Quantifying Shear Degradation and Fiber Breakdown in Mold Tooling
Quantifying shear rates and gate geometry limits prevents fiber length attrition, tool surface erosion, and mechanical strength loss in molded composite parts.
Fiber Breakdown
Meaning
Damage or degradation of the structural integrity of glass or carbon filaments within a thermoplastic matrix during processing. Such fiber breakdown occurs primarily during the high shear stages of injection molding, such as when the melt passes through a narrow gate. This reduction in fiber length diminishes the mechanical properties of the final molded part, including its tensile strength and impact resistance.
The phenomenon is most severe when using high injection speeds or complex mold geometries with sharp turns.
Length Reduction
Fragmentation of the fibers happens as they collide with each other and the walls of the processing equipment. While longer fibers provide better reinforcement, the mechanical stresses of the screw and barrel inevitably shorten them. When fiber breakdown is excessive, the resulting enclosure may fail to meet the structural requirements for a ruggedized device.
Monitoring the average fiber length in the finished part is a standard quality control step for high performance composites.
Process Optimization
Adjustment of molding parameters can mitigate the severity of the damage to the reinforcement. Lowering the screw speed and increasing the melt temperature reduces the viscosity and the associated shear stress. Because fiber breakdown cannot be entirely eliminated, engineers must design parts with a safety margin that accounts for this loss in strength.
Material Performance
Comparison of the initial fiber length in the pellets to the length in the molded part reveals the extent of the degradation.