The Numerical Simulation On Multilayer Coextrusion Blow Molding Mechanism

Multilayer coextrusion blow molding (shortened as MCBM) is is an advanced polymer molding technique by which high performance,low cost polymer multilayer composite container and goods are produced and the future important problems of engineering material will be resolved. Up to the present, few of researches on MCBM technique have been reported at home and abroad. Based on the inconsistency between the advance and research shortage of MCBM, The research on the theory and numerical simulation on polymer MCBM process are implemented in this paper, which yield the following main achievements:Based on process characteristics of MCBM and the theories of rheology and hydrokinetics etc., under reasonable assumption, the full 3-D steady isothermal viscoelastic theoretical model was established to describe polymer MCBM process.On the basis of the mixed finite element stable discrete techniques of Galerkin method, Penalty function method, EVSS method and Mini-Element method etc., the corresponding stable finite element numerical models with fast convergence were established for high non-linearity 3D viscoelastic theoretical models of MCBM. Finally, with above numerical model systems, direct finite element numerical simulations of MCBM process were implemented.The influencing rules of the viscoelastic property parameters and process parameters on the die swell of parison formation process were systematically studied by means of direct finite element numerical simulation, and the mechanism of die swell was disclosed by theoretical analysis. The research results showed that the die swell of parison mainly depent on the intensity and direction of the melt secondary flow. For the double layer cannular parison mold, when the direction of the out-normal of the face that be researched is same to the positive direction of coordinate axis, if the second normal stress difference is negative, the direction of the melt secondary flow is outward. And if the second normal stress difference is positive, the direction of the melt secondary flow is inward; when the direction of the out- normal of the face that be researched is opposite to the positive direction of coordinate axis, if the second normal stress difference is negative, the direction of the melt secondary flow is inward, the direction of the melt secondary flow is outward. The die swell of core melt, skin melt mainly depend on the difference of melt secondary flow effect between internal surface and outside surface. If the outward secondary flow speed of outside surface is faster than the speed of internal surface, the die swell of parison was caused, on the opposite, the die contraction of parison was caused.Based on the obove result of the die swell of parison formation process for MCBM process, the method of establishing inflation process of no die swell parison was put forward, and the influencing rules of the die swell of parison on the inflation process were systematically studied by means of a Contrastive discourse analysis of the inflation process without die swell of parison formation and the inflation process with die swell of parison formation, the theoretical method to evulate the influencing degree of the die swell of parison on the inflation process was established, the relation on the wall thickness and the radius influence-factor to describe the influence of the die swell of parison on the wall thickness and shape of multilayer coextrusion blow molding good and the die swell of parison formation process. Finally the scientific theoretical basis on the theoretical design method of MCBM technology and mold wan estabilished in which parison formation process and parison inflation process were integrated together.