By Tom Kleeman, Spartanics

There are a growing number of card applications requiring not only die cutting of parts from a sheet or web, but also precise die cutting of internal features within the part. Examples are the GSM cards that have die cut tabs for internal SIM modules used across Europe, the loyalty cards and accompanying keychain tags now ubiquitous in the United States.

Internal Holes
The punch used to create internal holes and similar clean edge cut out sections of the part’s interior can be made with standard types of punch tools and involves the same technical considerations as in standard blanking of parts. In such blanking and internal hole punching, the key technical consideration is the clearance between the punch and die, and how far it penetrates into the die cavity. These are the same technical considerations one needs to address when cutting the outside perimeter of a part.

To achieve the required quality, one correlates the clearance between the punch and the die with the thickness of the material to be cut. If the substrate is tissue paper thin, a very tight fit of the punch and die is required, such that there is essentially no clearance. If one is cutting thicker materials, such as 0.030 inch (0.76 mm) plastic cards, there is instead a need for considerable clearance. As a general rule of thumb, the punch should be smaller than the die cavity hole diameter by as much as 1/10 the material thickness, no matter what the substrate is; whether it is a metal, plastic, paper, magnet or other material. That is a first approximation, and it is the extremely rare application that does not then need fine tuning of clearance to achieve optimum quality and the exact hole dimensions required. In fact, the hole one ends up with is always smaller than the punch diameter because the material tends to close up. This is because you are relieving stresses in the material that existed before the hole was punched out. In applications where the size of the hole is critical, numerous iterations in punch diameter may be needed to achieve the desired dimension. There are also some subtleties in hold-downs and strippers that are typically addressed in tight tolerance applications, which are beyond the scope of this discussion. Reputable manufacturers of precision die cutting equipment will not only have experience in fine tuning tool design but will have stocks of representative tools available that can be used for sample production and to determine the ultimate tool design for a specific application.

Scoring
To achieve a score line with the desired characteristics one needs to adjust the thickness of the blade and the angle of the knife, i.e. the chisel point at the end. The sharpness of the knifepoint and the speed with which the tool is cycled will affect the end result. The spacing of the teeth on the perforating knife is another variable that is adjusted to achieve the desired end result, and the many ways in which one can vary the teeth shape and spacing is reflected in the varieties of score lines that can be created.

The mechanics of scoring are quite different from that of a standard punch. Unlike standard male/female tooling where a punch fits into a die, the scoring uses a knife blade to cut the material against an anvil. Usually the anvil is made of a soft steel that enables it to “give” a little, i.e. 0.00010 inches (0.0025 mm), such that the blade is not dulled too quickly. On a microscopic level one would see that the zero clearance goal is never quite achieved, but rather the blade cuts the anvil a little bit with each stroke. Unlike the soft steel anvil, the die block itself is made of tool steel, which would dull the blade very quickly if the anvil were not in place.

Typically, the blade has a 60° angle. The tooth pattern used for scoring is varied both by pitch (number of teeth per inch or mm) and depth (the relief or groove between the teeth). For a straight perforation, the tooth depth would be the thickness of the perforation such that one is essentially jabbing holes in the material. Or, to get a score line, the teeth would have a depth that is less than the material thickness. In some applications, such as with a material as strong as PVC, there is often a combination of scoring and perforating. More sophisticated tooling also arranges the blade itself so that it creates partial cuts; in other words, is actually a knife without a tooth pattern.

Blade thickness is also varied for different applications. Thickness of blades is conventionally described in point sizes, in the same dimensions that points are defined for a font. This is because the earliest scoring and perforating applications were done on letterpresses and the blades were created in specific point sizes just like movable type. A thickness of two points (i.e. approximately 0.028 inches or 0.7 mm) is quite common in applications such as specialty plastic cards.

The sequence and timing of how the punch and blade cuts are orchestrated is a critical part of die design. The punch always leads the blade because you want the punch to go all the way through the material to the die block, whereas the blade only kisses the anvil. The difference in height from the two cutting operations, i.e. the standard punch and the scoring, is called the punch lead. It is important to note that the punch lead is built into the design of the tool and is not something that one can adjust while the tool is on the press, and therefore examples the importance of sourcing tools from die cutting system suppliers who are well-versed in a specific application and will guarantee the tool’s performance vis-à-vis punch lead and similar parameters.

The type of punch press being utilized will also impact performance when it comes to these more complicated tools. For a blank through plastic card application a press of 15 tons (14,000 kg) might be adequate. However, presses with tonnage in excess of 30 tons (27,000 kg) are required to get the stability and horsepower needed for maximum performance when one is scoring or perforating. Otherwise, the added tool features needed to create internal die cut shapes will push the lighter weight presses beyond their capacity. Scoring, for example, takes a fair bit of power in a punch press because it is easier to cut through material than to push cut it against an anvil. A scoring blade works when the material is deformed, pushing it to the left or right. As a rule of thumb, a half ton of press capacity (500 pounds or 220 kg) is needed to make an inch along a score line.

In scoring tools, or any blade type tools such as steel rule dies, a die stop is also built in for protection. This stop, which is essentially a block of steel on both the top and bottom of the tool, ensures that the tool cannot close farther than the distance needed to allow the blade to work. This keeps the knife blade intact even should the press be slightly misadjusted. This feature is required when one is working with blades but is not a part of standard punch tooling, which are usually just adjusted in terms of how much the press closes during a job set up.

Steel Rule Dies
Steel rule dies have an entirely different construction than hard tooling. Typically, a hole punch is created by a part that is machined separately, sometimes it is just a section of tubing that has been sharpened, or sometimes it is made by bending the cutting knife material. Typically steel rulle die boards are made of high quality plywood, although some use synthetic materials.

The overriding advantage of a steel rule die is its cost, which is about 95% less than the standard blanking tool. This means that a US$150 steel rule die can be used in lieu of a US$30,000 hard tool. Another advantage is the relatively quick turnaround time to create a steel rule die. If one has both the part design and a working relationship with an experienced steel rule die maker, the turnaround time for a steel rule die is typically 48 hours or less. Moreover, job changes are very quick with steel rule dies. One simply slides a wooden board in and out of the press and a completed job changeover can be expected in 5 – 10 minutes. The better optically-registered gap press systems will align steel rule dies in the correct position and do so in a manner that is fully repeatable with each job setup.

With steel rule dies as with many things, the adage “You Get What You Pay For” is quite relevant. The advantages of steel rule dies are counterbalanced by several disadvantages or limitations that make them inadequate for many applications. The main disadvantage of steel rule dies is their low mechanical accuracy, due to the tendency of the cutting blades to bend or deflect while cutting. While the cutting boards may be precisely engineered, the cuts they create due to the instability of the blades are not as repeatable. For example, a part with a desired diameter of 1 inch (25 mm) may be off by as much as +/-0.002 inches (0.05mm), which is not very good dimensional stability. For applications where dimensional stability is not critical, steel rule dies are likely to be fine. However, for applications such as the standard CR80 cards used for financial card applications, they cannot be used, because varying card dimensions may be such that the cards jam in card readers that depend on cards’ dimensional stability. Another inherent disadvantage of steel rule dies is their short life. In the best cases, a steel rule die can be cycled 150,000 times. Most of the time, depending on the material being cut, the cycle number is far less than 150,000, perhaps as low as 10,000.

Steel rule dies are return-to-web dies, meaning you need the expense of some sort of parts extraction process and the labor time needed to operate machines to effect parts knockout.
There are basically two main schemes for parts knockout. One way is to use a pneumatic punch to knock out parts. A drawback with this method is that the knockout station needs to match the tool one uses, or has to somehow be adjustable to accommodate different shapes and parts arrangements, which can be quite a drag on production for plants doing many job changeovers for shorter runs.

An alternative method uses the principle of extracting cut parts by moving the web through a sharp bend over a small radius, via pulling it over a roller or equivalent. As the web turns, the cut parts are freed and can be stacked up or collected. This solution is easier for web applications, but there are also parts extractors that are able to grab new strips in sheet-fed applications that use the same principle of web deflection to separate parts. Web deflection is sometimes a superior method because of its versatility to handle different shaped parts without any adjustment of extraction tooling. On the other hand, the web deflection method may have difficulty with certain part shapes and the need to keep them properly oriented in order to reliably extract the parts. For many complex parts with internal features, a combination of different parts extraction methods used in sequence is often recommended.

Tom Kleeman is CEO of Spartanics (www.spartanics.com), which engineers and manufactures a range of automated equipment for die cutting, punching, laser cutting, counting, and inspection used by global card manufacturers among others finishing flat stock material. Its worldwide service organization also maintains offices and spare parts in Germany. Questions and comments can be forwarded to tkleeman@spartanics.com.