- September 01, 1995, Abderholden, Mark
A dual rotary knife retrofit on an older sheeter can be a cost-efficient way to upgrade your operation with many of the benefits of new equipment.
Today, more and more converting plants are seeking ways to improve their sheeting operation. With recent dual rotary knife technology, there is significant potential to produce better quality sheets, improve productivity and reduce waste.
Although the cost of these new sheeters is low and can be justified, they still may be too expensive for many small to mid-size converting plants. These plants have an option to improve sheet quality by installing a dual rotary, direct-drive knife retrofit into their existing sheeter.
Some sheeter manufacturers now offer a high-performance, stand-alone integral knife retrofit for virtually any existing sheeter. This new development provides many of the benefits of a complete new sheeter at a substantially reduced price. A knife retrofit can result in improved cut quality and accuracy, reduced waste, increased production and the ability to run higher calipers (greater than 0.028 in. or multiple webs) and short sheets.
Start with Two Questions
To determine if a knife retrofit is right for your company, you must consider the following two questions:
1. Is the existing sheeter still in good condition?
2. Is your production limited by your current sheeter?
Let's look at the first question. To determine if your existing sheeter is in good condition, be certain that the layboy/plier area remaining after the knife retrofit is mechanically sound. To improve stack quality upgrades can be made to this area, such as pallet center/offset piling, tape belt replacement and the addition of improved side tampers and backstops.
However, you don't want to spend a great deal of money trying to update a machine that is too old or worn out. Knife retrofit can be done on the following sheeter models:
Clark Aiken: Models K, D and C. Older models (1930 and 1940) are usually not worth rebuilding;
Moore & White: Models R75 and R100;
Hamblet: Models HHD, MHD and SHD;
Lambs Gray: most models.
Other sheeters no longer being manufactured may also be good candidates for a knife retrofit if they are in good mechanical condition.
Now for the second question: Is your production limited by your current sheeter? There is no one set rule that can be applied that says a knife retrofit will meet or exceed your production requirements. You have to investigate each application based on specific order mix and material run; however, depending on the sheeter type and configuration, you can experience a 30%-40% increase in production in most cases.
Adding a new knife section will increase production without sacrificing quality by allowing you to run both long and short sheets faster, although in some old sheeters the overlap design of the layboy may limit the maximum line speed. Some manufacturers offer high-speed tape belt outfeed upgrades to replace this section, allowing for higher-speed capabilities on the existing sheeter. This may also simplify the physical interface to the existing sheeter during installation.
What Modifications will be Needed
A dual rotary knife retrofit will require minor modifications to the existing sheeter. P.I.V. mechanical drives cannot be driven off the main knife motor. A small motor and drive are needed for the lower-speed tape belt sections after the knife. These changes are relatively simple and inexpensive.
You cannot achieve the best possible cut accuracy without proper web tension control. Either a load cell or swing arm dancer can provide web tension, the latter being preferred because it will compensate for egg-shaped rolls. The addition of good chucks and pneumatic disc brakes on the unwind stand is essential to improving web tension control.
Flat sheets are becoming much more critical with new high-speed, sheet-fed presses. The faster the press, the more sensitive it becomes to curl in the sheet. Additional de-curling (multiple stations) should be added directly before the knife if curling is a problem.
The benefits of a dual rotary knife are many:
Cut quality - Dual rotary knives create a cleaner cut that reduces dust and fiber pull.
Cut accuracy - With proper web tension, cut accuracy to [+ or -]0.015 in. (0.038 mm) is possible on a dual rotary, direct-drive knife. Cut accuracy is held not only at constant speed but also during acceleration and deceleration of the sheeter. This is not the case for most older fixed-bed (single rotary) knives.
Short sheet runnability - The minimum and maximum sheeted length for a fixed-bed sheeter is a function of the knife cylinder diameter. Although the maximum practical range is 3 to 1, most manufacturers incorporate at least a 4-to-1 range. If a 4-to-1 range is used, significant speed reductions are encountered on the shorter sizes due to the tendency of the single rotary, fixed-bed knife to kick the trailing edge of the sheet. On the longer sizes, the speed is reduced to eliminate excessive dust on the cut edge.
Dual rotary, direct-drive knives with synchronous cut speed eliminate both problems.
Performance - Dual rotary knives are faster and quieter than single rotary, fixed-bed knives
Quick makeready - On older sheeters gears have to be changed and P.I.V. pulleys need to be adjusted along with the squareness on all single rotary, fixed-bed knives. This usually takes five to 10 minutes per makeready and wastes 30 to 40 sheets in the square adjustment process. On most dual rotary knives that are di-rect-driven, ready change can take seconds (with virtually no waste), eliminating the need to adjust for squareness.
Maintenance - Older single rotary, fixed-bed knives require a great deal of blade maintenance, especially when running high calipers (greater than 0.028 in.) or recycled paperboard. It may be normal to replace the knife blades on a monthly basis, if not more frequently. Blade replacement usually takes eight hours. Dual rotary knives typically last nine to 12 months or longer before they need replacement.
What About Payback?
A simple payback analysis is shown in Table 1 for a typical converting plant with a single rotary, fixed-bed sheeter. These parameters for a small to mid-size converting plant show a financial justification of $136,977/year for retrofitting a dual rotary knife into an existing sheeter.
In terms of total sheets produced, assuming a sheet length of 36 in., you can expect to produce 8,000 to 9,000 sheets/hour after installing a high-performance, dual rotary, direct-drive knife onto an existing single rotary, fixed-bed sheeter.
With a dual rotary knife retrofit there is significant potential to produce better quality sheets, improve productivity and reduce waste. The retrofit for your existing sheeter may be a low-cost option that provides a significant improvement in productivity and quality while reducing waste in your sheeting operation.
Mark E. Abderholden has worked for Marquip for 11 years. He has been field service engineer and European field service manager; and he is currently head of the Sheeting Div. as sheeter marketing manager. Mark has authored papers for numerous trade magazines, including TAPPI's. He has a B.S. in mechanical engineering.