Fiber orientation and mechanical properties of short‐fiber‐reinforced injection‐molded composites: Simulated and experimental results

Abstract

A numerical simulation is presented that combines the flow simulation during injection molding with an efficient algorithm for predicting the orientation of short fibers in thin composite parts. Fiber-orientation state is represented in terms of a second-order orientation tensor. Fiber-fiber interactions are modeled by means of an isotropic rotary diffusion. The simulation predicts flow-aligned fiber orientation (shell region) near the surface with transversely aligned (core region) fibers in the vicinity of the mid-plane. The effects of part thickness and injection speed on fiber orientation are analyzed. Experimental measurements of fiber orientation in plaque-shaped parts for three different combinations of cavity thickness and injection speed are reported. It is found that gapwise-converging flow due to the growing layer of solidified polymer near the walls tends to flow-align the fibers near the entrance, whereas near the melt front, gapwise-diverging flow due to the diminishing solid layer tends to align the fibers transverse to the flow. The effect of this gapwise-converging-diverging flow is found to be especially significant for thin parts molded at slower injection speeds, which have a proportionately thicker layer of solidified polymer during the filling process. If the fiber orientation is known, predictions of the anisotropic tensile moduli and thermal-expansion coefficients of the composite are obtained by using the equations for unidirectional composites and taking an orientation average. These predictions are found to agree reasonably well with corresponding experimental measurements.

Injection moulding of long glass fibre reinforced polyamide 6-6: guidelines to improve flexural properties

“…existence of a fountain flow) [5], the core layer is thicker when the fibre length increases [6,7,12]. In addition, other studies have shown that whatever is the reinforcement length (short or long) the skin layers thickness remains roughly the same whereas the core layer thickness increases when the cavity thickness is higher [13,14]. This typical multi-layered structure also greatly influences the mechanical performances of the manufactured parts [10].…”

Anisotropy and compression/tension asymmetry of PP containing soft and hard particles and short glass fibers

“…In most shell like parts, a pronounced layer structure in terms of preferred orientations develops during melt injection. In this context it is well known, that typically particles and fibers in tool wall vicinity are oriented mainly in or at small angles to the flow direction due to shear gradient effects and mostly perpendicular (again with angle variations) to the flow direction in the core layer due to extensional flow effects [14,17,18,21,22]. The orientation of the filler in turn has an effect on the preferred crystallization direction due to trans-crystallization effects on the filler surface, displaying higher values for higher degrees of particle orientation [21,23,24].…”

“…temperature and shear rate), the composition of the composite, the interface interaction andin the case of rubber additions -the ratio of rubber and matrix viscosity [13][14][15][16]. Especially for discontinuous fiber reinforced polymers numerous efforts have been undertaken to analytically and numerically investigate and describe the orientation distribution by means of melt flow analysis [17][18][19][20]. In most shell like parts, a pronounced layer structure in terms of preferred orientations develops during melt injection.…”

Assessment of the Thermoelastic Properties of an Injection molded short-fiber Composite: Experimental and Modelling

“…This is a limitation of the present two-step approach. Many authors [10,11] prefer stiffness averaging to compliance averaging, because of a better agreement with experimental elastic constants, but without discussing the effect on thermal properties. In the present work, both the elastic and thermal properties predicted for the composite are considered, and they are compared with experimental results.…”