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Cammda Corporation

Why Friends Don’t let Friends use Gear Pumps for Metering Resins (December 2010) Not without satisfaction and a touch of pride, Sam looked around and saw literally thousands of lineal feet of filled and shaved saw-cut joints. It was a good job, fast and clean. We made some good money on this one, he thought. “What do you think, George? Sam said, “…great job, huh?” “Oh, yeah!” George replied, “…and we saved a bundle on material too. Next time we only need to buy resin, we still have half the hardener left in the machine!” This scenario, unfortunately, is all too commonplace. The automatic metering mixing and dispensing of two component resin systems is a tricky business. It seems simple, but when it comes down to it, the successful use of these materials involves a thorough understanding and application of several relatively new technologies by the machinery manufacturer. Back in the forties, the first commonly used dual component sealants were used for sealing aircraft fuel tanks. That marked the onset of a thermo-set polymer revolution. Polysulphides were followed by epoxies, polyurethanes, dual component silicones and methyl-methacrylates. These are all super adhesives; sealants and coatings that before the middle to the end of the last century didn’t exist. Each of these materials is similar in that its processing involves the accurate metering of each part and then the two parts have to be combined and thoroughly blended. Initially, two components polymers were weighed-out into a mixing vessel, mechanically mixed and then applied. This process was not safe, messy, cumbersome and wasteful. The working time was limited because materials had to have sufficient pot-life to enable the applicator to transfer the mixed compound into a dispensing device and get it applied before it started to set. This problem was compounded by the fact that thermo-sets are mass sensitive, meaning the more there was of them, the faster they reacted. Afterwards, the dirty equipment had to be thoroughly cleaned before the material cured. This was generally done with flammable or environmentally unfriendly solvents. People weren’t concerned about the environment. The contaminated solvents were generally evaporated into the atmosphere with the remaining sludge being dumped in landfill sites. For this technology to flourish advancements in processing technology had to occur; enter the static or motionless mixer. This mixer was invented in the early fifties in response to the need to process dual component materials in-line. It is a brilliant device that has no moving parts. It accepts two fluid streams, divides them in half, orients them 90 degrees and divides them again. Every element in the mixer effectively doubles the layers of liquid passing by it. If you start with two streams (layers), by the time you get past the fourth element

App ly i n g Expe r i e nce … D is pens i n g Sol u t i o ns


you have 16 layers and by the time you get to the twenty-fourth element there are almost 17 million layers. Initially, static mixers were fabricated from steel or stainless steel and were very expensive. Solvents needed to be used to wash them out. Once the initial patents on them expired, plastics manufacturers took up the call and started producing inexpensive disposable units. While these devices have for the most part made in-line processing of thermo-sets practical, they have their own limitations that must be considered when building a system that incorporates them. Static mixers have a tremendous surface area for their length. Therefore, there is a substantial amount of friction acting upon the liquids passing through them. We call this ‘pressure drop’. Pressure drop causes back pressure that must be overcome and accommodated in any dispensing system. The second consideration is called flow synchronization or ‘Flo-Sync’. Both components must enter the inlet of the mixer simultaneously for them to mix together. If one component enters the mixer before the other it will be divided and reintroduced to itself but the other component will not catch up with it and a slug of unmixed material will emerge from the outlet. So, now we understand something about the most common in-line form of mixing, let’s think about the metering technology required to process these super polymers. The most accurate form of metering is called ‘positive displacement’. This is where a volume of liquid is displaced by a solid. The volume of the solid exactly displaces the volume of the liquid (assuming the liquid is non-compressible) so it is precise, predictable and repeatable. Single acting positive displacement pumps are the most accurate devices for metering liquids. They can be sized to provide the power necessary to overcome back pressure and they can be mechanically tied together to provide accurate flow synchronization. Unfortunately, these are primarily “shot pumps” that need to recharge after each cycle making continuous flow difficult. Where continuous flow is desired people have tried to use gear pumps for metering. Gear pumps consist of a housing that holds two gear wheels. These gears are meshed in the middle. The housing fits tightly around the gears so that the spaces between the gear teeth and the housing form pockets. Liquid introduced at the inlet fills the pockets. When the gears are rotated the liquid is carried around to the outlet area where the teeth mesh together and it is expelled from the pockets, exiting through the outlet. A typical gear pump meter- mix-dispense system consists of two pumps connected usually with sprockets and a chain and driven with a common drive motor. In order for the gears to rotate there must be some clearance between the gears and the housing. Otherwise there would be too much friction and the system would seize-up. Therein lies the problem. Some of the liquid that should have been going out escapes through the clearance space back into the inlet area. This is called ‘slippage’. The amount of liquid lost in this manner is a function of the tightness of the tolerances to which the pumps are built and the back pressure of the system. The back pressure is a function of the viscosity of the liquid, its flow rate and the pressure drop in the system. The viscosity of the liquid is a function of the amount of heat in it. If the viscosity of the liquid is constant, ie. the temperature of the material is tightly controlled and the liquid flow rate is constant, the pumps are well made, there is no wear and the pumps do not heat up while running (changing the clearance gaps), then there is a reasonable chance that the output of the pump will be consistent. Coupling two of these pumps together compounds the problem as the two liquids may have totally different characteristics and may flow differently with respect to each

App ly i n g Expe r i e nce … D is pens i n g Sol u t i o ns


other as they have different rates of slippage under identical situations. Unless these conditions are carefully controlled, trying to accurately meter two liquids with a dual gear pump system is virtually hopeless. Every meter-mix-dispense equipment manufacturer knows this. BUT… The most seductive feature of a gear pump system is its low cost. The pumps are mass produced for many applications so they are inexpensive. The drive systems are also simple and cheap. Because there are companies that demand low cost dispensing systems and because there are companies willing to take advantage of an uninformed market, there are many inadequate systems available. Any gear pump system that does not incorporate material temperature control, closed loop independently monitored and controlled pumps is inadequate. This unfortunately includes all low cost bare-bones gear pump systems. Our friends Sam and George have one. It’s a shame that the “great job” that they did must be redone. Even though the material cured, because it was not mixed on ratio the characteristics of the finished product will not be what the architect or engineer specified. If they don’t re-do it there will probably be repercussions down the road. And that is why….

/written by : Greg McEwan, President CAMMDA Corporation 1-888-372-0123

App ly i n g Expe r i e nce … D is pens i n g Sol u t i o ns

Why Friends Don't Let Friends Use Gear Pumps  

The automatic metering mixing and dispensing of two component resin systems is a tricky business. It seems simple, but when it comes down to...

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