Weld-lines, also referred to as spider-lines or knit-lines, are regions in the wall of an extruded plastic pipe, running the whole length of the pipe, at which there is a change in dimension and a possible reduction in mechanical properties. Weld-lines result from the flow of the polymer melt around the supports (spiders) that hold the metal core to the outer die assembly in an in-line annular die. This work examines the fatigue life and the tensile strength of weld-lines created by a conventionally shaped 'torpedo" spider and four modified spider shapes. It is demonstrated that the shape of the spiders has a dramatic effect on the fatigue life of weld-lines while having a minimal effect on the tensile strength in static loading. The experimental results show that the fatigue life of the weld-line produced by one of the four new shaped spiders is significantly higher than that of a conventionally shaped spider. Numerical simulations of the flow field around this new and conventional spiders were carried out using the package FIDW@ based on the finite element method (FEW. The simulation results provide a good explanation for the experimental observations and show that the newly designed shape of spider can give rise to a higher relative fatigue life of the weld-line when compared with that of the bulk material than does the conventional spider.

An understanding of flow behaviour of polymer melts through a slit die is extremely important for optimizing die design and, consequently, for die performance in processing polymer sheets and films. In view of the complex nature and the physical properties of polymer melts as well as of die geometries, such as coat-hanger dies, no simple mathematical formulae can be used to compute the flow regimes within dies. This paper illustrates the development of a three-dimensional (3-D) computer model of an example of a coat-hanger die design using the computational fluid dynamics package, FIDAP, based on the finite element method. A difference of only 3.7% was found when comparing the velocity distribution at the die exit obtained from the 3-D simulation with that calculated using a twodimensional analytical design procedure, indicating that full 3-D analysis seems to be unnecessary. However it has been shown that unwanted flow phenomena and production problems can be ameliorated by means of visualization and the detailed information obtained from computer simulations. Comparative simulation results with polymers of different rheological properties in the same die are also described. The comprehensive analyses provide a means of interpretation for flow behavior, which allows modification of the die geometry for optimal design.

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The present study describes the results of a pilot study to carry out the initial development and testing of a new class of high-pressure all thermoplastic pipe, using recycled PET as reinforcement. A prototype pipe design has been manufactured and initial qualification testing carried out demonstrating a new high added value product application for recycled materials.