Screen changers, pumps adapted for PVC profiles
PSI-Polymer Systems introduced a screen changer and a gear pump at NPE2018 that can be used when extruding PVC profiles.
“As long as I have been coming to shows, we’ve always been asked the question, ‘What do you have for PVC?’ ” PSI GM Don Macnamara said.
One of the problems with designing screen changers and gear pumps for extruding PVC is the temperature sensitivity of the resin; the equipment can apply too much heat. Chemically stabilizing PVC can reduce the problems, but “even the ones that are stabilized eventually will degrade,”
PVC, especially rigid PVC, is particularly challenging to work with because it has a narrow processing window and requires a constant, steady flow, he said. That keeps many processors of rigid PVC from using screen changers or gear pumps. However, PSI’s two new products are designed especially for those applications.
“Both of these products are the culmination of over a year’s development work to come up with products that can run rigid PVC, the most sensitive of the PVCs, without seeing the type of degradation that normally leads to failure of this type of equipment in the process,” Macnamara said.
EXPANSION PLATE SCREEN CHANGER
PVC processors have tended to avoid screen changers because shear heating can occur and often will trigger localized burning of material. Most screen changer designs use transitions or steps in the
flow path that are compatible with the chemistry of most polymers. However, for heat-sensitive polymers, like PVC, these steps create hang-up areas that increase shear heating. That, in turn, increases the potential for carbon generation and material degradation.
“There are areas in the screen changer where you may have a change in the flow rate, and once you have a change in the flow rate, you’re introducing shear heat. If you get into a slow-flow area, now you’ve got that time element working against you. It’s in that slow-flow area where heat can trigger degradation,” Macnamara said.
PSI has designed an expansion plate screen changer that can work with PVCs that can tolerate temperature increases of 10 to 15 degrees Fahrenheit. The company has applied for a patent on the technology, which minimizes shear heating.
“With our [expansion plate screen changer], we solved the chemistry issues relative to heat, flow and thermal conductivity that make the use of a screen changer with PVC formulations so challenging,” Macnamara said.
The design also avoids the use of polymer seals that can degrade over time. Instead, steel-on-steel contact inside the screen changer forms a tight seal.
“We had to come up with a screen changer that didn’t have a seal but could change screens,” Macnamara said. “Engineering came up with a technology that would allow the body of the screen changer itself to be a seal.”
The expansion plate screen changer comes standard with corrosion-resistant stainless-steel components. Straight-through, chrome-plated flow boreholes reduce surface friction and eliminate hang-up areas, and the lack of mechanical seals reduces the likelihood of damage to the screen changer.
Generally used in sheet and profile extrusion, the device requires only a momentary shutdown for a screen change, which is a significant improvement over manual screen changes, Macnamara said.
In the absence of an automatic screen changer, an operator would need to open the extruder, remove the die, peel out the screen, replace the screen, put the die back and bolt the system back together, he said.
“The problem you have with it is that PVC can be so sensitive that if you stop the extruder beyond a short window, you’ve got to purge the extruder,” Macnamara said. “You’ve got to empty the entire barrel. They don’t like having to do that.”
While a manual screen change could shut down an extruder for a couple of hours, using an automatic screen changer reduces the downtime to only a minute and a half, he said.
CHLORINATED GEAR PUMP
Processors also typically avoid using melt pumps because heat is generated in the axial clearances of the gears and the lubrication zones of the bearings, Macnamara said. The temperatures become unsuitable for thermosensitive materials like rigid PVC.
However, PSI’s new chlorinated gear pump (CGP) is designed to more tightly control the temperature shifts typical in a gear pump.
“Our new CGP is highly effective at managing the temperature profile across the melt pump, which is so critical for PVC,” Macnamara said. “It took some hard work and several modifications, but our product will now allow our customers running a thermally sensitive polymer to take advantage of the die-pressure stabilization, increased output and reduction of extruder-pressure benefits of a gear pump without worrying that their polymer will carbonize or degrade because of it.”
PSI modified several components to improve heat transfer and limit temperature increases common in gear pumps, he said. The body and side plates of the CGP are jacketed for a heat-transfer oil, which travels through the entire pump in a single pass. To help push heat out of high-load areas of the gears, the CGP has dual-extended gear shafts that help transfer heat outward from the bearing journals into the cooler seal journals, and then on through the seals into the jacketed side plates.
The gear pump has cooling channels that provide further heat control; in addition, the jacketed side plates provide heat transfer from the bearings. A polymer used for lubrication of the bearings and gears is evacuated from the process flow via internal ports to the outside of the pump in a constant bleed, which amounts to about 0.01 percent of the flow.
Bruce Geiselman, senior staff reporter
PSI-Polymer Systems Inc.
Conover, N.C., 828-468-2600,