Meaning
Phenomenon where the measured density of a polyethylene resin increases over time due to continued secondary crystallization. Occurrences of polyethylene density drift shift the physical properties of a molded part long after it has reached room temperature. This change is most pronounced in high-density grades where molecular mobility remains high enough for gradual chain rearrangement.
Crystalline Growth
Rapid cooling during the extrusion or injection process traps the polymer chains in a less than optimal arrangement. Polyethylene density drift results as these chains slowly settle into a more compact crystalline lattice. This densification increases the stiffness and tensile strength while reducing the impact resistance.
Dimensional Variance
Shrinkage continues as the material becomes more dense, potentially moving the part out of the specified tolerance range. Monitoring polyethylene density drift is vital for precision components that must maintain a specific fit within an assembly. Standard testing protocols often require a period of conditioning to ensure the density has stabilized before final measurements are recorded.
Environmental Influence
Storage temperature accelerates the rate at which the molecular structure reorganizes. Elevated heat provides the energy needed for polyethylene density drift to occur more rapidly. Controlled annealing can be used to force this stabilization early in the production sequence.
