It is common to find a shredder or granulator on the floor of a plastics facility today, and measures must be in place to ensure the safety of operators and anyone who may be near the equipment.
The machines are essential for the recycling of plastic materials. However, they can be dangerous due to sharp rotating knives that, in some cases, can chew up material as large as car bumpers.
There are typically two kinds of granulators in shops today that either accept whole scrap or are designed to be secondary and handle pre-shredded material. They are efficient machines that can run from 125 RPM to 250 RPM. They rely on momentum and use solid, heavy rotors.
There also are a number of blades attached to the rotor shaft shear against stationary blades or anvils attached to the granulator frame. A sizing screen allows the material to recirculate until it is small enough to pass through the screen.
Shear-type shredders are slow-speed, high-torque machines that are fairly economical to operate.
Multi-shaft shredders deliver a defined particle size and reasonable throughput based on the screen opening size. Care must be used when choosing the cutter thickness and screen size since throughput can be reduced by excessive recirculation.
With options that feature blades and crushing rotors, safety is always a concern. Manufacturers in recent years have looked at ways to make shredders and granulators safer for operators.
As a safety option, companies today will install zero-speed solenoids that deny access to the chamber until a shredder or granulator is fully stopped.
The chamber should not be accessed for at least three minutes to ensure the machine is turned off. The zero-speed solenoids can help with this issue.
Another safety option is a positive-locking solenoid, which can prevent operators from opening access points. With the positive-locking solenoid, operators cannot open access points.
Another trend on shop floors today is the installation of dual-channel redundant safety interlocks. With this option, an operator can’t access the machine while it is running if even one channel fails.
The cutting chamber can usually only be accessed through the hopper area, or the front door of the machine. With an interlock in place, if that area is open, the machine can’t be turned on.
Granulators can be designed with custom hoppers to allow large pieces of plastic to be fed safely by hand when a conveyer cannot be used, due to space concerns or other issues.
Shredders offered today feature a design with a door on the front of the unit that can be accessed without an operator reaching inside the machine.
Manufacturers also offer shredders with a rotor motion sensor that ensures the rotor is completely stopped before a delay timer permits access to the cutting chamber. Rotors are somewhat sluggish, revolving for several seconds after power is turned off. Operators can slip and make contact with sharp knife edges while handling rotors.
In yet another nod to safety, a removable handle or tool permits the shredder rotor to be rotated safely through the gearbox externally. The rotor does not have to be touched.