Correlations between molecular weight, mechanical properties and morphology of micro-injection molded polyoxymethylene (POM)

Polyoxymethylene (POM) is a fast crystallizing polymer, whose structure is highly dependent on the processing conditions and is showing a broad range of mechanical properties. Three POM materials with different molecular weights were selected and a design of experiments (DoE) was performed varying melt temperature, mold temperature, and injection speed. In combination with increased viscosity at higher molecular weights, the flow resistance and shear stresses will also increase at a certain injection speed and geometric conditions. Thereby, the range of morphological differences depends not only on the process boundary conditions but also on the rheological conditions. This aspect is particularly relevant for micro-injection molded parts, as the acting cooling and shearing rates are much higher than in standard injection molding. A specially designed tensile rod with a radially symmetric cross section was utilized for the experiments, which offers advantages in terms of a symmetric flow and cooling behavior. The morphology was studied with thin sections from the center of the sample. Differential Scanning Calorimetry (DSC) was used to study the crystallinity of the samples and the mechanical properties were determined by a tensile test using an adopted optical extensometer. The mechanical properties of low molecular weight POM are only to a minor extent affected by the process variations. However, higher molecular weight POM is greatly affected in terms of its skin layer formation and improved mechanical properties favored by a low injection velocity.