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针对黏弹性包围引起的复合共挤制品层厚均匀性控制的技术难题,通过数值模拟,研究了黏弹性流变性能参数对黏弹性包围的影响规律和机制。研究结果表明,黏弹性包围是由多相分层流动的第二法向应力差驱动的二次流动诱发,主要取决于成型流动过程中二次流动的方向与强度。熔体二次流动的方向与第二法向应力差的正负号有关,而熔体二次流动强度则与第二法向应力差大小成正比。黏弹性包围随着熔体松弛时间的增加而增大。消除黏弹性包围的理论前提是消除其二次流动,而通过使其流动的第二法向应力差趋于零方可消除其二次流动。第二法向应力差趋于零的前提条件是使无滑移黏着共挤多相分层剪切流动转化为气垫非黏着完全滑移共挤多相分层柱塞流动,而气辅共挤成型工艺的气垫壁面滑移是实现这一转变的有效技术。
Aiming at the technical problems of layer thickness uniformity control of composite coextrusion products caused by viscoelastic surrounding, the influence law and mechanism of viscoelastic rheological parameters on viscoelasticity surrounding are studied through numerical simulation. The results show that the viscoelasticity surrounding is induced by the second normal stress difference driven multiphase stratified flow and mainly depends on the direction and intensity of secondary flow in the forming flow. The direction of melt secondary flow is related to the sign of the second normal stress difference, whereas the melt secondary flow strength is proportional to the second normal stress difference. Viscoelastic envelopes increase with increasing melt relaxation time. The theoretical precondition to eliminate the viscoelastic encirclement is to eliminate its secondary flow, and eliminate its secondary flow by making the second normal stress difference of the flow zero to zero. The second normal stress difference tends to zero prerequisite is to make non-slip co-extruded multi-phase shear flow laminar flow into a non-slip air-cushion co-extrusion fully co-extruded multiphase layered plunger flow, and gas co-extrusion Air-wall slip of the molding process is an effective technique to achieve this change.