Progressive cavity pumps allow for controlled dispensing of both abrasive and highly viscous materials
Figure 1. Metering Rod Pump
Historically, when dispensing single- and plural-component adhesives and sealants, the metering technologies of choice have been rotary gear or rod and piston pumps. These metering technologies - while still having a place in a variety of markets and applications - have limitations, including their ability to meet process cycle times, continuous flow outputs, material compatibility and maintenance costs. (See Figures 1-2)
Figure 2. Rotary Gear Pump
As an example, if an application requires a long, continuous bead of an adhesive or sealant material, a single-acting rod or piston metering pump may not have the volumetric capacity to complete the cycle within one stroke, and the cycle time may not allow for metering chamber reload. While dual-acting metering pumps increase output capacity, their design and sequencing often lead to decreased bead profile accuracy. These technologies, however, are successfully used for highly viscous and abrasive materials used in sealant and gasket applications. (See Figures 3-4)
Figure 3. Single-Acting Piston Pump
Gear pumps provide continuous material flow, meaning they are continually reloading while metering the materials. Their design is not suitable for processing abrasive materials. Abrasive materials will grind between gear surfaces and pump housings, thus quickly widening the tolerances and thereby losing efficiency or volumetric output. (See Figure 5)
A new metering technology is available that allows for controlled dispensing of both abrasive and highly viscous materials, while being continuous duty and simple to operate. This technology is referred to as progressive cavity (PC) pumps. Until recently, PC pumps were too large to adapt to most industrial dispensing processes. The PC pump has been redesigned to be a metering pump and downsized to be integrated with industrial automation for applying adhesives and sealants.
Figure 4. Dual-Acting Piston Pump
Progressive cavity pumps have been around for many years. They are used in applications ranging from wastewater treatment facilities and oil fields to the food and pharmaceutical industries. The recent downsizing and tighter manufacturing tolerances allow for accurate and repeatable metering as well as the transferal of materials for industrial adhesive and sealant dispensing applications. (See Figure 6)
Figure 5. Formed in Place Gasket
Cavities that contain the material to be dispensed are created by a 180° offset of the rotor and stator. As the rotor is turned counterclockwise by the motor, the cavities are advanced toward the outlet port. The rotor and stator create a continuous line of contact - just as a piston or rod pump does - that forces the material in the direction of the turning rotor. Material suck back is achievable by reversing the rotor direction to a clockwise rotation. Figure 7 shows the cavities and the relative positions of the rotor to the stator as the rotor is turned.
Figure 6. Progressive Cavity Metering Pump Components
As with all dispensing technologies, the wetted metering pump components must be compatible with the chemistry and fillers of the materials to be dispensed. Stators and seals are commonly made of EP, Viton, fluorinated silicone or PTFE. Rotors are typically made from stainless or hard-chromed steel. As part of the equipment selection process, the dispensed chemical materials and filler components are essential in order to select the correct pump components. (See Table 1)
Figure 7. Progressive Cavity Metering Pump Rotor and Stator
Progressive cavity pumps are versatile in their ability to process a range of materials with a variety of characteristics. PC pumps can process materials with viscosities ranging from water-thin to several million cps. Both soft and abrasive fillers are also processed with the PC pump. The fillers embed themselves in the elastomeric stator and decrease the wear tendency when compared to metal-to-metal surfaces found in rod and gear pumps. Another advantage of the PC pump design is that it is a low-shear, low-pressure metering system. The gentle action of the rotor, as compared to the reciprocating action of a piston pump, reduces material stress. In addition, the pressures required to meter the materials are substantially less that those required in a reciprocation piston-type metering pump.
PC pumps are rated in reference to their flow rate capability. For example, a 15mm diameter PC pump is rated at 14 ccs per revolution and can be rotated between 20 and 180 rpms, depending on the material being dispensed. To determine the PC pump required for any given application, both the material characteristics required and the application flow rate requirements must be specified. Table 2 shows PC pump ratings.
As with all metering pumps, a constant supply of material from an external source is required. PC pumps may be gravity fed from reservoirs as long as the viscosity and flow rate characteristics of the dispensed materials are sufficient to keep the pumps supplied with material. If gravity feed is not feasible, a variety of pressure-feed methods are available. PC pumps may be fed from cartridges, pressurized tanks and transfer pumps.
A PC pump may be used as a transfer pump to supply material to another smaller PC metering pump used at the point of dispense. PC transfer pumps are available in 5-, 30- and 55-gal sizes. They offer the same advantages as PC metering pumps in that they provide material flow at low pressure and low shear as well as transfer highly abrasive and viscous materials.
Two PC pumps may be linked together and driven by one motor to create a fixed ratio meter, mix, and dispense system. More commonly, two PC pumps are independently controlled, allowing for ratio adjustments by increasing or decreasing the rotor rpms of one or both PC pumps; this setup is known as a variable ratio meter, mix and dispense system. The control system typically includes two AC or servo motor controllers, a PLC, and an HMI touchscreen control panel. Through the HMI, the rpms are monitored and changed as required. Control systems are available with the ability to automatically monitor and correct ratios. These require the use of gear or corialysis-type flow meters with the proper software. Pressure switches (an additional feature) are incorporated at the outlet of the material supply system and at the PC pump inlet to monitor material supply pressures to assure the PC pumps do not run dry. (See Table 3)
When considering equipment for applications requiring high flow rates and continuous bead extrusion with highly viscous or abrasive materials, PC metering pump technology is a valuable alternative to traditional metering systems.
For more information on progressive cavity pumps, contact Liquid Control; 8400 Port Jackson Ave. NW, North Canton, OH 44720; phone 330-494-1313; fax 330-494-5383; e-mail firstname.lastname@example.org ; or visit www.liquidcontrol.com.