3-D Coupling Numerical Simulation On Fluid-Structure Interaction In Slit Die For Wide Casting Film

With the widespread applications of wide casting film, to develop high-performance andmore wide plastic film has become one of the trends. With the advantages of high productionefficiency, good product thickness uniformity and stable performance, casting method is oneof the widely used methods for producing casting film of thermoplastics at present. Ensuringexit flow rate uniformity is one of the key factors in designing of slit die. The melt pressurewill cause die deformation during extrusion, and it will directly affect thickness uniformity ofproduct. This phenomenon is ubiquity in slit die. The flow channel designed by mathematicalanalysis method and numerical simulation without regard of die deformation cannot givegood exit flow rate uniformity due to die deformation. The thickness uniformity of castingfilm is generally adjusted with choked block or flexible die lip in extrusion. Adjusting exitflow rate uniformity with choked block easily causes melt staged. The gap of slit will affectflow resistance therefore adjusting flexible die will change extrusion pressure. In order to betterreveal the melt flow characteristic in slit die and to improve the extrusion performance,3-Dnumerical simulation of fluid-structure interaction must be coupled.3-D coupling numericalsimulation on fluid-structure interaction in slit die can reveal flow characteristics andinfluence of die deformation on melt flow. In order to guide the die design, it’s important tostudy the effects of die’s main structure parameters on melt flow characteristic and exit flowrate uniformity.In this paper,3D melt flow analysis coupled with3D die deformation is used to revealthe melt flow characteristics in slit die with POLYFLOW software. The results show that themelt flow in slit die can be considered as the combination of pressure flow along the widthdirection of die in manifold and along the extrusion direction in slit. The die deformationoccurs mainly on the flexible side of the die. Slit deformation in the thickness direction showsapproximately linear increase along extrusion direction, and nonlinear variation along widthdirection. The exit flow rate uniformity is codetermined by the uniformity of damping zoneoutlet pressure along the die’s width direction and slit thickness uniformity after thedeformation.On this basis, the influence of extrusion output, die thickness and gap of slit on melt flowcharacteristic and exit flow uniformity were studied. With the increasement of extrusionoutput, melt exit flow rate uniformity keeps invariant without regard to deformation.However，when considering the die deformation, the variation of the outlet pressure along the die’s width direction of damping zone is very small, the slit thickness uniformity afterdeformation decreases, and consequently the exit flow rate uniformity decreases. Whenadopting die with flexible die lip, the even increasement of thickness has a small effect on theuniformity of damping zone outlet pressure along the die’s width direction. Meanwhile, theslit thickness uniformity after deformation increases, and the exit flow rate uniformityincreases. When using the contour die, the slit thickness uniformity increases and the exitflow rate uniformity increases. In addition, increasing the die thickness deviation along thedie’s width direction is more effective to improve exit flow rate uniformity. Geometricparameters of flow channel is one of the most important factors influencing exit flow rateuniformity. With the decreasement of Hs, the uniformity of damping zone outlet pressurealong the die’s width direction improves obviously, and slit thickness uniformity afterdeformation is improved.Consequently, the exit flow rate uniformity increases.