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FAQ: Steel Rule Die CuttingThere are many methods of stamping parts out of materials, but none are as generally cost effective, versatile, and precise as steel die cutting. With few exceptions, whenever steel rule die cutting is suitable for an application, it is also the best available method, usually by a large margin of cost and quality considerations. Simply put, steel rule dies work like a cookie cutter. They are made using razor-sharp steel rule that is cut, bent, and mounted in various substrates forming a stamping tool according to the desired shape. Substrates -- often called die boards -- include plywood, acrylic plastic, aluminum, and steel sheets that are generally cut using computer aided design (CAD) and precision laser cutting machines. Other components of steel rule dies typically include machined punches for internal holes and ejection material (rubber or springs) mounted around cutting edges to clear cut material from the die area. When put together, the result is a precision tolerance, knife-edged, ready-to-run tool suitable for efficient production of a wide variety of part configurations and materials. Typically, steel rule dies are made to +/- .010 dimensional tolerances, which can generally be maintained on many more stable materials (like thin gauge plastic films and various fibre gasket) and packing materials. Greater precision can be obtained by using steel substrates rather than standard hard plywood die boards and other means. Hole sizes and relative locations can often be easily held to higher precision tolerances without any additional costs, depending on configuration and materials. Tolerance affecting variations can enter in when cutting less stable materials (like rubber, sponge, foam, and cork) because of material reaction to pressures applied to materials during the impression and cutting cycle. Tooling wear and tear plus inherent post-fabrication instability (like expansion and shrinkage) on some materials can also affect do-able dimensional tolerances. Of course, in most cases, the very same factors that increase dimensional variations tend to also decrease actual tolerance requirements since such parts will easily conform to mating components. n general, die cutting is an inherently unidirectional
process, making it a suitable means to produce two-dimensional shapes.
While this process usually involves stamping parts out of flat rolled or
sheeted materials, it can be combined with other methods for three-dimensional
configurations in some cases. Steel
rule dies can be used to stamp parts out of most non-brittle, non-metallic flat
materials, plus softer and thinner gauge foils, screen or wire cloth.
Steel rule dies can withstand hundreds of tons of pressure, so if the
material can be cut cleanly (rather than shatter or break) by impression
stamping and is softer than the hardened steel cutting edges of steel rule dies,
it can generally be cut using this method. As mentioned above, steel rule dies are not suitable for cutting very hard, brittle, or shatter prone materials (like steel, glass, and some epoxyglass laminates). Material thickness can also affect the prospects of steel rule die cutting, though there are often economical ways to overcome this limitation on materials otherwise suitable to steel rule die cutting. Some part configurations and tolerance requirements can also effectively eliminate steel rule die cutting. The simple answer is a LOT LESS than hardened steel male-female dies. While a specific answer can only be given based on specific part configurations, Metro Industries' average charges for steel rule dies during 2001 was around $120. For comparative purposes, a conservative estimate for the same array of steel tooling would easily average over $5,000. The costs of steel rule dies include overall part size, length of cut edges, number and size of punches, and complexity of bends and joints in the part configuration. Other factors too numerous to list may enter in also, but even in extreme cases, steel rule tooling costs over $1,000 are as rare as hardened steel tooling under $1,000. Of course, the number of die cavities is also a major factor in die costs, but this is generally an option dependent on straight forward cost benefit calculations.
One of the best features of steel rule dies is the low cost of maintaining them. Cutting rule and punches can be easily replaced when necessary, or an often more cost effective option is simply to replace worn tooling. In either case, both cost and lead time are usually a fraction of typical hard tooling maintenance.
Because steel rule dies work like a cookie cutter, that is, stamping parts out of material rather than shearing them as in male-female hard tooling, there are some occasionally unavoidable surface and cut edge effects that may limit their viability in some applications. The most common of these are concave/convex edges or "hourglass effect." This is caused by the compression and extrusion of material as the die impression is stamped, especially on rubber and closed-cell sponge material. Although this all happens in a split second, the material "flows" under the pressure of the intruding force of the die before and during the cutting cycle. The die cuts straight through, but the momentary shift and subsequent recovery of the material once the part is ejected from the die leaves a curved rather than straight edge on a cross-sectional view. Because this is directly relative to the squeezing, stretching, and extruding of material under pressure, the "hourglass affect" increases with the softness and thickness of elastomer materials. It can be diminished through various die features, but generally not eliminated. For similar reasons, some softer plastic materials will react to the pressures of die cutting, often leaving slightly beveled cut edges, but this effect can usually be overcome through die design features. |
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