The exact strength and flexibility of plastics can be easily determined with finite element analysis (FEA) techniques. The FEA process subdivides the product or part into finite-sized units of simple shape. Mathematical equations are used to test each unit for displacement, from which the stresses and strains can be calculated.
Key FEA Calculation Requirements
One of the key requirements used in the analysis 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 is not only dependent on the material, but also the temperature of the material 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?
Effects of Plastic Materials Used
Plastics can be unreinforced or reinforced. Unreinforced plastics have a very non-linear stress-strain line (curvy, not straight) 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. Some molders, however, don’t analyze the stress-strain curve of the plastic they’re evaluating and 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.
Glass-reinforced plastic parts may be easily 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. Typically yield strength and ultimate strength are equivalent so the stress-strain curve stays linear. Glass fiber is much stiffer than the base resin and overwhelms the nonlinear properties of the resin. Fiber-reinforced materials behave much like a metal, but rather than stretching/yielding at a certain point, the material just breaks—there is no true yield point.
Another reinforcement issue is knit-line strength. A plastic melt flow front splits to go around a core pin or opening in the part during the molding process. When this flow front meets back up with the other half of the split on the back side of the opening, a knit line occurs. A butt-type knit joint is the worst case since the flow fronts meet “face-to-face”. A flow front will push any smoke, trapped air, or mold surface contamination in front of it. This all gets concentrated at the knit line and weakens the bond between the two fronts. It’s even a much weaker situation with fiber-reinforced materials, since the fiber cannot cross over the knit line. A similar condition exists whereby the flow fronts meet and then flow side-by-side to finish filling the part. This is a stronger situation since any contamination may still be pushed along in front of this flow front.
Using FEA Moldflow Packages
Several FEA “moldflow” packages on the market today predict plastic flow in a mold. Results from this flow analysis are used to infer fiber orientation and knit-line locations. Structural FEA can then be performed using mechanical properties of “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 addition of strengthening features to be designed into the part.
It is paramount not to have a high-stress area coincide with a knit line location. If this is the case, the mold gating may be relocated to force knit line locations to be in a lower stress area or the part is re-designed to strengthen the knit line area.
Relying on Plastics Engineers
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.