Deriving Ogden Constitutive Parameters from Biaxial Stretch Testing Data
Deriving Ogden constitutive parameters pairs non-linear biaxial stretch data with Drucker stability constraints to ensure reliable finite element simulations.
Stretch Blow Molding
Meaning
Biaxial orientation of a heated thermoplastic preform takes place via mechanical rod extension followed by high-pressure air inflation inside a mold cavity. Executing stretch blow molding produces lightweight bottles and hollow containers with enhanced mechanical strength, clarity, and gas barrier properties. The manufacturing process applies primarily to polyethylene terephthalate and polypropylene polymers processed within their rubbery glass transition window, remaining unsuited for non-orientable or highly crystalline thermoset resins.
Process Kinematics
An internal stretch rod pushes axially against the preform base while high-pressure compressed air expands the polymer radially against cavity walls. Performing stretch blow molding requires precise synchronization between mechanical rod movement and multi-stage air injection pressures. Simultaneous axial stretching and radial blowing align polymer chains in orthogonal directions, inducing strain hardening that halts local necking and produces uniform container wall thickness.
Improper stretch rod timing causes preform puncture or uneven material distribution along container bases.
Preform Temperature
Conditioning preforms within a tight thermal window between glass transition and melting temperatures ensures proper chain orientation. Cold preforms resist stretch and rupture under blowing pressures, while overheated preforms haze and lose mechanical strength.
Container Quality
Consistent wall thickness distribution determines top-load strength and carbonation retention in beverage bottles. Process drift in preform reheating yields uneven material allocation, increasing scrap rates during high-speed production runs.