Perforating tear-resistant materials takes a big step.
- Published: September 01, 1996, By Ulvila, Richard
How do you perforate tough materials that are constructed not to tear? New perforating technology reportedly has the answer.
When we attempt to perforate tear-resistant materials such as plastic films, we are trying to create controlled tearing of materials that have been intentionally designed not to tear. A new technology has been developed specifically to deal with this production paradox facing many converters.
In simple terms, perforating is controlled, partial cutting. The usual purpose of perforating is to provide a particular desired strength (or hold) that keeps a material intact until the separation of that material is required. At that time, a force is applied in order to separate, or tear, the material.
One of the most commonly perforated materials is, of course, paper. By using paper perforating as a starting point, we can better understand the related problems of perforating many tear-resistant materials, such as plastic films, foils, and tear-resistant plasticized papers.
Let's start with some of the most popular steel rules used for paper perforating. Table 1 shows rules that would satisfy most of the common requirements for paper perforations.
Now consider the following common variables:
1. The general and specific application requirements outlined by the customer.
2. Process requirements, such as multiple handling, folding, etc.
3. The type of stock and its special properties: paper, plastic, foil, cardstocks, thickness, layers, virgin, recycled, etc.
4. Bevel choice, such as centerface or sideface.
5. Recognition of the difference between tear strength and burst strength.
Other possible variables may include age of machinery, runout, web tension, operator proficiency, type of perf holder being used, etc.
Percent of Hold
"Percent of hold" is the percent of material remaining uncut after a stock is perforated. It is calculated by multiplying the number of teeth per inch (TPI) by the tie (space) in decimals. For example, 8 TPI x .032 in. = 26%. Burst strength is a function of this percentage.
Determining the optimum percent of hold is a function of the first three variables listed above. For example, recycled paper has shorter fibers and may tend to burst more easily. If a customer using 8Tx.032-in. perforations with virgin paper switched to recycled paper, we might suggest a 10Tx.032- or 12Tx.032-in. perforation, depending on the extent of the shift in burst characteristics. (Note that in these examples, tie size, .032 in., remains the same, but the number of ties changes. This approach to rule selection can minimize appearance change, while providing the desired percent of hold.)
Tear strength is a function of the individual tie's burst strength, so that an 8Tx.032-in. perforation and a 4Tx.064-in. perforation have the same percent of hold and, therefore, the same burst strength, but the 8Tx.032-in. perforation has one-half the tear strength, because the tie is half as large. This is a simple model to be used solely for approximation.
Plastic Film Versus Paper
Plastic films have specific characteristics that make perforating them extremely difficult. Actually, the properties that create plastic's usefulness and versatility are the same properties that make it so difficult to perforate - specifically, strength, ductility, and plasticity.
The new technology developed for solving this dilemma is UltraMicroperf. The problem was easy to identify but difficult to solve: It required the development of the world's smallest hold or space between the teeth. Previously, the smallest space or tie available was approximately .007 in.
Innovative, proprietary technology was developed to reduce that dimension to as little as .002 in. As a result, the perforation of plastic films and other tear-resistant materials is now possible, allowing these materials to be perforated in such a way that they perform in much the same manner that conventional perforations perform on standard paper stocks. This capability provides many product and production opportunities.
Perforating tear-resistant materials requires precise cutting to be combined with ultra-small ties. How small a tie is a function of the ductility of the material. The more ductile the material, the smaller the tie required.
Table 2 includes examples of several Ultra-Microperf configurations. The applications are listed where there has been specific success with a particular pattern.
Practical Applications
Let us examine several specific examples of what this new perforating technology makes possible, many of which are demonstrated on the New Technology Sampler that accompanies this article.
A major government agency had a serious problem with a special p-s paper label pulling away from the carrier sheet at the lead and/or trail edges of the label. This problem occurred as the labels were being coiled into tight rolls, and it required a perforating solution. Among the approaches considered were a 77-tooth, a 100-tooth, a 120-tooth, and a 160-tooth. All the patterns created a virtually "invisible" and effective perforation, but the 120-tooth with a .0045-in. tie (54% of hold) proved to be the ideal solution.
Several methods of stamp perforations have been proposed to the US Postal Service, including several "invisible" patterns, such as 70-, 100-, and 120-tooth, all of which provide a super-clean, crisp perforated edge.
To emulate the classical appearance of a round-hole perforated stamp, a mock round-hole cutting edge was produced and then micro-toothed with various patterns of perforated edges. All of the perforations separated precisely as designed, with unwanted tearing into the stamp itself virtually eliminated - not to mention the elimination of scrap between stamps.
Also, by perforating any material, such as paper, film or foil, with dense patterns of UltraMicroperf, the perforated areas cannot be disturbed without showing evidence of tampering.
A major pharmaceutical company has approved UltraMicroperf for blood sample vials, a critical and demanding application. The technology also permits fan folding of plastic films, tear-resistant materials, and foils, allowing these types of materials to perform like standard paper stocks.
With a much sleeker bevel minimizing stock displacement and distortion, UltraMicroperf can also be effectively used with laser-printed products, because tenting is dramatically reduced, and copy can be placed closer to the perforations - yet still print clearly.
Possibly the most promising and exciting use of this technology is its ability to produce labels without carrier sheets. Simply explained, this process involves the microperforating of the label stock (which would often employ a future-activated adhesive). At point of application, the microperforated label would be removed from the surrounding stock (looking like it had been cleanly die-cut), [TABULAR DATA FOR TABLE 2 OMITTED] the adhesive would then be activated, and the label would be applied. The obvious advantages of carrierless labels include economy, weight reduction, and the elimination of nonrecyclable carrier sheets - the latter providing an important ecological benefit.
Richard Ulvila, director of research and development at Zimmer Industries Inc., has been a recognized leader in high-technology engineering and R&D for over 30 years. At Zimmer he contributed to the development of a host of advanced cutting and perforating rules and flexible dies for the converting industry. He can be reached at 800/225-0108 (in NJ 201/427-8000).
