The right way to convey powders
For plastics processors, conveying powders involves a completely different set of challenges compared to pellets. Here’s how to handle them.
September 1, 2019 by Canadian Plastics
For companies that manufacture or — supply products, a lot hinges on delivery. Which is why Amazon, for example, is spending millions to develop drones that can get packages to customers in 30 minutes or less. For plastics processors, meanwhile, the importance of good delivery begins earlier, before the product is even made — conveying material from a central point to the processing machine efficiently can jump-start the molding process. Unlike plastic pellets, however, moving powders can be especially demanding for processors since no two powder conveying systems are exactly alike. With powders, the checklist of considerations is particularly long, and includes factors such as conveying rates and distances, material characteristics, flow aids, pick-up and discharge, filtration, and safety concerns about combustible materials.
To recommend the right solution, it’s critical for the conveying system supplier to first understand the powder in question, beginning with its bulk density. “We want to see and touch a sample of the powder that’s going to be conveyed,” said Brian Davis, general manager of Maguire Products Canada Inc. “Understanding the powder’s bulk density is key to calculating the size of the conveying system. The bulk density also factors into calculating the system parameters and conveying velocity, since higher bulk density materials require higher pick-up velocities.” The flow characteristics of powders are affected by the bulk density of the material and the size, shape, and cohesiveness of individual particles. Blends of multiple powders may include particles having various origins, sizes, shapes, and bulk densities.
In general, powders used in the plastics industry can be either free flowing, sluggish or non-free flowing, and this distinction can impact conveying needs. Making it tougher, one product might consist of several grades, with each behaving differently than the other. Additional characteristics include being abrasive or combustible; whether the powder absorbs moisture; whether there potentially could be chemical compatibility issues with conveying hoses, gaskets, filters, or process equipment; and materials that are “smoky” like talcum powder, which has a high fines content, requiring more filter area. Powders that are combustible should obviously be treated with extreme caution. “Even powders that aren’t normally combustible can explode if the particles are a certain size and in a certain concentration, for example PVC dust,” said Janice Xiong, lead systems engineer with Hamilton Plastic Systems Ltd. “Since these materials, and more traditionally combustible powders, can catch fire and explode when mixed with air, the conveying system needs to be a closed system.” Some equipment suppliers avoid jobs that involve conveying combustible powders period, and for a simple reason. “I’d say that any equipment maker that makes pellet conveyors is by definition not making powder conveying equipment that’s explosion-proof,” Brian Davis said.
For non-free flowing powders, meanwhile, the key is to move the material at a constant rate from the pick-up point to the material line and at a constant rate from the discharge point to the material receiver. These materials often require the most equipment modifications, and there are a variety of dis-charge and feeder methods to handle these powders. Maguire Products’s MPF powder feeder, for example, is designed to handle non-free flowing powders; the hopper features an integral bridge breaker bar that sweeps the bot-tom of the hopper once every two turns of the auger to keep the flights of the auger full and to provide accurate metering. And Metro P powder hopper loaders from Motan Inc. have been developed for reliable, service-free continuous operation for conveying both free flowing and non-free flowing materials. Carbon black and titanium dioxide powders are difficult to handle as they pack or smear, bridging or “rat-holing” in the process stream, greatly reducing efficiency; both are also notorious for potentially binding or jamming moving components, causing time-consuming cleanup efforts. They also tend to be abrasive — almost like sand — as does PVC powder. Knowing in advance that abrasive materials are going to be conveyed allows the system supplier to make adjustments. “Materials of construction are a very important consideration in the design and functionality of a powder conveying system, especially with abrasive material that can damage the bends and the receiver itself, where the velocity is highest,” said Jan Rickenbach, Motan’s extrusion sales manager. “You can convey at a slower rate to avoid damaging the system, but only to a certain extent. A better option is to have the conveying system made with stainless steel pipes, deflector plates, and ceramic or glass elbows and bends.”
At the other end of the spectrum, some powders flow almost too easily. “Polyethylene, especially as used in rotational molding, is a very soft powder and flows like water,” said Rob Miller, president of Wittmann Battenfeld Canada Inc. “With a material like that, velocity becomes a potential problem because conveying the powder too quickly can generate angel hair or streamers, just as with pellets.”
KEEP IT MOVING
Once they have a handle on the compound being conveyed, knowing the conveying distance — which can comprise both horizontal and vertical factors — is a second important factor the system supplier will consider. “A typical up-and-in system offers a vertical lift from floor-level, conveying up to a receiver over an extruder or loss-in-weight feeder,” said David Kennedy, sales manager with Vac-U-Max. “It’s important to know the number of 45° or 90° sweep elbows, keeping in mind that one sweep elbow is equivalent to 20 feet of linear tubing.” The end goal is to reduce the number of elbows in the system, since powders don’t always flow as cleanly as pellets. “If vertical vacuum conveying runs exceed 12 to 15 feet in batch mode, the material in that line will stop conveying and fall back down,” Kennedy said. “For this scenario, it’s important to design a line-clearing valve to make sure the vertical leg is clear before the batch process stops.” Third, a properly designed powder conveying system will take into account how the materials are received and introduced to the process. Material can be received in paper bags, fibre drums, barrels, bulk bags, super sacks, rigid intermediate containers, silos, day bins, railcars, bulk trucks, or upstream process equipment.
The process of silo selection, to cite one example, is different for powders than for pellets. “Silos designed for powders should have a minimum cone angle of between 60° to 70° for better flowability, compared to 45° and 60° cone designs typically used in pellet silos; and if the powder is non-free flowing, the cone should have pro-visions like air cannons to prevent build-up,” Brian Davis said. “These design features should be specified before the silo is ordered, because trying to retrofit a silo involves cutting into the lining, which disrupts its effectiveness.” Also, as opposed to an atmospheric vent for a pellet silo, a silo designed for powders typically requires a bin vent filter, which is a dust filtration unit on top of the silo. “The bin vent filter prevents powder from being discharged into the outside air when the silo is being filled,” said Rob Miller. Pick-up points might also require attention. “The pick-up point can be configured with a wand for a simple up-and-in system, bag dump station, bulk bag unloader, and in some cases silo or railcar, all dependent on the container holding the material at the beginning of the process,” David Kennedy said. The pick-up point also poses a challenge for powder when it comes to efficient material transfer. “There must be a means to aerate the powder so that it flows freely into the tubing from start to finish,” said Doug Brewster, conveying product manager with Conair Group. “Non-free flowing powders in particular require aeration at the pick-up point or agitation to keep them moving properly.” Gates, which open and close a path of material flow, play a role in powder conveying too, but can cause problems when material leaks from the gate orifice to the body along the blade. The Series E Slide Gate from Lorenz Conveying Products Corp. is used to convey powders in vacuum applications, and can be air-purged to eliminate the chance of powder passing into the body cavity, thereby avoiding gate failure.
And at the end of the conveying process, there are a variety of discharge methods to handle difficult non-free flowing powders, such as over-sized receiver discharge openings as well as 70° and other cone-less, jam-proof vacuum receivers. “For difficult-to-move powders, the key is to move the material at a constant rate from the pick-up point to the material line and at a constant rate from the discharge point to the material receiver,” David Kennedy said. A tip: Lighter weight powders need larger receivers in order for the material to fall out of the air stream, the experts say. Powder systems handle particles measured in microns, some of which are invisible to the human eye at less than 10 microns and capable of staying suspended in the air for days. This is why filters, which separate air from particulate, are another key consideration when designing a powder conveying system. “Filtration is one of the biggest challenges in powder conveying, and is largely dictated by the type of powder being conveyed and its characteristics,” Rob Miller said. Filtration is based on the air-to-cloth ratio, which is the volumetric flow rate of air flowing through a dust collector’s inlet duct divided by the total cloth area in the filters. The air-to-cloth ration for pellets is usually 5:1 or 6:1, the experts say, whereas for powders it can 2:1 or 3:1. “Knowing the air-to-cloth ratio is critical to sizing the filter area to do the job,” Miller continued. “Factors such as the type of powder, particle size, conveying rate and distance, conveying pressure, and air volume used in the conveying circuit all contribute to calculating the air-to-cloth ratio.”
Other filtering factors to consider include the number of filters, filter placement, the cleaning cycle, and the material of the filter. Wittmann Battenfeld’s PDR central powder conveyors, for example, have a spun woven polyester dacron blend pleated filter media; and Conair’s PR series of powder receivers — which have a 3:1 air-to-cloth ratio — use up to three high-capacity, pleated polyester cartridge filters, with an optional filter vent available that holds in pressurized air and dust.
PUTTING IT ALL TOGETHER
The considerations listed above should determine which of the two primary methods for transferring bulk materials — vacuum and mechanical — is the better choice for a particular powder conveying application. And the equipment suppliers lean towards using vacuum in most cases. “Vacuum conveying is the best method for handling powders because it requires less maintenance, has less spillage and dust leak-age, minimizes dust exposure, and gives enhanced flexibility,” said Janice Xiong. “And because it’s a closed process that protects against ambient air, it’s the preferred technology for trans-porting powders that are combustible.” As a subset of that question, there are two main categories of vacuum conveying technologies to select from: dilute phase, which uses constant vacuum pressure to lift the material to get it flowing through the system; and dense phase, where the vacuum pressure is higher but is modulated on and off to move the material through the lines in small batches. Again, the final choice will come down to the specifics of each powder conveying situation. “Dilute phase is less expensive and is the industry standard with powders,” said Jan Rickenbach. Typical conveying rates in dilute phase may go up to 2,500 lbs per hour, with typical conveying distances of less than 300 feet. “But dense phase can be the better choice when higher conveying rates, longer conveying distances, or fragile or abrasive materials are involved,” Rickenbach continued. Some technologies are essentially hybrids of the two. “Conair’s Wave system is a dilute phase system that can also operate in a dense phase mode,” Doug Brewster said. “Wave conveying technology lets the powder flow like a wave on the ocean, and can increase through- put nearly two-fold, allows conveying over longer distances, eliminates clogging, and reduces the build-up of powder residue near destination receivers.”
And speaking of clogging and build-up, cleaning a powder conveying line of residual material is vital to process efficiency and quality control for applications requiring change-over, and — depending on the powder — might have to be done more frequently than with a pellet conveying system. “A PVC powder system will wear much faster than most pellet systems, and if you don’t maintain the filters and the filter blowback systems you’ll develop material problems,” Rob Miller said. “And even with a blowback system, powders can accumulate in the filters, signalled by slowdown of material.” The good news is that maintenance is getting easier. Hamilton Plastics Systems’s Vectra powder loader has quick-release latches for no-tools disassembly, and a unique, removable filter plate module designed to make filter changes faster and cleaner; Wittmann Battenfeld’s PMX series powder loaders have a blowback system designed to extend the operational life of the filter media; and the VLP series powder vacuum loaders from Novatec Inc. use spun bond polyester filter elements that are washable for years of use with only simple cleaning and an occasional washing and air dry.
In the end, not all powders require complicated, custom-made equipment — some materials can be accommodated using standard “plug-and-play” components. But whatever the situation, working with your equipment supplier to achieve an understanding of material properties and operational needs is the best way to get the right powder conveying system for your needs. Unfortunately, they can’t deliver it by Amazon drone — at least not yet.