Extrusion, Gear Pumps

What is a Gear Pump? or Resin Metering System?

EXTRUSION TECHNOLOGY

What is a Gear Pump? or Resin Metering System?

This months Extrusion Technology column explores Gear Pumps also known as Resin Metering Systems, or Melt Pumps. Gear Pumps have been used for many years for a variety of applications. I was recently surprised to find a bronze Gear Pump available in my local Orchard Supply Hardware store. This $100 unit could be used for pumping water or oil on farms. For a $100, believe me, the thought crossed my mind as to whether I could use this for polymer extrusion. But Polymer Gear Pumps have special requirements, they work with hot polymer melt, and offer highly accurate volumetric metering.

A Gear Pump literally consists of two gears which intermesh. These gears rotate within the pump housing. Polymer melt is fed into one side of the gear pump. Contrary to your first impression, the fluid does not flow where the gears intermesh. The fluid fills the interstices of the gear teeth and is conveyed like a waterwheel circumferencially between the gear and the pump housing. The accuracy of the pump depends on having closely mating gears, so that no fluid leaks though the intermesh. There must also be a very small clearance between the gears and the pump housing to prevent leakage over the top of the gear teeth. The output from a pump with large clearances is affected by the differential pressure across the pump and the ability therefore to accurately meter resin output is not good.

So now that we know what a Gear Pump is, why use one? The two main reasons for using a gear pump are:

- to accurately meter resin volumetric output from the extruder

- to generate die pressure

These sound like two expected functions of the extruder screw, and indeed if you can obtain your desired output stability and pressure through screw design, you don't need a Gear Pump.

For certain resins, and certain applications, screw design alone has not been able to match the output stability and pressure generation offered by a Gear Pump. This is because of the positive displacement method of pumping afforded by a Gear Pump, which has found wide application, particularly in the manufacture of communications cable.

There are significant downsides to the use of Gear Pumps and one should be aware of these:

- increased demand on extruder technician skill

- additional equipment cleaning and assembly

- gear pump assembly and disassembly requires skilled technicians

- gear pumps require on-going upkeep and maintenance

You can reduce the heart ache associated with Gear Pumps if you apply good engineering in four key areas:

- Invest in elegant, well designed adapters for mating the pump to your extruder and extrusion die

- If you can avoid using a pressure feed back loop on the extruder, don't use one. As long as barrel pressure does not drop below a few hundred PSI, when you would risk starving the pump, or rise above the barrel pressure limit, just let the barrel pressure fluctuate. A 1000 psi barrel pressure fluctuation with an accurate pump will only create about a 10 psi die pressure fluctuation down stream of the pump, so why sweat the small stuff, there are bigger fish to fry.

- Gear Pump output is directly proportional to Pump RPM, so invest in an accurate drive. Most pumps will require less than 1HP, so a DC drive with Tach Feed Back is a good way to go (0.1% set speed accuracy is common these days)

- Match the size of the pump to your volumetric output requirements. Pumps are specified as to their volumetric output per revolution. For example, a 10 cc/rev. pump will deliver 10 cubic centimeters of melt every revolution. The normal working speed of a Gear Pump is 10 to 60 RPM. So a 10 cc/rev gear pump will comfortably deliver 100 cc to 600 cc of melt per minute.

There are a number of manufactures of Polymer Gear Pumps, including Zenith, Normag and LCI. I recommend McNew and Associates in San Rafael for good customer service (McNew represent Zenith).

Mark Carter, Polymer Extrusion Engineer, Fremont CA

SPE