Meaning
Composite materials containing discrete reinforcing fibers distributed in a thermoplastic matrix provide increased stiffness and strength compared to unfilled resins. These short fiber reinforced polymers typically incorporate glass or carbon fibers with lengths of less than one millimeter. This reinforcement increases the tensile modulus and heat deflection temperature of the compound.
The resulting material is widely used in automotive and industrial components that require high structural load capacity.
Fiber Orientation
The distribution and alignment of the fibers within the moulded part determine the directional properties of the material. In short fiber reinforced polymers, the flow of the melt through the gate and cavity creates a complex orientation pattern. Fibers tend to align parallel to the flow direction near the part walls but perpendicular to it in the core.
This anisotropy results in different shrinkage rates and mechanical strengths along different axes. If the orientation is not controlled, the part can suffer from severe warpage upon cooling due to these differential stresses. This non-uniform behavior requires careful gate placement to ensure the fibers align in the direction of the expected mechanical loads.
Processing Effect
Shearing forces during compounding and injection moulding can break the fibers, reducing their effective reinforcing capability. High shear rates within the screw and nozzle reduce the average length of the fiber in short fiber reinforced polymers. This length reduction decreases the efficiency of stress transfer from the polymer matrix to the fibers.
Moulders must optimize screw speed and back pressure to minimize fiber breakage during plastication.
Defect Prevention
Injection moulding of filled materials introduces challenges such as increased tool wear and poor surface finish. To handle short fiber reinforced polymers, the injection mould must utilize hardened steel cavities and gates to resist the abrasive nature of the fibers. The part design must also avoid sharp corners that can cause stress concentration or uneven fiber distribution.
Proper processing is essential to prevent part warpage and weld line weakness.
