How to convey regrind and powder

A dust collector from Entek Adaptive. Photo Credit: Entek Adaptive

The expression “move it or lose it” applies in many contexts, but maybe never better than resin conveying: whatever kind of molding you’re doing, you’re out of business if you’re not distributing resin pellets quickly and reliably from a starting point to the processing machines in your plant.

Historically, most shops specialized in molding standard virgin resin pellets, and so that’s what they conveyed through enclosed, airtight pipelines. But times have changed. Powder – whether resin or additives – is being increasingly used, and therefore conveyed. And regrind is even more popular – almost everyone now is trying to integrate some sort of recycled resins into their products, and regrind and recycled material are often requisite in projects these days.

Here’s the potential problem: Powder and regrind can both present special, yet different, conveying challenges compared to virgin pellets. Understanding them, and if necessary, overcoming them, is critical.

This understanding begins with knowing the differences between standard pellets, powder, and regrind. As a rule, typical virgin resin pellets are clean, uniformly sized, and lightweight, which makes them fairly easy to convey, with little to no dust, although modern blends such as PET or glass-filled nylons can be highly abrasive. Powders, by contrast, are much finer in size than pellets – and some are extremely fine, submicron, powders – and many can also be abrasive in nature, and with variations that may be either lighter-weight or sticky and heavier. And powder is obviously dusty – actually, powder is dust – but like virgin pellets, powders tend to have a consistent particle size. Finally, regrind can be a mixed bag consisting of differing sizes and thicknesses of chipped or flaked plastics, and the quality can depend on the source. In-house regrind– which includes rejected parts as well as tails, moils, runners, and flashings that result from the injection molding and extruding processes which naturally produce waste – is at least a known quantity to the molder, but it can be dusty. Regrind bought from a third-party vender can be a true unknown quantity, probably obtained from the seller from multiple industrial sources and with the possibility of foreign contamination like metal fragments, dirt, dust, and oil in the mix. “There can be a lot more risk associated with conveying this kind of regrind,” said Justin Carter, conveying products manager with Conair Group.

NEW AND IMPROVED

Usually, processors looking to introduce powder or regrind into a conveying system break down into either shops with conveying systems already installed for moving traditional pellets, or shops that want new systems designed with powders and/or regrind in mind.

Let’s take this second scenario first, because it’s becoming the norm. “From our perspective, we’re definitely seeing an increase in requests for regrind conveying systems, more so than powder,” said Don Wood, vice president, systems business development, North America with Novatec. “Our customer base is allowing and/or requiring more regrind to be used in their processes, which ultimately increases regrind conveying system demand.”

The OEMs agree that it’s critically important to identify all of the materials that you process in your plant, and where each material comes from, when designing a new conveying system. That said, whether the customer wants to convey pellets, powder, or regrind, the system supplier will in most cases recommend dilute phase vacuum conveying, which is what most molders have traditionally used anyway. “Dilute phase is the bread and butter of resin conveying for the plastics industry,” said Justin Carter. With a dilute phase system, constant vacuum pressure “lifts” the material to get it flowing through the system and then pulls it to the discharge point, all at velocity fast enough to insure the material remains “suspended” throughout the convey pipeline. Dilute phase systems are easy to control and maintain, OEMs say.

An alternative method is dense phase, where the vacuum pressure is modulated on and off to move the material through the lines in small batches, with air pockets in between. This method is much gentler on the material, OEMs say – meaning less wear and tear on pipeline components – and particularly efficient at moving heavier materials. But dense phase also tends to be more expensive.

Aside from price, a big determining factor is the conveying distance – how far does material have to travel from storage to point of use? “In general, dilute phase conveying is recommended practice especially with a mild conveying distance +/- 500 feet horizontal and reasonable throughput rate,” said Ron Yap, senior territory manager with Novatec. “Dense phase conveying is ideal for longer distance and multiple drop pots with elevated throughput rates.”

A second factor when choosing either dilute or dense phase might be the flowability of the material to be conveyed, which is determined by its size, shape, and weight/bulk density. “Different sizes and thicknesses of material affects the flow,” said Rick Buschini, vice president of material handling sales with Entek Adaptive. “When it comes to regrind, for example, larger sizes tend to interlock and bridge much more.” Regrinds are almost always considered non-free flowing, he continued. “Only occasionally with a very fine size would regrind be considered even close to free flowing,” he said. A big exception in the flowability discussion is film regrind, some OEMs say. “Regrind film flake has low bulk density, which makes it very difficult to convey because the air doesn’t get behind the material as much and can’t create the same pull,” said Jan Rickenbach, extrusion sales manager with Motan. “It requires a good infeed situation to make sure the film flake is picked up properly.” According to Rob Miller, president of Wittmann Battenfeld Canada, another problematic regrind is bottle flake, which has unique flow characteristics and will probably need an extended discharge. “And PVC siding has a thin sheet-like profile that can also cause conveying issues,” Miller said.

The range of flowability for powders, meanwhile, is very wide. “Some powders can be free flowing if the particle size is consistent, doesn’t condense or pack, isn’t sticky, and is usually on the mid-range of density – 25-40 pounds per cubic feet [PCF],” said Rick Buschini. “Others are hard to flow, especially with any hint of moisture content. In general, powders aren’t considered free flowing.”

Differences in flowability notwithstanding, dilute phase is still generally considered better for most powders and regrinds – as well as pellets – for the milder conveying distances. “It’s very rare to use dense phase conveying on regrind, and dense phase is also difficult with powders, especially sticky powders,” Buschini said.

FILTRATION IS KEY

In any new conveying system designed for either powder or regrind, filtration is a top concern. Many loaders and receivers are equipped with filters which prevent the material from moving beyond the receiver. The filter media is sized so that conveying air can escape to the vacuum source, while the material loses velocity and falls by gravity into the body of the receiver. “Different types of filtration are required depending on the material properties,” said Joe Dziedzic, manager of technical sales with AEC, a division of ACS Group. “The level of dust or fines in the material stream will determine the level of protection in the material receivers.” When conveying powders, OEMs say, a specialized loader and filter should be used. “Powder loaders utilize a longer, larger filter area which contains either a ‘bag’ filter made of coated, finely woven filter media, or a cartridge-style filter, to separate powders from the airstream,” said Justin Carter. “The larger filter area is needed because fine powders are tougher to separate from the airflow, so they accumulate more readily on the filter media, where they can reduce airflow and vacuum. To keep powders flowing smoothly and keep the filters clean, Carter continued, powder loaders generally use a burst of compressed air after each conveying cycle. “This air blows backward through the filter to loosen and remove remaining powder, which falls into the base of the receiver,” he said.

Dust collected in filters from regrind or powder collected might actually have a use. “Some shops want to keep the dust and extrude or mold it whenever possible,” Jan Rickenbach said. “They just have to remember that dust particles melt differently than pellets, regrind, or virgin powders.”

The customer may also need special powder valves for conveying and sometimes special technologies for sticky powders, such as conveying lines with bypass lines that blow additional air in the conveying pipe to keep the powder moving. “And some powder has a chemical reaction with aluminum, and in this situation, we recommend using stainless or carbon steel material contact,” said Ron Yap.

Vacuum systems for pellets can just pick up from the bottom of bins, and while this can be done for some powders, it’s not usually recommended, as smaller, lighter particles will pick up first and leave the heavier particles behind. “The other concerns are sanitary in blowback from the filter powders falling out through the make-up air,” said Rick Buschini. “Some materials can be designed with this in mind but many can’t, so in general airlocks are used to introduce powders into material streams.”

For regrind, meanwhile, the conveying system may require lower micron filtration, but this is influenced by the regrind itself. “Depending on the regrind quality and the amount of dust, the system could require same filter elements as with powder in special cases, but could in most cases use flat screen filters and larger discharges,” said Ryan Ismirlian, conveying product engineer with Novatec. For regrind with rough or inconsistent cuts, additional system accessories may be required to help get the material moving or prevent bridging or ratholing. “We might recommend pickup material agitators and/or receiver discharge bridge breakers to influence initial flow,” Ismirlian continued. “Larger discharges on receivers also help reduce bridging and influence the initial flow after a loading cycle.” These bridge breakers, along with pulse blowback, are almost mandatory for light-gage regrind such as sheet or bottle-flake, some OEMs say, where the bulk density could be as low as 18 PCF.

The quality of in-house regrind, it should be noted, depends largely on the quality of a shop’s size reduction equipment. The closer the inhouse regrind is to a clean pellet shape, the better for conveying, and dull or misaligned knives and screens prevent this. “Dusty in-house regrind is a grinder issue, not a conveying issue,” said Rob Miller. “If a customer is using a reasonable beside-the-press granulator or central grinding area and the knives are sharp, they’ll get quality regrind.” Beyond keeping a granulator in good working trim, a processor can adjust the screen size before the material gets conveyed. “This will change the size of the flake and give better flow properties,” said Joe Dziedzic.

GO WITH THE FLOW

Once they can get the material moving, OEMs will focus on designing pipes and destination points. Normally, the diameter of the material line tube is determined by the rate and distance. But in the case of regrind with square particles, large particles, irregular shapes or soft materials, the tube size may need to be larger to prevent clogging. “Powders and regrinds are less predictable and can even surge on a system, so pipe sizing is definitely a key,” said Rick Buschini.

And since powders and some regrinds can be very abrasive, high wear-resistant tubing and bends might be recommended to increase life span. “Aluminum elbows will wear quickly, ceramic-lined or glass elbows are tougher and will last longer, and the HammerTek brand could last for many years, but is also the most expensive option,” said Brian Davis, general manager of Maguire Canada. HammerTek deflection elbows have a spherical vortex chamber that extends partially beyond the 90° flow path, causing a ball of pellets to rotate in the same direction as the air stream, gently deflecting incoming pellets around the bend. Powder conveying along conventional aluminum lines can eventually discolour the material, Davis continued, leading to discoloured parts. “Stainless steel pipes might be the better option for powder conveying,” he said.

At the end of the dilute phase process, the material is typically separated from the conveying air at the point of use and then drained by means of slide valves, butterfly valves, and rotary airlock valves into loaders or receivers. One big challenge in moving regrind, OEMs say, is that flakes with irregular sizes and thicknesses can slip between the rotor and the housing on rotary valves, locking the valves when used for conveying or discharging the material. Some OEMs have developed special rotary valves with combinations of features that prevent locking of the valve.

“Filterless receivers are a great maintenance-saving alternative for dusty materials,” Justin Carter said. “The steeper cone angle can even be advantageous for lighter, harder-to-flow materials. Care must be taken with dusty and low-bulk-density materials to not overfill the receiver in order to prevent material escaping back through the vacuum header to the central dust collection system.” Filterless receivers work well for materials with significant bulk densities like coarse regrind, OEMs say, because they rely in part on gravity for separation. “They’re less effective for extremely light, dusty or low-bulk-density materials, some of which might escape back through the vacuum header to your central dust collecting system,” Carter said.

Both powder loaders and receivers, meanwhile, require a much larger filter area roughly three to five times as large as for resin loaders and receivers, and therefore feature a larger vessel that is typically elongated to accommodate one or multiple filter bags or cartridges.

UPGRADING IT

A second option for molders whose end-customers are demanding regrind and/or recycled material in their products, or that are expanding into molding powders, is to keep their existing pellet conveying systems and possibly add some component upgrades. “Usually, existing conveying equipment will work and can be modified to function with the new material selection, but it will have to be audited by a qualified system engineer to verify what modifications are required and determine any system throughput limitations that can’t be rectified within simple modifications,” said Ron Yap. As with a new installation, a lot will depend on the material being conveyed – the closer the regrind is to a pellet as far as being clean and uniformly shaped, as mentioned, the better the chance a standard pellet conveying system can move it without difficulty. “In a typical application, we’re not going to adjust air volumes or pump horsepower or pipe size or any other conveying parameters just because it’s regrind,” said Rob Miller. “We might get a slightly different rate of conveying with regrind because the particles won’t flow as well through the airstream as round pellets, but otherwise there’s no real difference.”

One exception, OEMs say, might be abrasive regrind, which can damage elbows designed for a non-abrasive pellet application. “It may be necessary to replace these elbows with tougher elbows – ceramic, for example, or even the HammerTek brand,” Miller said. “And depending on the particle size and shape of some regrinds, we might recommend a larger line size to facilitate easier conveying.”

If a pellet conveying system is going to be used for powders, meanwhile, the filtration will have to be upgraded – at the very least – and changes perhaps made for getting the material into the airflow. “The conveying system for pellets won’t work on the receiver side for powders,” said Rick Buschini. “The vacuum loader or pressure blower may be capable but the pellet receiver will allow a significant amount of material to pass into the blower or secondary filter, and the system won’t perform for very long. So, sometimes it can be modified, but often the only piece that can be used is the vacuum loader or filter receiver.”

Powder conveying systems, meanwhile, may have an easier transition to handing regrind than standard pellet systems. “A powder system will already have the filtration system in-house to handle any regrind, as long as it’s granulator regrind and not pulverized regrind,” said Brian Davis. “Equipment-wise, a powder system probably won’t need any new hardware to convey regrind other than small changes relating to how it’s introduced at the machine – it might need a proportioning valve, with one inlet tied to virgin powder and a second tied to the regrind source; or we might have to add a diverter valve to accomplish the same thing.”

MASTERING MAINTENANCE

Along with design and operation, maintenance has a lot to do with how well any resin conveying system meets production goals and objectives. In a standard pellet system, a regular maintenance schedule is recommended – daily, weekly, monthly, and annually depending on what you should check and when you should check it – and must be followed to prevent potential equipment disruption or failure. “With conveying systems that handle regrind or powders, more emphasis needs to be placed on filtration and filter cleaning,” said Joe Dziedzic. “In some cases, wear is an issue, and routine maintenance must be performed to keep the system running smoothly.”

One of the prices of conveying regrind and powders, maintenance-wise, is that elbows and pipes may require particular care. “Powders and powdery regrinds will cake at the elbows, sometimes very quickly, narrowing the inside diameter, so these need to be regularly cleaned,” said Brian Davis. “And with horizontal lines that convey powders or regrind – especially glass-filled regrind – it’s a good practice to rotate them to avoid wear, although not many processors do this.”

As with standard pellet conveying systems, signs of trouble in regrind and powder conveying systems include increased throughput time; drops in dosing or in dispensing volume accuracy; leaks anywhere in the system, especially the joints, connections, lines, and tubes; and an increase in operating pressure or vacuum levels.

Resin conveying is the lifeblood of a plastics processing facility, and powders and regrind are becoming big components of that lifeblood. Conveying them as efficiently as pellets is a necessity to your business, not an option.