Die Cutting Parts with Internal Die Cut Features–Tool Selection and Other Technical Considerations by Tom Kleeman, CEO

There are a growing number of 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, or the slots one sees on luggage tags that enable the tags to be affixed to luggage straps. When a die cut part has internal features such as holes or breakaway tabs, standard tooling usually needs to be replaced with one of three options—progressive dies, compound dies, or variants of steel rule dies. In addition to simple holes or other internal cutaway shapes, internal features often involve scoring or perforating. In this white paper, we will discuss how one selects and configures tooling for die cutting such complex parts with internal die cut features, and related technical considerations for successful implementation.

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, which are beyond the scope of this discussion that are typically addressed in tight tolerance applications. 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

A scoring blade as shown in (See Figure 2: Score and Perforate) presents new issues that are likely to be unfamiliar to those who have only used simple punch type tooling. Among the areas that require adjustment in order to achieve a score line with the desired characteristics are 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 60o 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. In this figure, the blade is shown on the right side of the diagram in blue, and is barely penetrating the material as it meets the anvil (depicted in white). On the left hand side of the diagram the punching operation is shown, with the blue punch penetrating the material to knock a hole in it. The arrows show the difference in height from the two cutting operations, i.e. the standard punch and the scoring. This height is referred to as 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, these 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.

Separate Dies

One might ask why complex tooling is required if all features could be created in two distinct operations by two separate tools. In other words, why not first use a tool for perforation or scoring, and subsequently pass the material through a standard blanking system to cut out the finished part perimeter?

While this is not unheard of, generally speaking it is not recommended nor practical. Such a two-tool-two-step process would inherently double the material handling, requiring a re-stacking operation between cutting operations. This means also that the chances of making bad parts are multiplied because if either operation is out of adjustment, the finished product will be off. Scrap rates therefore become prohibitive, as does the labor and costs of machine time to run two different operations.

Progressive Dies

The progressive die in which two or more tools are built side by side in one physical housing, can be used in lieu of separate dies. With the progressive die, as the material cycles through the press different features are created in
different stations as the tool cycles. For example, (see PDF copy with illustrations) in this figure one sees the right side of the tool blanking a part while the left side of the tool is creating a score line and perforation. So, for example, as the material moves through, part 4 is being blanked out while part 3 is getting perforated. Note that there can be more than two stations within a progressive die set up, depending upon the complexity of the desired finished part.

In the progressive die set up, tool progression must be carefully considered. The tool progression is defined by the center-to-center spacing between two stations in a tool, and the tool is built to the specifications of the parts’ step up. The tool must be very accurately built to match this step up, and the artwork similarly must be properly printed to enable – this matching.

There are numerous advantages to the progressive die set up. For simple shapes, progressive dies have a moderate cost, perhaps only 50% more than an ordinary blanking tool for that application. Progressive dies can be used flexibly such that one can opt to use them only as blanking tools if an application does not require score lines or perforations. This ability for double duty allows one to handle diverse applications with one tool. For example, one could be making luggage tags and credit cards with the same tool, simply by removing the tool part that creates the internal slots of the luggage tag when credit cards are being run. Or, one could design the heights of the different tool elements so that adjustments in the press shut height result in presence or absence of certain die cut features in the finished part. Because one can potentially modify the penetration of a tool in an infinite number of ways, it means that the tool can be as versatile as the forethought in its design allows. This means that for an incremental cost increase a tool can be built that is flexible enough for a wide range of applications.

There are distinct disadvantages of the progressive die set up, however. Registration can be problematic because printing must absolutely match the dimensions of the tool, or one has station-to-station positioning errors that result in bad parts. If printing does not match the tool step-up (progression) to specification, one has to choose between positioning material for correct tolerances in the blanking along the outside perimeter or choose to have the internal features positioned correctly with the blanking off kilter. If there are cut-to-print registration problems the best a progressive die that cuts at two stations will do is create two times the errors. Thus, a progressive die set up can never match the accuracy of a tool system that only has to position material once.

Another inherent problem is the throughput of this type tool since a progressive die set up means there is always at least one extra press cycle to complete a part. For example, if one has a strip with 10 parts drawn in it, the tool will need to cycle 11 times to make the parts. This means that for every 10 parts made one is losing 10% of machine throughput. Or, if there were five parts per strip, the loss would be 20% of machine throughput, and so on, according the relative size of the machine and parts. For plants that are operating close to capacity, this cost factor can make other types of tooling for complex parts more economical, even if they have higher upfront costs.

Compound Dies

Compound dies are essentially two or more tools built inside each other.

Putting all these pieces together the red substrate successfully die cut with all internal features in one press stroke.

Unlike progressive dies where finished die cut parts fall away from the cutting area, a compound die is always a return-to-web type die.

There are many ways to design a compound die, but since there is no place for the finished part to go during a compound die’s operation, the part must be pushed back into the scrap web such that it can then be carried out of the tool and extracted in one or another fashion later in the die cutting operation. This necessity for a separate parts extraction process is one downside of the compound die system.

There are numerous advantages of a compound die system, first and foremost being the high and unsurpassed mechanical accuracy of a single step process. The relative locations of punched hole, score lines, and perimeter cuts are exceedingly repeatable and as accurate as your die maker’s skill. Using the better optically-registered gap presses, for example, one can count on 0.1 mm accuracy in cut-to-print registration, which is highly important to complex parts.

A second advantage of a compound die set up is its throughput. Because all internal and perimeter features of the part are created in one cycle there are no lost cycles. That means that if a strip is designed to create 10 parts, these 10 parts will be created in 10 press strokes.

Compound dies typically have many extra moving parts such as shedders and knockouts and require very high tolerance machining work to create. For that reason, compound dies are typically 2 ½ time the cost of standard blanking dies (i.e. an additional 150% cost), and if an application does not require the type of accuracy that a compound die affords, they typically cannot be cost-justified.

The complexity of the compound die also means that they are inherently less reliable. For example, they are typically more sensitive to hitting double thicknesses of material or similar jamming or feeding problems with material positioning, as compared to standard blanking dies. This in turn usually translates into a slightly higher maintenance schedule, and complex dies cannot typically achieve the standard of the best blanking dies that only need to be re-sharpened every 3 million cycles.

The required knockout systems needed for parts extraction also involve more cost, both for the knockout mechanism and for the time it takes to do the knockout step of the operation.

Steel Rule Dies

Steel rule dies, have an entirely different construction than hard tooling. The stripper section of the steel rule die, shown here in green, is made of a rubbery material that can collapse when the part is being cut and then spring back effectively pushing the part off the blade. 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 the 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 U.S. $150 steel rule die can be used in lieu of a U.S. $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.

Like compound dies, 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 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 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.

Summary – Tooling Options

Selecting tooling for complex die cut parts with internal features boils down to knowing the real requirements for mechanical accuracy in an application and the expected run lengths. For long runs of applications with moderate requirements for mechanical tolerances, the progressive dies are usually the best match. If, however, there are very tight mechanical tolerances in a long run job, a compound die can be cost-justified. For short runs (e.g. 5, 000 or 10,000 parts) of applications with loose mechanical tolerances, very inexpensive steel rule dies are the tooling of choice. Because of their low cost and the short time it takes to make them, steel rule dies are often used for prototyping jobs that might ultimately use one or another hard tooling because for any job on the order of 1 or more million parts it is often less expensive to pay for one hard tool than to continuously replace steel rule dies.

Free consultations on the selection of tooling type and tooling design are typically provided free of charge by manufacturers of die cutting systems. A word of caution is to ensure that consultations are only done with manufacturers that are equally versed in all of the tooling types discussed in this white paper—progressive dies, compound dies, and steel rule dies—in order to ensure that there is objective unbiased input that will find best match technology for the production challenge at hand.

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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 printers, card manufacturers, label manufacturers and other converters, among others finished flat stock material. Its worldwide service organization also maintains offices and spare parts in Germany.

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