Amidst the high energy price environment in Europe in the past two years, finding efficient ways to reduce consumption and increase yield has become even more important in the energy-intensive plastics industry.
Machinery has a key role to play in this endeavour. At Fakuma 2023, there were plenty of innovations highlighting their low-power consumption, for example, Hellweg’s wet grinder with 70 kWh to 90 kWh energy consumption, or Arburg’s Allrounder 520 H injection moulding machine.
Today we are looking at gear pumps and their role in energy saving during the extrusion process. PSI-Polymer Systems, a US-based manufacturer of extrusion equipment, has shared data on how gear pumps enable the industry to meet growing sustainability demands.
"As processors struggle to meet today's sustainability demands, the gear pump has emerged as an eco-friendly, go-to solution proven to reduce overall energy load for the extrusion line while simultaneously minimising the amount of material required for the product,” said Don Macnamara, general manager of PSI-Polymer Systems. “This allows for 100% use of regrind without compromising output consistency and minimising scrap from start-up or off-spec dimensions. Plastics are not going to go away, and it falls to us as industry participants to produce more responsibly. The gear pump helps get us there."
Gear pumps use the actions of rotating cogs or gears to transfer fluids, melted plastic in this case. The rotating element develops a liquid seal with the pump casing and creates suction at the pump inlet. Fluid, drawn into the pump, is enclosed within the cavities of its rotating gears, and transferred to the discharge.
By building up pressure before the melted plastic reaches the die, the gear pump allows the extruder to process the same, or often more volume, at a lower revolutions per minute (RPM). This translates to less energy required to produce a specific volume of output. Whilst this energy amount is arguably small, it adds up, says Macnamara. Consider an extruder running at 100 RPM, 24 hours/day, 5 days/week, 50 weeks/year. That equates to 36 million RPM/year and a gain in yield of 10% to 25% at a lower RPM and a drop in corresponding extruder amps which significantly helps reduce the product's energy footprint.
A gear pump can also minimise head pressure, i.e. the flow resistance of the die and the pipping that connects the die to the end of the barrel. Lowering the head pressure reduces the extruder motor load, leading to significant long-term energy savings. Lower head pressure also minimises wear and tear on the screw and barrel, which leads to fewer rebuilds and therefore a lower carbon footprint.
Gear pumps also allow processors to reclaim material more efficiently.
“Many processors limit the percentage of regrind in the extruder due to pressure surge which results from variable feed and melt rates of the differing feedstock bulk densities,” PSI explained. “The tight clearances in the gear pump effectively dampen the extruder surge at the pump inlet by up to 50:1. In many cases, this results in die pressure control of +/- .25%. Consistent die pressure holds wall/gauge variation within a tight tolerance. Running true to the lower end of the dimensional specification eliminates scrap caused by out-of-spec production (over/under).”
In turn, eliminating or reducing scrap saves energy by avoiding the energy impact oof reprocessing.
“Running a tighter tolerance also means using less polymer per product length,” PSI added. “For example, one conduit producer extruding 1.5” SDR 13.5 pipe had an average weight of 151 grams per length. Adding a gear pump minimised the overage and resulted in a constant weight per length of 145 grams. At 65 ft/min, 20 hours/day, 5 days/week and 50 days/year, the 6-gram savings eliminated 257,940 lb from the product footprint. That is roughly one million pounds taken out of production in four years while consistently making high-quality pipe.”
Finally, a gear pump can reduce the demand for additional cooling during the extrusion process by reducing backpressure-induced shear heating. This can lead to further energy savings by reducing the load on the cooling system.