and laser deliver heat to the joint in different ways. Each method offers different advantages, but all require careful parameter optimization to produce consistent, fatigue-resistant welds. In a recent study, EWI used both IR and laser welding to intentionally under-weld half of the sample joints while properly welding the remaining half. To evaluate the resulting weld quality, heated after cross-section (HACS) analysis — a technique perfected by EWI that reheats and polishes a weld cross-section to reveal internal material flow — was used to assess whether intermolecular diffusion has occurred at the weld interface.
The underwelded joints, which represent conditions manufacturers may encounter when heat input, pressure, or other variables are inconsistent, may appear acceptable externally but often lack the internal bonding required to withstand repeated loading. Optimized welds on the other hand, which achieve intermolecular diffusion, consistently demonstrate improved fatigue life and higher load capacity.
From Lab Testing to Production Reality
After creating the weld samples described above, a three-point bend fatigue test, based on ASTM D790 and D7774, was used to determine the maximum strain the welds could withstand. In this test, welded specimens are repeatedly flexed at controlled strain levels until failure occurs, and the number of cycles to failure provides a direct measure of durability under realistic loading conditions.
Fatigue testing demonstrated that welds with full intermolecular diffusion consistently survived more cycles and tolerated higher loads than those without. Linking weld structure to performance highlights the need for analytical tools that can confirm diffusion early, and the only method capable of confirming intermolecular diffusion is HACS analysis.
Additionally, while fatigue testing confirms long-term performance, it is time-consuming and impractical to apply broadly across a large quantity of parts or multiple design iterations. Processes that appear stable at prototype volumes may behave differently as cycle times shorten, tooling wears, or material lots change, and even minor changes in energy input or alignment can significantly affect weld quality without producing obvious visual defects. Tools like HACS analyses can help manufacturers identify these risks early, define meaningful process windows, and reduce the risk of costly redesigns or revalidation later.
Designing for Reliability and Compliance
Ensuring fatigue-resistant polymer welds is essential for patient safety, regulatory confidence, and performance over the life of the device. While mechanical fatigue testing remains an important validation method, it is not always practical or efficient as a primary tool for evaluating weld quality during product development and manufacturing optimization. HACS analysis offers a more direct and efficient approach by revealing whether a weld achieved intermolecular diffusion, which predicts fatigue resistance. By using HACS analysis to assess weld quality, manufacturers can rapidly refine welding parameters, identify better processes, and reduce reliance on extensive fatigue testing.
As medical devices continue to evolve, the durability of polymer welds will remain a critical factor in product performance. Early assessment of fatigue resistance using methods like HACS analysis, combined with an understanding of materials and processes, will allow manufacturers to create more robust, reliable devices.
This article was written by Jeff Ellis, PhD, Senior Technology Leader for EWI’s polymers group. EWI, headquartered in Columbus, OH, is an independent engineering and technology consultancy that helps manufacturers develop, validate, and implement advanced manufacturing processes and innovations. For more information, contact Ellis at
