Finite element analysis (FEA) is a computerized method that models how products behave, using a virtual environment. By using FEA, the performance of a material or product can be tested under a range of conditions. Variables that can be manipulated include pressure, vibration, heat, and fluid flow. Results of these tests can accurately determine if the product will operate the way it was designed, or if adjustments need to be made.
For example, the exact strength and flexibility of plastics can be easily determined with FEA techniques. The FEA process subdivides the product or part into thousands (or even hundreds of thousands) of finite-sized units of simple shape, such as cubes. Areas that are expected to have high stress are usually divided into a higher mesh density of smaller units, compared to those areas that experience little stress.
Mathematical equations are then used to test each unit for displacement, from which the stress and strain can be calculated. The cumulative effect of the performance of each unit is also calculated, resulting in an appraisal of the intended strength and function of the product.
FEA is ideal for determining which material is best for a particular design or application.
Stress responses can be modeled for different factors such as mechanical stress and vibration, loading, acceleration, material fatigue, torque, motion, fluid flow, heat transfer, and electrostatics.
One of the key elements of FEA is the stress-strain curve or plot, which is distinctive for each material. This is a reflection of the amount of deformation (strain) that is caused by tensile/compressive loading (stress, or pressure). The shape of this curve depends on several conditions, including material composition, material temperature, and the speed of loading. The final curve reveals the critical properties of the material-will it deliver the properties that it must have for its intended use?
Plastics can be unreinforced or reinforced. Glass-reinforced plastic parts are analyzed with linear FEA techniques. Linear FEA assumes “small displacement” of the part being analyzed and uses an appropriate equation to solve the calculations more quickly. Unreinforced plastics (more flexible) have a very non-linear stress-strain line up to the yield point and must be analyzed with equations derived for nonlinear materials, not linear materials.
This is an important distinction-some molders use what they know and can afford. Nonlinear FEA software is more expensive, takes more time to set up, and takes more time to run. Molders also sometimes don’t personally analyze the stress-strain curve of the plastic they’re evaluating and instead rely on the published Young’s modulus value in a plastic supplier’s data package. This can provide very misleading results because the modulus value represents just a single point on the stress-strain curve. A non-linear FEA analysis incorporates all the actual stress-strain information to provide accurate results.
FEA analysis can also be used to predict knit-line strength. The knit line occurs where flow fronts join. Flow fronts will push any smoke, trapped air, or mold surface contamination in front of it, which can be trapped at the knit line, weakening the bond of plastic along that line. It’s important, of course, to also use the best injection-molding science to eliminate any contamination during the process. FEA can be used to model both “in-flow” and “cross-flow” directions to get a better understanding how the part will perform/react. Knowing knit-line locations ahead of time also allows engineers to better design strengthening features for the part.
Designers/engineers must understand the material they are evaluating to get the most out of the FEA simulation. This requires going beyond the single published values in a plastic vendors spec sheet. Stress-strain plots-at various temperatures and strain rates-should be evaluated to choose the properties most appropriate for the part being designed.
Post time: 03-28-2017