Plastics Technology

JUL 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Gear Pump & Screen Changer for Rigid PVC PSI-Polymer Systems Inc., Conover, N.C., introduced its first-ever screen changer and gear pump designed specifically for rigid PVC extrusion at NPE2018. Target areas for the two new products include sheet and profiles. Generally, the devices can be used with PVC formulations that can tolerate melt-temperature variations of 10-15° F. Screen changers are generally not applied to rigid PVC processes because of flow transitions in the sealing areas exposed to the melt stream. As resin travels over these transi- tions and steps, shear heat is introduced, which can trigger localized burning of the material. But PSI has developed an Expansion Plate Screen Changer (ESC) that reportedly solves "the chemistry issues relative to heat, flow and thermal conductivity" for PVC processes tolerant of 10-15° F swings in melt temperature. The screen changer requires a momentary shutdown for screen changes. This eliminates the need to disassemble the die, which on short runs results in high scrap rates. To change screens on the ESC, the PLC control electrically heats the expansion spacer bars, which separate the body halves above and below the slide plate between the body halves. The spacers are far from the bore to avoid raising the melt temperature. The expansion of the spacers pushes the bodies apart, freeing the slide plate to move unimpeded. Once the new screen pocket is in position and the spent screen is outside the body, shop air or a cooling fluid is directed through the spacer plates, cooling and contracting the steel. This effectively shrinks the body halves back against the slide plate, recreating a seal through intimate steel-on-steel contact. The ESC has chrome-plated flow bores that run straight through the screen changer to eliminate hangup areas. The unit has no mechanical seals, so galling is eliminated. Moreover, the upstream and downstream bodies are identical. These interchangeable halves are field repairable—if a body surface in contact with the slide plate gets scratched, the body can be surface ground by a local machine shop and placed back in service. Components are made from stainless steel. The bodies have electric heating standard and can be jacketed for more consistent heat transfer. Processors of rigid PVC generally avoid melt pumps, according to PSI, due to heat generated in the axial clear- ances of the gears and lubrication zones in the bearings. But PSI's Chlorinated Gear Pump (CGP) is said to be ideal for thermally sensitive polymers and brings all of the benefits of gear pumps—die-pressure stabilization, increased output, and extruder-pressure reduction—to rigid PVC sheet and profiles. The new design utilizes modified components and improved heat transfer to reduce temperature excursions that can trigger carbonization/degradation of sensitive materials. The gear- pump body and side plates are jacketed for steam or oil heat transfer. These media travel through the entire pump in a single pass. Dual extended gear shafts aid in conductive transfer of heat out of the high-load areas of the gears. Heat transfers outward from the bearing journals into the cooler seal journals and then through the seals into the jacketed side plates. Seals are contained within and are integral to the side plates for best conductivity. Cooling channels in the body wick heat away from the bearing ODs to control heat buildup. Large, jacketed side plates provide additional heat transfer from the bearings. Bleed material used for lubrication of the bearings and gears (approx. 0.01% of flow) is evacuated from the process flow via internal ports to outside the pump in a constant bleed. 828-468-2600 • psi-polymersystems.com EXTRUSION 48 JULY 2018 Plastics Technology PTonline.com Extrusion With Technology Keeping Up PRODUCT FOCUS

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