Meaning
Analytical measurements describe the variation in density across different regions or polymer fractions within a plastic part or resin batch. In semi-crystalline polymers, polymer density distribution indicates the local level of crystallinity resulting from varying cooling rates during processing. This distribution directly influences the mechanical and optical properties of the finished product.
Density Variation
Thicker sections of a moulded part cool more slowly than thinner walls, allowing more time for polymer chains to organize into crystalline regions. This difference in cooling rate creates a wide polymer density distribution across the part’s cross-section. Differential shrinkage occurs as a result, which often leads to warpage or sink marks if the moulding parameters are not adjusted.
Mechanical Property
High-density regions provide stiffness and chemical resistance, while lower-density regions offer impact strength and flexibility. If the polymer density distribution is too broad, the part may suffer from localized weak spots that are prone to premature failure under load. Virgin resins are formulated to produce a predictable distribution, whereas regrind introduces mixed molecular weights that broaden the density profile and decrease mechanical reliability.
Process Control
Optimizing the pack pressure and cooling time helps narrow this distribution by ensuring more uniform solidification across different wall thicknesses. When these variables are held constant, moulders can maintain tight dimensional tolerances across long production runs.
