After the product has been designed and approved, after the tools have been specified and fabricated, after the press make-ready has generated a quality diecut part, the goal is to maximize productive output. Diecutting productivity is a classic race against the clock, where the speed of the press must be optimized; waste minimized and yield the key measurement. But this simple objective is made endlessly complex by a simple reality. As the press is accelerated, the diecut sheet breaks apart, the press has to be stopped, the jammed material cleared and the operator seems to have two options, before restarting.
The first is to slow the press down, as this seems a logical move to reduce stress and strain on the diecut sheet.
The second is to increase the size of the tags or nicks holding everything together, with the idea of making the diecut layout stronger and more resistant to tensile stress.
The problem with both these options is they are dealing with the symptoms of the problem, rather than the underlying cause of the problem. This means no matter how effectively these options are implemented, they will never come close to maximizing productive output. The key factor everyone is ignoring is contained in the following statement:
Diecutting is a converting manufacturing process
This simply means diecutting is an alignment of integrated but separate diecutting or processing disciplines. The correct method of analyzing the sheetfed system is to recognize there are several manufacturing operations, which are linked as the material is transported from the feeder to delivery. See illustration 1.
Therefore, to maximize productive output, it is essential to consider the impact of each type of diecutting manufacturing, and the stress and obstacles presented by the high-speed transportation of the diecut sheet from processing centre to processing centre.
Why do nicks fail on-press?
The critical tool to hold the collection of diecut parts together is the connecting tag of uncut material we call the nick. See illustration2 .Unfortunately the problem with this essential transportation device is it is a permanent disfigurement of the finished product. This requires the professional toolmaker and diecutter to understand all of the forces acting upon each nick in processing and transportation, and to take action to eliminate or significantly lower the forces stressing, fracturing, and prematurely breaking each nick.
So, what are the factors impacting on-press nick performance? There are 12 different forces undermining nick holding power:
– Cutting knife bevel angle and displacement force
– Excess diecutting pressure and knife edge damage
– Keeping the diecut sheet flat and diecut parts aligned
– Diecutting draw and knife-to-knife and crease-to-knife competition – Material parameters and grain direction
– A poorly integrated nicking and ejection pattern
– Simultaneous stripping and blanking stress and tool flexing
– Acceleration speed tensile stress
– Deceleration speed compression and the concertina effect
– Press, gripper and chain condition
– Air trapping and compression
– Gravity, friction, static and drag
Platen diecutting is a displacement process, where the bevel faces of the knife wedge generate 75% of the splitting force. See illustration3. Therefore, the most aggressive force splitting the nick or tag apart is the knife itself. See illustration 4. To eliminate this first force, it is essential to reduce the bevel angle of the knife in the areas where nicks will be placed. However, it is not simply just a recommendation to change all of the blades to a low bevel angle, but to change the knives where nicks are positioned. The lower the bevel angle of the knife, the easier to penetrate and to diecut, however, there is less metal to protect the sensitive tip of this knife-edge, and the lower bevel angle would suffer more damage. See illustration 5.
Excess diecutting pressure and knife edge damage
The entire focus of the platen diecutting process is to convert with the lowest pressure possible; however, most of the damage is done to the knives in the die, during the on-press make-ready sequence. In practical terms this means the knife makes contact with the steel
cutting plate and the tip of the knife is swaged and damaged. See illustration6. This type of damage has several disadvantages, but in terms of nicking, the width of the wedge penetrating the material has increased, the lateral displacement force has increased, and the force splitting the nick or tag apart is now significantly higher. See illustration 7.
This additional displacement force is why so many nicks must be added, or the width of existing nicks increased as the production run progresses. Naturally, the solution is to calibratethepressandthesteelruledie,andtoexecuteaprecisepress levellingprocedure, suchas two-sheetpatch-up.Thegoalis todiecutwithminimalpressure,to eliminate knife-edge damage, and to generate a very rigid cutting action.
Keeping the diecut sheet flat and diecut parts aligned
When a nick fails, the assumption is the width of the nick must be increased, to give it greater tensile strength and holding power. While this is not an entirely illogical conclusion, it does little to attack the source of the nick failure problem. The challenge of diecutting is one of transportation, as the diecut sheet must be moved at high speed from press unit to press unit. Most nick failure and sheet break-up occur during the transportation phase, from the platen into and out of the stripping unit, and from the stripping unit into the blanking or delivery unit. The problem in this movement of the sheet is if the diecut parts are not perfectly flat and aligned with every other diecut part in the sheet, they will snag and catch on the lowertools and press components. See illustration 8. No matter how strong the nick or how many nicks are used, these are two opposing forces and the result will be sheet break-up every time. The solution is to keep the diecut sheet flat, eliminate flexing and part misalignment, see illustration 9, and implement devices such as lifters,
brakes and a press funnel, to smooth the diecut layout and keep the diecut sheet flat. See illustration 10.
Diecutting draw and knife-to-knife and crease-to-knife competition
If we make the mistake of examining the forces acting upona single knife, it would not give a true picture of what is happening in diecutting. Every other knife is initially pulling the material toward the valley created by the pressure of the knifeedge, see illustration 11, and then pushing from the centreline of the knife as soon as the tip has penetrated the surface of the material. In the same way other components, suchas creasingrules are driving the material into the female tool,the counter or matrix channel, at the same time. See illustration 12.
These are called ‘Draw’ forces in platen diecutting and although invisible, they have a dramatic impact on the entire converting process. In reality, platen diecutting is the conversion of a vertical force into a lateral action, see illustration 13, and in practice the lateral pull and push of the steelrule die components, has the greatest impact on performance. This knowledge drives the use of ejection material to isolate, and protect the nick during and immediately after the diecutting cycle is complete.
Material parameters and grain direction
There are clearly many factors undermining the strength of the nick tag, however, it is the material and the material characteristics which provide the resistance to fracturing and failure. The habit of using the calliper or thickness of the material to determine the size of the nick is obviously an important factor, but it is more important to recognize the relative strength of each material and the fibre content and fabrication methods, which created each paperboard. For example, although a recycled paperboard, virgin hardwood fibre material, and a Kraftfibre content,mayhave identical thickness, the difference in holding powerforthe same size nick ortag is significant. See illustration 14.
In assessing the impact the fibre strength of each material has on the size of the gap ground into the steel rule knife, it is important to recognize the impact of the major fibre orientation or grain direction of each material. For example, in a nick alignment parallel to the grain direction the cutting action is splitting the fibres apartlaterally, while nicks aligned atright angles to the fibre direction are attempting to literally pull each fibre apart. See illustration 15. Therefore, fibre characteristics and the orientation of the fibre to the nick are critical factors in on-press failure or success.
A poorly integrated nicking and ejection pattern
Ejection performs a key role in diecutting. First, it clamps thematerial againstthe cutting anvil to stabilize it in preparation for knife penetration. Second, it isolates the material to improve the action of the knife. Third, it minimizes the impact of accumulated draw forces puling and tugging at the newly formed nick or tag. And, fourth, the rubber provides balanced ejection to keep the diecut sheet flat and all theparts aligned.Itis impossible tonickwithout integrating ejection tools oneither side of the nicked knife to stabilize, to isolate, to minimize, and to balance all of the forces conspiring to undermine the formation of the nick ortag. The ejectionmaterial shouldbe a ‘profile’rubber speciallydesignedto collapse in toward the knife as diecuts the material. See illustration 16. Unfortunately, one action,whichwill always undermine nicking performance, is to nick the knife and the rubber at the same time. See illustration 17. It is vital to recognize the key role of ejectioninevery facet ofplatendiecutting,butparticularly importanttounderstand the relationship between ejection and the nicking pattern.
Diecutting manufacturing is a race against the clock and every second and every impression are critical to success and to commercial survival. A fast, precise make-ready is essential; however, the ultimate test of any converting organization is the amount of quality product produced for every hour of operation. One of the most critical disciplines in maximizing on-press productive throughput is the selection and execution of a precise nicking pattern. To design and implement the most effective nicking pattern it is essential to understand all of those forces which act upon the nick ortag to undermine its strength and holding power. This is the first article in a two part series, designed to focus upon those factors, which make or break diecutting productivity.
Kevin Carey is the technical director of Die-Cutting Works. He can be reached at 1-
518-932-7856 or by email at firstname.lastname@example.org. For more information, visit www.dieinfo.com .
This article is reprinted with permission from the International Association of Diecutting and Diemaking’s monthly magazine, The Cutting Edge, July 2002. The IADD is an international trade association serving diecutters, diemakers and industry suppliers worldwide. IADD provides conferences, educational and training programs, a monthly magazine, online resource library of 450+ technical articles, industry experts to answer technical questions, publications and training manuals, recommended specifications, onlineused equipment marketplace, videos andmore. IADD also co-presents Odyssey, a bi-annual trade show and innovative concept in technical training featuring a hands-on Techshop where training programs come alive in an actual working diemaking and diecutting facility inside the exhibit area. Visit www.iadd.org or call 1-815-455-7519 for more information aboutIADD.