Thermal Drift Mechanics in Multi Cavity Hot Runner Manifolds
Engineered cold pitch offsets prevent valve tip shear, flash, and nozzle leakage in multi cavity manifolds operating at elevated molding temperatures.
Linear Dimensional Growth
Meaning
Post-moulding expansion occurs when molded polymeric components absorb ambient moisture or undergo secondary crystallization over time. Measuring linear dimensional growth quantifies the physical size increase of a plastic part after ejection from the mold cavity. The metric governs final part fit and assembly clearances across the post-processing lifespan of semi-crystalline and hygroscopic resins.
Calculations apply strictly to post-ejection structural changes and exclude mold shrinkage that occurs while the polymer remains inside the cavity tooling.
Material Mechanisms
Polyamides and other hydrophilic engineering plastics absorb atmospheric water molecules, which insert between polymer chains and expand the intermolecular spacing. This internal swelling induces measurable linear dimensional growth while simultaneously reducing glass transition temperature and modifying mechanical stiffness. In semi-crystalline polymers like polypropylene, ongoing post-moulding crystallization causes subtle dimensional shifts that depend on mold temperature and cooling rates.
Moulders must differentiate between initial mold shrinkage and subsequent dimensional swelling to achieve tight assembly tolerances.
Environmental Impact
Storage relative humidity and ambient operating temperatures dictate the rate and ultimate magnitude of post-moulding swelling. Uncontrolled linear dimensional growth causes press-fit assemblies to jam, warps planar surfaces and alters clearance gaps in precision automotive or electronic housings. Conditioning parts in controlled humidity chambers accelerates moisture equilibrium before final quality inspection.
Failure to account for environmental absorption leads to field failures when dry-as-moulded components expand in humid operating environments.
Tolerance Specification
Engineering drawings must state whether specified dimensions apply immediately after ejection or after full environmental conditioning. Tracking linear dimensional growth establishes realistic quality thresholds for hygroscopic polymers.