Rotary Die Types:
In contrast to solid dies constructed from a solid cylinder of tool steel, flexible dies are comprised of a thin sheet of metal with integral cutting blades. Flexible dies are wrapped around a solid base cylinder embedded with magnets that hold the die in place during production. Magnetic cylinders are expensive and require initial investment in various sized rolls to facilitate the use of flexible dies. Once the initial investment is made, the advantages of flexible dies can be realized. Flexible dies are relatively inexpensive to produce and can also be made very quickly, often within one or two business days. Low cost means they are readily replaced when they become dull or damaged, and ease of handling allows them to be changed out on press with little down time. Because flexible dies are also relatively light and thin, they are much less expensive to ship, handle and store.
Flexible dies are most often used to cut simpler, less abrasive materials, and for shorter runs. For suitable applications, such as standard-sized, pressure sensitive adhesive (PSA) label cutting, the low cost of replacing flexible dies may outweigh the significant upfront investment in magnetic cylinders over the long term.
Rotary press technology advancements including servo motors and sophisticated sensors allow for use of smaller flexible dies on larger magnetic cylinders, reducing the need for a large magnetic cylinder inventory to be maintained. Advances in flexible die manufacturing technology, including special coatings, surface treatments, and production methods have increased the durability of flexible dies, but have still not approached the life expectancy of solid dies.
Advantages of flexible dies
- Low manufacturing cost
- Short lead time
- Thin, lightweight
- Easy to ship, handle and store
- Quick changeover on press
Disadvantages of flexible dies
- Initial magnetic cylinder investment cost
- Limited to thin, easy to cut materials
- No ability to resharpen – must be replaced
- Lower durability – more frequent replacement
Steel Type Selection
We’ve discussed the two main rotary die types, solid and flexible, but what are the key die design characteristics that affect performance in rotary die cutting? For solid tooling, steel type selection has a significant impact on die performance, especially for abrasive or difficult to cut substrates. Tool steel material grades vary greatly in terms of chemical composition, and thus material properties. For tool steel to be effective as a cutting die, the die must undergo heat treatment after being engraved.
Heat treatment of steel is prescribed according to a specific profile based on the steel type. The heat treatment profile defines the temperature to which the die is heated (austenitizing temperature), the length of time the material is held at the prescribed temperature (soak time), and the speed and method by which the material is cooled (quenched) to impart specific desirable properties to the material such as hardness. The heat/quench cycle is followed by a series of heating and slow-cooling cycles, known as tempering. After tempering, the die steel has ideally achieved the appropriate balance between hardness and toughness, which is key to optimum die cutting performance.
Tool steel materials vary in composition based on presence and concentration of various alloying elements such as Carbon (C), Manganese (Mn), Molybdenum (Mo), Silicon (Si), Cobalt (Co), Vanadium (V), and Chromium (Cr), among others. A wide range of materials are used in the construction of rotary cutting dies based on their specific properties and their ability to adequately cut a given substrate. Material cost must be weighed against die cutting performance to avoid extra cost for minimal incremental benefit. In general, premium tool steel grades are often specified for cutting applications requiring longer die life, or when difficult or abrasive materials are being cut.
Cutting Blade Geometry
Arguably the most important part of a rotary die is the cutting blade, the feature most directly responsible for the quality of the finished die cut. As new substrate materials are continually developed across all converting industry segments, cutting blade geometries are continually developed and adjusted to provide optimum cut quality. Blade geometry is often treated as a trade secret among die makers, as are the various processes used to produce those geometries.
Blade geometry includes characteristics such as blade height measured from the die surface, blade angle relative to the die surface, and width of the flat area at the peak of the blade, known as land width. Generally, a blade geometry that cuts flawlessly through one material may fail miserably when cutting another, which makes it important that the die manufacturer has experience and knowledge of how blade geometry affects cut quality over a wide range of applications.
Various treatments or coatings may be applied to provide additional benefits and performance characteristics for rotary dies. Treatments may include various coatings designed to improve resistance to corrosion, non-stick coatings to prevent waste material or die cut parts from sticking in the die cavities, or special coatings designed to increase the life of cutting blades.
Cryogenic treatment, which can be applied as part of the overall heat treatment process, is designed to improve longevity and increase run times for specific tool material types. The best confirmation of improved performance is to conduct a trial between a treated and non-treated tool, if the converter has the ability to adequately control and measure each critical factor in their die cutting process.