Meaning
Radiant heating and plug-assisted pneumatic force shape heated semi-crystalline polymer sheets into thin-walled packaging containers inside cooled mold cavities. Performing polypropylene thermoforming requires exact thermal control due to the sharp melting point and narrow processing window of semi-crystalline polyolefins. The scope of this converting process includes forming solid phase and melt phase sheets into food trays and industrial packaging.
This process stops once the formed sheet cools below the crystallization temperature and moves to the mechanical trimming press.
Sag Control
Heating semi-crystalline sheet above its melting point causes rapid loss of melt strength, resulting in central sheet drop prior to forming. Precise oven zone programming prevents excessive center sag during polypropylene thermoforming cycles. High melt strength resin grades modified with long-chain branching resist sagging, permitting wider processing windows on high-speed thermoforming lines.
Infrared temperature sensors measure sheet surface temperatures to trigger sheet transfer before gravity distorts material thickness distribution.
Plug Assistance
Syntactic foam or aluminum plug assists enter heated sheet cavities before pneumatic pressure forces polymer against mold walls. Mechanical prestretching distributes polymer material uniformly into deep-draw container corners, preventing localized wall thinning. Mold temperature control maintains consistent part clarity and minimizes post-molding shrinkage in rigid polyolefin packaging.
Crystallization Shrinkage
Post-mold shrinkage in semi-crystalline containers continues for hours after trimming as polymer chains organize into crystalline structures. Non-uniform mold cooling induces differential shrinkage, warping flat container flanges and disrupting lid sealing surfaces. Nucleating agents added during resin compounding accelerate crystallization, reducing cycle times and improving part stiffness.
Variable regrind ratio in thermoforming sheet alters shrinkage rates, forcing operators to adjust trim die clearances.
