Strong and Reliable Couplings for Plastic Shredding

A growing concern for the environment and waste reduction is driving the growth of the demand for plastic shredders and recycling systems. These machines are a challenge for component suppliers such as R+W, the coupling manufacturer, as they have to match very demanding standards of reliability and strength.

by Stefano Vinto

Cutting down on the waste of raw materials, plastic first and foremost, is a must when it comes to environmental sustainability. This is one of the reasons for the remarkable growth in the demand of plastic shredders and recycling systems; the segment is highly relevant for the high-precision mechanical component industry.
According to the 2018 report on recycling in Italy “L’Italia del riciclo”, global plastic production added up to 335 million tons last year. In Europe packaging accounts for 40% of the total and almost 80% of this amount is regularly recovered and recycled.
In Italy, the national Consortium for the collection, recycling and recovery of plastic packaging estimated that the weight of waste collected by means of sorting adds up to a million tons.

Shredding machines in the plastic industry: an increasingly important segment
Italian companies which produce machines for plastic shredding can offer plants suitable for processing not just new polymers, but also increasingly high percentages of recycled materials, with reduced energy consumption and high-performance finished products.
Therefore, the transition towards a circular economy, which at a first glance could seem to be a threat for plastics, is in actual fact a growth opportunity for the makers of machines, equipment and moulds.
In modern plastics industries, recycling is a fundamental activity instrumental both as regards the type of product manufactured and the processes used.
The choice among the different methods of plastic recycling (mechanical, chemical etc) is linked to the degree of contamination of the plastics being processed, as a consequence of the presence of organic materials (other polymers) or inorganic substances (various impurities). Another factor which determines the choice of the process is the molecular structure of the plastics involved, just like the presence of transversal chemical bonds, as in the case of thermosetting resins or rubbers.
A shredding machine or shredder can reduce the size of the plastic: there are different types of machines, equipped with different cutting systems (with one shaft, two shafts or four shafts). The cutting system also affects the choices of the designer when it comes to protecting the kinematic chain.

Single shaft configuration for high speed shredding
This type of machine is characterized by high levels of reliability and versatility and by reduced machine down times.
This configuration is most often present in those shredding machines used in primary mechanical recycling activities and is typical in those applications which envisage a high shredding speed.
The material pushing system drives the usher which moves on the profiled slide on the cutting chamber. The pushing system is conceived so as to prevent the loss of material when the pusher is activated. The profile of the slide prevents the pusher from being blocked and makes it easier to process materials in sheet or thin foil form. Such a protection system does not, however, represent the only solution for the protection of the kinematic chain. It is therefore advisable to proceed with an accurate choice of the kinematic couplings, so as to protect the electric motor from accidental blocks caused by the random introduction of impurities. For best results, torque limiters are used, which protect simultaneously the motor and
the gear unit, with extremely fast action items and with rapid and practical reassembly operations. One of the most frequent design solutions for this type of machine, is the integration of the gear unit within the cutting rotor.
Another design choice, frequent in this type of shredder, concerns the cutting rotor equipped with bolted tool docks.
The special shape of the tool is conceived so as to provide different choices both in terms of the cutting edge (by means of rotation of the tool and its repositioning using the bolt) and of the optimization of the withdrawal services, on which the material is conveyed.
Other fundamental elements of the machine are the perforated screens, whose purpose is making the granulometry of the shredded materials as uniform as possible.

Shredding machines with two and four cutting shafts
In shredders with two shafts, the cutting
system is obtained using high strength steel alloys; the cutting chamber is equipped with suitable stripping systems, to prevent the processed material from ending up wound around the cutting rotors.
Machines with four cutting shafts, also meant for other types of materials, represent the optimal shedding choice in those contexts where secondary mechanical recycling is carried out. These machines are characterized by a lower functioning speed with respect to those previously seen. Among the other highliths there are the low maintenance requirements and by a lack of sensitivity to the presence of foreign objects contained in the material which needs to be shredded.

Strong and reliable couplings to satisfy the requirements of shredding machines
Working in such a dynamic industry implies the capability of satisfying its requirements; first and foremost, as regards the essential waste shredding machines, the utmost strength and reliability. R+W products meant for this type of machines must be very strong, to bear relevant efforts, but at the same time must provide an element which can preserve all mechanical components concerned from motion in case of overload so as to protect the entire kinematic chain and prevent machine down times due to the breakage of some component or even of the shedding tools.
For this reason, for this type of application, the ideal solutions are provided by the use of couplings for high torque values with an integrated torque limiter. R+W Italia offers different solutions which vary depending on the complexity of the machine. In shredders or plants destined to plastic recycling high torque coupling s are used which belong to the series dedicated to power transmission. Specifically, the couplings used are the EK series elastomer couplings, the LP series disc pack coupling and the BZ series of gear couplings with limited backlash. When a torque limiter is needed, the optimal choice is a limiter from the ST series, which may be integrated into any one of the above-mentioned couplings.