The potential advantages and benefits of the spiral mandrel distribution system have basicallyrnmade it the standard design for dies used to produce blown or tubular film. Figure 1 is an imagernof a spiral mandrel die with the body and outer die lip cut away to show the spiral mandrel andrninner die lip. The polymer melt normallyrnenters the die from the bottom center and isrndistributed to the spirals via channels oftenrnreferred to as ports. The spirals channels arernsimply helical grooves cut into the mandrelrnthat direct the polymer around therncircumference of the die. A small gap isrnformed between the internal diameter of thernbody and the external diameter of thernmandrel that allows the polymer to leak outrnof the spirals. The spiral channel depthrnnormally reduces along the channel lengthrnforcing more and more polymer out into thernannular gap and ultimately producing anrnannular flow in the axial direction. After thernspiral distribution section, the materialrngenerally flows through a relaxationrnchamber and then through the die lips andrnexits the die.rnHowever, despite the popularity of this typernof die, it can suffer from some particular andrneven peculiar problems that are very difficultrnto diagnose. For example, if the geometry ofrnthe spiral distribution system is not designed properly in relation to the polymer flow rate andrnpolymer properties, the die may not distribute the polymer adequately resulting in die poorrnperformance and ultimately poor product quality. In an extreme case, the design can result in arnback flow that essential creates a stagnation point in the flow field. The problem is that thisrnstagnation point cannot be seen and the only indication is a visual defect that appears as faintrnlines in the product that equal in number to the number of spiral channels. With prolongedrnrunning, the stagnation area may start to create a build-up of degraded material that makes therndefect appear even worse.rnThankfully, the recent advance in computing power has made 3D Finite Element Method (FEM)rnflow analysis a very viable and practical tool for analyzing and designing spiral mandrel dies.
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