Even modern grades of rigid PVC are less heat stable and have a smaller “processing window” than many thermoplastic materials. For this reason, most grades of rigid PVC require a special machine application to ensure optimal processing conditions. Negri Bossi, the Italian supplier of injection moulding machinery, has been developed a PVC technology to provide consistent and cost-effective production of visually and dimensionally perfect parts. Rather than offering a slightly modified standard machine, the machines producer has developed a more bespoke solution that provides the low shear, high torque, greater temperature regulation and resistance to chemical attack needed.
Negri Bossi’s high level of modularity allows perfect matching of injection unit to clamp size. This is most important for PVC, where residence times are particularly critical. At the same time, typical items such as drainage products require a large platen area to accommodate multiple moving cores. The ST servo-hydraulic technology is particularly suited to the long static phases associated with the moulding of PVC components. This provides high performance whilst maintaining low energy consumption. These benefits are also available on the larger Vector models when specifying the smart energy solution.
Negri Bossi has developed specific injection unit solutions for rigid PVC. This includes low compression ratio screw profiles and the option of monolithic screw design to reduce material “hang-ups”. Barrel cooling fans and mica bands aid the removal of excess shear heat. On screw sizes of 80 mm and above internal screw cooling is an option. PVC ready models offer a high torque hydraulic screw drive, or a reduction gearbox option for the electric screw drive. This specification provides the torque and screw speed stability required for optimum material processing.
To demonstrate the degree of thought that goes into their PVC optimised machines, anti-corrosive components such as chrome plated tie-bars are also specified, making sure that mechanical integrity and cosmetic appearance are preserved in the longer term.