The role of tension control in the slitter rewinder
Among the most common packaging materials, we can find: BOPP, OPP, CPP, PET, PVC, LDPE, HDPE, complex materials such as PET + ALU + LDPE, paper.
In most cases, these are flexible, non-stretchable materials that normally need to be unwound, processed (cut, laser-etched, perforated, …) and rewound into reels of different diameters and widths.
During unwinding and subsequent rewinding, it is important to maintain optimum material tension in order to achieve uniform, defect-free finished reels.
How to ensure optimal film tension control
As far as slitter rewinders are concerned, it is important that they are designed, constructed and equipped with the right equipment to stick to this requirement from the moment when the material enters the machine.
During the unwinding of the parent reel, it is important to entrust the management of the material tension to a servo motor (to be precise, an asynchronous vector A.C. motor managed by a digital inverter), rather than simply to a pneumatic brake. The latter is still present in the machine, but only for the management of a hypothetical electrical “black-out” or an “emergency” stop.
The servo motor in the unwinding zone is an additional motor to the two normally present on the slitter rewinders: the pull motor and the rewinding motor.
The additional servo motor in the unwinding zone, where provided, guarantees specifically this extremely precise and high-performant control of the automatic unwinding tension, which is thus controlled directly by the machine’s computer, with no time waste and no operator interaction.
Functional advantages of the servo motor in the unwinding system
Having this additional servo motor also means “regenerating energy” during operation, energy that would otherwise be dissipated into the environment in the form of heat from the pneumatic brake, resulting in wear on the pads and the emission (albeit limited) of dust due to the wear of the pads themselves.
All our machines, whether compact or with an independent unwinder, are suitable to support the third servo motor, the one dedicated to controlling the unwinding of the parent reel.
In particular, the SRP 162/182/SVI series of slitters also features a shaftless type of independent unwinding carriage as standard, a configuration that is particularly well-suited to being complemented by the servo-motor.
Advantages of the independent shaftless unwinding station
The independent unwinder configuration typical of SRPs also offers other advantages, for example:
• It allows much easier access to the cutting units to replace them if broken or worn, to reposition them, etc.;
• It is equipped with a bonding bridge for the aerial passage of the film;
• the film splicing table on the unwinding unit is also “standard”. This detail is not to be overlooked when perfect film splices are to be made on the new parent reels being loaded into the machine;
• possible sources of vibration are consequently placed away from the machine body.
High-performance friction shafts for perfect winding
Another element that makes a difference when it comes to keeping the tension of the processed material constant is the rewinding shafts, which must be equipped with state-of-the-art clutches.
Our slitters and rewinders are equipped with clutched shafts with special, newly patented rings, which are assembled on ball bearings in order to minimise the moment of inertia during activation/anchoring.
In addition, they are extremely light in weight as they are made of a special aeronautical aluminium alloy, with anti-wear heat treatment: the weight per unit is exactly one third of that of the comparable traditional steel ring.
The intrinsic lightness of these special friction rings is also fundamental for their “handling” during spindle size changeover (from 70 to 76.2 mm and vice versa). All this results in a moment of inertia reduced to a minimum, which makes these rings extremely high-performance, guaranteeing exceptional control over the stresses transmitted to the processed materials.
