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Question and Answer

Strategies for Optimal Diecutting

by Kevin Carey, DieInfo

February-March, 2009
How Do I Run Faster with Less Nicks?
Nicking always will be a controversial subject because each nick or tag is a permanent disfigurement to a diecut product, which often is used as a point of purchase display or sales tool. A nick is simply a gap ground into the profile of a steel rule die to create an uncut tag of material, which is designed to link one diecut part to an adjoining diecut part (illustration 1a). By creating a distributed pattern of nick/tags/links, the entire loose collection of diecut parts can be moved at high speed from one processing unit of the diecutting press to the next.

Illustration 1a Illustration 2a

In answering the question of running at higher speed with fewer nicks, it is important to define what are the forces and/or stresses that cause each nick to fracture and fail? These forces would include friction and drag, tensile stress and static, and acceleration and deceleration forces. However, the primary cause of nick/tag failure is because the diecut part snags or catches a lower female converting tool or press component as it is moved forward at high speed (illustration 2a). This can be the female crease tool or matrix, the lower female stripping tool apertures, or cavities in the lower blanking grid.

As the entire process relies upon the smallest connecting tags possible, it is obvious that any part of the diecut sheet catching or snagging any obstacle as it is accelerated forward will cause a disastrous sheet break-up. Compounding this inevitable problem is the difficulty of discerning the sources of the break-up, as the debris is cleared from the press.

Illustration 3a

To eliminate all of these problems, the solution is to "fly-the-sheet" (illustration 3a). This is a very simple but highly effective technique, which requires adding or building in resilient lifters or ramps onto or into the lower tools. These flexible ramps, which can be made from thin metals, plastic, or paperboard, support the sheet as it is drawn across the lower tools into position. On impression the lifters collapse; however, as soon as the platen opens these flexible ejectors gently lift the diecut parts/sheet clear of the lower tools. As the diecut sheet is accelerated forward, the smooth ramp profile of each lifter ensures the diecut parts clear any obstructions in the lower tools and smoothly flow into the next press section. This simple technique ensures maximum speed with minimal nick/tags and provides optimal productive output.

How Do I Eliminate Diecut Part Flaking?
One of the more common failures in diecutting is called flaking or edge chipping. This is generally a series of tears in the paperboard liner or flakes of material, which although obvious to see, still remain attached to the diecut product through secondary fiber (illustration 1b). This is a serious issue, because there are no practical methods of removing the flakes of material and/or of disguising or hiding the problem, and the carton or the diecut part generally has to be scrapped.

Illustration 1b Illustration 2b

Although the term ‘cutting’ is used to describe the diecutting process, it is more accurate to describe platen diecutting as a displacement process. This is because the material is split or separated by driving a knife into the material; however, the separation of the material is achieved not through primarily knife edge pressure, but by the action of the knife bevel angles, pushing the material away from either side of the central edge of the knife cutting edge (illustration 2b). This lateral splitting or displacement force drives the material at right angles away from the vertical stroke of the blade, causing the material to fracture before the knife has fully penetrated the material, and often the force causes the material to prematurely shear and split laterally (illustration 3b).

Illustration 3b

The most obvious solution to this problem is to reduce the lateral force of the bevel angles of the knife, by selecting a knife with a lower bevel angle. For example, if a material and/or a design shape were susceptible to flaking, the chance of flaking would be far less likely if a knife with a bevel angle of 40 degrees were selected rather than a knife with a bevel angle of 50 degrees.

Illustration 4b

As this often happens on-press when there was no preparation in the die to use a lower bevel angle, the most effective option is to use ejection material to stabilize and prevent premature lateral displacement of the material. To do this, use a standard band saw to cut narrow strips of a dense ejection material with the walls of the strip at a specific angle. By placing these strips on either side of the blade where flaking is occurring, the angle shape of the material will cause it to collapse inward, toward the knife as it is compressed (illustration 4b). This lateral push toward the knife clamps prevents the material from being pushed away from the knife until the cutting edge has completely penetrated the material.

Why is Steel Rule Die Calibration so Important?
The foundation of effective platen diecutting is the ability to quickly set a precise kiss-cut impression, which can be sustained for the entire production run. One of the problems with platen diecutting is the excessive amount of patching, shimming, or paper tape application to a paper image of the die layout positioned behind the steel rule die (illustration 1c). This is the primary focus of press makeready, and it is often the cause of considerable lost productive time, as the steel rule die knives get progressively further damaged and more and more patch-up is required.

Illustration 1c

Is there a solution to this problem? Yes, there is. There are two key disciplines required in platen diecutting to ensure optimal kisscut performance every time. These are press calibration and steel rule die calibration. One of the dangers of being complacent is the mistaken acceptance that a diecutting press is perfectly flat, precisely parallel, and resistant to mechanical deflection under load (illustration 2c). The second critical discipline is steel rule die calibration. This is a simple but sometimes challenging discipline to implement.

Illustration 2c

The steel rule knife, which is commonly inserted into the steel rule dieboard, is an extraordinarily precise and flexible cutting tool. The important height dimension is maintained to an exacting tolerance of plus or minus 0.001." This begs the question: If the press cutting surfaces are flat, parallel, and deflection-free, and the knife is accurate to within one thousandth of an inch, why is it necessary to apply several layers of patch-up tape to the underside of the knife to get it to cut?

Illustration 3c

As you can imagine the press is never perfectly flat, parallel or deflection-free, and the dieboard is rarely delivered to the press with every cutting blade seated perfectly and set at precisely the same height, in a warp-free dieboard (illustration 3c). So what is the alternative?

The solution is to calibrate or footprint the press. This is a simple discipline that takes only a few hours and will permanently eliminate all potential press cutting variables. The second solution is to calibrate the steel rule die. As with press calibration, this is a simple discipline, which adds a small amount of time to making the steel rule die, but which results in optimal kisscut performance.

Calibrating the steel rule die will minimize patch-up; it will improve quality and yield; it will enhance diecut part quality and consistency; and it will reduce makeready and lower operating cost.

DieInfo is an education, training, and publishing organization focused upon optimizing productive performance in diemaking and in diecutting. Its innovative program "Diecutting Works" integrates weekly individual and team coaching to implement fast press changeover and to sustain optimal throughput in diecutting. For more information, contact Kevin Carey at (206) 427-9297.