Meaning
Low molecular weight chain fragments consisting of a small, finite number of repeating monomer units are the common byproducts of incomplete polymerization or thermal degradation during extrusion. These polymer oligomers possess distinct physical properties compared to their high molecular weight counterparts. They often remain trapped within the amorphous regions of the resin matrix.
When exposed to heat, they can vaporize or migrate to the surface of the finished part.
Material Migration
Migration of these short chains to the polymer surface causes blooming, which appears as a white, powdery residue on the molded component. Because polymer oligomers have high mobility in the resin matrix, they escape easily from the bulk material over time. This migration is particularly problematic in food packaging applications where the substances can contaminate the food.
The leaching behavior limits the use of certain recycled plastics.
Rheological Effect
Viscosity drops when a high concentration of low molecular weight chains is present in the polymer melt. While these polymer oligomers act as internal lubricants that improve flowability during injection molding, they also reduce the mechanical strength of the solidified polymer. This reduction in viscosity can lead to flashing at the parting line of the tool.
Moulding Complication
Volatilization of these light compounds inside the hot barrel leads to the accumulation of plate-out on the mold cavity. This buildup of polymer oligomers blocks the vents and causes cosmetic defects on the molded parts. Frequent cleaning of the tool is required to prevent these defects.
