CompositesWorld
Issue link: https://cw.epubxp.com/i/517026
JUNE 2015 10 CompositesWorld DESIGN & TESTING » Adding fber reinforcement to plastic parts creates unique challenges for structural analysis. Variations in material properties and fber orientations during injection molding processes can be signifcant but often are overlooked. Te result is an over-simpli- fcation of the model that neglects key infuences, such as weak areas due to weld lines, residual strains that cause warpage, and property variations due to fber alignment. Terefore, parts are often overdesigned or must be optimized through trial and error. In a market where molders are looking to increase performance while reducing cost, those approaches are just not acceptable. Te efect of fber orientation on material properties is a key way the injection molding process impacts mechanical performance. Te following illustrates just two ways fber orientation infuences the structural behavior of fber-flled parts and discusses the need for a bi-directional approach to design and analysis. As the injected material fows through the mold, fber alignment is afected by the direction of fow and the mold cavity geometry and can vary greatly throughout the part. Te resulting fber orien- tation has a direct correlation with mechanical properties. In Fig. 1, Example A illustrates how areas with highly oriented fbers have a high modulus in the direction of orientation and a much lower one (one-third as high) in the cross-fow. In comparison, Example B illustrates that if fbers are largely 45° to the fow direction, the moduli are equal. Fig. 2 shows how variation in fber alignment results in signif- cant variation in mechanical properties in even simple part geometry. With the gate location at the left point, the frst area experiences expanding fow, and fber orientation is largely perpendicular to the fow. Tis is followed by an area with more random alignment and, fnally, areas where fow is predominantly in the fow direction. Te result is a complex distribution of aniso- tropic mechanical properties, as shown in Views A and B. Under mechanical loading, plastic parts typically exhibit a signifcant plasticity prior to rupture. When fbers are added, both plasticity and rupture loads are infuenced by fber orientation and loading direction. Tis is illustrated in the tensile test results shown in Fig. 3, where all three load directions show signifcant plastic response prior to rupture, but the stifness and strength of the material show high dependence on the relative fber direction. Te plasticity of the specimen loaded in the fber fow direction is clearly diferent than that of the fbers loaded at 45° or perpendicu- larly (90°). Te strain-to-rupture loads are also diferent, with the specimen loaded in the cross-fow direction rupturing 33% sooner. Tese examples demonstrate the need for structural simu- lation with insight into the after-molding or as-manufactured condition of the part and an ability to model the nonlinear material response. Simulation solutions are making great strides in providing these capabilities. For example, Autodesk Moldfow Understanding the infuence of fber orientation on structural analysis of fber-flled parts FIG. 3 Example stress-strain curve for Extron 3019 (30% glass fber) showing plasticity and rupture (last measurement). 80 70 60 50 40 30 20 10 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 Fibers at cross flow Fibers in x direction Fibers under 45 ° E x (MPa) 12,000 6,000 E y (MPa) 4,000 6,000 FIG. 2 VIEW B: This shows the variation in the modulus perpendicular to the dominant (y) direction. It is shown to measure low (indicated by blue regions) in the aligned fow and high (yellow/red) in crossfow and randomly aligned regions. FIG. 2 VIEW A: This shows the variation in the modulus in the dominant, or x, direction. It is shown to measure high (indicated by yellow/red regions) in the aligned fow and low (blue regions) in crossfow and randomly aligned regions. A B FIG. 1 Fibers at fow direction Δ Fibers at 45° Uniaxial Strain (mm/mm) Uniaxial Stress (MPa) fow direction 0° 45° cross fow direction 90°