Experimental analysis and predictive modeling of the tensile and flexural strength of FDM-printed PLA for clinical devices

Imagine a world where the tools doctors use to heal are not mass-produced in factories, but crafted one by one, tailored perfectly to the unique shape of a patient's body. This is the promise of additive manufacturing, often called 3D printing. Unlike traditional methods that carve away material from a solid block, this technology builds objects layer by layer, like stacking thin slices of dough until a loaf takes shape. One of the most popular materials for this process is a plastic derived from cornstarch or sugarcane, known as polylactic acid, or PLA. It is favored because it is safe for the human body, breaks down naturally over time, and is relatively easy to print. However, for these printed objects to be used as medical devices, such as surgical guides or custom implants, they must be strong enough to withstand the forces of the human body without snapping or bending. The challenge lies in the fact that the way a 3D printer lays down this plastic creates tiny weaknesses inside the material, making it behave differently depending on which direction you pull or push it.

A team of researchers at Tunis El Manar University and the University of Monastir set out to solve this puzzle by treating the 3D printing process like a precise recipe. They wanted to understand exactly how three specific settings—how thick each layer of plastic is, how fast the printer moves, and how much extra plastic is pushed through the nozzle—change the strength of the final object. While scientists have long known that layer thickness and speed matter, the amount of plastic squeezed out, known as the extrusion ratio, has been a mystery. The researchers suspected that pushing out slightly more plastic than the printer's standard setting might fill in tiny gaps between the layers, creating a denser, stronger part. To test this, they did not rely on guesswork. Instead, they printed hundreds of small test pieces, each with a different combination of these three settings, and then subjected them to rigorous physical tests. They pulled the pieces apart to measure their tensile strength, which is the force required to break them, and bent them to measure their flexural strength, which is how well they resist snapping under pressure.

The results revealed a clear and surprising truth about how these printed parts hold together. The researchers found that the amount of plastic pushed through the nozzle was the single most important factor in determining strength. When they increased the extrusion ratio to 110 percent, meaning they pushed out ten percent more material than the standard setting, the internal gaps and tiny holes within the plastic almost completely disappeared. This simple adjustment transformed the material, making it significantly tougher. They discovered that the printed parts were naturally anisotropic, meaning they were stronger in some directions than others. When tested by bending, the pieces could withstand between 34 and 51 megapascals of force, but when pulled straight apart, they held up under only 12 to 24 megapascals. This difference happens because the layers are fused together on the outside but can separate more easily when pulled directly apart.

To find the perfect balance for creating the strongest possible medical devices, the team used a mathematical approach to analyze all their data and predict the ideal settings. They determined that the best combination for maximizing both strength and stiffness was to use a layer thickness of 0.20 millimeters, a printing speed of 40 millimeters per second, and that crucial 110 percent extrusion ratio. When they tested these specific settings on new samples, the real-world results matched their predictions with remarkable accuracy, with errors as small as 1.89 percent and no more than 6.32 percent. This level of precision confirms that by carefully controlling how much material is deposited, engineers can eliminate the weak spots that usually plague 3D printed parts. The study concludes that this optimized method provides a reliable blueprint for creating high-performance PLA components, offering a path forward for manufacturing custom medical tools, implants, and other devices that must be both lightweight and incredibly durable.