Meaning
Positioned between melt plastification and final bottle trim, hollow article fabrication converts continuous molten tubes into hollow containers within clamped mold halves. Industrial container production using extrusion blow molding forms a downward parison that is captured by tool halves and expanded with air against chilled aluminum or steel cavities. The process governs bottle geometry, handle formation, and wall thickness distribution for high-density polyethylene and polypropylene containers, reaching its limit when part complexity requires internal ribs or strict wall tolerances achievable only through injection molding.
Parison Behaviour
Gravitational forces pull the molten tube downward during cycle pauses, causing wall thinning near the top of the container. Equipment running extrusion blow molding utilizes hydraulic program rings to vary the die gap during extrusion, placing more material in high-stretch zones like corners and handles. Resins with low melt strength sag excessively, producing thin top walls and inconsistent neck dimensions.
Cooling Constraint
Heat transfer from the blown polymer to the mold wall controls overall cycle duration. Because polyolefins exhibit low thermal conductivity, thick pinch-off sections remain hot longer than thin sidewalls, forcing extended mold contact times to prevent post-molding distortion. Inadequate cooling leads to warpage and sink marks along handle seams in finished bottles.
Scrap Economics
Flashing generated at the head and tail pinch-offs represents up to thirty percent of the total shot weight. Reclaiming this flash as regrind reduces raw material expenditure, but excessive re-processing degrades the polymer chain length and broadens the melt flow index. Continuous re-introduction of regrind requires careful blending with virgin resin to maintain consistent parison swell and prevent parison drop speed variation.
Uncontrolled regrind usage introduces thermal history variations that cause wall thickness instability across long production runs.
