Fibre orientation and mechanical behaviour in reinforced thermoplastic injection mouldings

Abstract

Process variables, fibre orientation distribution and mechanical properties were inter-related for injection-moulded short-glass fibre-reinforced polypropylene and polyamide and long-glass fibre-reinforced polyamide. The properties of the reinforced grades were also contrasted with those of the base polymer. A rectangular mould with triple pin-edge gates on the same side to facilitate a single melt flow-front or double flow-front advancing adjacently, was employed. Mouldings were evaluated for fibre orientation distribution, and tensile, dynamic mechanical and fracture properties. The relative magnitudes of the shell and the core fibre-orientation persuasions depended on the melt and the mould temperatures, and the injection ram speed. An increase in Go, Kc, ultimate tensile strength (UTS) and elastic moduli values and a decrease in tan ~5 values were observed with fibre reinforcement. The properties showed marked sensitivity to the position of the specimens in the moulding because of the associated variations in the fibre orientation. The prediction of UTS based on arute of mixture relationship for strength and the Halpin-Tsai equations for elastic moduli had limited success. The predictions were improved by employing measured data representative of regions of high fibre orientation, e.g. knit-line. The estimation was weakened in the material systems containing significant production-induced voids. An inference of the fracture toughness values was that the composites contained flows of the order of 0.2 mm in size, which is conceivable, either as voids or as defects introduced in the machining of the specimens.

Micromechanical parameters from macromechanical measurements on glass reinforced polypropylene

“…In the very centre of the melt the rate of shear is low and the transverse fibre alignment present at the gate is retained. These general features are apparent in studies of fibre orientation distribution found in the (1,3,6,17,23) literature. Figure 7 shows the values of < cos 2 (φ) > obtained from a series of micrographs taken across the thickness of injection moulded tensile bars containing 30% w/w glass fibres.…”

“…The properties of thermoplastic composites result from a combination of the fibre and matrix properties and the ability to transfer stresses across the fibre-matrix interface. Variables such as the fibre content, aspect ratio, strength, orientation and the interfacial strength are of prime importance to the final balance of properties exhibited by injection moulded thermoplastic composites (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Fibre strength may be reduced significantly after fibre formation by damage caused during both the fibre and composite production processes (16,17).…”

A Novel &ldquo;Fiber-Spacing&rdquo; Model of Tensile Modulus Enhancement by Shortening Fibers to Sub-Millimeter in an Injection-Molded Glass Fiber Reinforced Polymer Bulk Molding Compound (GFRP-BMC)

“…(18) is claimed to be valid for (0.42 < © o < 0.54) using the molding conditions given in the experimental section. Since it is reported at higher injection speeds, melt and molding temperatures polymer composites exhibit a thicker core with a more square flow front, 21) this analysis was beyond the scope of this study since it would require specimen fabrication at many molding conditions.…”

Characterization of Velocity Profile of Highly-Filled GFRP-BMC through Rectangular Duct-Shaped Specimen during Injection Molding from SEM Fiber Orientation Mapping

“…9) In short and long glass fiber polyamide injection-molded rectangular cuboid-shaped plaques, Akay and Barkley correlated longitudinal and transverse fiber flow orientations with tensile and dynamic mechanical and fracture properties. 10) Taking cross sections parallel to the flow line at various depths, they found a 3-layer [skin-core-skin] flow pattern appearing to transition into 5-layers with depth. Core thickness also increased with injection speed, melt and mold temperatures.…”

“…Core thickness also increased with injection speed, melt and mold temperatures. 10) A typical model used for injection molding simulation is by Hele-Shaw 12) that gives simplified equations for nonisothermal, non-Newtonian, inelastic flows in a thin cavity. Normally the following assumptions are proposed by HeleShaw prior to mathematical modeling: 1) cavity thickness is much less than width; 2) incompressible polymer melt behaves as viscous with no elastics where viscosity is sheardominated; 3) viscous force is much higher than inertia or gravitational forces; 4) in thickness direction, velocity and pressure gradient are zero; 5) no fountain flow exits at flow front; 6) velocity is zero at mold walls; and 7) heat transfer between mold and paste is dominated by conduction then heat transfers into the core by convection.…”