Meaning
Chemical breakdown of low molecular weight ring-shaped polymer molecules during high temperature processing leads to the release of volatile compounds and surface deposit formation. The process of cyclic oligomer degradation occurs predominantly in polyesters and polyamides when the material is subjected to excessive thermal energy or long residence times in the extruder. These breakdown products often migrate to the surface of the melt.
Defect Generation
Condensation of the liberated cyclic structures on the cool metal surfaces of the mould tool creates a sticky residue known as plate-out. This residue causes cyclic oligomer degradation to directly ruin the surface finish of the moulded parts, which produces cosmetic blemishes or surface pitting. Frequent tool cleaning is required to prevent these defects from repeating across a production run.
Mechanical Influence
Polymer chains lose their cohesive strength when these ring structures break down and plasticize the matrix, or when they initiate wider chain scission. This reduction in molecular weight alters the viscosity of the melt, which causes inconsistent cavity filling and fluctuations in part weight. Extruded films and thin-walled parts are especially vulnerable to the pinholes and tear lines caused by these localized viscosity changes.
Process Prevention
Keeping the melt temperature within the resin manufacturer’s recommended limits reduces the rate of this thermal breakdown. Dry nitrogen purging in the hopper and vent-port vacuum extraction help to remove volatile cyclic species before they can degrade or deposit on the tooling.
