- February 01, 1996, Boyle, Edward
Neglecting to give the appropriate consideration to the unwind and unwind roll handling can be a critical mistake.
The unwind stand can be one of the most overlooked, yet critical, pieces of equipment utilized in the converting process, according to John D. Wilkes, chief executive officer of John Dusenbery Co., Randolph, NJ. "The unwind and unwind roll handling comprise a much neglected area of focus that warrants more attention than it is generally given."
That is because the unwind must not only be capable of bearing weights of hundreds or even thousands of pounds, but it must work precisely with your printing or converting equipment. And, while an unwind is typically purchased in conjunction with your printing or converting line, changes in the type of material being converted may result in the need to purchase a different type of unwind stand.
Wilkes notes that, in general, there are five different types of unwinds: shafted, shaftless, center driven, surface driven, and turret. A typical shafted unwind has three main advantages, says Wilkes: It is cost-effective; it can handle varying width rolls; and it provides the maximum possible grip between shaft and core. In contrast, shaftless unwinds avoid the need for the operator to handle shafts, while turret shaftless unwinds minimize changeover times since a new roll can be loaded into the stand while the other roll is unwinding.
Taking the Unwind Plunge
How do you know just which type of unwind is right for you?
Robert Rodemich, president of International Machinery Inc., Alpharetta, GA, says that the type of unwind offered with any particular piece of equipment is usually determined by the size and weight specifications of the material that the machine is designed to handle.
For example, film is generally sold in small roll diameters to 32 in.; paper is sold in 40- to 60-in. rolls; and board is sold in diameters of 60 to 80 in. These roll-diameter limitations reflect the various weights and yields associated with the thickness of the various substrates. In essence, says Rodemich, a 60-in. roll of board may have less lineal feet or yards of material than a 32-in. roll of film. So the size and weight of the material to be run tend to dictate the size and, to some degree, the type of unwind chosen for any application.
A 32-in. roll of film weighing several hundred pounds would easily fit on an 80-in.-diameter unwind stand-but an 80-in.-diameter roll of board weighing a ton or more could not possibly fit a smaller-diameter film unwind.
"We are fond of telling our customers that the easiest way to tell what materials an existing machine was designed to handle is by the diameter of the unwind and rewind," says Rodemich.
Just as the type of material to be processed dictates the physical size of the unwind, it also dictates the tension control and/or the braking aspects of the unwind. Large-diameter rolls of reasonably heavy material require heavy-duty tension and braking systems to control the weight of the roll. If this same heavy-duty tension control is applied to lightweight extensible film products, the products will stretch or break.
Wilkes notes that driven unwinds are used when the weight of the unwind roll is too great for the roll to be accelerated by the tension in the web alone. "This is the case for very light materials or for very wide webs in which the steel core must be of massive construction," says Wilkes.
Large-diameter unwinds normally require larger core shaft and core sizes to accommodate the weight of the roll. Most films and papers are produced on 3- and 6-in. cores, whereas board is produced on 8-, 10-, and 12-in. cores. This is reasonably easy to understand if you can envision an 80-in.-diameter roll of board weighing a ton or more, says Rodemich. Suspending the weight of this large roll on a 3-in.-diameter shaft is likely to bend or break the shaft, hence the use of larger core shafts and core sizes on heavier grades of material or roll weights that are quite heavy.
"Once you understand that the nature and weight of the material to be run dictates the physical size of the unwind, its tension and braking system, and the size and type of core shaft to be used, it is reasonably easy to determine the type and configuration of the unwind stand that you choose," explains Rodemich.
Single Versus Multiple Positions
For example, single-position unwinds have the capacity for one roll at a time and require the stopping of the associated machine after each roll has been run off. Multiple-position unwinds with splice capabilities allow continuous machine operation.
"In terms of expense," says Rodemich, "single-position unwinds are the least expensive type of unwind - if only initial cost is to be considered. If the cost of lost production time and material waste are considered over the anticipated useful life of the machine, the economics may well swing in the direction of multiple-position splicing unwinds with the advantage of continuous operation."
Unwinds are usually offered in shafted or shaftless models. The advantage of shaftless models is less operator fatigue and injury, says Rodemich. Most of the shaftless unwinds are fitted with roll-lifting devices that position the roll prior to engaging the chucks to hold the roll. This eliminates the need for the machine operator to lift or maneuver heavy core shafts prior to setting the roll into the unwind. As the industry grows more conscious of the cost of employee injury, both Rodemich and Wilkes note, more companies are moving in the direction of shaftless unwinds with roll lift devices.
An alternative to the shaftless design is the new lightweight carbon fiber shafts associated with newer shaft designs. (Please see the related feature on core shafts.) In many cases, says Rodemich, the carbon fiber shafts are less than half the weight of the earlier solid steel or pneumatic core shafts without any loss of roll weight capacity. The lighter shafts, therefore, handle the traditional roll weights without the operator having to deal with excessive shaft weights. Supplier Information:
Regular shaft inspection and maintenance should be part of the manufacturing routine.
Of all the variables that can affect the performance of your core shaft - such as roll weight, core diameter, and web speed - the most common problem is often the most preventable.