Selecting the Right Unwind System
Unwind Stands can be supplied in different arrangements and modifications. To facilitate the selection process for either Surface or Centre Unwind Stands, the advantages and limitations are discussed:
Surface Unwind Stands- Manual Splice
The basic surface unwind stand incorporates an air shaft and canvas strap with weight, for a surface brake. This unwind is used for flexible packaging under low tension, low speeds and does not need additional tension controls. The tension is regulated by the amount of weight provided in one canvas pocket.
Advantages: Very simple, efficient and inexpensive system. Tension is applied to one surface. Therefore, it is constant during the unwinding cycle, regardless of roll diameter.
Limitations: Tensioning weights must be handled by the operator. Tension selection is limited to weight inventory. Roll momentum causes film to continue unwinding during line stops which limits maximum line speeds to 45-60 MPM (150-200FPM). Canvas drag weight will scratch sensitive films, especially high quality, printed material.
Surface Driven Unwind
An improved version of this unwind is a surface driven unwind for thin, extensible films. A driven roller is applied on the surface of the unwind roll and the speed of the roller is controlled by a brake or a variable speed motor. This unwind is not suitable for rigid slippery films, high speed unwinding or frequent stopping.
Surface Unwind Stand- Double Drum
The roll of the film is placed on two rollers and the rollers act as a brake. A centre shaft through the roll or other means must be used as a guide.
Advantages: Suitable for wide films where handling of the shaft is difficult or where the shaft would bend under the weight of the material.
Limitations: Not suitable for slippery films and high speeds. Problems with marking film due to heavy rolls.
Surface Unwind with Centre Assist
Advantages: Centre Assist takes advantage of the support that the double roll Surface Unwind provides. It uses the centre brake on the core of the roll to maintain tension with one of the methods ( to be further discussed). This unwind stand is suitable for the majority of films.
Limitations: Since the entire roll is supported on the surface, the risk of marking increases with soft films or heavy rolls.
Centre Unwind Stands
Centre unwind stands can be driven or non-driven. The roll is supported by either an air shaft, mechanical or pneumatic chucks. A cantilevered shaft is available as an optional feature eliminates operator handling of the heavy air shafts. The unwind stand with centre shaft is restricted to the weight and deflection limitations of the air shaft.
Advantages: Suitable for most applications, even for high-speed, slippery films. For frequent starts, especially with thin extensible films, a driven unwind shaft is recommended. The Shaft less Unwind Stand with centre-locking chucks can be mechanically or pneumatically locked, for one width of core. Only a small rotational movement of the chucks is required to lock it. For variable widths, one side or both sides can move line early. Ideal for most applications where the roll is self-supporting and does not bend in the centre. Most unwind stands can be driven and are self-loading. Self-loading eliminated over-head cranes, saves time and provides safe operation.
Limitations: Relatively expensive. More difficult to control tension. Limited by air capacity.
There are various combinations of unwind stands for specific applications. Macro custom-designs unwind stands & unwind/rewind systems for special materials or unique applications.
Automatic Splice Unwind Stands
Automatic Splice Unwind Stands can be comprised of a combination of any of the single stands, previously mentioned. They are typically used where stopping the web for roll change-over is not economical or where the process requires continuous operation.
Most of the basic configurations of the auto unwind are : Zero Speed Splice and Full Speed Splice. For the zero speed splice, the web is spliced at zero or low speed during which the film is drawn from an accumulator under full speed during splicing.
For the full speed splice, the new roll is pre rotated to the full web speed and the tail end of finished roll is spliced together using adhesive, to the leading edge of the new roll. The remaining film of the finished roll is cut off.
Both methods, if properly designed caad do a good job of auto-splicing. For high speed applications, the full speed splice is preferred. This eliminates the need for a large accumulator (Note: the greater the running speed, the larger the accumulator that is required). The zero speed splice method is best suited for some rigid and difficult-to-splice materials.