Meaning
Physical reduction in the length of reinforcing fibers occurs during the compounding and injection molding of thermoplastic composites. During high-shear processing, glass fiber attrition limits the reinforcement potential by breaking long fibers into shorter fragments. It takes place as the polymer melt travels through the extruder screws, runners, and gates.
Molders must minimize this damage to retain the mechanical properties specified on the resin datasheet. This mechanical degradation is restricted to fiber-reinforced systems and does not affect unfilled polymers.
Breakage Mechanism
Fibers fracture due to intense shear forces and collision with other fibers or metal surfaces. As the polymer pellets melt, the viscous resistance transmits stress to the fiber bundles. Once the stress exceeds the tensile strength of the glass, the fibers break.
This reduction is irreversible and worsens with increased screw speed.
Processing Factor
Molding parameters control fiber damage. High backpressures and restrictive gate designs increase the shear, leading to shorter fiber lengths. Molders select wide gates and low screw speeds to preserve fiber integrity.
Property Degradation
When fibers fall below the critical length required for stress transfer, the composite loses stiffness and impact strength. Finished parts molded under high shear will exhibit lower tensile strength than those produced under optimized conditions. This disparity requires molders to test part performance rather than relying solely on datasheet values.
Preserving fiber length is essential for load-bearing applications.
