Plastics comprise a significant portion of the total volume of most municipal waste streams. According to recent surveys, discarded bottles, bags, packaging and other end-of-life plastic products can represent as much as 20% of the solid waste streams in many communities across the country. More recent estimates suggest that content is on the rise. While plastic recovery rates vary, the wide variety of post-consumer plastic waste holds great potential for recycling.

Once materials have been pulled from the waste stream, processing plastics for recycling is neither complicated nor difficult – provided you have the right equipment. For most plastics recycling operations, the process begins with a carefully chosen granulator. Like primary reducers, shredders or grinders, plastics granulators perform the essential act of size reduction, turning full sized bottles, jugs and other post-consumer products into a uniform blend of flakes or granules that are sold to molders and manufacturers of plastic products as recycled raw material.

How they work
Granulators are essentially electric motors turning a rotor with cutting knives attached inside a closed chamber. While there are several variations in both chamber design and positioning of the cutter knives, most plastic granulators perform the same function. “There are basically three types of rotors in most granulators today. The scissor cut (or double scissor cut); the chevron or V-type rotor; and the helical rotor design,” said Mike Cyr, vice president – sales for Concord, Ontario Canada based Rotogran International, Inc. “Each has their own pros and cons,” he added.

As material enters the cutting chamber, the rotor sweeps by and cuts the plastic between a fixed position knife and the rotating blades. Material remains in the cutting chamber until it is sized to pass through a changeable screen. Pressurized air provides cooling and facilitates material flow through the cutting chamber, exhausting the properly sized flakes or granules through the sizing screen and out of the chamber. Finished particle size can range from .125” to .375” with a widely accepted industry standard nominal dimension of .3125”.

Choosing the right granulator
Plastics granulators are sized according to the feed opening in the cutting chamber. Jack Bowne, vice president – sales for Hosokawa Polymer Systems of Berlin, Connecticut said, “There’s a granulator for every size job, from as small as four by six inches, to as large as 38” by twelve feet. Our largest model is rated at over 600hp. Nearly all granulators are powered by electric motors. But there are a few hydraulic units, and even fewer diesel powered machines in the field,” he added. Hosokawa has been in the size reduction business for more than 105 years.

Choosing the right plastics granulator for your operation begins with understanding your customers’ needs. Specifications for granule size and required volumes will narrow the field right away.

Throughput is another important consideration for choosing a granulator. MRF systems need to be balanced to achieve maximum throughput and efficiencies. Undersized components can create costly bottlenecks that demand considerably more time and attention from operators than is otherwise necessary. When there’s a bottleneck, it’s more than discarded plastic that backs up. Operators typically think of it as money.

One of the most important aspects in choosing a granulator is how quickly the blades can be accessed and changed. Modern designs take into account the need to keep downtimes at a minimum – often by allowing customers to pre-set blade tolerance outside the cutting chamber.

Most granulators are rated in terms of pounds per hour. Inside the cutting chamber, volume and rotor speed are obvious factors impacting throughput, but according to Mr. Cyr, “The real key is the number of cuts per revolution of the rotor. Unless the granulator is designed to cut quickly and efficiently, you just wind up moving material around in the cutting chamber without producing saleable product. It’s activity without productivity. All other things being equal, the granulator that cuts the most material per revolution will reduce and size more plastic per hour,” he pointed out.

Maintaining your granulator
While keeping a granulator in top working condition isn’t necessarily tough to do, there are several precautions that operators should take to ensure minimal downtime, long service life and of course, high quality output. Common sense plays a part.

“Keeping inbound material as clean as possible is important for maintaining sharp cutting edges on stationary knives and rotor blades,” said Mike Cyr. “It’s a harsh environment in the cutting chamber of any plastics granulator. Excessive dirt and other contaminants will reduce blade life, requiring more frequent maintenance on the granulator,” he said.

In addition, processing dirty material may cause premature wear on shaft bearings, which ultimately affects the gap between stationary knives and rotating blades. With worn bearings, the rotor develops excessive play, reducing the quality of cut and throughput capability of the machine.

“When there’s an improper tolerance between the blades, material tends to be torn rather than cut,” said Mr. Cyr. “This means less consistency in finished products and it can eventually lead to more maintenance. It’s very important to maintain the proper blade tolerance,” he said.

To help keep things clean, Hosokawa Polymer Systems offers an optional “de-dusting” system. This air driven attachment removes significant quantities of dust from the cutting chamber and exhausts it away from the rotor and blades. According to Jack Bowne, “The de-duster can extend blade life and ensure better quality cuts while reducing operating costs in the long run.”

Other granulator maintenance considerations include keeping electric motors clean and lubricated, as well as inspecting sizing screens for damage, clogs or other signs of wear.

As recovery rates for recycling plastic increase, more and more materials recovery facilities will rely on granulators to convert bulky, discarded plastic containers and other recyclable products into saleable raw materials.