Sizeable question: Choosing shredders or granulators

Photo Credit: WEIMA America Inc.

We’re calling it: Given the high prices of resin, the supply chain problems hampering delivery, and the trend toward circular economy initiatives, the days of plastics processors treating their scrap as an afterthought are over. Scrap – both startup waste and imperfect product – is just too valuable these days to throw away, and reclaiming it through size reduction might just make the difference between profit and loss.

But in order for a plastics processor to get the most from its scrap – whether it’s turned into process-ready regrind for in-house production, or sold to others for recycling – it has to be reduced to a manageable and uniform size. Which is where granulation and shredding equipment come in.

Problem is, since shredders and granulators both reduce the size of scrap plastic, it’s easy to think of them interchangeably – to think that any of them are equally up to the task of size-reducing plastic in the same manner. And just to make it more confusing, much of the different kinds of scrap that might be generated in a plastics processing plant – pipe and profiles, sprues and runners, film and sheet, and small and large molded parts – can theoretically be size-reduced by both granulators and shredders.

But confusing the two is a mistake; worse is actually using them interchangeably, which could cost your company the thousands of dollars you’d hoped to save by reclaiming scrap in the first place, because usually you won’t get the particle size you want. It’s not the most common problem in size reduction, the OEMs say, but it does happen, mostly with used machines that are either inherited or bought at auction or from a broker. “The used market for size reduction equipment is very extensive, and you can often find a granulator or shredder available for half the price of a new machine,” said Joe Platek, business development manager for size reduction with ACS Group, which owns the Cumberland brand of granulators and shredders. “But just because it’s a great deal doesn’t make it the right equipment for the job.”

So, how do you determine which of the two size reduction technologies is the right one for your your needs? You begin by a) understanding the working principles of each; which then allows you to b) understand the differences between them, which are significant. Granulators, which are a common sight in plastics processing plants, primarily work by shearing material into fine, uniform particles the size of virgin pellets, just like with scissors. The range of granulators available is extremely broad, and can be classified into two main groups: beside-the-machine models used to grind relatively small volumes of sprues, runners, off-spec parts, and edge trim from film lines for immediate recycling back into the process; and bigger, more powerful central granulators that are often located in a room separate from the production floor, and which are used to chop large volumes of scrap, often from multiple processing lines or molding cells. Both granulator types, though, operate at high speeds with relatively low torque – even so-called “low-speed” granulators have rotors that turn at upwards of 190 rpm and standard-speed granulators operate at 400 to 500 rpm or more.

Shredders are the opposite in that respect: they tend to operate at lower speeds, usually between 100 to 130 rpm, with high torque. Shredders tear the plastic materials apart, and can chew through almost anything, generally reducing big chunks into smaller, manageable pieces – but not to a small, uniform particle size comparable to virgin pellets. “Shredders in single-shaft designs, which cut down against one or more stationary bed knives, are preferred for plastics,” said Greg Parent, the Canadian sales agent for Vecoplan. “Dual-shaft shredders aren’t used much in plastics anymore.”

KEY QUESTIONS

When deciding between a shredder and a granulator, a series of questions need to be considered, centred around the volume and type of scrap to be processed, the feeding method, and the final required condition and end-use of the shredded or granulated bits.

Typically, granulators don’t have a minimum throughput rate, whereas shredders do. A properly sized granulator can chop thousands of pounds of scrap as easily as a few pounds, with the only limiting factors being the size and configuration of the feed opening and cutting chamber and the need to avoid over-feeding and jamming the rotor. “The bigger granulators today – 150 to 200 horsepower – can handle large parts provided they’re not too thick,” said Dave Miller, general manager for size reduction with Conair Group. “With granulators, it’s more about the thickness of the scrap.”

Shredders, on the other hand, don’t normally work efficiently – and sometimes won’t work at all – at extremely low throughputs, especially single-shaft shredders, which use a horizontal hydraulic ram to drive scrap material into the cutting area at the intersection of the rotor and the stationary knives. “The more scrap there is in the feed bin and the heavier it is, the easier it is for the ram to push it forward into the rotor,” Miller said. “And unlike granulators, whose rotors are fed scrap by gravity, shredders monitor and control both ram speed and rotor speed to assure that scrap feed rates are optimized for aggressive shredding without overloading.”

And the type of scrap material being size-reduced matters too. Granulators can handle a range of small, medium, and even large parts, provided the parts are thin-walled and the scrap fits in the feed opening. “Good materials for granulators are low to medium volumes of blow molded and injection molded parts, thermoformed materials, runners, sprues, bottles, profiles, and trim scrap,” said Andrew White, inside sales manager for plastics with WEIMA America. “For shredders, meanwhile, all the above-mentioned are suitable – as long as they’re being batch-fed – as well as large blow molded, injection molded, rotational molded, whole or half gaylord boxes, broken bales and even full bales, film, and plastic purge.” When it comes to these off-spec rolls of plastic film and fibre, OEMs say, special cutters are available for these shredder applications to ensure that long strips of film and fibre strands don’t wrap themselves around the rotor. A problem with shredders, however, is that they can have a tough time with loose, undensified objects. “Pipes, bottles, and other lightweight objects have the tendency to bounce around in the shredder, away from the blades, so that cutting efficiency goes way down,” said Mike Cyr, president of Rotogran International. “This type of loose, lightweight scrap doesn’t pose a problem if it’s manually or conveyor-fed to the cutting chamber of a granulator.”

THE PURGE

Plastic purge, mentioned above, is another dividing line between granulators and shredders. Usually generated during injection molding startup or as a waste product during extrusion, purgings and other similar lumps should definitely not be fed into a granulator, most OEMs say – even a central granulator with a hog rotor and plenty of horsepower – for the simple reason that most are simply too big and thick: they can be up to several inches thick and weigh thirty or forty pounds. “Putting purgings into a granulator reduces throughput and increases wear at a minimum, and can potentially cause power spikes,” Andrew White said. “Worst-case, they can damage the granulator – I’ve seen instances where the frame/housing has fractured and they have to be welded back together.” To avoid these problems, companies that recycle cold purgings with granulators often cut them into smaller pieces using a band saw or a similar tool. “The caveat is, some shops don’t allow band saws on the floor because they can be dangerous,” said Joe Platek. “For them, a shredder may be a necessary investment if they want to reuse the purgings.”

Plastic injection lumps in a shredder. Photo Credit: WEIMA America Inc.

There’s a bit of grey area, though, when it comes to regrinding purgings of a certain size and temperature, some OEMs say. “Small, warm purgings can be put into a granulator that’s been customized to handle them, such as with an open rotor to allow air flow and water-cooling jackets around it,” said Mike Cyr. “But if the purgings are either too warm or actually hot, taken right after the purge, they can smear on the granulator’s side walls, causing fines.” This same modified granulator could also handle size reduction of blow molded tabs and tails, Cyr continued, which are warm and often roughly equal in size to purgings.

Shredders, on the other hand, are ideal for purgings of any size, period.

FEEDING TIME

When it comes to feeding, the difference between granulators and shredders is stark. Granulators are not built to be shock loaded – instead, the material has to be metered in. “One of the compromises with granulators is that they need to be fed smaller amounts of scrap continuously, either by hand or by an automated feeding system, such as a robot, conveyor, or some other special feeding mechanism,” said Greg Parent. “Overfeeding it with scrap can flood the cutting chamber and jam the rotor.”

Shredders are just the opposite: Designed for aggressive shredding, they require only the very simplest feeding systems, and actually work best when heavy, dense scrap is simply dumped into the feed hopper, with the hydraulic ram that’s built into the bottom of the hopper then pushing the scrap into the rotor – often called the “dump-and-run” strategy. “Most single-shaft shredders control the introduction of the material to the cutting rotor rather than relying on gravity,” Parent said. “By having the material sit on the shredder floor and using a ram that can speed up, slow down, stop, or reverse based on the amp load of the rotor, jams and operator intervention isn’t required.” And if the rotor does jam, he continued, it can quickly reverse to clear the problem. “Shredders not only dispose very easily of one or even two full gaylords of material at time, they also save on labour because the operator can then walk away and do other jobs,” Parent said.

THE ENDGAME

Of all the differences between granulators and shredders, probably the biggest is the end-product: the form of the scrap after size reduction. Shredders, as mentioned, tear the scrap up, with the sizing determined in part by the size of the holes in the shredder classifying screen – which can be as large as two inches, or 50.8 millimeters, in diameter to less than half that size – but with even the smallest hole diameter still producing a shred that’s larger than granulated material. “A common complaint is that customers use a shredder with a very small screen hoping for finer particles, but they just get smaller shredded material, with rough edges, that’s still larger than virgin pellet-size,” Mike Cyr said. “Using smaller screen sizes with the shredder also causes production values to drop dramatically.” With granulators, sizing screen holes are usually one-quarter to three-eighths of an inch, or 6.35 to 9.5 millimeters, in diameter, so the particles passing through tend to be granular, close to the size of virgin pellets.

All of which matters if the goal is to recycle the material back into the process. “Shredded material usually needs a secondary granulation process to have the optimal size and uniformity needed to flow and blend with virgin material and other additives in the processing machine,” said Dave Miller. “On the other hand, shredding alone is usually very effective if the material is going to be shipped off to a recycler, because uniformity doesn’t matter quite as much as long as the size is manageable.”

In short, shops looking to reduce small plastic scraps into valuable, process-ready regrind definitely need granulators, since a shredder won’t cut it for that job. “Anytime a processor is using only a shredder, it’s probably the wrong machine, unless they’re recouping purgings,” said Mike Cyr. They may even want the two-stage solution of a shredder and granulator paired together, in order to avoid the labour-intensive prep work needed to get heavy, dense scrap such as purgings and thick-wall pipe or sheet ready for granulation – with this set-up, the shredder performs the coarse size-reduction work, with minimum operator involvement, and the scrap is then fed into the granulator for final sizing to yield a consistent, uniform feed material. In this very common scenario, the processor doesn’t have to choose one or the other while stilling processing at a good rate. “Pairing the shredder with a granulator ultimately gives you the best result, and we’re seeing more interest in these two-stage systems from our customers as the labour force decreases,” said Andrew White.

And if floor space constraints are a factor, OEMs say, consider stacking the machines, with the shredder on top. “Putting a shredder on top of a granulator saves space and also allows the granulator to reach and maintain its maximum throughput capacity,” Mike Cyr said. “The only downside to a stacked system is that it doesn’t allow for metal detection, but this is more of an issue for recyclers – it shouldn’t be a problem when size-reducing in-house scrap, which is a known quantity and won’t have foreign materials like tramp metal.” Some stacked systems are sold as a single unit from a single vendor. Zerma’s ZCS series stacked shredder-granulator combination, for example, provides a compact size reduction system in which the shredder and granulator are individually driven, allowing for better control of the complete process, Zerma said – the shredder is driven via a gearbox on the shaft end on one side of the rotor, and the granulator is driven by a separate belt drive.

In the end, when choosing a granulator or a shredder – or a granulator and a shredder – it’s critical to examine the working principles and characteristics of each piece of equipment as well as the needs of your factory. “There are some applications where a granulator will handle large parts and that’s sufficient for the process, and other times where it can’t and a shredder should be brought in for pre-sizing,” Joe Platek said. “Either way, it’s always best to discuss the full application with a technical expert so you can weigh the options and find the best fit.”