Longevity Driven Pump Design Reshapes Polymer Handling
The importance of so-called post-consumer trash as a raw material supply is growing. Since it frequently gets combined with tougher contaminants like sand, glass splinters, and metal foil residues, it normally needs to be sorted first. Either contact surfaces or extremely narrow spaces are used to seal throughout the pump pressure building process. At those locations, the foreign particles cause significant wear or seizing-up, which can reduce durability and result in unscheduled stoppages. New solutions increase the range of applications, increase robustness, and prolong service life. Additionally, the solutions work well with heavily loaded polymers.
The circular economy is subject to enormous cost pressure, while the basic raw materials are currently often cheaper. Beyond that the processing of post-consumer waste is relatively costly. This is primarily because it is difficult to filter out the impurities to a sufficiently clean quality. Consequently, the final purity of the recyclate has a massive influence on the achievable price.
High-quality recycling of recovered raw materials
There are basically two different recycling concepts that make up a circular economy. The first is pure filtration, or mechanical recycling of the polymer mix, whereby the foreign particles are removed from the existing polymer melt. The achievable purity is however limited in this process. It is therefore often unsuitable for direct contact with foodstuffs, and for applications imposing higher demands.
The second concept, chemical recycling, depolymerizes the polymer chains in the recycling process. This process either produces a low-viscosity intermediate product or breaks the raw material down directly into partially gaseous streams. Polymer generated from these recovered raw materials is of high quality, and can be used without restriction. The process sequence is more complex than mechanical recycling, however.
Extending service life, avoiding sudden stoppages
Both process concepts are the same initially. The post-consumer waste must first be shredded, and manually precleaned to remove coarse impurities. The more tolerant the downstream process chain, the leaner the precleaning can be implemented. Significant wear along the entire process chain is a familiar aspect of post-consumer waste recycling, and it is completely normal for many components to need replacement every year or two. The end of life of wearing parts is usually signaled by changing process parameters such as rotation speeds or temperatures. So their replacement can be planned and coordinated with other maintenance work. Alongside the general requirement to extend service life, it is extremely important to avoid sudden stoppages and unplanned maintenance interventions.
Filler agglomerates block flow channels
In highly filled mixes there are hardly any gaps between the fillers, and the interaction of the solids severely impedes the flow. Solids are repeatedly ground up in the flow, and the mix has limited flowability. Filler agglomerates behave similarly to impurities in recyclate, and can cause blockages. This is because the filler agglomerates tend to block flow channels – especially converging channels.
Higher concentrations can settle in converging flow channels, and the pressure pushes the polymer further out of the gaps between the filler particles, which further compacts the fillers, practically solidifying them. The pump is in effect blocked, usually without causing major damage. After cleaning – entailing a major disruption of the process – the pump can be run normally again.
More robust design
The latest-generation of pumps from Maag feature a design that no longer collects dirt in an annular channel between the bearing and shafts. This means the new design is inherently less susceptible to damage from solid impurities, oversized fillers or high filler concentrations. This attribute has proved its worth right from launch.
Traditional gear pumps that have not yet been optimized for such applications have some problem areas that are at high risk if the pumped material is contaminated. The worst impact is from medium-sized ductile metal parts about 0.2 mm in size or larger (depending on pump size) being drawn into the slide bearing. This then leads to seizing-up of the shaft in the bearing: crushing, and ultimately melting, of the particle, which consequently melts and tears open the bearing and the shafts. In the worst case, cold welding occurs; the pump comes to an abrupt stop, and can no longer be used. In the best case, the surface is broken up, merely impairing the operating limits to some extent. This does, however, mean that even minor faults can stop the pump running.
Fewer teeth mean that the product is less compacted in the final meshing phase.
Gear pumps are generally optimized in terms of their bearings to work very reliably with a wide range of clean polymers. Because pure polymers in some instances have very extreme properties, good filling of the bearings is the main challenge in order to maintain a stable lubricating film. Recycling plants are always fed with a mixture, so the most extreme polymer properties such as melt breakage are not encountered. In particular, the mix contains various processing aids that have been added to counteract such phenomena. This opens up entirely new possibilities for the design of bearings. Instead of allowing open access to the bearing, the manufacturer has incorporated a local filtration at the bearing lubrication inlet. A special geometry ensures that the bearing is still supplied with melt, but the larger particles are no longer drawn into the bearing gap. Whatever still gets in can cause only limited, minor wear. This does result in the bearing geometry being washed out over time, though without causing serious unforeseeable damage.
The service life of the interior components is normally limited by general wear, and end of life is signaled by a gradual decrease in efficiency. This means that the replacement of wearing parts can be planned and carried out together with other maintenance work. As opposed to traditional filtration systems with filter surfaces, the gap does not clog up, and the teeth carry the particles into an area of the bearing where, almost like a backwash filter, the dirt is pushed back out of the gap.
Service life extended
In an application in which the pump had a service life of around two to three weeks, the manufacturer was able to increase it to between 18 and 24 months by incorporating the filtering bearings. Above all, abrupt severe failure due to bearing seizure is now completely eliminated. This represents a massive improvement in terms of reliability and robustness, makes production plannable, and is essential to a cost-effective operation.
When processing highly filled polymers, filtration reduces the concentration of fillers in the bearing. The reduction in filler significantly improves the convergent bearing flow. This bearing design enables a higher filler concentration in the conveyed polymer melt than would otherwise be possible.
Greatly reduce pre-cleaning
If users are forced to generate very high pres – sures at very high temperatures, the filtering bearings are no longer sufficient for stable operation with the impurities in the post-con – sumer waste. In such applications, a solution involving external lubrication of the bearings is deployed. A small amount of clean polymer (about 0.5 – 1 percent) that is compatible with the process and of sufficient viscosity is injected into the bearings. This ensures that the shaft is supported securely. Mixing the lubricant into the process slightly increases the downstream flow rate. At first glance, such an approach appears costly. But this process can, in some cases, substantially reduce the required pre – cleaning, and so is economical for that reason alone.
Gear toothing made more robust
Another area in gear pumps that causes significant wear or blockages is the tooth meshing. When the second flank meshes, the remaining product stream must flow out via longer routes and through a narrower gap. Larger pieces of contaminant, in particular, can stick to the tooth root at this point, while the opposing tooth con – tinues to engage further into the tooth space. These larger pieces are then pressed together into the remaining gap between the tooth head and root. The more the trapped volume has to be reduced, the greater the risk of blockage. To prevent this effect, the manufacturer uses a special toothing design with fewer teeth for recycling applications. Fewer teeth not only mean that the product is less compacted in the final meshing phase; there is also much more room for the product to flow out in the first meshing phase while the opposing tooth is engaging in the tooth space. This additionally reduces the loading on the gears due to lower squeezing pressures.
Wear usually occurs on the contacting tooth flanks where the driving shaft drives the rotat – ing shaft. However, although the flank pressure increases due to the increased curvature of the optimized geometry, this new process means that far fewer particles are effectively trapped and ground between the flanks, and so the ser – vice life is extended.
Tooth flank wear significantly reduced
If the recyclate is so contaminated that the wear between the tooth flanks is unacceptable, users can install a timing gear between the reduction gear and the pump. This intermediate gear then drives both shafts simultaneously so that no torque transmission takes place in the pump, and the tooth flanks run practically contact-free. This significantly reduces tooth flank wear. The introduction of filtering bearings has extended service life from 2 or 3 months to between 18 and 24 months.
Changing to an optimized gear geometry also has the advantage that the teeth are thicker and more robust. This means that even larger particles can only cause dents and deformations in the teeth; they will not usually break.
Flexible modular system improves circular economy
Pumps that prevent accumulation of dirt in the annular channel between the shaft and bearing are suitable as a basic system. Moreover, the filtering bearings in conjunction with the optimized gear toothing on the shafts cope well with medium degrees of contamination and medium filler concentrations, provided the required pressure is not too high.
If contamination is higher, users can counteract the primary wear in the tooth meshing by installing a timing gear. This makes sense where users want to maximize service life, or where replacement part costs exceed the investment in the timing gear.
There is also an option to lubricate the bearings externally. A small additional extruder is integrated into the system to continuously feeds the clean polymer melt to the pump in order to lubricate the bearing. This is the only way to enable applications with higher pressures and thinner viscosities that would otherwise simply be inconceivable. With these measures, users can make much more out of post-consumer waste, and so contribute to the circular economy. Even heavily contaminated waste collected from nature can increasingly be included, which increases the incentive for collection for recycling.
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