塑料技術(shù)手冊(cè):Volume 1:吹塑成型:Blow molding 節(jié)選

　　《塑料技術(shù)手冊(cè) 吹塑成型（1 影印版）》：　　Parison Swell The die flow influences parison behavior through molecular orientation， which manifests itself at the die exit as extrudate swell.Flow at the inlet of a die， where the streamlines are converging rapidly， involves a high rate of stretching in the flow direction.This produces a high degree of molecular orientation， and if the melt is permitted to exit immediately， as for flow through an orifice plate， there would be a very high degree of swell.However， if a long straight section，for example a capillary or straight annular die， follows the entrance， then molecular relaxation processes will lead to a disappearance of the orientation generated at the entrance， and as the die is made longer， the degree of swell is reduced.At the same time， however， the shearing in the die produces some axial orientation， and for a very long die， there is still significant swell resulting from this shear flow.If the die has an expansion or contraction section， this will introduce stretching， and this willintroduce orientation whose direction depends on the details of the die design.For these reasons， parison swellis very sensitive to die design.　　The melt leaving the die may exhibit sharkskin or melt fracture， which are irregularities in the surface of the extrudate that can affect the surface finish of an extrusion blow—molded container.This effect occurs above a critical shear stress in the die and is often the factor that limits the speed of an extrusion process.Extrudate distortion is most severe in the case of narrow molecular weight distribution （MWD） （chapter 1） high—viscosity plastics.Increasing the temperature or reducing the extrusion rate can sometimes eliminate it， but either of these actions will increase the cycle time.The use of an internal heater in the mandrelis a method to reduce the tendency toward melt fracture on the inside of the parison.The detailed origins of this phenomenon are not fully understood， but the shape and material of construction of the die and the formulation of the plastic are known to be contributing factors.　　……