Meaning
Mechanical stress induced by high velocity gradient flow breaks high molecular weight polymer chains during melt processing. In injection moulding hot runners and high-compression screws, polymer shear degradation reduces average molecular mass when mechanical forces exceed covalent bond energy along the polymer backbone. This degradation mechanism governs processing limits for shear-sensitive resins like ultra-high molecular weight polyethylene and polycarbonate.
The phenomenon stops when melt temperatures rise sufficiently to lower shear stress below critical chain scission thresholds or when screw speeds are reduced.
Shear Rate
Narrow gate geometry and thin-wall mould cavities subject polymer melts to extreme shear rates exceeding 50000 reciprocal seconds. Excessive stress induces polymer shear degradation near runner walls, severing high molecular weight polymer chains. Reducing injection speed lowers local shear stress, protecting delicate polymer molecular weight distributions.
Viscosity Loss
Chain scission alters fundamental rheological behavior by shortening average macromolecular chain length. Severe polymer shear degradation causes drastic drops in melt viscosity, leading to flash at parting lines and uneven cavity filling. Recycled resins undergo cumulative viscosity loss over repeated processing cycles due to ongoing mechanical damage.
Mechanical Failure
Moulded components manufactured from shear-damaged resin exhibit reduced impact strength and premature environmental stress cracking. Detecting polymer shear degradation prior to part distribution requires gel permeation chromatography to measure molecular weight distribution shifts. Lowering back pressure during plastication preserves polymer impact performance in demanding structural applications.
