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,”
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
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
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
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,”
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,”
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.