Viscoplastic Strain Rate Sensitivity Optimization for Short Glass Reinforced Components
Dynamic viscoplastic yield in short glass components depends on screw-retained fiber length and gate-driven orientation tensors under high strain rates.
PM Inserts
Meaning
Threaded metal fasteners installed into pre-molded plastic bosses provide durable, high-strength connection points for subsequent product assembly. Post-mould inserts, often abbreviated as pm inserts, avoid the long cycle times and tooling complexity associated with moulding-in metal components. These fasteners are widely used in automotive dashboards and electronic enclosures, terminating where the boss wall fractures or the insert strips under torque.
Installation Methods
High-frequency vibration from ultrasonic horns or direct heat application softens the plastic surrounding the pre-molded hole, allowing the insert to be pressed in. As the polymer cools, it solidifies around the metal knurls and grooves, locking the insert in place. Press-in and expansion inserts provide alternative cold-installation methods, though they generally offer lower retention values than thermal or ultrasonic options.
Stress Generation
Insertion forces and material displacement generate hoop stresses within the surrounding plastic boss. If the boss wall is too thin or the insert is oversized, these stresses can exceed the yield strength of the polymer, causing the boss to crack over time due to stress relaxation or environmental stress cracking. Careful calculation of the boss diameter is required to balance the insert pull-out force against the risk of boss failure.
Designers must also ensure that the weld lines do not run through the boss, as this would create a weak plane.
Material Compatibility
Amorphous polymers like polycarbonate are highly sensitive to residual stresses and are more prone to cracking around pm inserts. High-flow resins are preferred for the boss structure to ensure that the polymer fills the grooves during installation.